Addition Reactions

An addition reaction is the characteristic reaction of an unsaturated compound -- one containing a localised C=C double bond or C$\equiv$C triple bond. In an addition reaction, two molecules combine to give a single product: one $\pi$ bond of the substrate breaks, and two new sigma bonds form in its place. Correspondingly, the substrate's hybridisation changes during the reaction -- from $sp^2$ to $sp^3$ for the addition reactions of alkenes, and from $sp$ to $sp^2$ for the addition reactions of alkynes.

Like substitution, addition reactions are classified by the nature of the initiating reagent into three types:

1. Electrophilic addition. The textbook example is the bromination of ethylene: $Br_2$ polarises as it approaches the electron-rich double bond, and the electrophilic end ($Br^+$) adds first, generating a bromonium-ion-like intermediate that the released $Br^-$ then captures, giving the addition product 1,2-dibromoethane,

   $H_2C=CH_2 + Br_2 \longrightarrow BrCH_2\text{-}CH_2Br$

2. Nucleophilic addition. The textbook example is the addition of HCN to acetaldehyde: the nucleophile $CN^-$ attacks the electrophilic carbonyl carbon while the C=O $\pi$ electrons shift onto oxygen (the $-E$ electromeric effect from the earlier section), and the resulting alkoxide picks up a proton to give acetaldehyde cyanohydrin, $CH_3CHO + HCN \longrightarrow CH_3\text{-}CH(OH)(CN)$ …