Question

For the compound having molecular formula C$_4$H$_9$Br, write : (a) the isomer which is most reactive towards SN1 displacement. (b) the isomer which, on reacting with Na metal in the presence of dry ether, gives 2,5-Dimethylhexane.

Hint:

For SN1 reactions, always identify the most stable carbocation. For Wurtz reaction, split the product alkane symmetrically into two identical alkyl fragments and then attach halogen to the terminal carbon of each fragment.

Answer 1

Step 1: Isomers of $\text{C}_4\text{H}_9\text{Br}$.
Four structural isomers: 1-bromobutane ($1^\circ$), 2-bromobutane ($2^\circ$), 1-bromo-2-methylpropane ($1^\circ$, isobutyl bromide), 2-bromo-2-methylpropane ($3^\circ$, tert-butyl bromide).
Step 2: (a) Most reactive towards SN1.
SN1 rate depends on carbocation stability: $3^\circ > 2^\circ > 1^\circ$. 2-Bromo-2-methylpropane $(\text{CH}_3)_3\text{CBr}$ forms the stable tertiary carbocation $(\text{CH}_3)_3\text{C}^+$ and is most reactive towards SN1.
Step 3: (b) Isomer giving 2,5-dimethylhexane (Wurtz reaction).
Wurtz: $2\text{RBr} + 2\text{Na} \rightarrow \text{R-R} + 2\text{NaBr}$. 2,5-Dimethylhexane $(\text{CH}_3)_2\text{CHCH}_2\text{CH}_2\text{CH}(\text{CH}_3)_2$ is two isobutyl groups joined. Required bromide: 1-bromo-2-methylpropane $(\text{CH}_3)_2\text{CHCH}_2\text{Br}$. \[ \boxed{\text{(a) 2-Bromo-2-methylpropane, (b) 1-Bromo-2-methylpropane}} \]