Chemical reaction

A thermite reaction using iron(III) oxide. The sparks flying out-wards are globules of molten iron trailing smoke in their wake.

A chemical reaction is a process that leads to the trans-formation of one set of chemical substances to another.[1]Classically, chemical reactions encompass changes thatonly involve the positions of electrons in the formingand breaking of chemical bonds between atoms, with nochange to the nuclei (no change to the elements present),and can often be described by a chemical equation.Nuclear chemistry is a sub-discipline of chemistry thatinvolves the chemical reactions of unstable and radioac-tive elements where both electronic and nuclear changesmay occur.The substance (or substances) initially involved in achemical reaction are called reactants or reagents. Chem-ical reactions are usually characterized by a chemicalchange, and they yield one or more products, which usu-ally have properties different from the reactants. Re-actions often consist of a sequence of individual sub-steps, the so-called elementary reactions, and the infor-mation on the precise course of action is part of thereaction mechanism. Chemical reactions are describedwith chemical equations, which graphically present thestarting materials, end products, and sometimes interme-diate products and reaction conditions.Chemical reactions happen at a characteristic reactionrate at a given temperature and chemical concentration,and rapid reactions are often described as spontaneous,

requiring no input of extra energy other than thermal en-ergy. Non-spontaneous reactions run so slowly that theyare considered to require the input of some type of ad-ditional energy (such as extra heat, light or electricity) inorder to proceed to completion (chemical equilibrium) athuman time scales.Different chemical reactions are used in combinationsduring chemical synthesis in order to obtain a desiredproduct. In biochemistry, a similar series of chem-ical reactions form metabolic pathways. These reac-tions are often catalyzed by protein enzymes. These en-zymes increase the rates of biochemical reactions, so thatmetabolic syntheses and decompositions impossible un-der ordinary conditions may be performed at the temper-atures and concentrations present within a cell.The general concept of a chemical reaction has been ex-tended to reactions between entities smaller than atoms,including nuclear reactions, radioactive decays, and re-actions between elementary particles as described byquantum field theory.

1 History

Chemical reactions such as combustion in the fire,fermentation and the reduction of ores to metals wereknown since antiquity. Initial theories of transformationof materials were developed by Greek philosophers, suchas the Four-Element Theory of Empedocles stating thatany substance is composed of the four basic elements –fire, water, air and earth. In the Middle Ages, chemi-cal transformations were studied by Alchemists. They at-tempted, in particular, to convert lead into gold, for whichpurpose they used reactions of lead and lead-copper al-loys with sulfur.[2]

The production of chemical substances that do not nor-mally occur in nature has long been tried, such as thesynthesis of sulfuric and nitric acids attributed to the con-troversial alchemist Jābir ibn Hayyān. The process in-volved heating of sulfate and nitrate minerals such ascopper sulfate, alum and saltpeter. In the 17th century,Johann Rudolph Glauber produced hydrochloric acid andsodium sulfate by reacting sulfuric acid and sodium chlo-ride. With the development of the lead chamber pro-cess in 1746 and the Leblanc process, allowing large-scaleproduction of sulfuric acid and sodium carbonate, respec-tively, chemical reactions became implemented into theindustry. Further optimization of sulfuric acid technol-

1

2 2 EQUATIONS

Antoine Lavoisier developed the theory of combustion as a chem-ical reaction with oxygen

ogy resulted in the contact process in 1880s,[3] and theHaber process was developed in 1909–1910 for ammoniasynthesis.[4]

From the 16th century, researchers including Jan Baptistvan Helmont, Robert Boyle and Isaac Newton tried to es-tablish theories of the experimentally observed chemicaltransformations. The phlogiston theory was proposed in1667 by Johann Joachim Becher. It postulated the exis-tence of a fire-like element called “phlogiston”, whichwascontained within combustible bodies and released duringcombustion. This proved to be false in 1785 by AntoineLavoisier who found the correct explanation of the com-bustion as reaction with oxygen from the air.[5]

Joseph Louis Gay-Lussac recognized in 1808 that gasesalways react in a certain relationship with each other.Based on this idea and the atomic theory of John Dal-ton, Joseph Proust had developed the law of definiteproportions, which later resulted in the concepts ofstoichiometry and chemical equations.[6]

Regarding the organic chemistry, it was long believedthat compounds obtained from living organisms were toocomplex to be obtained synthetically. According to theconcept of vitalism, organic matter was endowed witha “vital force” and distinguished from inorganic materi-als. This separation was ended however by the synthe-sis of urea from inorganic precursors by Friedrich Wöh-

ler in 1828. Other chemists who brought major contri-butions to organic chemistry include Alexander WilliamWilliamson with his synthesis of ethers and ChristopherKelk Ingold, who, among many discoveries, establishedthe mechanisms of substitution reactions.

2 Equations

As seen from the equation CH4 + 2 O2 → CO2 + 2 H2O, a coefficient of 2 must be placed before the oxygen gas on thereactants side and before the water on the products side in orderfor, as per the law of conservation of mass, the quantity of eachelement does not change during the reaction

Main article: Chemical equation

Chemical equations are used to graphically illustratechemical reactions. They consist of chemical or structuralformulas of the reactants on the left and those of the prod-ucts on the right. They are separated by an arrow (→)which indicates the direction and type of the reaction; thearrow is read as the word “yields”.[7] The tip of the arrowpoints in the direction in which the reaction proceeds. Adouble arrow () pointing in opposite directions is usedfor equilibrium reactions. Equations should be balancedaccording to the stoichiometry, the number of atoms ofeach species should be the same on both sides of the equa-tion. This is achieved by scaling the number of involvedmolecules (A, B, C and D in a schematic example below)by the appropriate integers a, b, c and d.[8]

a A+ b B −→ c C+ d D

More elaborate reactions are represented by reactionschemes, which in addition to starting materials and prod-ucts show important intermediates or transition states.Also, some relatively minor additions to the reaction canbe indicated above the reaction arrow; examples of suchadditions are water, heat, illumination, a catalyst, etc.Similarly, some minor products can be placed below thearrow, often with a minus sign.Retrosynthetic analysis can be applied to design a com-plex synthesis reaction. Here the analysis starts from

3

An example of organic reaction: oxidation of ketones to esterswith a peroxycarboxylic acid

the products, for example by splitting selected chemicalbonds, to arrive at plausible initial reagents. A specialarrow (⇒) is used in retro reactions.[9]

3 Elementary reactions

The elementary reaction is the smallest division intowhich a chemical reaction can be decomposed to, it hasno intermediate products.[10] Most experimentally ob-served reactions are built up from many elementary re-actions that occur in parallel or sequentially. The ac-tual sequence of the individual elementary reactions isknown as reaction mechanism. An elementary reactioninvolves a few molecules, usually one or two, because ofthe low probability for several molecules to meet at a cer-tain time.[11]

Isomerization of azobenzene, induced by light (hν) or heat (Δ)

The most important elementary reactions are unimolec-ular and bimolecular reactions. Only one molecule is in-volved in a unimolecular reaction; it is transformed byan isomerization or a dissociation into one or more othermolecules. Such reactions require the addition of en-ergy in the form of heat or light. A typical example ofa unimolecular reaction is the cis–trans isomerization, inwhich the cis-form of a compound converts to the trans-form or vice versa.[12]

In a typical dissociation reaction, a bond in a moleculesplits (ruptures) resulting in two molecular fragments.The splitting can be homolytic or heterolytic. In the firstcase, the bond is divided so that each product retains anelectron and becomes a neutral radical. In the secondcase, both electrons of the chemical bond remain withone of the products, resulting in charged ions. Dissocia-tion plays an important role in triggering chain reactions,such as hydrogen–oxygen or polymerization reactions.

AB −→ A+ BDissociation of a molecule AB into fragments A andB

For bimolecular reactions, two molecules collide and re-act with each other. Their merger is called chemical syn-thesis or an addition reaction.

A+ B −→ AB

Another possibility is that only a portion of one moleculeis transferred to the other molecule. This type of reactionoccurs, for example, in redox and acid-base reactions. Inredox reactions, the transferred particle is an electron,whereas in acid-base reactions it is a proton. This typeof reaction is also called metathesis.

HA+ B −→ A+ HB

for example

NaCl(aq) +AgNO3(aq) −→ NaNO3(aq) +AgCl(s)

4 Chemical equilibrium

Main article: Chemical equilibrium

Most chemical reactions are reversible, that is they canand do run in both directions. The forward and re-verse reactions are competing with each other and dif-fer in reaction rates. These rates depend on the concen-tration and therefore change with time of the reaction:the reverse rate gradually increases and becomes equalto the rate of the forward reaction, establishing the so-called chemical equilibrium. The time to reach equilib-rium depends on such parameters as temperature, pres-sure and the materials involved, and is determined by theminimum free energy. In equilibrium, the Gibbs free en-ergy must be zero. The pressure dependence can be ex-plained with the Le Chatelier’s principle. For example,an increase in pressure due to decreasing volume causesthe reaction to shift to the side with the fewer moles ofgas.[13]

The reaction yield stabilizes at equilibrium, but can be in-creased by removing the product from the reaction mix-ture or changed by increasing the temperature or pres-sure. A change in the concentrations of the reactants doesnot affect the equilibrium constant, but does affect theequilibrium position.

5 Thermodynamics

Chemical reactions are determined by the laws ofthermodynamics. Reactions can proceed by themselvesif they are exergonic, that is if they release energy. The

4 6 KINETICS

associated free energy of the reaction is composed oftwo different thermodynamic quantities, enthalpy andentropy:[14]

G = H− T · S.G: free energy, H: enthalpy, T: temperature, S: en-tropy, Δ: difference(change between original andproduct)

Reactions can be exothermic, where ΔH is negative andenergy is released. Typical examples of exothermic re-actions are precipitation and crystallization, in which or-dered solids are formed from disordered gaseous or liq-uid phases. In contrast, in endothermic reactions, heatis consumed from the environment. This can occur byincreasing the entropy of the system, often through theformation of gaseous reaction products, which have highentropy. Since the entropy increases with temperature,many endothermic reactions preferably take place at hightemperatures. On the contrary, many exothermic reac-tions such as crystallization occur at low temperatures.Changes in temperature can sometimes reverse the signof the enthalpy of a reaction, as for the carbon monoxidereduction of molybdenum dioxide:

2CO(g)+MoO2(s) −→ 2CO2(g)+Mo(s); Ho = +21.86 kJ at 298 KThis reaction to form carbon dioxide and molybdenum isendothermic at low temperatures, becoming less so withincreasing temperature.[15] ΔH° is zero at 1,855 K, andthe reaction becomes exothermic above that temperature.Changes in temperature can also reverse the directiontendency of a reaction. For example, the water gas shiftreaction

CO(g) +H2O(v) ⇌ CO2(g) +H2(g)

is favored by low temperatures, but its reverse is favoredby high temperature. The shift in reaction direction ten-dency occurs at 1,100 K.[15]

Reactions can also be characterized by the internal energywhich takes into account changes in the entropy, volumeand chemical potential. The latter depends, among otherthings, on the activities of the involved substances.[16]

dU = T dS − p dV + µ dnU: internal energy, S: entropy, p: pressure, μ: chem-ical potential, n: number of molecules, d: smallchange sign

6 Kinetics

The speed at which a reactions takes place is studied byreaction kinetics. The rate depends on various parame-ters, such as:

• Reactant concentrations, which usually make the re-action happen at a faster rate if raised through in-creased collisions per unit time. Some reactions,however, have rates that are independent of reac-tant concentrations. These are called zero order re-actions.

• Surface area available for contact between the reac-tants, in particular solid ones in heterogeneous sys-tems. Larger surface areas lead to higher reactionrates.

• Pressure – increasing the pressure decreases the vol-ume between molecules and therefore increases thefrequency of collisions between the molecules.

• Activation energy, which is defined as the amount ofenergy required to make the reaction start and carryon spontaneously. Higher activation energy impliesthat the reactants need more energy to start than areaction with a lower activation energy.

• Temperature, which hastens reactions if raised,since higher temperature increases the energy of themolecules, creating more collisions per unit time,

• The presence or absence of a catalyst. Catalysts aresubstances which change the pathway (mechanism)of a reaction which in turn increases the speed of areaction by lowering the activation energy needed forthe reaction to take place. A catalyst is not destroyedor changed during a reaction, so it can be used again.

• For some reactions, the presence of electromagneticradiation, most notably ultraviolet light, is neededto promote the breaking of bonds to start the reac-tion. This is particularly true for reactions involvingradicals.

Several theories allow calculating the reaction rates at themolecular level. This field is referred to as reaction dy-namics. The rate v of a first-order reaction, which couldbe disintegration of a substance A, is given by:

v = −d[A]dt

= k · [A].

Its integration yields:

[A](t) = [A]0 · e−k·t.

Here k is first-order rate constant having dimension1/time, [A](t) is concentration at a time t and [A]0 isthe initial concentration. The rate of a first-order reac-tion depends only on the concentration and the proper-ties of the involved substance, and the reaction itself canbe described with the characteristic half-life. More thanone time constant is needed when describing reactions of

7.2 Oxidation and reduction 5

higher order. The temperature dependence of the rateconstant usually follows the Arrhenius equation:

k = k0e−Ea/kBT

where Eₐ is the activation energy and kB is the Boltzmannconstant. One of the simplest models of reaction rateis the collision theory. More realistic models are tai-lored to a specific problem and include the transition statetheory, the calculation of the potential energy surface,the Marcus theory and the Rice–Ramsperger–Kassel–Marcus (RRKM) theory.[17]

7 Reaction types

7.1 Four basic types

A B+ A B

A B+A B

C+A B A C B+

+A B C D A C + B D

Representation of four basic chemical reactions types: synthesis,decomposition, single replacement and double replacement.

7.1.1 Synthesis

Main article: Synthesis reaction

In a synthesis reaction, two or more simple substancescombine to form a more complex substance. These reac-tions are in the general form:

A+B −→ AB

Two or more reactants yielding one product is anotherway to identify a synthesis reaction. One example of asynthesis reaction is the combination of iron and sulfur toform iron(II) sulfide:

8Fe+ S8 −→ 8FeS

Another example is simple hydrogen gas combined withsimple oxygen gas to produce a more complex substance,such as water.[18]

7.1.2 Decomposition

Main article: Decomposition reaction

A decomposition reaction is when a more complex sub-stance breaks down into its more simple parts. It is thusthe opposite of a synthesis reaction, and can be writtenas[18][19]

AB −→ A+B

One example of a decomposition reaction is theelectrolysis of water to make oxygen and hydrogen gas:

2H2O −→ 2H2 +O2

7.1.3 Single replacement

In a single replacement reaction, a single uncombined el-ement replaces another in a compound; in other words,one element trades places with another element in acompound[18] These reactions come in the general formof:

A+BC −→ AC +B

One example of a single displacement reaction iswhen magnesium replaces hydrogen in water to makemagnesium hydroxide and hydrogen gas:

Mg + 2H2O −→ Mg(OH)2 +H2

7.1.4 Double replacement

In a double replacement reaction, the anions and cationsof two compounds switch places and form two entirelydifferent compounds.[18] These reactions are in the gen-eral form:[19]

AB + CD −→ AD + CB

For example, when barium chloride (BaCl2) andmagnesium sulfate (MgSO4) react, the SO4

2− anionswitches places with the 2Cl− anion, giving the com-pounds BaSO4 and MgCl2.Another example of a double displacement reaction is thereaction of lead(II) nitrate with potassium iodide to formlead(II) iodide and potassium nitrate:

Pb(NO3)2 + 2KI −→ PbI2 + 2KNO3

6 7 REACTION TYPES

Illustration of a redox reaction

Sodium chloride is formed through the redox reaction of sodiummetal and chlorine gas

7.2 Oxidation and reduction

Redox reactions can be understood in terms of transferof electrons from one involved species (reducing agent)to another (oxidizing agent). In this process, the formerspecies is oxidized and the latter is reduced. Though suf-ficient for many purposes, these descriptions are not pre-cisely correct. Oxidation is better defined as an increasein oxidation state, and reduction as a decrease in oxida-tion state. In practice, the transfer of electrons will alwayschange the oxidation state, but there are many reactionsthat are classed as “redox” even though no electron trans-fer occurs (such as those involving covalent bonds).[20][21]

In the following redox reaction, hazardous sodium metalreacts with toxic chlorine gas to form the ionic compoundsodium chloride, or common table salt:

2Na(s) + Cl2(g) −→ 2NaCl(s)

In the reaction, sodiummetal goes from an oxidation stateof 0 (as it is a pure element) to +1: in other words, thesodium lost one electron and is said to have been oxi-dized. On the other hand, the chlorine gas goes from anoxidation of 0 (it is also a pure element) to −1: the chlo-rine gains one electron and is said to have been reduced.Because the chlorine is the one reduced, it is consideredthe electron acceptor, or in other words, induces oxida-tion in the sodium – thus the chlorine gas is consideredthe oxidizing agent. Conversely, the sodium is oxidizedor is the electron donor, and thus induces reduction in theother species and is considered the reducing agent.

Which of the involved reactants would be reducing or ox-idizing agent can be predicted from the electronegativityof their elements. Elements with low electronegativity,such as most metals, easily donate electrons and oxidize– they are reducing agents. On the contrary, many ionswith high oxidation numbers, such as H2O2, MnO−4, CrO3, Cr2O2−7, OsO4 can gain one or two extra electrons and are strong oxi-dizing agents.The number of electrons donated or accepted in a re-dox reaction can be predicted from the electron config-uration of the reactant element. Elements try to reachthe low-energy noble gas configuration, and therefore al-kali metals and halogens will donate and accept one elec-tron respectively. Noble gases themselves are chemicallyinactive.[22]

An important class of redox reactions are theelectrochemical reactions, where electrons from thepower supply are used as the reducing agent. Thesereactions are particularly important for the productionof chemical elements, such as chlorine[23] or aluminium.The reverse process in which electrons are released inredox reactions and can be used as electrical energy ispossible and used in batteries.

7.3 Complexation

Ferrocene – an iron atom sandwiched between two C5H5 ligands

In complexation reactions, several ligands react with ametal atom to form a coordination complex. This is

7.5 Precipitation 7

achieved by providing lone pairs of the ligand into emptyorbitals of the metal atom and forming dipolar bonds.The ligands are Lewis bases, they can be both ions andneutral molecules, such as carbon monoxide, ammoniaor water. The number of ligands that react with a cen-tral metal atom can be found using the 18-electron rule,saying that the valence shells of a transition metal willcollectively accommodate 18 electrons, whereas the sym-metry of the resulting complex can be predicted with thecrystal field theory and ligand field theory. Complexa-tion reactions also include ligand exchange, in which oneor more ligands are replaced by another, and redox pro-cesses which change the oxidation state of the centralmetal atom.[24]

7.4 Acid-base reactions

In the Brønsted–Lowry acid–base theory, an acid-basereaction involves a transfer of protons (H+) from onespecies (the acid) to another (the base). When a proton isremoved from an acid, the resulting species is termed thatacid’s conjugate base. When the proton is accepted by abase, the resulting species is termed that base’s conjugateacid.[25] In other words, acids act as proton donors andbases act as proton acceptors according to the followingequation:

HA+B ⇌ A− +HB+

HA: acid, B: Base, A−: conjugated base, HB+: con-jugated acid

The reverse reaction is possible, and thus the acid/baseand conjugated base/acid are always in equilibrium. Theequilibrium is determined by the acid and base dissocia-tion constants (Kₐ and K ) of the involved substances. Aspecial case of the acid-base reaction is the neutralizationwhere an acid and a base, taken at exactly same amounts,form a neutral salt.Acid-base reactions can have different definitions de-pending on the acid-base concept employed. Some ofthe most common are:

• Arrhenius definition: Acids dissociate in water re-leasing H3O+ ions; bases dissociate in water releas-ing OH− ions.

• Brønsted-Lowry definition: Acids are proton (H+)donors, bases are proton acceptors; this includes theArrhenius definition.

• Lewis definition: Acids are electron-pair accep-tors, bases are electron-pair donors; this includes theBrønsted-Lowry definition.

Solution Supernate

PrecipitateSuspension

Precipitation

7.5 Precipitation

Precipitation is the formation of a solid in a solution orinside another solid during a chemical reaction. It usu-ally takes place when the concentration of dissolved ionsexceeds the solubility limit[26] and forms an insolublesalt. This process can be assisted by adding a precipi-tating agent or by removal of the solvent. Rapid precipi-tation results in an amorphous or microcrystalline residueand slow process can yield single crystals. The lattercan also be obtained by recrystallization from microcrys-talline salts.[27]

7.6 Solid-state reactions

Reactions can take place between two solids. However,because of the relatively small diffusion rates in solids, thecorresponding chemical reactions are very slow in com-parison to liquid and gas phase reactions. They are accel-erated by increasing the reaction temperature and finelydividing the reactant to increase the contacting surfacearea.[28]

7.7 Reactions at the solid|gas interface

Reaction can take place at the solid|gas interface, sur-faces at very low pressure such as ultra-high vacuum. Viascanning tunneling microscopy, it is possible to observereactions at the solid|gas interface in real space, if the timescale of the reaction is in the correct range.[29][30] Reac-tions at the solid|gas interface are in some cases related tocatalysis.

8 9 REACTIONS IN ORGANIC CHEMISTRY

In this Paterno–Büchi reaction, a photoexcited carbonyl group isadded to an unexcited olefin, yielding an oxetane.

7.8 Photochemical reactions

In photochemical reactions, atoms and molecules absorbenergy (photons) of the illumination light and convertinto an excited state. They can then release this en-ergy by breaking chemical bonds, thereby producing rad-icals. Photochemical reactions include hydrogen–oxygenreactions, radical polymerization, chain reactions andrearrangement reactions.[31]

Many important processes involve photochemistry. Thepremier example is photosynthesis, in which most plantsuse solar energy to convert carbon dioxide and waterinto glucose, disposing of oxygen as a side-product. Hu-mans rely on photochemistry for the formation of vita-min D, and vision is initiated by a photochemical reactionof rhodopsin.[12] In fireflies, an enzyme in the abdomencatalyzes a reaction that results in bioluminescence.[32]Many significant photochemical reactions, such as ozoneformation, occur in the Earth atmosphere and constituteatmospheric chemistry.

8 Catalysis

Main article: CatalysisFurther information: Reaction Progress Kinetic AnalysisIn catalysis, the reaction does not proceed directly, but

Reaction path

Energ

y

X

Y

(X→Y)Ea(Y→X)Ea

∆H

Reaction without catalyst

Reaction with catalyst

Schematic potential energy diagram showing the effect of a cat-alyst in an endothermic chemical reaction. The presence of acatalyst opens a different reaction pathway (in red) with a loweractivation energy. The final result and the overall thermodynam-ics are the same.

Solid heterogeneous catalysts are plated on meshes in ceramiccatalytic converters in order to maximize their surface area. Thisexhaust converter is from a Peugeot 106 S2 1100

through reaction with a third substance known as catalyst.Although the catalyst takes part in the reaction, it is re-turned to its original state by the end of the reaction andso is not consumed. However, it can be inhibited, de-activated or destroyed by secondary processes. Catalystscan be used in a different phase (heterogeneous) or inthe same phase (homogeneous) as the reactants. In het-erogeneous catalysis, typical secondary processes includecoking where the catalyst becomes covered by polymericside products. Additionally, heterogeneous catalysts candissolve into the solution in a solid–liquid system or evap-orate in a solid–gas system. Catalysts can only speed upthe reaction – chemicals that slow down the reaction arecalled inhibitors.[33][34] Substances that increase the activ-ity of catalysts are called promoters, and substances thatdeactivate catalysts are called catalytic poisons. With acatalyst, a reaction which is kinetically inhibited by a highactivation energy can take place in circumvention of thisactivation energy.Heterogeneous catalysts are usually solids, powdered inorder to maximize their surface area. Of particularimportance in heterogeneous catalysis are the platinumgroup metals and other transition metals, which are usedin hydrogenations, catalytic reforming and in the syn-thesis of commodity chemicals such as nitric acid andammonia. Acids are an example of a homogeneous cat-alyst, they increase the nucleophilicity of carbonyls, al-lowing a reaction that would not otherwise proceed withelectrophiles. The advantage of homogeneous catalysts isthe ease of mixing them with the reactants, but they mayalso be difficult to separate from the products. Therefore,heterogeneous catalysts are preferred in many industrialprocesses.[35]

9 Reactions in organic chemistry

In organic chemistry, in addition to oxidation, reductionor acid-base reactions, a number of other reactions cantake place which involve covalent bonds between carbonatoms or carbon and heteroatoms (such as oxygen, nitro-gen, halogens, etc.). Many specific reactions in organic

9.1 Substitution 9

chemistry are name reactions designated after their dis-coverers.

9.1 Substitution

In a substitution reaction, a functional group in a particu-lar chemical compound is replaced by another group.[36]These reactions can be distinguished by the type of substi-tuting species into a nucleophilic, electrophilic or radicalsubstitution.

SN1 mechanism

SN2 mechanism

In the first type, a nucleophile, an atom or molecule withan excess of electrons and thus a negative charge or partialcharge, replaces another atom or part of the “substrate”molecule. The electron pair from the nucleophile at-tacks the substrate forming a new bond, while the leavinggroup departs with an electron pair. The nucleophilemay be electrically neutral or negatively charged, whereasthe substrate is typically neutral or positively charged.Examples of nucleophiles are hydroxide ion, alkoxides,amines and halides. This type of reaction is found mainlyin aliphatic hydrocarbons, and rarely in aromatic hydro-carbon. The latter have high electron density and enternucleophilic aromatic substitution only with very strongelectron withdrawing groups. Nucleophilic substitutioncan take place by two different mechanisms, SN1 andSN2. In their names, S stands for substitution, N for nu-cleophilic, and the number represents the kinetic order ofthe reaction, unimolecular or bimolecular.[37]

The three steps of an SN2 reaction. The nucleophile is

green and the leaving group is red

SN2 reaction causes stereo inversion (Walden inversion)

The SN1 reaction proceeds in two steps. First, the leavinggroup is eliminated creating a carbocation. This is fol-lowed by a rapid reaction with the nucleophile.[38]

In the SN2 mechanism, the nucleophile forms a transi-tion state with the attacked molecule, and only then theleaving group is cleaved. These two mechanisms dif-fer in the stereochemistry of the products. SN1 leadsto the non-stereospecific addition and does not result ina chiral center, but rather in a set of geometric isomers(cis/trans). In contrast, a reversal (Walden inversion) ofthe previously existing stereochemistry is observed in theSN2 mechanism.[39]

Electrophilic substitution is the counterpart of the nu-cleophilic substitution in that the attacking atom ormolecule, an electrophile, has low electron density andthus a positive charge. Typical electrophiles are the car-bon atom of carbonyl groups, carbocations or sulfur ornitronium cations. This reaction takes place almost ex-clusively in aromatic hydrocarbons, where it is calledelectrophilic aromatic substitution. The electrophile at-tack results in the so-called σ-complex, a transition statein which the aromatic system is abolished. Then, the leav-ing group, usually a proton, is split off and the aromatic-ity is restored. An alternative to aromatic substitution iselectrophilic aliphatic substitution. It is similar to the nu-cleophilic aliphatic substitution and also has two majortypes, SE1 and SE2[40]

Mechanism of electrophilic aromatic substitution

In the third type of substitution reaction, radical substi-tution, the attacking particle is a radical.[36] This pro-cess usually takes the form of a chain reaction, for exam-ple in the reaction of alkanes with halogens. In the firststep, light or heat disintegrates the halogen-containingmolecules producing the radicals. Then the reactionproceeds as an avalanche until two radicals meet andrecombine.[41]

X·+ R−H −→ X−H+ R·R·+ X2 −→ R−X+ X·

10 9 REACTIONS IN ORGANIC CHEMISTRY

Reactions during the chain reaction of radical sub-stitution

9.2 Addition and elimination

The addition and its counterpart, the elimination, are re-actions which change the number of substitutents on thecarbon atom, and form or cleave multiple bonds. Doubleand triple bonds can be produced by eliminating a suitableleaving group. Similar to the nucleophilic substitution,there are several possible reaction mechanisms which arenamed after the respective reaction order. In the E1mechanism, the leaving group is ejected first, forming acarbocation. The next step, formation of the double bond,takes place with elimination of a proton (deprotonation).The leaving order is reversed in the E1cbmechanism, thatis the proton is split off first. This mechanism requiresparticipation of a base.[42] Because of the similar condi-tions, both reactions in the E1 or E1cb elimination alwayscompete with the SN1 substitution.[43]

E2 elimination

The E2 mechanism also requires a base, but there theattack of the base and the elimination of the leavinggroup proceed simultaneously and produce no ionic in-termediate. In contrast to the E1 eliminations, differentstereochemical configurations are possible for the reac-tion product in the E2 mechanism, because the attack ofthe base preferentially occurs in the anti-position with re-spect to the leaving group. Because of the similar condi-tions and reagents, the E2 elimination is always in com-petition with the SN2-substitution.[44]

Electrophilic addition of hydrogen bromide

The counterpart of elimination is the addition where dou-ble or triple bonds are converted into single bonds. Simi-lar to the substitution reactions, there are several types ofadditions distinguished by the type of the attacking parti-cle. For example, in the electrophilic addition of hydro-gen bromide, an electrophile (proton) attacks the doublebond forming a carbocation, which then reacts with thenucleophile (bromine). The carbocation can be formedon either side of the double bond depending on the groups

attached to its ends, and the preferred configuration canbe predicted with the Markovnikov’s rule.[45] This rulestates that “In the heterolytic addition of a polar moleculeto an alkene or alkyne, the more electronegative (nucle-ophilic) atom (or part) of the polar molecule becomes at-tached to the carbon atom bearing the smaller number ofhydrogen atoms.”[46]

If the addition of a functional group takes place at theless substituted carbon atom of the double bond, then theelectrophilic substitution with acids is not possible. Inthis case, one has to use the hydroboration–oxidation re-action, where in the first step, the boron atom acts as elec-trophile and adds to the less substituted carbon atom. Atthe second step, the nucleophilic hydroperoxide or halo-gen anion attacks the boron atom.[47]

While the addition to the electron-rich alkenes andalkynes is mainly electrophilic, the nucleophilic addi-tion plays an important role for the carbon-heteroatommultiple bonds, and especially its most important rep-resentative, the carbonyl group. This process is oftenassociated with an elimination, so that after the reac-tion the carbonyl group is present again. It is thereforecalled addition-elimination reaction andmay occur in car-boxylic acid derivatives such as chlorides, esters or an-hydrides. This reaction is often catalyzed by acids orbases, where the acids increase by the electrophilicityof the carbonyl group by binding to the oxygen atom,whereas the bases enhance the nucleophilicity of the at-tacking nucleophile.[48]

Acid-catalyzed addition-elimination mechanism

Nucleophilic addition of a carbanion or anothernucleophile to the double bond of an alpha, beta unsat-urated carbonyl compound can proceed via the Michaelreaction, which belongs to the larger class of conjugateadditions. This is one of the most useful methods for themild formation of C–C bonds.[49][50][51]

Some additions which can not be executed with nucle-ophiles and electrophiles, can be succeeded with free rad-icals. As with the free-radical substitution, the radical ad-dition proceeds as a chain reaction, and such reactions arethe basis of the free-radical polymerization.[52]

9.3 Other organic reaction mechanisms

+

11

The Cope rearrangement of 3-methyl-1,5-hexadiene

Mechanism of a Diels-Alder reaction

Orbital overlap in a Diels-Alder reaction

In a rearrangement reaction, the carbon skeleton of amolecule is rearranged to give a structural isomer of theoriginal molecule. These include hydride shift reactionssuch as the Wagner-Meerwein rearrangement, where ahydrogen, alkyl or aryl group migrates from one carbonto a neighboring carbon. Most rearrangements are asso-ciated with the breaking and formation of new carbon-carbon bonds. Other examples are sigmatropic reactionsuch as the Cope rearrangement.[53]

Cyclic rearrangements include cycloadditions and, moregenerally, pericyclic reactions, wherein two or more dou-ble bond-containing molecules form a cyclic molecule.An important example of cycloaddition reaction is theDiels–Alder reaction (the so-called [4+2] cycloaddition)between a conjugated diene and a substituted alkene toform a substituted cyclohexene system.[54]

Whether or not a certain cycloaddition would proceeddepends on the electronic orbitals of the participatingspecies, as only orbitals with the same sign of wave func-tion will overlap and interact constructively to form newbonds. Cycloaddition is usually assisted by light or heat.These perturbations result in different arrangement ofelectrons in the excited state of the involved moleculesand therefore in different effects. For example, the [4+2]Diels-Alder reactions can be assisted by heat whereas the[2+2] cycloaddition is selectively induced by light.[55] Be-cause of the orbital character, the potential for developingstereoisomeric products upon cycloaddition is limited, asdescribed by the Woodward–Hoffmann rules.[56]

10 Biochemical reactions

Biochemical reactions are mainly controlled by enzymes.These proteins can specifically catalyze a single reaction,so that reactions can be controlled very precisely. Thereaction takes place in the active site, a small part of the

Substrate enteringactive site of enzyme

Enzyme/substratecomplex

Enzyme/productscomplex

Products leavingactive site of enzyme

Products Substrate

Active site

Enzyme changes shapeslightly as substrate binds

Illustration of the induced fit model of enzyme activity

enzyme which is usually found in a cleft or pocket linedby amino acid residues, and the rest of the enzyme isused mainly for stabilization. The catalytic action of en-zymes relies on several mechanisms including the molec-ular shape (“induced fit”), bond strain, proximity and ori-entation of molecules relative to the enzyme, proton do-nation or withdrawal (acid/base catalysis), electrostaticinteractions and many others.[57]

The biochemical reactions that occur in living organismsare collectively known as metabolism. Among the mostimportant of its mechanisms is the anabolism, in whichdifferent DNA and enzyme-controlled processes result inthe production of large molecules such as proteins andcarbohydrates from smaller units.[58] Bioenergetics stud-ies the sources of energy for such reactions. An impor-tant energy source is glucose, which can be produced byplants via photosynthesis or assimilated from food. Allorganisms use this energy to produce adenosine triphos-phate (ATP), which can then be used to energize otherreactions.

11 Applications

Thermite reaction proceeding in railway welding. Shortly afterthis, the liquid iron flows into the mould around the rail gap

Chemical reactions are central to chemical engineeringwhere they are used for the synthesis of new compoundsfrom natural raw materials such as petroleum and min-eral ores. It is essential to make the reaction as efficientas possible, maximizing the yield and minimizing the

12 14 REFERENCES

amount of reagents, energy inputs and waste. Catalystsare especially helpful for reducing the energy required forthe reaction and increasing its reaction rate.[59][60]

Some specific reactions have their niche applications. Forexample, the thermite reaction is used to generate lightand heat in pyrotechnics and welding. Although it is lesscontrollable than the more conventional oxy-fuel weld-ing, arc welding and flash welding, it requires much lessequipment and is still used to mend rails, especially inremote areas.[61]

12 Monitoring

Mechanisms of monitoring chemical reactions dependstrongly on the reaction rate. Relatively slow processescan be analyzed in situ for the concentrations and iden-tities of the individual ingredients. Important tools ofreal time analysis are the measurement of pH and anal-ysis of optical absorption (color) and emission spectra.A less accessible but rather efficient method is introduc-tion of a radioactive isotope into the reaction and mon-itoring how it changes over time and where it moves to;this method is often used to analyze redistribution of sub-stances in the human body. Faster reactions are usu-ally studied with ultrafast laser spectroscopy where uti-lization of femtosecond lasers allows short-lived transi-tion states to be monitored at time scaled down to a fewfemtoseconds.[62]

13 See also

• Chemical reaction

• Substrate• Reagent• Catalyst• Product

• Chemical reaction model

• Chemist

• Chemistry

• Limiting reagent

• List of organic reactions

• Organic reaction

• Reaction progress kinetic analysis

• Combustion

• Mass balance

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[9] Corey, E. J. (1988). “Robert Robinson Lecture. Ret-rosynthetic thinking?essentials and examples”. ChemicalSociety Reviews 17: 111. doi:10.1039/CS9881700111.

[10] IUPAC, Compendium of Chemical Terminology, 2nd ed.(the “Gold Book”) (1997). Online corrected version:(2006–) "elementary reaction".

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15

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