Addition Reactions Explained: Electrophilic and Nucleophilic Addition

Electrophilic Addition to Alkenes

The carbon-carbon double bond in alkenes is a region of high electron density due to the pi electrons above and below the bond axis. This electron-rich site attracts electrophiles (electron-poor species), initiating electrophilic addition. The general mechanism involves two steps: first, the electrophile attacks the pi bond to form a carbocation (or a cyclic intermediate like a bromonium ion), then a nucleophile attacks the carbocation to complete the addition.

Hydrohalogenation adds HX (HCl, HBr, HI) across the double bond. The proton (electrophile) adds first, forming a carbocation, then the halide ion (nucleophile) attacks. Markovnikov rule governs the regiochemistry: the proton adds to the less substituted carbon of the double bond, and the halide adds to the more substituted carbon. This selectivity arises because the reaction proceeds through the more stable (more substituted) carbocation intermediate.

Halogenation adds X2 (Br2 or Cl2) across the double bond through a cyclic halonium ion intermediate rather than an open carbocation. The halonium ion (a three-membered ring with the halogen bridging the two carbons) prevents nucleophilic attack from the same face, so the second halide attacks from the opposite face. This produces anti addition: the two halogen atoms end up on opposite sides of the former double bond.

Hydration adds water across the double bond to produce an alcohol. Acid-catalyzed hydration follows Markovnikov rule (the OH ends up on the more substituted carbon) and proceeds through a carbocation intermediate. Oxymercuration-demercuration achieves the same Markovnikov regiochemistry without carbocation rearrangements. Hydroboration-oxidation produces the anti-Markovnikov alcohol (OH on the less substituted carbon) with syn addition stereochemistry.

Catalytic hydrogenation adds H2 across the double bond using a metal catalyst (typically platinum, palladium, or nickel) to produce an alkane. Both hydrogen atoms add to the same face of the double bond (syn addition) because the reaction occurs on the catalyst surface where the alkene is adsorbed. This is one of the most important industrial reactions, used to hydrogenate vegetable oils (making margarine) and to reduce unsaturated intermediates in pharmaceutical synthesis.

Nucleophilic Addition to Carbonyls

The carbonyl group (C=O) has the opposite electronic character from alkenes: the oxygen is more electronegative than carbon, making the carbonyl carbon electrophilic (partial positive charge) and the oxygen nucleophilic (partial negative charge). Nucleophiles attack the electrophilic carbon, breaking the pi bond and pushing the electron pair onto oxygen to form an alkoxide intermediate.

Grignard reagents (RMgBr) and organolithium compounds (RLi) add carbon nucleophiles to carbonyls, forming new carbon-carbon bonds. Adding a Grignard reagent to formaldehyde gives a primary alcohol, to an aldehyde gives a secondary alcohol, and to a ketone gives a tertiary alcohol. This carbon-carbon bond-forming ability makes Grignard reactions indispensable in organic synthesis.

Hydride reducing agents (NaBH4 and LiAlH4) deliver a hydride ion (H-) to the carbonyl carbon. Sodium borohydride (NaBH4) is a mild reagent that selectively reduces aldehydes and ketones to alcohols without affecting esters or carboxylic acids. Lithium aluminum hydride (LiAlH4) is more powerful and reduces aldehydes, ketones, esters, carboxylic acids, and amides.

Aldehydes and ketones react with alcohols to form hemiacetals and acetals, with water to form hydrates (geminal diols), with amines to form imines (Schiff bases), and with hydrogen cyanide to form cyanohydrins. Each of these reactions follows the same fundamental pattern: nucleophilic attack on the electrophilic carbonyl carbon, followed by proton transfer steps that complete the transformation.

Addition to Alkynes

Alkynes undergo the same types of addition reactions as alkenes but can react with one or two equivalents of reagent. Adding one equivalent of HBr to a terminal alkyne gives a vinyl halide following Markovnikov selectivity. Adding two equivalents gives a geminal dihalide. Selective partial reduction of alkynes is a valuable synthetic tool: Lindlar catalyst (poisoned palladium on calcium carbonate) reduces an alkyne to a cis-alkene, while sodium in liquid ammonia (dissolving metal reduction) gives a trans-alkene.

Stereochemistry of Addition

Addition reactions can be classified by their stereochemical outcome. Syn addition places both new groups on the same face of the double bond (examples: catalytic hydrogenation, hydroboration, dihydroxylation with osmium tetroxide). Anti addition places the groups on opposite faces (examples: halogenation through a halonium ion, epoxidation followed by ring opening). Understanding these stereochemical patterns is essential for predicting the three-dimensional structure of products, especially when the reaction creates new stereocenters.