Question

Which of the compounds will give ${S_n}1$ reaction?
a.
b.$C{H_3} - Cl$
c.$C{H_3} - C{H_2} - Br$
d.

Answer 1

Basically there are two types of nucleophilic substitution reactions, that is, ${S_n}1$ and ${S_n}2$ reaction. Here we are asked about the ${S_n}1$ reaction, so this reaction is a unimolecular reaction and the rate of this reaction is dependent on the concentration of only one reactant.

Complete Step By Step Answer:
Nucleophilic substitution reactions are those reactions in which a nucleophile which is electron-rich, attacks electrophile which is positively charged, in order to replace the leaving group. There are two types of nucleophilic reactions, that is, ${S_n}1$ and ${S_n}2$ reactions. So, ${S_n}1$ reaction is known as a unimolecular reaction, so its rate is dependent upon the concentration of only one reactant. This is a two-step reaction. Whereas ${S_n}2$ reaction is a one-step process and is bi-molecular, so its rate is dependent upon both the concentration present in the reaction.
In ${S_n}1$ reaction, tertiary carbocations are considered to be most stable, therefore tertiary carbocations are most reactive towards ${S_n}1$ reaction as compared to primary and secondary. As more is the stability of carbocation, more is the reactivity.
So, as per the stability of the carbocation,
option (A) will give ${S_n}1$ reaction, because here, the formation of stable tertiary carbocation will take place, so this will give us ${S_n}1$ reaction. This molecule will always go for ${S_n}1$ reaction.

Therefore, the correct option is option (A).

Note :
Since, we know that ${S_n}1$ is a unimolecular reaction, so its rate will be dependent upon halo alkenes only not on nucleophiles. There are several other factors on which ${S_n}1$ reaction depends, such as polar protic solvent is required here in this, the better-leaving group should be there, that increases the rate of the reaction, weak nucleophiles are preferred.