Question

For the compound having molecular formula \(C_4H_9Br\), write:
(a) the isomer which is most reactive towards S_N1 displacement.
(b) the isomer which, on reacting with Na metal in dry ether, gives 2,5-dimethylhexane.

Hint:

For \(S_N1\), choose the halide giving the most stable carbocation. For Wurtz reaction, split the product into two identical alkyl groups.

Answer 1

Concept:
For \(S_N1\) reactions, reactivity depends on carbocation stability. \[ 3^\circ \gt 2^\circ \gt 1^\circ \gt CH_3 \] Wurtz reaction involves reaction of alkyl halides with sodium metal in dry ether. \[ 2R-X+2Na \rightarrow R-R+2NaX \]

Step 1: Find the isomer most reactive towards \(S_N1\).
Among isomers of \(C_4H_9Br\), tert-butyl bromide forms the most stable carbocation. \[ (CH_3)_3CBr \rightarrow (CH_3)_3C^+ \] The carbocation is tertiary and highly stable. Therefore, it is most reactive towards \(S_N1\). \[ \boxed{(CH_3)_3CBr} \]

Step 2: Find isomer giving 2,5-dimethylhexane in Wurtz reaction.
The product is: \[ CH_3-CH(CH_3)-CH_2-CH_2-CH(CH_3)-CH_3 \] This product is formed by coupling of two isobutyl groups. So the alkyl halide must be isobutyl bromide: \[ (CH_3)_2CHCH_2Br \] Wurtz reaction: \[ 2(CH_3)_2CHCH_2Br+2Na \rightarrow (CH_3)_2CHCH_2CH_2CH(CH_3)_2+2NaBr \] Hence: \[ \boxed{(a)\;(CH_3)_3CBr} \] \[ \boxed{(b)\;(CH_3)_2CHCH_2Br} \]