section 1_structure and bonding in hydrocarbons (1)

7/26/2019 Section 1_Structure and Bonding in Hydrocarbons (1)

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• Hydrocarbons are compounds that are onlycomposed of hydrogen and carbon.

• Which of the molecules above is saturated withhydrogen atoms?

Alkanes

• Saturated hydrocarbons do NOT contain any pi bonds.

• When communicating about molecules, each uniquemolecule must have a unique name.

– The suffix ane is used for saturated hydrocarbons.

• How are the above hydrocarbons different?

Alkanes



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1. Identify the parent chain, the longest consecutivechain of carbons.

2. Identify and name substituents (side chains).

3. Number the parent chain and assign a locant (andprefix if necessary) to each substituent.

– Give the first substituent the lowest number possible.

4. List the numbered substituents before the parent

chain name in alphabetical order. – Ignore prefixes (except iso) when ordering alphabetically.

Naming Compounds – AlkanesSummary

• Name the following molecule.

• Draw the bond-line representation for 1-tert-butyl-2-ethyl-4-methyl-3-cyclopentylcyclohexane.

(e.g.)



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• There are many compounds with two fused ringscalled bicyclic compounds.

• To name a bicyclic compound, include the prefix“bicyclo” in front of the normal name ending in -ane.For example, the compounds below could both benamed, bicycloheptane.

Naming Compounds –Bicyclic Compounds

• We know that if two molecules are not identical, theycannot have the same exact name.

• What is the difference between the two compoundsbelow?

– The number of carbons connecting the bridgeheads isdifferent. Count them.




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1. To number the bicyclo parent chain, start at abridgehead carbon and number the longest carbonchain connecters first.

– What numbering would result if we started from the otherbridgehead?

2. Give the substituents the lowest numbers possible.


• For molecules that are large and complex, there areMANY more rules to follow using the IUPAC system.

– Just pronouncing the name of this molecule is difficult.

• Often trade names are used for the sake of simplicity.

– The molecule above is also known as esomeprazole, orNexium, which is a drug for acid reflux.

Naming Compounds –Larger Molecules



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• Recall that isomers are different structures made fromthe same atoms.

– Isomers are NOT identical, but they have the same formula.

• Constitutional isomers differ in connectivity.

– Consider two of the five constitutional isomers for hexane.

• Draw the other three C6H14 constitutional isomers.

Constitutional Isomers

• The number of different possibleways to connect atoms increasesdramatically when there aremore atoms.




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• How can you recognize if two molecules are isomers? – Are these two structures

isomers? Do they havethe same formula?

– If they have the same chemical formula, they may beisomers, or they may be identical.

• You can test if they are identical using two methods:

1. Flip one of the molecules in 3D space and rotate around its

single bonds until it is superimposable on the othermolecule.

2. Name them. If they have the same name, theyare identical.


• To rationalize and predict the outcomes of chemicalreactions, it is helpful to assess the stability ofcompounds.

• Remember:

– Stable = low potential energy = low reactivity = little energy

will be released upon reacting• If you drove a car today, what chemical reaction with

alkanes did you perform?

• What is the general reaction equation for acombustion?

Relative Stability of Isomers



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• Compare the heats of combustion for three octane

isomers.

• Which is most stable? Which is the best fuel?

Relative Stability of Isomers

• Petroleum, which literally means rock oil is the mainsource of alkanes.

• Petroleum is a mixture of hundreds of hydrocarbons,mostly alkanes, with varying numbers of carbons andvarying degrees of branching.

• The alkanes in petroleum with 5 to 12 carbons permolecule are most valuable, and they can be separatedfrom the rest of the oil by distillation.

– HOW does distillation work?

Sources and Uses of Alkanes



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• Table 4.5 shows the various components of petroleum.

• The gasoline fraction of crude oil only makes up about19%, which is not enough to meet demand.


• Gasoline is a mixture of straight, branched, andaromatic hydrocarbons (5 –12 carbons in size).

– Large alkanes can be broken down into smaller molecules byCRACKING.

– Straight chain alkanes can be converted into branchedalkanes and aromatic compounds through REFORMING.

– After using these processes, the yield of gasoline is about47% rather than 19%.




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• At room temperature: – Small alkanes with 1 –4 carbons are gases.

– Medium size alkanes with 5 –12 carbons are liquids.

– Large alkanes with 13 –20 carbons are oils.

– Extra large alkanes with 20 –100 carbons are solids like tarand wax.

– Supersized alkanes called polymers can have thousands ormillions of carbon atoms in each molecule.

• What type of properties would you expect suchpolymers to possess?

• Why? Consider London forces.


• Carbon atoms in alkanes are sp3 hybridized.

• What bond angles are optimal for such carbons?

• If cycloalkanes were flat, what bond angles would beexpected?

• To optimize the bond angles, most cycloalkanes areNOT flat in their most stable conformation.

Cyclic Alkanes



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• Why are heatsof combustionreported perCH2 group?

• Considering theprevious slidewhich ring hasthe least RING

STRAIN?

Cyclic Alkanes

• Cyclopropane is 44 kJ/mol lessstable than cyclohexane perCH2 group. It is highly strainedand very reactive due to:

1. Angle strain:

• Bond angles of 60° cause

electron pair repulsion inadjacent bonds

• Inefficient sigma bond overlap

2. Torsional strain:

• Eclipsing C –H bonds all the wayaround the ring—see Newmanprojection

Cyclic Alkanes – Cyclopropane



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• Cyclobutane is 27 kJ/mol less stable than cyclohexaneper CH2 group. It is also strained and reactive:

1. Angle strain results from bond angles of 88°, although it isnot as severe as the 60° angles in cyclopropane.

2. Slight torsional strain results because adjacent C –H bondsare neither fully eclipsed nor fully staggered.

• Why does it adopt a puckered conformation?

Cyclic Alkanes – Cyclobutane

• Cyclopentane is only 5 kJ/mol less stable thancyclohexane per CH2 group:

1. Angles are close to the optimal value.

2. Identify the minimal but significant torsional strain in thestructure. It is very helpful to use a handheld model.

• Why does it adopt the envelope conformationrather than a flat conformation?

Cyclic Alkanes – Cyclopentane



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Alkenes

• C=C double bonds are found in a variety of compoundsincluding pheromones and many other classes ofcompounds.

Alkenes



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• Why might it be helpful to know the chemical structureof pheromones such as those below?

Alkenes

• Alkenes are also important compounds in the chemicalindustry.

• 70 billion pounds of propylene (propene) and200 billion pounds of ethylene (ethene) are both madefrom cracking petroleum each year.

Alkenes



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Alkenes

• Alkenes are named using the same procedure we used toname alkanes with minor modifications.

1. Identify the parent chain, which should include the C=C doublebond.

2. Identify and name the substituents.

3. Assign a locant (and prefix if necessary) to each substituent. Give

the C=C double bond the lowest number possible.4. List the numbered substituents before the parent name in

alphabetical order.

5. The C=C double bond locant is placed either just before the parentname, or just before the -ene suffix.

Alkene Nomenclature



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• For the pi bond to remain intact, rotation around adouble bond is prohibited.

• As a result, cis and trans structures are not identical.

• Is there a difference between cis-butane and trans-butane?

• What specific type of isomers are cis- andtrans- butene?

Alkene Isomerism

• In cyclic alkenes with less than 8 atoms in the ring, onlycis alkenes are stable. WHY?

• Draw the structure for trans-cyclooctene.

• When applied to bicycloakenes, this rule is calledBREDT’S RULE.

Alkene Isomerism



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• Apply Bredt’s rule to the compounds below.

• The carbons of the C=C double bond and the atomsthat are directly attached to them must be planar to

maintain the pi bond overlap.• A handheld model can be used to help visualize the p

orbital overlap and the resulting geometry.

Alkene Isomerism

• Cis and trans modifiers are strictly used to describeC=C double bonds with identical groups on eachcarbon. Where are the identical groups in trans-2-pentene?

• For molecules with different groups attached to theC=C double bond, the E / Z notation is used instead ofcis/trans notation.

Alkene Isomerism



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• Assigning E or Z to a stereoisomers:1. Prioritize the groups attached to the C=C double bond based

on atomic number.

Alkene Isomerism

• Assigning E or Z to a stereoisomers:

1. Prioritize the groups attached to the C=C double bond basedon atomic number.

2. If the top priority groups are on the same side of the C=Cdouble bond, it is Z (for zusammen, which means together).

3. If the top priority groups are on opposite sides of the C=Cdouble bond, it is E (for entgegen, which means opposite).

Alkene Isomerism



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• Because of steric strain, cis isomers are generally lessstable than trans.

Alkene Stability

• The difference in stability can be quantified bycomparing the heats of combustion.

• How does heat of combustion relate to stability?

Alkene Stability



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Alkene Stability

• Consider the following stability trend:

• What pattern do you see?

Alkene Stability



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• List the following molecules in order of increasing heatof combustion.

– 2,3,4-trimethyl-1,3-pentadiene

– 2-isopropyl-1,4-pentadiene

– 3,3-dimethyl-1,5-hexadiene

– 4,5-dimethylcyclohexene

Alkene Stability

• In 1866, August Kekulé proposed that benzene is a ringcomprised of alternating double and single bonds.

• Kekulé suggested that the exchange of double andsingle bonds was an equilibrium process.

Structure of Benzene



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• We now know that the aromatic structures areresonance contributors rather than in equilibrium.

• HOW is resonance different from equilibrium?

• Sometimes the ring is represented with a circle in it

• WHY?

Structure of Benzene

• The stability that results from a ring being aromatic isstriking.

• Recall that in general, alkenes readily undergo additionreactions.

• Aromatic rings are stable enough that they do notundergo such reactions.

Stability of Benzene



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• Heats of hydrogenation can be used to quantifyaromatic stability.


• Molecular orbital (MO) theory can help us explain aromaticstability.

• The six atomic p-orbitalsof benzene overlap tomake six MOs.




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• The locations of nodesin the MOs determinestheir shapes based onhigh-level mathematicalcalculations.


• The delocalization of thesix pi electrons in thethree bonding molecularorbitals accounts for thestability of benzene.




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• Does every fully conjugated cyclic compound havearomatic stability? NO.

• Some fully conjugated cyclic compounds are reactiverather than being stable like benzene.


• AROMATIC compounds fulfill two criteria:

1. A fully conjugated ring with overlapping p-orbitals

2. Meets HÜCKEL’S RULE: an ODD number of electron pairs or4n+2 total π electrons where n = 0, 1, 2, 3, 4, etc.

• Show how the molecules below do NOT meet the

criteria.




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• We can explain Hückel’s rule using MO theory.• Let’s consider the MOs for cyclobutadiene.

• The instability of the unpaired electrons (similar to freeradicals) makes this ANTIAROMATIC.


• A similar MO analysis for cyclooctatetraenesuggests that it is also ANTIAROMATIC.




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• However, if the structure adopts a tub-shapedconformation, it can avoid the antiaromatic instability.

• The conjugation does not extend around the entirering, so the system is neither aromatic norantiaromatic.


• Predicting the shapes and energies of MOs requiressophisticated mathematics, but we can use FROSTCIRCLES to predict the relative MO energies.




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• Use the FROST CIRCLES below to explain the 4n+2 rule.

• Note that the number of bonding orbitals is always anodd number; aromatic compounds will always have anodd number of electron pairs.


• AROMATIC compounds fulfill two criteria:

1. A fully conjugated ring with overlapping p-orbitals

2. Meets HÜCKEL’S RULE: an ODD number of electron pairs or4n+2 total π electrons where n = 0, 1, 2, 3, 4, etc.

• ANTIAROMATIC compounds fulfill two criteria1. A fully conjugated ring with overlapping p-orbitals

2. An EVEN number of electron pairs or 4n total π electronswhere n = 0, 1, 2, 3, 4, etc.

Aromatic Compounds Other ThanBenzene



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• Annulenes are rings that are fully conjugated.

• Some annulenes are aromatic, while others areantiaromatic.

• [10]Annulene is neither. WHY?





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• The pKa value for cyclopentadiene is much lower thantypical C-H bonds. WHY?


vs.

• Heteroatoms (atoms other than C or H) can also bepart of an aromatic ring.




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3

• If the heteroatom’s lone pair is necessary, it will beincluded in the HÜCKEL number of pi electrons.


• If the lone pair is necessary to make it aromatic, theelectrons will not be as basic.


pKa=5.2

pKa=0.4



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• The difference in electron density can also be observedby viewing the electrostatic potential maps.


• Will the compounds below be aromatic, antiaromatic,or non aromatic?




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3

• Many polycyclic compounds are also aromatic.

• Such compounds are shown to be aromatic using heatsof hydrogenation. HOW?





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• Alkynes are molecules that incorporate a CC triple

bond.

Alkynes

• Given the presence of two pi bonds and their associatedelectron density, alkynes are similar to alkenes in theirability to act as a nucleophile.

• Converting pi bonds to sigma bonds generally makes amolecule more stable. WHY?

Alkynes



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• Acetylene is the simplest alkyne. – It is used in blow torches and as a precursor for the synthesis

of more complex alkynes.

• More than 1000 different alkyne natural products havebeen isolated.

– One example is histrionicotoxin,which can be isolated from SouthAmerican frogs, and is used on

poison-tipped arrows by SouthAmerican tribes.

Alkyne Uses

• An example of a synthetic alkyne is ethynylestradiol.

• Ethynylestradiol is the activeingredient in many birth controlpills.

• The presence of the triple bond increases the potencyof the drug compared to the natural analog.

• How do you think a CC triple bond affects themolecule’s geometry? Its rigidity? Its intermolecularattractions?

Alkyne Uses

section 1_structure and bonding in hydrocarbons (1)

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