Question

1. Why alcohols need to be activated in order to have the hydroxyl group substituted?

2. What is carbocation rearrangement? Why in some particular reactions there is no carbocation rearrangement even if there is a carbocation?

Answer 1

Suppose, we are going to prepare chloroalkanes from alcohols.   When primary alcohols react with HCl,we require anhydrous ZnCl2 as an activator because primary alcohol is quite stable and unreactive,without an activator substitution of -OH by -Cl is very difficult. However,with tertiary alcohols, the reaction with HCl occurs at room temperature,without the use of any activator because tertiary alcohol is highly reactive among the alcohols giving tertiary-chloroalkane.    2) Carbocation rearrangement is the formation of most stable carbocation intermediate through migration of certain groups or atoms based upon their migratory aptitude to give the mist stable priduct,because most stable intermediate could result in most stable product. The migratory aptitude is as per the increasing order CH3- < Ph-< H-. However inspite of presence of carbocation in some cases e.g in vase of benzyl carbocation, carbocation rearrangement doesnt occur since,the benzyl carbocation is itself stabilised through resonance.So,there is no need for carbocation rearrangement. Hence,the reaction of benzyl chloride with aqueos KOH occurs wuthout rearrangement via SN1 reaction to give benzyl alcohol.