Addition Reactions: Electrophilic Addition, Cycloaddition, and Catalytic Hydrogenation of Alkenes and Alkynes
Addition reactions are the characteristic transformations of alkenes and alkynes. The pi bonds of unsaturated hydrocarbons are electron-rich and react with electrophiles, restoring the sigma bond framework while incorporating new atoms or groups. Addition reactions are among the most important in organic chemistry, used industrially to produce polymers, alcohols, halogenated compounds, and hydrogenated fats. The global production of ethanol through acid-catalyzed hydration of ethylene exceeds 50 million metric tons annually. Understanding addition reactions provides the foundation for predicting the behavior of unsaturated compounds in synthesis and biological systems.
Electrophilic Addition to Alkenes
General Mechanism
Electrophilic addition to alkenes proceeds through a two-step mechanism. First, the electrophile attacks the pi bond, forming a carbocation intermediate. Second, the nucleophile attacks the carbocation to give the addition product. The reaction is typically exothermic because two sigma bonds replace one pi bond. The pi bond energy is approximately 250 kJ/mol, while two sigma bonds provide approximately 300 to 400 kJ/mol. The reaction is governed by Markovnikov’s rule — the electrophile adds to the less substituted carbon, forming the more stable carbocation intermediate.
Hydrogen Halide Addition
Addition of hydrogen halides — HCl, HBr, HI — follows Markovnikov’s rule. The proton adds to the less substituted carbon, and the halide adds to the more substituted carbon. The reaction rate increases with increasing carbocation stability — tertiary alkenes react faster than secondary, which react faster than primary. Carbocation rearrangements can occur, leading to unexpected products. Addition of HBr in the presence of peroxides gives anti-Markovnikov addition through a radical mechanism — the bromine radical adds first, followed by hydrogen abstraction.
Acid-Catalyzed Hydration
Addition of water to alkenes requires acid catalysis. The proton adds to form the carbocation, and water attacks the carbocation to form the protonated alcohol. Deprotonation gives the alcohol product. The reaction is reversible — the equilibrium favors the alkene for simple alkenes but favors the alcohol for more substituted alkenes. The equilibrium constant for hydration of isobutylene to tert-butanol is 1.0 at 25 degrees Celsius. Industrial hydration of ethylene to ethanol requires high pressure and phosphoric acid catalyst on a solid support.
Halogen Addition
Bromine and chlorine add to alkenes through a cyclic halonium ion intermediate. The halogen molecule is polarized as it approaches the pi bond, forming a bridged halonium ion with the halogen bridging both carbons. The second halide anion attacks from the opposite side of the halonium ion, giving anti addition — the two halogen atoms end up on opposite faces of the original double bond. The reaction is stereospecific — cis alkenes give racemic mixtures of dl pairs, and trans alkenes give meso compounds. Bromine in carbon tetrachloride is a qualitative test for unsaturation — the red-brown bromine color disappears as the colorless dibromide forms.
Oxymercuration-Demercuration
Oxymercuration-demercuration achieves Markovnikov addition of water without carbocation rearrangement. Mercury(II) acetate adds to the alkene to form a mercurinium ion, which is attacked by water. Reduction with sodium borohydride replaces mercury with hydrogen. The reaction proceeds under mild conditions with excellent yields and no rearrangement products. The method is preferred over direct acid-catalyzed hydration for most synthetic applications.
Hydroboration-Oxidation
Hydroboration-oxidation achieves anti-Markovnikov addition of water with syn stereochemistry. Borane adds to the alkene in a concerted, four-centered reaction — the boron adds to the less substituted carbon and the hydrogen adds to the more substituted carbon. The reaction proceeds through repeated additions — one borane molecule reacts with up to three alkenes. Oxidation with hydrogen peroxide and sodium hydroxide replaces boron with hydroxyl, retaining the stereochemistry. The overall transformation adds water across the double bond with anti-Markovnikov regiochemistry and syn stereochemistry.
Addition to Alkynes
Hydrogen Halide Addition
Alkynes add one or two equivalents of hydrogen halide. The first addition gives the vinyl halide, and the second addition gives the geminal dihalide. Both additions follow Markovnikov’s rule. The reaction with terminal alkynes gives the product with the halide on the more substituted carbon. HBr addition to alkynes can also proceed through a radical mechanism to give anti-Markovnikov products.
Halogen Addition
Bromine and chlorine add to alkynes to give trans-dihaloalkenes through a vinyl halonium ion intermediate. Further addition produces tetrahaloalkanes. The stereochemistry is trans because the halide attacks the halonium ion from the opposite face. Bromine addition to alkynes is slower than to alkenes, allowing selective addition to alkynes in the presence of alkenes under controlled conditions.
Hydration
Alkyne hydration with mercury(II) sulfate and sulfuric acid gives enols that tautomerize to carbonyl compounds. Internal alkynes give ketones. Terminal alkynes give methyl ketones — the hydration follows Markovnikov’s rule with the hydroxyl adding to the more substituted carbon. Anti-Markovnikov hydration is achieved with hydroboration-oxidation, giving aldehydes from terminal alkynes.
Hydroboration of Alkynes
Hydroboration of alkynes with dialkylboranes or disiamylborane gives vinylboranes. Oxidation with hydrogen peroxide produces enols that tautomerize to aldehydes for terminal alkynes. The reaction proceeds with syn stereochemistry — the boron and hydrogen add from the same face. Bulky borane reagents give high regioselectivity, placing boron at the less hindered terminal carbon.
Catalytic Hydrogenation
Heterogeneous Hydrogenation
Hydrogenation of alkenes and alkynes over metal catalysts — palladium, platinum, nickel — adds hydrogen across the multiple bond. The reaction is heterogeneous, occurring on the metal surface. Both hydrogen atoms add from the same face of the alkene or alkyne — syn addition. The reaction is highly exothermic and often requires careful temperature control. Lindlar’s catalyst — palladium on calcium carbonate poisoned with lead — selectively hydrogenates alkynes to cis alkenes without further reduction to alkanes.
Homogeneous Hydrogenation
Wilkinson’s catalyst — RhCl(PPh₃)₃ — catalyzes hydrogenation in solution. The mechanism involves oxidative addition of hydrogen, coordination of the alkene, and reductive elimination of the alkane. The stereochemistry remains syn addition. Homogeneous catalysts can achieve higher selectivity and operate under milder conditions than heterogeneous catalysts. Asymmetric hydrogenation using chiral catalysts achieves high enantioselectivity — the Noyori asymmetric hydrogenation of beta-keto esters is used industrially to produce enantiopure alcohols.
Stereochemistry of Addition
Addition of hydrogen, borane, osmium tetroxide, and potassium permanganate proceeds with syn stereochemistry. Addition of halogens, peroxyacids, and hydrohalogenation proceed with anti stereochemistry. The stereochemistry is determined by the reaction mechanism — whether the two groups add through a cyclic transition state giving syn addition or through an intermediate that blocks one face giving anti addition.
Cycloaddition Reactions
Diels-Alder Reaction
The Diels-Alder reaction between a conjugated diene and a dienophile is the most important cycloaddition reaction. The reaction forms a six-membered ring with up to four stereocenters in a single step. The reaction is stereospecific — the diene and dienophile maintain their stereochemistry in the product. The endo rule predicts that the major product has the dienophile’s electron-withdrawing groups oriented toward the diene. The reaction is accelerated by electron-donating groups on the diene and electron-withdrawing groups on the dienophile.
Epoxidation
Epoxidation of alkenes with peroxyacids — typically meta-chloroperoxybenzoic acid — forms epoxides in a single step. The reaction is stereospecific — the configuration of the alkene is preserved in the epoxide. The mechanism involves direct transfer of oxygen from the peroxyacid to the alkene through a butterfly transition state. Epoxides are valuable synthetic intermediates because they undergo regioselective ring-opening reactions with a wide range of nucleophiles.
Dihydroxylation
Osmium tetroxide catalyzed dihydroxylation adds two hydroxyl groups across the double bond with syn stereochemistry. The reaction proceeds through a cyclic osmate ester intermediate that is cleaved with sodium sulfite or sodium bisulfite to release the diol. Asymmetric dihydroxylation using chiral ligands — the Sharpless asymmetric dihydroxylation — achieves high enantioselectivity. Potassium permanganate also gives syn dihydroxylation but is less controlled and often cleaves the diol to carbonyl products.
Ozonolysis
Ozonolysis cleaves alkenes to carbonyl compounds. Ozone adds to the double bond to form a primary ozonide, which rearranges to a secondary ozonide. Reductive workup with dimethyl sulfide or triphenylphosphine gives aldehydes. Oxidative workup with hydrogen peroxide gives carboxylic acids. Ozonolysis is valuable for determining the position of double bonds in unknown structures and for cleaving cyclic alkenes to open-chain dicarbonyl compounds.
Industrial Addition Reactions
The largest-scale addition reaction is the hydration of ethylene to ethanol, catalyzed by phosphoric acid on silica at high temperature and pressure. Approximately 50 million metric tons of bioethanol are produced annually through fermentation, with about 5 million tons produced synthetically through hydration. Polymerization of ethylene and propylene — coordination polymerization using Ziegler-Natta catalysts or metallocene catalysts — produces polyethylene and polypropylene with annual production exceeding 150 million metric tons.
Hydrogenation of vegetable oils converts unsaturated fats to saturated fats, producing margarine and shortenings. The process uses nickel catalysts at moderate temperature and pressure. Partial hydrogenation produces trans fats, which are now recognized as health hazards and are being phased out from food production. Selective hydrogenation using modified catalysts minimizes trans fat formation while achieving the desired melting characteristics.
Frequently Asked Questions
What is the difference between Markovnikov and anti-Markovnikov addition? Markovnikov addition places the hydrogen on the less substituted carbon and the other group on the more substituted carbon. Anti-Markovnikov addition places the hydrogen on the more substituted carbon. Markovnikov addition proceeds through carbocation intermediates, while anti-Markovnikov addition typically involves radical or metal-mediated mechanisms.
How do I convert an alkyne to a cis alkene selectively? Use Lindlar’s catalyst — palladium on calcium carbonate poisoned with lead or quinoline. The poisoned catalyst prevents over-reduction to the alkane and gives exclusively the cis alkene. Alternative methods include hydrogenation with nickel boride.
What controls the stereochemistry of addition reactions? The mechanism determines stereochemistry. Concerted addition through a cyclic transition state gives syn addition. Stepwise addition through a cyclic onium ion gives anti addition. Catalytic hydrogenation on metal surfaces gives syn addition because both hydrogen atoms come from the metal surface.
Alkanes, Alkenes, and Alkynes — Reaction Mechanisms — Organic Synthesis Strategies