Which would undergo $S_{N} 1$ reactions faster in the following pair?
$C H_{3} - C H_{2} - C H_{2} - B r$ and $C H_{3} - C H - C H_{3}$
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$B r$

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Solution

In the $S_{N} 1$ reaction, the big barrier is carbocation stability. Since the first step of the $S_{N} 1$ reaction is the loss of a leaving group to give a carbocation, the rate of the reaction will be proportional to the stability of the carbocation. Carbocation stability increases with increasing substitution of the carbon ( $t e r t i a r y > s e c o n d a r y > p r i m a r y$ ) as well as with resonance.
$C H_{3} C H (B r) C H_{3}$ is faster than $C H_{3} C H_{2} C H_{2} B r$ .

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