Question

 Give reasons for the following:

(CH₃)₃C–O–CH₃ on reaction with HI gives (CH₃)₃C–I and CH₃–OH as the main products and not (CH₃)₃C–OH and CH₃–I.

Answer 1

Usually, iodide, being a big nucleophile, attacks on the group with low steric hindrance and the reaction proceeds by S_N2 mechanism.

However, in this case, methanol, on leaving generates a tertiary carbocation, which is more stable. Hence, this reaction proceeds by S_N1 mechanism and therefore, we get (CH₃)₃C-I and CH₃-OH as the major products.

Answer 2

CH₃)₃-C–O–CH₃ is an ether with two different alkyl groups, of which (CH₃)₃-C-, a tertiary alkyl group, on reaction with hydrogen halide (HI) forms a tertiary halide. This occurs as the reaction is an S_N1 reaction. The reaction involves the formation of a stable carbocation. If the ether has a primary alkyl group, then the reaction follows the S_N2 mechanism.