Alcohols to alkyl halides with H-X

(substitution reactions - S_N1 and S_N2 mechanisms)

 

S_N1 mechanism (nucleophilic substitution unimolecular)···

For secondary and tertiary alcohols

(carbocation intermediates, 3 steps)

Relative order of alcohol reactivity = carbocation stability order = 3^o > 2^o > 1^o > methyl

Relative HX reactivity = acidity order = H-I > H-Br > H-Cl

 

S_N2 mechanism (nucleophilic substitution bimolecular)···

For primary alcohols

(1^o carbocations are unstable; 2 steps)