Nucleophilic Substitution Reactions of Alcohols

Hydroxide is a poor leaving group, so before any substitution the alcohol's -OH is first activated -- protonated by an acid to the good leaving group -OH2+, which a nucleophile such as Br- can then displace, giving an alkyl halide. With hydrohalic acids: CH3-CH2-OH + HBr gives CH3-CH2-Br + H2O (tertiary alcohols additionally need heating). Primary alcohols react by the SN2 mechanism, with simultaneous nucleophilic attack of Br- and departure of water in one step; tertiary alcohols react by the SN1 mechanism via a carbocation (protonation, then slow loss of water to the cation, then fast capture by Br-; the intermediate carbocation can also lose a proton to give an alkene as a side product, which can itself re-add HBr). Alcohols are also converted to alkyl halides with PCl3/PBr3 (an SN2 attack on phosphorus), and with thionyl chloride (SOCl2, in the presence of py …