1.2 typology and structure of hydrocarbons

9VOLUME V / INSTRUMENTS

1.2.1 Aliphatic hydrocarbons

The first important class of hydrocarbons is aliphatichydrocarbons, from the Greek ¨leifar, ‘oil, fat’, whichcomprises the alkanes, alkenes and alkynes.

Alkanes

StructureAlkanes are hydrocarbons with sp3 hybridized carbon

atoms and the general formula CnH2n�2. They are describedas ‘saturated’ because in their molecules, the four possiblebonds of carbon – arranged in space according to a regulartetrahedral structure – are simple and saturated with hydrogenatoms or other carbon atoms. The bond angles are identical toone another and measure 109.5°. The alkane series isdescribed as ‘homologous’ because the molecules differ fromone another by a constant amount: to move from one alkaneto the subsequent one a CH2 unit is always added.

Generally speaking, hydrocarbon molecules are describedby various equivalent descriptive systems: all the atomsbelonging to the molecule may be reported explicitly, or onlythe carbon atoms, implying that all the free valences of theseatoms are saturated with hydrogen atoms. Alternatively, onlythe skeleton of the intramolecular bonds may be shown:

IUPAC (International Union of Pure and AppliedChemistry) nomenclature entails the use of a commonsuffix to refer to any given class of compounds; for thealkanes this suffix is -ane. The first four alkanes arenamed methane, ethane, propane and butane; whendealing with compounds containing 1, 2, 3 or 4 carbonatoms, the prefixes meth-, eth-, prop- and but- arealways used. Starting from alkanes with 5 carbonatoms, prefixes are used which simply indicate thenumber of carbon atoms present in the molecule:pentane, hexane, heptane and so forth. Thenomenclature for alkanes with a large number ofcarbon atoms is reported in Table 1.

Molecules with an identical molecular formula butdifferent structure are known as structural isomers whichhave different chemical-physical properties and chemicalreactivity.

For non-straight chain alkanes, IUPAC nomenclature setsa series of rules for their identification:• Identify the longest straight chain containing only

carbon atoms in the molecule and all the alkyl residuesbound to it.

• Number each carbon atom in this chain progressively sothat the substituents are given the lowest numbers. If asubstituent recurs several times in the structure, theprefixes di-, tri-, tetra-, penta- and so forth are used.

• Prefix the alkyl residues with the number of the carbonof the longest chain to which they are attached.

• If two chains of identical length can be identified, theone with the most substituents is used.

C4H10

CH3

CH3H3C

CH

)

C

C

C C

1.2

Typology and structureof hydrocarbons

Table 1. IUPAC nomenclature for alkanes as a function of the number of carbon atoms

Carbon atoms Name Carbon atoms Name Carbon atoms Name

10 decane 22 docosane 60 hexacontane

11 undecane 23 tricosane 70 heptacontane

12 dodecane 30 triacontane 80 octacontane

20 icosane 40 tetracontane 90 nonacontane

21 henicosane 50 pentacontane 100 hectane

Often, however, IUPAC nomenclature is not used,and many compounds are named in accordance withthe rules in force before its introduction. In the case ofthe molecules shown above, for example, the mostcommon names are isobutane for 2-methylpropane,neopentane for 2,2-dimethylpropane and isooctane for2,2,4-trimethylpentane; the latter is the chemicalcompound used to determine the octane number offuels (specifically, isooctane is conventionally given anoctane number of 100).

The residues formed by alkyl groups with a freevalence deriving from alkanes lacking a hydrogen atomare described with the suffix -yl. We therefore havemethyl, ethyl, propyl and butyl residues, etc. The firstfour residues are also indicated by the symbols Me, Et,Pr and Bu. The carbon atoms in alkane molecules canbe classified according to the number of hydrogenatoms to which they are attached: those attached tothree hydrogen atoms are known as primary, thoseattached to two hydrogen atoms as secondary; finally,tertiary carbon atoms are attached to one hydrogenatom. Alkyl residues, therefore, can be classifiedaccording to the type of carbon on which the freevalence is found. In the case of C4H9� residues, adistinction can be made between: butyl, sec-butyl andtert-butyl, depending on whether the residue isprimary, secondary or tertiary:

A further distinction between alkane moleculescan be made if one or more chiral carbon atoms arepresent inside the alkane; a carbon atom is chiral if thefour substituents are all different. A chiral compoundhas the special property of having anon-superimposable mirror image. It therefore has twomirror structures – known as enantiomers – which,like right and left hands cannot be superimposed andmust therefore be considered different. Enantiomericcompounds have identical physical properties, exceptfor their opposite specific rotatory power, in otherwords the ability to rotate plane-polarized lightimpacting upon them to the right or to the left. Thanksto this particular property, a distinction can be madebetween two enantiomers which otherwise, accordingto IUPAC standards, would be identified by the samename. The symbol (R) (from the Latin rectus) is usedbefore the name of the alkane if the enantiomer rotates

polarized light to the right; the symbol (S) (from theLatin sinister) if it rotates light to the left. Theexistence of these isomers is extremely important forbiological compounds like aminoacids, but it is purelyacademic for alkanes (except in some specificconditions).

Methane The simplest alkane is methane, discovered in 1776 by

Alessandro Volta who, during a boat trip on Lago Maggiorenear Angera, noticed gas bubbles rising from the muddybottom of the lake. Volta later collected this gas, noting itsinflammable nature and naming it inflammable native swampgas (in this case, the methane was produced by anaerobicorganisms, known as methanogens, on the lake floor). With thegeneral formula CH4, it has angles between the H�C�Hbonds which are all identical and measure 109.5°; the C�Hdistances measure 1.091 Å, whilst the energy of each bond is104 kcal/mol. This is an apolar molecule as it is perfectlysymmetrical, and therefore has a dipole moment of zero.

Ethane The higher homologue of methane is ethane, with a

general formula of C2H6; the ethane molecule has a covalentC�C bond of s type formed by the overlap of sp3 orbitalsmeasuring 1.536 Å, and H�C�H bond angles of 109.3°.

The s bond allows for the relative rotation of the methylgroups without affecting the combination of sp3 orbitalsleading to their formation. This allows the molecule to takeon different arrangements, known as conformations, whichmay change into one another without cleaving any bond orexceeding a significant potential energetic barrier. The studyof energy changes in molecules as a result of a change inconformation is known as conformational analysis. Sincelittle energy is required to change the conformation, therelative rotation of the methyl groups is considered to befree. Fig. 1 shows the transition to different conformations ofan ethane molecule. It can be seen that ethane always returnsto the same condition after a rotation of 120° around theC�C bond. The conformation represented by the threehydrogen atoms superimposed on one another is known aseclipsed, whilst a staggered conformation is obtained by

H3C

CH3

CH3

C

tert-butyl

CH3 CH2 CH2 CH2 CH3

CH3

CH2 CH

butyl sec-butyl

NATURE AND CHARACTERISTICS OF HYDROCARBONS

10 ENCYCLOPAEDIA OF HYDROCARBONS

1 3 1 32 4

2

1 3 5

2

2-methylpropane 2,2-dimethylpropane 2,2,4-trimethylpentane

pote

ntia

l ene

rgy

rotation (°)0 60

3 kcal/mol

120

Fig. 1. Potential energy of the ethane molecule in its different conformations.

rotating the methyl groups 60° around the bond; betweenthese two structures there is an infinite number of otherconformations, generically described as skew. Fig. 1 showsthat the energetic barrier to rotation (known as torsionalstrain) for the ethane molecule is about 3 kcal/mol; this isdue to the repulsion of the electron clouds around thehydrogen atoms which, in the eclipsed conformation, areaffected to a greater extent by their mutual interaction. Thestaggered and eclipsed structures are known as conformers.

Propane and butane. Conformational analysisThe members of the alkane series above ethane are propane

(C3H8) and butane (C4H10). In propane, a free rotation aroundthe two C�C bonds can also be observed with a torsionalstrain slightly above the 3 kcal/mol of ethane due to thepresence of a methyl group instead of a hydrogen leading to agreater repulsion between the two mutually rotating groups. Ofparticular interest is the butane molecule

which, as well as being the first member of the alkane series tohave four conformers, also presents two different structuralisomers.

The conformational analysis of butane shows thatrotation around the C2�C3 bond leads to the formation ofthe conformers anti I, eclipsed II, gauche III, eclipsed IV,gauche V, eclipsed VI, returning to the conformer anti I whenthe bond has rotated through 360°. The eclipsed II andeclipsed VI structures have identical energy, as do the twogauche conformers. In butane, the maximum potentialenergy difference between the anti I structure, the moststable, and the eclipsed IV structure which is the mostunstable, is over 5 kcal/mol. Compared to the energeticbarrier of the various conformations of ethane, higher valuesare found for butane. This is due to the fact that in butane theelectronic repulsion is greater, since it occurs between ahydrogen atom and a methyl group in the eclipsed II and VIstructures and between two methyl groups in the eclipsed IV

conformation. Fig. 2 shows the variation in potential energybetween the different conformations of the butane molecule.

In order of molecular weight, butane is the first alkane topossess two different structural isomers: butane (also knownas normal butane, n-C4H10) and 2-methyl propane (alsoknown as isobutane, i-C4H10).

As a consequence of the different spatial arrangement ofthe atoms, the two molecules differ in the type of carbonatoms of which they are composed. In fact, butane has twoprimary carbon atoms and two secondary carbon atoms,whereas 2-methylpropane has three primary carbon atomsand a tertiary carbon atom.

As the molecular weight of alkanes increases, thenumber of isomers rises exponentially from 2 for butane to75 for C10H22, over 300,000 for C20H42 and above fourbillion for C30H62.

Higher alkanesAlkanes with a molecular weight from 70 to 240 u are

liquid under standard conditions (298 K and 1 atm);however, if their molecular weight reaches or exceeds 240 u,they are solid and described as waxes. It is also worth notingthe existence of some alkanes with a high molecular weight(over 1 million daltons) produced by polymerizationreactions. Polyethylene, for example, is an alkane consistingof an extremely long chain of �CH2� groups which, withthe exception of the two ends of the chain, is composed ofsecondary carbon atoms. Polypropylene, by contrast, is analkane which may have the following structures:

Each carbon atom in the main chain is linked to fourdifferent substituents and is therefore chiral; a distinctioncan thus be made between the different forms ofpolypropylene by the arrangement of the substituents as wellas by molecular weight. Specifically, if the methyl groupsare all oriented towards the same side of the chain, thepolymer is described as isotactic; however, if they areoriented in alternating directions or randomly, the polymer isdescribed as syndiotactic or atactic respectively. In this case,the alkane’s stereospecificity (the way in which its spatialmolecular structure conditions its properties) is extremelyimportant because the different polymers have differentchemical-physical and mechanical properties.

CycloalkanesAs well as alkanes with straight-chain and branched

structures, there are also alkanes which have a cyclic orpolycyclic structure. The nomenclature for cycloalkanesfollows that for alkanes, with the addition of the prefix

]]

]]

]]

isotactic polypropylene

syndiotactic polypropylene

atactic polypropylene

1

2

3

4

TYPOLOGY AND STRUCTURE OF HYDROCARBONS


pote

ntia

l ene

rgy

rotation (°)

anti I

gauche III

eclipsed II eclipsed IV eclipsed VI

gauche V

anti I

3.4kcal/mol

4.5kcal/mol

0.8kcal/mol

Fig. 2. Potential energy (not to scale) of the conformers of butane depending on the angle of rotation.

cyclo-. Below, the structures of cycloalkanes containing upto nine carbon atoms are reported:

As for alkanes, the carbon atoms in cycloalkanes arenumbered so that the lowest numbers are given to thesubstituents. By following this rule, the molecule shownbelow has the name 1,3-dimethylcyclopentane and not1,4-dimethylcyclopentane:

The simplest cycloalkane is cyclopropane. This isan extremely reactive compound since it has bondangles which are highly deviated from the equilibriumvalue for alkanes of 109.5°. In fact, in cyclopropane thethree carbon atoms are planar and form bond angles of60°. This variation in the bond angles entails areduction in the overlap between the sp3 orbitalsleading to the formation of the C�C bond, andtherefore a decrease in bond energy. The energyexpended to deviate the geometry of the bond angles incyclopropane, and more generally in all cycloalkanes,is known as angle strain. There is also a ring strain,known as torsional strain, resulting from the fact that,as has already been seen for ethane, the most stablemolecular conformation is that in which the hydrogenatoms are found in a staggered position with respect toone another, whilst the structure of cyclopropane forcesthe hydrogen atoms into an eclipsed conformation. Incyclopropane, the contribution of angle strain isnevertheless far greater than that of torsional strain,although generally speaking the energy associated withtorsional strain is significant. The carbon atoms in thecyclobutane molecule are not all on the same plane andhave C�C�C bond angles of 90°; to minimize thetorsional strain, one carbon atom is located slightly outof the plane whilst the bond angles have a value ofabout 88°.

The stability of cycloalkanes continues to increasefrom cyclopentane to cyclohexane, which has zeroangle strain (in this molecule, all the carbon atoms lieat the corners of a tetrahedron with bond angles of109.5°). However, the torsional strain in cyclohexane isa function of the conformation adopted by themolecule: in the chair conformation the torsional strainis zero since all the hydrogen atoms are staggered,whereas the value of the strain increases in the twist,boat and half-chair conformations. The maximum

energy difference between these four structures isabout 11 kcal/mol. If, on the one hand, this means thatat ambient temperature cyclohexane molecules pass atvery high frequency from one structure to anotherwithout impediments of energetic type, on the other itcan be said that the probability of finding the moleculein the chair conformation is over 90%. Fig. 3 shows thedifferent conformations of cyclohexane and thecorresponding energy levels.

In the chair conformation of cyclohexane, two differenttypes of hydrogen atoms can be identified:• Axial (shown in the structure below), where the covalent

bond with carbon is oriented perpendicular to the meanplane formed by the carbon atoms.

• Equatorial, where the hydrogen atoms are orientedroughly parallel to this plane.

Cycloalkanes in which one or more hydrogenatoms are substituted by alkyl groups are known asalkyl-cycloalkanes. The presence of the ring preventsthe rotation of the s bonds, making it possible todistinguish between two compounds which differ inthe position of the substituents with respect to thering, and which are known as cis diastereoisomers ortrans diastereoisomers (in Latin cis means «on thesame side» and trans «on the other side»). Unlikeenantiomers, cis and trans diastereoisomers havedifferent chemical-physical properties. For example,decalin – an alkane with the general formula C10H18,reported in Fig. 4 and consisting of two condensedcyclohexane rings which share two consecutivecarbon atoms – exists both as a cis isomer and a transisomer. However, cis-decalin, also known ascis-decahydronaphthalene, has a melting point of 242K and a boiling point of 460 K, whereas trans-decalinhas a melting point of 230 K and boiling point of 466 K.

Whereas alkanes such as decalin or decahydroazuleneare described as bicyclic, the fusion of several rings leadsto the formation of compounds which are genericallydescribed as polycyclic; these include

4

1

3

5 2

cycloheptane cyclooctane cyclononane

cyclopropane cyclobutane cyclopentane cyclohexane



pote

ntia

l ene

rgy

conformations

11 kcal/mol

7 kcal/mol5.5 kcal/mol

chair boat half-chairtwist

Fig. 3. Energy levels of the conformations of cyclohexane.

perhydrophenanthrene which consists of three condensedrings containing six carbon atoms:

The nomenclature for polycyclic alkanes obeys thefollowing rules: • The carbon atoms are numbered and the compound is

given the name of the straight-chain alkane with thesame number of carbon atoms.

• A prefix is added which indicates the number of ringswhich must be broken in order to obtain a hydrocarbonwithout cyclic chains (bicyclic, tricyclic, tetracyclic, etc.).

• The number of carbon atoms in between the sharedatoms is specified in square brackets.Using these rules, decalin can be given the systematic

name bicyclo[4.4.0]decane, since the shared atoms are C1and C6 and between these are two groups of four atomsand one of zero atoms, since C1 and C6 are consecutive.Below, the structures of some bicyclic compoundsconsisting of rings sharing two non-consecutive carbonatoms are shown:

If there are three rings, the nomenclature introducestwo numbers which indicate which of the shared atomsis the last shared carbon atom. In accordance with this

rule, the adamantane molecule is also known astricyclo[3.3.1.1(3,7)]decane:

Steroidal hydrocarbons are a particular category ofcycloalkanes. These saturated hydrocarbons contain thering of perhydrocyclopentanophenanthrene and areknown as steroids; most of these, in addition to thehydrocarbon chain, contain other oxygenatedsubstituents (e.g. cholesterol and testosterone) but thereare some which consist exclusively of carbon andhydrogen such as androstane and cholestane.

Alkenes

Structure Alkenes are compounds which have at least one double

C�C bond in their molecules. These compounds aregenerically known as olefins or unsaturated hydrocarbons(the latter name derives from the fact that the double bondcan be interpreted as the outcome of a dehydrogenationreaction of the corresponding alkane). The carbon atomsparticipating in the double bond are sp2 hybridized and, inaddition to the carbon atom with which they share the doublebond, are bonded to two other carbon or hydrogen atoms.The three bonds of the unsaturated carbon atom are planarand form bond angles of 120°.

Alkenes have the general formula CnH2n and areidentified by the suffix -ene. The first three members of theseries – in accordance with IUPAC rules – are ethene,propene and butene, but they are more commonly known asethylene, propylene and butylene. There are no exceptionsfor the other alkenes, which bear the double bond on the firsttwo carbon atoms of the chain, starting from pentene. Thenomenclature used for alkenes in which the double bond isfound inside the chain follows the rule stating that the nameof the molecule is preceded by the position (as low aspossible) of the carbon bearing the double bond. Inaccordance with this rule, the alkenes shown below are

12

H

H

HH

345

6

78

9

10

1

12 2

3

3

4 455

66

8

7

7

8

bicyclo[3.2.1]octane bicyclo[2.2.2]octane

1 12

2

334

4

5

5

6

6

7

bicyclo[1.1.2]hexane bicyclo[2.2.1]heptane

decaline decahydroazulene perhydrophenanthrene



1

6

2

5

3

4

10

7

9

8

cis

trans

Fig. 4. Structure of the cis andtrans isomers of decalin.

described by the IUPAC name of 1-pentene (or simplypentene) and 2-pentene. If substituents are present, the samerules used for alkanes apply. Finally, if two or more doublebonds are present, the position of the carbons bearing thedouble bond is indicated followed by the name of the alkenewith the addition of di-, tri- etc before the suffix -ene. Byfollowing this rule, the alkene shown in the diagram below isknown by the IUPAC name of 1,3-butadiene:

Under normal conditions, the presence of a double bondprevents the molecule from rotating around this bond. Thispeculiarity makes it possible to differentiate between twounsaturated molecules which, even though they contain thesame groups of atoms, differ in their arrangement around thedouble bond. However, for this distinction to be possible,neither of the two sp2 hybridized carbons must have twoidentical substituents; in this case, a distinction can be madebetween two different molecules bearing the two groups onthe same side (cis) or on opposite sides (trans); this is thecase for hexene:

The melting points of these two molecules are 136.6 Kand 159.6 K respectively, whilst the boiling points are 339.8K and 340.2 K. Given these properties, the two isomers canbe separated by fractional crystallization and not bydistillation, since the two boiling points are almost identical.However, this result cannot be generalized, since the twodiastereoisomers of 4,4-dimethyl-2-pentene, for example,which have boiling points of 353 K and 349 K, can beseparated by fractional distillation.

When the four substituents of the double bond aredifferent, the cis and trans conformations cannot bedistinguished. To render the nomenclature used for thediastereoisomers of alkenes universal, theCahn-Ingold-Prelog rule is therefore used; this assignsincreasing priority to the substituents linked to the doublebond (for saturated residues, priority is assigned on thebasis of the molecular weight of the residue, with thehighest priority being given to the residue with the highestmolecular weight). If the two sp2 carbon atoms bear thehighest priority substituents on the same side, the alkene isidentified with the symbol (Z) (from the Germanzusammen, «together»); otherwise it is given the symbol (E)(from the German entgegen, «opposing»).

Ethylene The first alkene is ethene or ethylene; under standard

conditions it is a colourless and odourless gas with thegeneral formula C2H4 which melts at about 104 K and boilsat about 169 K. Since it has two sp2 hybridized carbonatoms, its molecule has a planar structure whose bond anglesare slightly deviated compared to the 120° associated withthis type of hybridization. This is due to the greater bulk ofthe electron clouds involved in the formation of the doublebond, which squeeze the two hydrogen atoms linked to the carbon atom; as a result, the H�C�H bond anglesmeasure about 118°, the two C�C�H bond angles about

120° and the length of the C�C bond is 1.34 Å, less thanthe 1.54 Å observed for the ethane molecule.

Unlike alkanes, the residue obtained from an ethylenemolecule which is deprived of a hydrogen atom does notfollow the rules of IUPAC nomenclature and is known asvinyl:

Propylene Propylene is the higher homologue of ethylene, has a

general formula of C3H6 and, like ethylene, is a gas understandard conditions which, however, melts at 88 K and boilsat about 225 K.

The residue obtained from propylene deprived of ahydrogen atom, known as allyl, is of particular interest and isvery important in hydrocarbon chemistry:

The structure of this molecule will be discussed below(see Section 1.2.3).

DienesHydrocarbons with two double bonds are generically

known as dienes; they can be subdivided into allenes,conjugated dienes and isolated dienes.

Allenes bear the double bonds on the same carbon atomand are also known as cumulated dienes. The simplest ofthese compounds is 1,2-propadiene, a gaseous molecule witha melting point of about 240 K and a boiling point of about137 K (Fig. 5). In this molecule, the distance between thecarbon atoms is shorter than that of the simple double bondsof alkenes and measures 1.31 Å. When two cumulateddouble bonds are present, the p orbitals of the central carbonwhich form the two p bonds are perpendicular to oneanother. This peculiarity means that the molecule is notplanar and that the four substituents of the two carbon atomslie on two perpendicular planes. As a direct consequence ofthis, the mirror molecules of an allene which have differentsubstituents on the allenic carbon atoms are notsuperimposable and are therefore optically active. There arethus two different enantiomers even though there is no chiralcentre.

Conjugated dienes are thus called because they havealternating double and single bonds. The simplest of these is1,3-butadiene, a gaseous compound under standardconditions with a melting point of 164.3 K and a boilingpoint of 268.8 K. The distances between the atoms inpositions 1-2 and 3-4 are 1.34 Å, in line with the 1.337 Å ofthe simple double bond of ethene; the distance 2-3 is farshorter than the generic length of a single bond and

H2C CH CH2

H2C CH

cis-3-hexene trans-3-hexene

1-pentene 2-pentene 1,3-butadiene



(�)

(�)

(�)

(�)

Fig. 5. Structure of 1,2-propadiene and the perpendicular molecular orbitals which generate the two p bonds following the interaction of the p orbitals of the carbon atoms.

measures 1.47 Å; this is because the two carbon atomsforming the bond are both sp2 hybridized.

Conjugated dienes have a sequence of carbon atoms withp orbitals perpendicular to the plane of the molecule, eachoccupied by an electron. The presence of these unpairedelectrons in partially overlapping orbitals gives the resultingstructure a high degree of stability. It is as if each electron ina p orbital contributed to the formation of a bond with thetwo adjacent atoms. This peculiar delocalization, known ashyperconjugation, can be easily identified by observing thetotal electron density of a conjugated diene which, asreported in Fig. 6 for 1,3-butadiene, is uniform over atomswhich present conjugation. Due to the delocalization of the pelectrons occupying the pz orbitals along the skeleton of themolecule, the actual electronic structure of the 1,3-butadienemolecule forms a hybrid of two limit structures. Every timethe molecules can be represented in different ways simply bychanging the electron occupation of molecular orbitals withcomparable energy, we are dealing with resonancestructures, which represent limit arrangements of theelectrons in the molecule. The actual arrangement of theelectrons is a hybrid of all the resonance structures whichcontribute to the real description of the molecule. Theconcept of resonance, introduced to describe conjugatedsystems, will be dealt with in greater detail below (seeSection 1.2.2).

The stability which hyperconjugation confers onconjugated dienes can be measured indirectly through theenergetic analysis of hydrogenation reactions. Table 2 showsthe heats of hydrogenation for various unsaturatedhydrocarbons. It is interesting to compare the hydrogenation

energy of 1,3-pentadiene, which releases 226 kJ/mol, withthat of 1,4-pentadiene, which releases 252 kJ/mol. Thedifference in the energy required to hydrogenate moleculeswith the same number of atoms and double bonds can onlybe due to the conjugation of the double bonds in1,3-pentadiene, which therefore stabilizes this molecule by26 kJ/mol. However, by analysing the heat of hydrogenationof 1,3-hexadiene and 1,3,5-hexatriene compared to that of 1-hexene, it can be observed that the presence of two and threedouble conjugated bonds leads to a gain in energy of 24 and38 kJ/mol. This makes it possible to extend the concept ofthe stability of the delocalization of p electrons not only todienes but to all compounds which possess n double bondsalternating with single bonds.

A conjugated diene of considerable importance in thechemistry of unsaturated hydrocarbons is isoprene (or 2-methyl-1,3-butadiene according to IUPAC). Under standardconditions, it is a liquid which solidifies at 131 K and boilsat 307 K. The isoprene molecule, whose structure forms thebasis for a long series of unsaturated hydrocarbons, presentshyperconjugation of the p electrons in a similar way tobutadiene. The individual isoprene unit, condensed intostraight-chain, branched and cyclic structures, is found in alarge number of natural organic compounds known asterpenes, classified according to the number of isopreneunits of which they are composed: mono-terpenes(2 isoprene units), sesqui-terpenes (3 units), di-terpenes(4 units), tri-terpenes (6 units), tetra-terpenes (8 units),poly-terpenes (9 or more units).

Finally, isolated dienes have two double bonds which intheir molecule occupy positions not adjacent to a carbon



A B C

Fig. 6. 1,3-butadiene molecule: A, highest occupied molecular orbital (HOMO); B, lowest unoccupied molecular orbital (LUMO); C, total electron density.

Table 2. Hydrogenation reactions of unsaturated hydrocarbons

Compound Reaction �H°r (kJ/mol) Ref.

1,3-butadiene �2H2�� 236.7�0.4 Kistiakowsky et al., 1936

1,3-pentadiene �2H2�� 226.4�0.6 Dolliver et al., 1937

1,4-pentadiene �2H2�� 252.0�0.6 Kistiakowsky et al., 1936

1-hexene �H2�� 125.0�3.0 Linstrom and Mallard, 2003

(Z)-1,3-hexadiene �2H2�� 226.0�1.0 Fang and Rogers, 1992

(Z)-1,3,5-hexatriene �3H2�� 336.0�1.4 Turner et al., 1973

atom, or alternating with a single bond of s type. Below, thestructure of 2,6-octadiene is shown:

Higher alkenesFor compounds with more than four carbon atoms, the

structural rules outlined above for compounds with a lowermolecular weight hold true, bearing in mind that in the caseof alkenes, as for alkanes, the number of possible isomersand diastereoisomers grows exponentially as the molecularweight increases. Generally speaking, alkenes with morethan 4 carbon atoms are liquid under standard conditions,whilst those with more than 15 carbon atoms are solid.Below, the alkenes formed by the addition of a high numberof dienylic molecules will be analysed briefly. The mostimportant natural compound is the addition polymer ofisoprene consisting entirely of cis stereoisomers, in otherwords natural rubber. It has a high molecular weight(sometimes over 106 u) characterized by the sequence of adouble bond followed by two single bonds. The homologueof natural rubber, differing from it due to the presence ofdouble bonds in the trans configuration, is guttapercha.Interestingly, the difference between the cis and transconfigurations alone in the structure of the polymerproduces two compounds with very different properties. Theunsaturated polymer compounds most similar to naturalrubber are those derived from the polyaddition of1,3-butadiene. In this case, too, a material structurallysimilar to polyisoprene is obtained, which takes the genericname of elastomer.

CycloalkenesCycloalkenes are cyclic molecules which contain one or

more double bonds. The nomenclature follows that forstraight-chain alkenes, with the addition of the prefix cyclo-.The structures of some cycloalkenes are shown below:

The smallest of these hydrocarbons only exist in thecis form, since a trans structure would create excessivering strain. The first cycloalkene which can be isolatedin the trans form is cyclooctene. It is interesting to notethat the trans-cyclooctene molecule is optically active,since its peculiar structure makes the mirror moleculesnon-superimposable. There are thus two differentenantiomers of trans-cyclooctene, although there is nochiral carbon in the molecule. This peculiarity hasalready been seen in compounds with double cumulatedbonds.

As for linear dienes, there are also cyclic systems whichhave double conjugated bonds. Examples of this class ofcompounds are cyclobutadiene, cyclopentadiene andcyclooctatriene:

Cyclic alkenes with only double conjugated bonds arealso known as annulenes (see below).

Generally speaking, cycloalkenes with a high molecularweight and conjugated double bonds are not stable,transforming over time into compounds with condensedrings, consisting of 4, 5 or 6 carbon atoms per ring.

Alkynes

StructureAlkynes are unsaturated hydrocarbons containing a triple

C�C bond, in which the carbon atoms are sp hybridized,linked to only one other atom, of carbon or hydrogen, andform bond angles of 180°.

Alkynes have the general formula CnH2n�2 and areindicated by the suffix -yne. The first member of the alkyneseries, although known under the IUPAC nomenclature asethyne, is normally referred to as acetylene; this is by far themost important alkyne. At ambient temperature it is a gaswhich liquefies at 189 K; additionally, being an unstablecompound, it explodes easily, producing carbon andhydrogen. It is generally used to assign a name to the higheralkynes, considered derivatives of acetylene; following theserules, for example, propyne is also known asmethyl-acetylene.

For substances in which the triple bond is found insidethe molecule, for branched or cyclic molecules, the rules ofnomenclature described for alkenes hold true. Likeacetylene, propyne and 1-butyne are also gases understandard conditions (their boiling points are 250 K and 283K respectively), whilst (again under standard conditions)2-butyne is liquid, since its boiling point is 300 K. Startingfrom the alkynes containing four carbon atoms, positionaland chain isomers exist whereas, due to the triple bond, thereare no stereoisomers.

cyclobutadiene cyclopentadiene cyclooctatriene

cyclohexene cis-cyclooctene trans-cyclooctene

cyclopropene cyclobutene cyclopentene

]]

]]

]]

natural rubber

cis-1,4-polyisoprene

gutta-percha

trans-1,4-polyisoprene

elastomer

1,3-polybutadiene



1.2.2 Aromatic hydrocarbons

Aromatic hydrocarbons are the other large class of carbonand hydrogen compounds. The parent of this class isbenzene, discovered in 1825 by Michael Faradayimmediately after he became director of the chemicallaboratory at London’s Royal Institution. Faraday managedto isolate benzene from the distillation products of an oilobtained as a by-product of the manufacture of illuminantgas. The composition of the benzene molecule was found tobe six carbon atoms and six hydrogen atoms, but thearrangement of these atoms in the molecule was unclear. Theproblem remained unsolved until 1865, when FriedrichAugust Kekulé von Stradonitz realized that its structure hadto be cyclic with three double bonds. The German chemistthus described how he managed to identify the structure ofthe parent of aromatic compounds: “I was sitting intent onwriting my treatise, but the work was not progressing; mythoughts were elsewhere. I turned my chair towards the fireand fell asleep. Again, the atoms started to leap about beforemy eyes, but this time the smaller groups remained modestlyin the background. My mind’s eye, rendered more acute byrepeated visions of this type, was now able to distinguishlarger structures, of different sorts, arranged in long rowswhich in some places were fairly close to one another, alltwining and twisting like a pile of moving snakes. Thensuddenly one of the snakes, grasping its own tale, whirledironically before my eyes. As if by a flash of lightning Iawoke... I spent the rest of the night working out theconsequences of the hypothesis. Gentlemen, let us learn todream, and perhaps then we will see the truth”. For benzene,Kekulé proposed the existence of two equivalent structuresin equilibrium with one another:

Although it did not fully clarify the molecule’sproperties, this description of benzene’s structureremained in force until the mid-20th century, even afterLinus Pauling had introduced the concept of resonance in 1931-32. The structures proposed byKekulé, thus become two resonance limit structuresamong all those possible. As for conjugated systems,

even the actual structure of benzene is a hybrid of thepossible resonance limit-structures, with greaterstability than any one of these, in other words aresonance hybrid.

Each of the six carbon atoms which make up benzene issp2 hybridized; as a result, they lie on a plane, forming bondangles of 120°. Of the three sp2 orbitals, two are involved inthe formation of s bonds with the vicinal carbon atoms andone in the formation of a bond with the hydrogen atom. Eachcarbon atom has a p orbital perpendicular to the plane of thering, occupied by an electron, formed by two lobes placedabove and below the plane created by the benzene ring. Thefact that the p orbital of each carbon atom partially overlapswith the two p orbitals of the vicinal carbons and that thecarbon atoms are fused in a ring generates a continuumbetween the p orbitals, leading to the formation ofdoughnut-shaped molecular orbitals (Fig. 7) located bothabove and below the ring, inside which the six p electronsare delocalized.

The benzene molecule, therefore, is represented as ahexagon with six hydrogens linked to it, inside which acircle is drawn to represent the electron delocalizationof this structure:

As a consequence, the carbon atoms are bonded to oneanother by the formation of a s bond and half a p bond.

The delocalization of the six electrons leads to theformation of an unusually stable structure. Compared to anunsaturated compound with three double bonds, thebenzene molecule has a stability of 150 kJ/mol, a valuecalculated from the hydrogenation energy of an unsaturatedhydrocarbon (Table 3) as already seen for conjugateddienes. The heat of hydrogenation of a double bondgenerally has a value of about �120 kJ/mol; in line withthis value, the hydrogenation of cyclohexene releases 118 kJ/mol, that of 1,3-cyclohexadiene 224 kJ/mol and thatof 1,4-cyclohexadiene 233 kJ/mol (the difference between1,3-cyclohexadiene and 1,4-cyclohexadiene is due to thestability of the conjugation of the two double bonds in1,3-cyclohexadiene). These values would lead one topredict a value of about �358 kJ/mol for the



A B C

Fig. 7. Benzene molecule: A, highest occupied molecular orbital (HOMO); B, lowest unoccupied molecular orbital (LUMO); C, total electron density.

hydrogenation of benzene; however, experimentally, ithas been found that the heat released is only 205 kJ/mol.The difference between the two heats of reaction is dueto the stability conferred by the phenomenon ofaromaticity. Additionally, it is interesting to note thataromaticity also leads to an accentuation of stability incomparison to straight-chain conjugated systems.Referring to Table 2, in fact, it can be seen that thepresence of three conjugated double bonds in1,3,5-hexatriene gives the molecule a stability of40 kJ/mol, a modest value when compared to the150 kJ/mol found for benzene. This evidence allows usto conclude that the phenomenon of delocalization is anecessary but not sufficient condition to explain thearomaticity of benzene.

In addition to benzene, other molecules consistingof aliphatic groups linked to benzene rings, ofcondensed benzene rings and especially of compoundswith no similarities to the hexagonal ring of benzene,have the peculiarity of being aromatic. The ruleallowing for the identification of aromatic compoundswas proposed by the German chemist and physicistErich Hückel. Born in 1896 at Charlottenburg, in theBerlin suburbs, Hückel worked immediately after hisdoctorate alongside Peter Debye. Together, in 1923,they formulated the theory of electrolytic solutionsknown as the Debye-Hückel theory. During the 1930s,Hückel’s interests shifted towards quantum mechanicsand in 1931, at the Stuttgart Polytechnic, he formulatedhis famous rule for the identification of aromaticcompounds. According to this rule, in order for acompound to be aromatic, it must possess a specificnumber of p electrons, equal to 4n�2, in the twoclouds of delocalized electrons above and below theplane of its molecule. To understand the reason for thisrule, reference should be made to the simplest form ofthe LCAO (Linear Combination of Atomic Orbitals)theory, according to which molecular orbitals areformed by the linear combination of atomic orbitals.The combination of atomic orbitals gives rise tobonding and antibonding molecular orbitals. Thenumber of electrons allowing a molecule to bearomatic is that needed to completely occupy all themolecular bonding orbitals. This makes the cohesionbetween the atoms in the molecule as high as possible.

Using Hückel’s rule, it can be predicted that compoundswhich have 2, 6, 10, ... p electrons delocalized above andbelow the plane of the molecule will be aromatic (see alsoSection 1.2.3).

Benzene Under standard conditions, benzene is a liquid which

melts at 278.6 K and boils at 353.3 K. All its atoms arecoplanar with a C�C bond length of 1.39 Å, in between the1.47 Å of a single bond between two sp2 hybridized carbonatoms and the 1.33 Å of an isolated double bond in analkene. The length of the C�H bonds is 1.10 Å and thebond angles between three consecutive atoms in themolecules measure 120°.

ArenesArenes are compounds whose molecules contain both

aromatic and aliphatic groups. They can be divided intothree subclasses:• Alkylbenzenes, consisting of an aromatic group linked to

an aliphatic group.• Alkenylbenzenes, consisting of an aromatic part and a

group containing at least one double bond.• Alkynylbenzenes, consisting of an aromatic group linked

to a residue with a triple bond.Obviously, only some of the countless molecules

which can be formed by the combination of aromatic,aliphatic and unsaturated groups belong to thesesubclasses. The nomenclature for the simplestcompounds entails assigning names to the residueslinked to benzene, followed by the suffix -benzene.Some specific compounds, however, are generallyknown by their traditional names, as in the case ofmethylbenzene, known as toluene; isopropylbenzeneknown as cumene; vinylbenzene known as styrene:

If there is more than one substituent on a singlebenzene ring, the six aromatic carbon atoms arenumbered so as to give the most important substituentthe lowest number, following the rules ofnomenclature already described for aliphatichydrocarbons. Usually, once the most importantsubstituent has been identified, the positions of thevicinal carbons to which it is linked are described withthe term ortho- (indicated by the letter o-), thesubsequent carbons with meta- (indicated by the letterm-) and those immediately opposite with the term

toluene cumene styrene



Table 3. Hydrogenation reactions of unsaturated hydrocarbons


1-cyclohexene �H2�� 118.0�6.0 Linstrom and Mallard, 2003

1,3-cyclohexadiene �2H2�� 224.4�1.2 Turner et al., 1973

1,4-cyclohexadiene �2H2�� 233.0 Roth et al., 1991

benzene �3H2�� 205.3�0.6 Kistiakowsky et al., 1936

para- (indicated by the letter p-). As an example, thestructures of p-ethylethylbenzene andm-ethylvinylbenzene are shown below; thedimethylbenzenes, by contrast, are usually known bythe name xylenes (o-xylene, m-xylene and p-xylene):

Another way to assign names to arenes consists ofnumbering the hydrocarbon chains, as has alreadybeen shown for alkanes, alkenes and alkynes, and thenconsidering the benzene ring as a substituent of thishydrocarbon. The residue represented by the benzenemolecule with one fewer hydrogen atom and a freevalence takes the name phenyl; the residue formed bytoluene with a free valence instead of a hydrogen atom in the methyl position is known as benzyl. As an example, the structures of diphenyl,1,2-diphenylpropane and hexaphenylethane are shownbelow:

Among the arenes are compounds consisting of botharomatic and aliphatic rings such as, for example,cyclohexylbenzene and tetralin:

Condensed aromaticsThe parent of the condensed aromatic compounds

consists of two benzene rings which share a bond and takesthe name naphthalene; under standard conditions it is a solidwhich melts at 353 K and boils at 490 K. It consists of 10planar sp2 hybridized carbon atoms:

Its stability can again be studied with recourse to theheats of hydrogenation. The addition of two hydrogenmolecules leads to the formation of tetralin and theproduction of 125 kJ/mol. The further reduction of tetralin todecalin with the addition of three hydrogen moleculesreleases 318 kJ/mol. It can thus easily be seen that theaddition of each hydrogen molecule to the double bondreleases about 63 kJ/mol, a typical value for thehydrogenation of benzene (and thus aromatics in general).By contrast, the hydrogenation of 1,4-dihydronaphthaleneleads to the release of as much as 113.5 kJ/mol, a value moretypical of an alkene. The aromaticity of naphthalene resultsfrom the two electron clouds located above and below theplane of the molecule which, in accordance with Hückel’srule, are occupied by 10 delocalized p electrons which canbe considered as belonging to two distinct electron cloudsoccupied by six p electrons and thus characteristic ofaromatic systems which share a pair of p electrons. Thenomenclature for the derivatives of naphthalene is assignedby numbering all the carbon atoms in the compound (alsosubdivided into a or b atoms) and then indicating theposition of the relevant substituents. As an example, thestructures of 1,2-dimethylnaphthalene anda-phenylnaphthalene are shown below:

The members above naphthalene are anthracene andphenanthrene, whose molecules are characterized by thegeneral formula C14H10 and the presence of 14 delocalized pelectrons which give them aromaticity. In the nomenclature forthese compounds, the carbon atoms are numbered as follows:

a-phenylnaphthalene1,2-dimethylnaphthalene

1

4

8

5

2

3

7

6

naphthalene

hexaphenylethane

diphenyl 1,2-diphenylpropane

m-xylene p-xylene

p-ethyl-ethylbenzene

m-ethyl-vinylbenzene

o-xylene



cyclohexylbenzene tetralin

As the number of condensed rings increases, the number ofdifferent isomers also increases. There are five other isomers ofnaphthacene, formed by the condensation of four benzene rings,shown below: 1,2-benzanthracene, 3,4-benzophenanthrene,chrysene, 9,10-benzophenanthrene and pyrene.

All these compounds have the general formula C18H12and an H/C ratio of 0.67. The H/C ratio decreases inverselyto the size of polycondensate aromatic compounds. It passesfrom a value of 1 for benzene to 0.8 for naphthalene, 0.71for compounds with three aromatic rings, 0.67 for those withfour aromatic rings and 0.63 for those with five condensedrings. The H/C ratio continues to fall with subsequentcondensations, tending towards zero in graphite. As far ashydrocarbons in general are concerned, it can be observedthat the maximum H/C ratio is that of the methane molecule(equal to 4) and that in general this value ranges from 4 to 2in saturated aliphatic molecules, has a value of 2 inmolecules with a double bond and cycloalkanes, fallingbelow 2 for unsaturated and polyunsaturated compounds, to1 for benzene and below 1 for condensed aromatics.

AnnulenesIn addition to benzene and the condensed aromatic

compounds, there is a different class of cyclic aromatic

molecules, known as annulenes; to be considered aromatic,these must have delocalized electrons above and below theplane on which the atoms lie and satisfy Hückel’s rule.According to Hückel’s rule, two is the lowest number ofdelocalized electrons allowed for an aromatic compound, butthere are no neutral hydrocarbons with this number (as willbe seen in Section 1.2.3, the cyclopropenyl cation isaromatic). The subsequent compounds require six electrons,and are represented by benzene. On the other hand, there areno aromatic compounds with 10 delocalized p electrons. Infact, [10]annulene, although it satisfies Hückel’s rule, is notaromatic since the molecule is not planar; the interaction ofthe hydrogens at the centre of the cycle forces two doublebonds in the ring into the trans conformation, thuspreventing the molecule from adopting a planar geometry.However, in accordance with Hückel’s rule, we find that[14], [18] and [22]annulene are particularly stablecompounds thanks precisely to their aromaticity. Thediagram below shows the structures of some aromaticannulenes:

1.2.3 Cationic, anionic and radicalhydrocarbons

CarbocationsCarbocations are positively charged molecules which

have an electron gap on a carbon which has its threevalence electrons in sp2 hybridized orbitals and one emptyp orbital oriented perpendicularly to the plane describedby the three occupied orbitals. These are unstablecompounds which, except in some very specificcircumstances under which they can be studied directly,are usually chemical intermediates which cannot beisolated. The resulting geometry of the cation is, as foralkenes, trigonal planar.

In saturated systems the stability of carbocationsdecreases passing from cations localized on tertiary,secondary and primary carbon atoms until it reaches themethyl group CH3

�, which is the most unstable. This scale ofstability is due to the effect of the R electron-donatinggroups which tend to fill the cation’s electron gap, thusstabilizing it.

[14]annulene [18]annulene

[22]annulene

naphthacene 1,2-benzanthracene

3,4-benzophenanthrene chrysene

9,10-benzophenanthrene pyrene



1

14

42

23

3

88

5

5

9

9

10 10

7

7

6

6

anthracene phenanthrene

In unsaturated or polyunsaturated systems, there arecarbocation structures which are particularly stable, as is thecase for the allylic and benzylic carbocations. The allyliccarbocation

although characterized by the presence of a charge on aprimary carbon, has a stability between that of secondaryand tertiary carbocations. This is due to the presence of tworesonance hybrids. The positive charge is thus delocalizedonto the two carbon atoms, which are therefore equivalent.

The benzylic carbocation is a resonance hybrid of thefive structures shown below:

The delocalization of the positive charge onto fourcarbon atoms makes the benzylic carbocation more stablethan a tertiary radical, although in formal terms the charge isfound on a primary carbon.

Generally speaking, it can be stated that the stability ofcarbocations is due to two factors: the inductive effectand the conjugative effect. The former stabilizes acarbocation if the groups linked to the positive carbon areelectron-donating groups which stabilize the electron gap.The latter, on the other hand, stabilizes the carbocationsincreasingly the more the resonance effect delocalizes thecharge. For hydrocarbons specifically, the conjugative effectalways predominates over the inductive effect. In accordance

with this rule, the most stable carbocations are those whichhave a large number of conjugated double bonds which makeit possible to delocalize the positive charge as much aspossible.

Finally, it is worth describing some specific instances inwhich the presence of a positive charge makes the moleculesextremely stable, as in the case of aromatic carbocations. Ashas already been stated, there are no neutral aromaticmolecules with two delocalized p electrons. Thecyclopropenyl ion, on the other hand, is trigonal planar withthree sp2 hybridized carbon atoms and has two electronclouds occupied by two p electrons above and below theplane of the molecule. This cation is a resonance hybrid ofthree equivalent structures. As such, in addition to being anaromatic molecule, the ion is stabilized by the conjugativeeffect (Fig. 8 A).

A particularly important cation is cycloheptatrienyl, alsoknown as the tropylium ion (Fig. 8 B); it has seven planar sp2

hybridized carbon atoms and two electron clouds occupiedby a number of p electrons which satisfies Hückel’s rule.The positive charge is delocalized onto as many as sevencarbon atoms which, as also shown by Nuclear MagneticResonance (NMR) analysis, are equivalent.

CarbanionsHydrocarbons whose molecules have a negative charge

on a carbon atom are known as carbanions; likecarbocations, they are unstable and reactive. In saturatedsystems, the stability scale of carbanions is exactly theopposite to that of carbocations: the CH3

� group is the moststable, followed by carbanions with a charge localized onprimary, secondary and tertiary carbon atoms. This is due tothe electron-donating inductive effect of the alkyl

C�

H2 C�

H2

C�

H HC�

CH2

C�

H

CH2 CH2

H2C H2C� CH2

CHCH2

CH�



A B

C D

Fig. 8. A, total electron densityof the cyclopropenyl ion; B, total electron density of thetropilium cation; C, lowest unoccupiedmolecular orbital (LUMO) of the cyclopentadienyl anion; D, highest occupied molecularorbital (HOMO) of thetriphenylmethyl radical.

substituents of carbon which, by increasing the electrondensity of the already negatively charged atom, compromiseits stability. Moreover, the stability of anions increasespassing from sp3 hybridized carbon atoms to sp2 hybridizedatoms, up to anions with negative charges localized on sphybridized carbon atoms. Given the greater s characteristicof these orbitals – and thus their ability to attract electrons –they are the most stable. Experimental confirmation of thestability scale presented here has come from the analysis ofthe energy of the formation reaction of the anion and the H+

ion starting from the corresponding neutral hydrocarbon.The energy needed for the three reactions reported in Table 4,in fact, decreases constantly from an sp3 hybridizedmolecule (ethane) to an sp hybridized one (acetylene).

For carbanions, too, the most important factordetermining stability is the conjugative effect, which allowsthe negative charge to be delocalized onto several atoms.Aromatic compounds are among the most stable carbanions.The addition of an electron, and thus of a negative charge,allows some hydrocarbons to meet the requirements forbeing aromatic. The first aromatic anion is thecyclopentadienyl anion (Fig. 8 C), with the formula C5H5

� andfive equivalent hydrogen atoms (as shown by NMRanalysis); it also satisfies Hückel’s rule since it is planar andhas six electrons in p orbitals, as compared to five in theneutral molecule. Aromatic anions with 10 delocalized pelectrons include the cyclooctatetraenyl anion and thecyclononatetraenyl anion. In the molecule of the former, twonegative charges are delocalized onto eight atoms, whereasin the latter a charge is delocalized onto nine carbon atoms.

Free radicalsFree radicals are uncharged chemical species which have

at least one orbital containing a single unpaired electron. Inthe case of hydrocarbons, the unpaired electron is localizedon a carbon atom. The stability of free radicals follows thesame scale as carbocations: radicals on sp3 hybridizedcarbon atoms are more stable than those on sp2 and spatoms. For free radicals, too, the conjugative effect plays acrucial role in determining stability. Thus the allyl radical ismore stable than the tertiary radical because it delocalizesthe unpaired electron onto two carbon atoms; the benzylradical is yet more stable because it delocalizes the unpairedelectron onto four carbon atoms. Although free radicals areunstable and extremely reactive, some of them, by virtue ofinductive and conjugative stabilization effects, may exist inappreciable concentrations even at ambient temperature.This is true of the triphenylmethyl radical, where theunpaired electron is localized on a tertiary carbon stabilizedby the inductive effect and which is a resonance hybrid of asmany as 13 limit structures, in which the radical can bedelocalized onto as many as 10 carbon atoms (Fig. 8 D). The

high stability of this compound thus makes it possible toisolate it, keeping it stable for a relatively long time.Hexaphenylethane, even when kept at ambient temperature,has such a weak single C�C bond that it breaksspontaneously, giving rise to an equilibrium with twotriphenylmethyl radicals.

CarbenesHydrocarbons characterized by the presence of at least

one neutral divalent carbon atom, which thus forms only twobonds, are known as carbenes; the parent carbene ismethylene, whose electronic structure is anything butsimple. This compound has three electronic structures, twoof which are singlet states whilst one is a triplet state. Thetwo singlet states can be differentiated into a low energysinglet state (where the non-bonding electrons with pairedspin occupy an sp2 hybrid orbital leaving a p orbital empty)and an excited singlet state (where the electrons with pairedspin occupy separate p orbitals). The triplet state, which isthe most stable of the three, is characterized by the presenceof the non-bonding electrons with parallel spin in twodifferent p orbitals; in this state, therefore, the methylenemolecule is a biradical. In the low energy singlet state, thecarbon atom is sp2 hybridized; as a consequence, thegeometry of the molecule is planar with H�C�H bondangles of about 120°. By contrast, in the excited singlet ortriplet states, the molecule is linear with an H�C�H bondangle of 180°, since the carbon atom is sp hybridized.

1.2.4 Physical properties of hydrocarbons

The main physical properties of hydrocarbons, such as themelting point, boiling point, critical parameters or density,depend on their molecular and electronic structure, whichhas been discussed above in general terms for allhydrocarbons.

The boiling point is strictly correlated with themolecular weight of the hydrocarbon. For homologouscompounds – which differ in their molecular weight – theboiling point increases in line with the latter. For saturatedcompounds with the same number of carbon atoms, theboiling point increases from straight-chain to branchedmolecules, with cyclic compounds having the highestboiling points. In the presence of unsaturations (in otherwords double and/or triple bonds), both the melting pointand the boiling point are generally higher. This is due to thepresence of sp2 and sp hybridized carbon atoms which,being more electronegative than sp3 hybridized carbons,generate a charge imbalance which increasesintermolecular forces.



Table 4. Deprotonation reactions of hydrocarbons


ethane CH3�CH3��CH3�CH�

2�H� 1758.0�8 DePuy et al., 1989

ethylene CH2�CH2��CH2�CH��H� 1703.0�13.0 Graul and Squires, 1990

acetylene CH�CH��CH�C��H� 1580.0�20.0 Linstrom and Mallard, 2003

The melting point is not correlated in an equally directway with molecular structure. Generally speaking, the rulestating that the melting point rises as molecular weightincreases holds true but there are numerous exceptions; infact, comparing the melting point of hexene and4-methylpentene with the 2-butenes or 2-methylpropene, itcan be observed that alkenes with four carbon atoms actuallyhave a higher melting point than alkenes with six carbonatoms. Normally, the melting point for molecules with anidentical number of carbon atoms rises proportionally withthe compactness and symmetry of the molecule. In theseinstances, packing in the solid state is facilitated, with aconsequent increase in phase stability and therefore themelting point. For example, cyclobutane melts at 183 K ascompared to the 136 K of butane, and benzene at 278 K ascompared to the 120 K of 2-methylpentane.

Table 5 shows the electric dipole moment and thediamagnetic susceptibility of some hydrocarbons.

The dipole moment, in particular, identifies thedistribution of the electrical charge in the molecule anddepends on its structure and symmetry as well as on thepresence of carbon atoms with different hybridizations. Forexample, methane, ethane, cyclopropane, ethene,1,3-butadiene, ethyne, p-xylene are perfectly symmetricalmolecules which, as such, have a dipole moment of zero andare apolar compounds. By contrast, asymmetrical moleculessuch as propene or 1-hexyne, which additionally has acharge imbalance due to the presence of two sp hybridizedatoms, have dipole moments of 0.35 and 0.89 debye. Thedipole moment influences phase changes since, as itincreases, intermolecular attractive interactions increase andthe melting and boiling points rise.

The magnetic properties of a substance also provideinformation on the behaviour of the molecular electrons.Specifically, the study of diamagnetic properties(summarized in the value of diamagnetic susceptibility) dueto the emergence of an induced molecular magnetic momentand which therefore do not depend on either magnetic,orbital or spin moments, provide information on electronicconfigurations.

The viscosity of homologous series increases asmolecular weight rises; in alkanes, for example, the viscosity

of hexane, octane and decane at 20°C is 0.326, 0.542 and0.920 cP respectively. Cyclic hydrocarbons are more viscousthan the corresponding straight-chain hydrocarbons: forexample, hexane and cyclohexane have a viscosity of 0.326and 1.02 cP. Unsaturated compounds, on the other hand,usually have lower viscosity than saturated hydrocarbons:cyclohexane 1.02, cyclohexene 0.66 and benzene 0.652 cP.

Hydrocarbons are flammable organic compounds whichburn in the presence of a comburent. The ratio of fuel tocomburent, however, must fall within a certain range in orderfor combustion to take place; this range depends on bothconstituents and is identified by a lower and upper limit. Thelower and upper flammability limits indicate the minimumand maximum volume percentages of the fuel in thecomburent, above and below which, in the presence of atrigger, the mixture catches fire. Table 6 shows the limits forsome hydrocarbons in air. Table 7 reports the melting andboiling points, density and refractive index for variousalkanes, alkenes, alkynes and aromatic compounds. Forhomologous series, density increases as molecular weightincreases and, given an identical number of carbon atoms, it ishigher for cyclic than for straight-chain or branchedcompounds. Among the different classes of hydrocarbons,however, density increases, given an identical number ofcarbon atoms, from alkanes to alkenes, alkynes and aromatics.

Spectroscopic characterization methodsThe spectroscopic characterization methods used for

hydrocarbons are essentially mass spectrometry (or massspectroscopy), infrared spectroscopy and nuclear magneticresonance spectroscopy.

Mass spectra can be considered a sort of identity card formolecules. When molecules are bombarded with high energyelectrons they shatter, generating a characteristic distributionof positively charged fragments from which it is possible tomeasure mass and relative abundance, since they aredeflected by electric and/or magnetic fields. The results ofthese measurements are reported in a diagram, known as amass spectrum, whose x-axis shows the ratio of thefragment’s mass to its charge, generally equal to �1, whilstthe y-axis shows the relative intensity registered by theinstrument (mass spectrometer) proportional to theabundance of fragments detected. The spectrum presentspeaks with a very modest relative abundance, which in some



Table 5. Dipole moment and diamagnetic susceptibility(Weast, 1987)

MoleculeDipole moment

m (debye)

Diamagneticsusceptibility(�cm�106) CGS

methane CH4 0.0 12.2

ethane C2H6 0.0 27.3

ethene C2H4 0.0 1.0

cyclohexene C6H10 0.55 57.5

ethyne C2H2 0.0 12.5

propyne C3H4 0.72 –

benzene C6H6 0.0 54.84

toluene C7H8 0.36�0.03 66.11

Table 6. Flammability limits (vol. %)of some hydrocarbons in air, determined

under standard conditions(Weast, 1987; Hunter and Lias, 2003; Lias, 2003)

Molecule Lower limit Upper limit

methane CH4 5.00 15.00

hexane C6H14 1.18 7.40

ethene C2H4 2.75 28.60

1-butene C4H8 1.65 9.95

ethyne C2H2 2.50 80.00

benzene C6H6 1.40 7.10

toluene C7H8 1.27 6.75

cases may be due to fragments produced in very lowquantities but which more frequently indicate fragments inwhich the heavy isotopes of carbon and/or hydrogen arepresent. In fact, 98.89% of the carbon making uphydrocarbons consists of the 12C isotope with the remaining1.11% being of the 13C isotope; 99.985% of the hydrogen ispresent as the 1H isotope and 0.015% as deuterium D (the 2Hisotope). This means that in the mass spectrum of ahydrocarbon, peaks will also be found due to the presence offragments which contain these heavier isotopes. Thesepeaks, with a value of mass number incremented by one ormore units depending on the number of 13C and D present inthe molecule, are found immediately after the peaks createdby fragments without heavy isotopes; the relative abundanceof these fragments is nonetheless very low given the tinypercentage of these isotopes present in nature.

Infrared spectroscopy is a powerful tool for classifyingand identifying hydrocarbons. It is based on the ability ofInfraRed (IR) radiation impacting on a chemical compoundto excite the vibrational frequencies of some chemicalbonds; as a result, depending on the bonds present in themolecule, a sample being examined absorbs specificfrequencies of the infrared spectrum. More generally, the IRspectra obtained give the wave number rather than thewavelength absorbed; the relationship between these twomagnitudes is given by the equations n�c�l, n

_�1�l, where l

is the wavelength, c is the speed of light, n is the frequencyand n

_is the wave number.

As in the case of the mass spectrum, each IR spectrumcan be correlated with a single compound. The range ofwavelengths which defines the infrared spectrum ofelectromagnetic radiation lies between 400 and 4,000 cm�1.The main vibrational frequencies which absorb in the infraredfield are bond stretching vibrations and bond bendingvibrations. Two different types of bond stretching can be

identified: symmetrical (or in phase) and asymmetrical (outof phase); in both cases, bond stretching is indicated by theletter n. There are two different types of bending: one in theplane (indicated with the letter d) and one outside the plane(indicated with the letter g). Bending in the plane may leadthe bonds to converge in the same direction, in which case wespeak of rocking; alternatively, the bonds may diverge inopposite directions, in which case we have scissoringvibration. Bending outside the plane is classified as eithertwisting or wagging, depending on whether the bonds movein two different opposing directions or in the same direction.Generally speaking, in the IR spectrum of hydrocarbons, aseries of peaks are observed which present mediumabsorption intensities of infrared radiation, others whichpresent high absorption intensities and a series of lowintensity absorptions which it is difficult to assign to specificbond vibrations in the molecule since they are due toparticularly complex internal torsional vibrations.

The medium-intensity absorption peaks refer tostretching vibrations; specifically, at decreasing wavenumbers we find the stretching of C�H bonds, triple C�Cbonds, double C�C bonds and finally single C�C bonds.The peaks which have high density absorption are correlatedwith the bending in and out of the plane of C�H bonds.

The NMR spectroscopy of a hydrocarbon providesdetailed information on the bond state of the hydrogen andcarbon atoms, through which it is possible to clarify oridentify the structure of the chemical compound analysed.Briefly, this technique is based on exciting the nuclear spinstates by using radio frequency impulses and on measuringthe energy absorbed as a function of the frequency applied. Asfar as hydrocarbons are concerned, the atoms which can bestudied using NMR are hydrogen and the 13C isotope. Sincethe frequency of radiation absorption depends on the densityof the electrons surrounding the nucleus, the resonance



Table 7. Physical properties of hydrocarbons (Weast, 1987)

Molecule M.P. (°C) B.P. (°C) Density* Refractive index*

methane CH4 �182.5 �161.5 0.415�164 –

ethane C2H6 �182.8 �88.63 0.572�108 1.07690

hexane C6H14 �95 68 0.6594 1.0749

cyclopropane C3H6 �126.6 �33 0.720�79 –

cyclohexane C6H12 6.5 81 0.7791 1.4266

ethene C2H4 �169.2 �104 0.001260 1.363100

1,3-butadiene C4H6 �108.9 �4.4 – 1.4292�25

cyclohexene C6H10 �103.5 82.9 0.8110 1.4465

ethyne C2H2 �81.8 �83.6 0.6181�82 1.00050

benzene C6H6 5.5 80.1 0.878715 1.501120

toluene C7H8 �95 110.6 0.8669 1.4961

cumene C9H12 �96 153 0.86420 1.491120

diphenyl C12H10 70 255.9 1.989677 1.58877

* The superscript numbers indicate the temperature in °C at which the measurement was made

frequency reflects the state of hybridization and bonding ofthe atom under investigation. In the benzene molecule, forexample, there is only one type of carbon and hydrogen atom;as a consequence, in the 13C NMR spectrum there is a singletype of signal due to the six equivalent nuclei of the C atoms.Similarly, in the PMR (Proton Magnetic Resonance)spectrum, there is a single signal due to the six equivalentprotons of the hydrogen atoms. By contrast, in the 13C NMRspectrum, propane has two different signals due to thesecondary carbon and the two primary carbons (which in thiscase are equivalent). From the examples provided, it can bededuced that the number of peaks shown by the NMRspectrum corresponds to the different number of atomspresent, whilst the position of these absorption peaks in thespectrum is a function of the electronic structure characteristicof these atoms, which differs significantly depending onwhether they are aromatic, aliphatic, benzylic, vinylic, allylic,primary, secondary, tertiary, etc. This is due to the fact that themagnetic field applied produces a shift of the electrons insidethe molecule, so that the nuclei are subjected to an effectivefield which is greater or less than that actually applied,leading to a shift in the absorption frequency which wouldhave been obtained had the atomic nucleus not been shielded.This shift, known as chemical shift, is measured with respectto a reference signal (in the case of PMR the absorptionfrequency of the hydrogen atoms of tetramethylsilane);generally reported as a relationship to the frequency of thespectrometer, it is in the order of parts per million (ppm).

Bibliography

Atkins P.W. (1994) Physical chemistry, Oxford, Oxford UniversityPress.

Atkins P.W., Friedman R. (1997) Molecular quantum mechanics,Oxford, Oxford University Press.

Graham Solomons T.W. (1993) Chimica organica, Bologna,Zanichelli.

Morrison R.T., Boyd R.N. (1969) Chimica organica, Milano, CasaEditrice Ambrosiana.

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DePuy C.H. et al. (1989) The gas phase acidities of the alkanes,«Journal of American Chemical Society», 111, 1968-1973.

Dolliver M.A. et al. (1937) Heats of organic reactions. V: Heats ofhydrogenation of various hydrocarbons, «Journal of AmericanChemical Society», 59, 831-841.

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Hunter E.P., Lias E.G. (2003) Proton affinity evaluation, in: LinstromP.J., Mallard W.G. (editors) NIST Chemistry WebBook, NationalInstitute of Standards and Technology, Standard reference database,69.

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Weast R.C. (editor in chief) (1987) CRC Handbook of chemistry andphysics. A ready-reference book of chemical data, Boca Raton(FL), CRC Press.

Carlo Cavallotti

Davide Moscatelli

Dipartimento di Chimica, Materiali e Ingegneria chimica ‘Giulio Natta’

Politecnico di MilanoMilano, Italy



1.2 typology and structure of hydrocarbons

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