CBSE Class 12 · 2010 · All India · Haloalkanes and HaloarenesShort Answer

S${}_{\mathrm{N}}$1: a two-step mechanism — slow ionisation to a planar carbocation, then fast attack by the nucleophile from either face.
Rate = k[RX] (first order); partial \textbf{racemisation}; order of reactivity 3° > 2° > 1°; favoured by polar protic solvents.
Example: hydrolysis of tert-butyl bromide, (CH${}_3$)${}_3$CBr + OH${}^{-}$ → (CH${}_3$)${}_3$COH.
S${}_{\mathrm{N}}$2: a single-step concerted backside attack through a 5-coordinate transition state.
Rate = k[RX][Nu] (second order); complete \textbf{inversion} of configuration (Walden); reactivity 1° > 2° > 3°.
Example: CH${}_3$Cl + OH${}^{-}$ → CH${}_3$OH + Cl${}^{-}$.