Question

Which of the following is not true for $S_{N}1$ reactions ?

• A. These occur through a single step.
• B. These are favoured by polar solvents.
• C. $3^o$ alkyl halides generally react through this mechanism.
• D. Concentration of nucleophile does not affect the rate of such reactions.

Answer 1

The correct option is A These occur through a single step.

The mechanism above shows a $S_{N}1$ reaction.

The first step involves the slow loss of leaving group $L$ giving a carbocation. In the second step, attack by nucleophile $\left(N{u}^{-}\right)$ occurs on the carbon centre of carbocation. $S_{N}1$ mechanism proceeds through carbocation formation and so, it is a two-step process and like the stability of carbocation increases, reaction favours $S_{N}1$ mechanism. This is because the formation of the carbocation is a slow and rate-determining step. Thus, the greater is the stability of carbocation, greater will be the possibility of its formation and hence, greater will be the rate.

$3^o$ carbocations are more stable than others due to the electron donating effect of three alkyl groups. Polar solvents also increases the stability of carbocation by surrounding it through negative ends. So, they both favour $S_{N}1$.Rate of $S_{N}1$ mechanism depends on nucleophile concentration only. So, as the concentration of nucleophile increases, the reaction favours $S_{N}1$ mechanism.

Hence, the statement that the $S_{N}1$ reactions occur through a single step is incorrect.

$\therefore$ Option A is the correct answer.