Question

Allyl chloride is more reactive than n-propyl chloride towards nucleophilic substitution reaction. Explain why?

Answer 1

Hint: In nucleophilic substitution reaction the electron rich nucleophile attacks the electrophile which is positively charged. There are two types of Nucleophilic substitution reactions which are faster for a compound with a stable carbocation.

Complete step by step answer:
Nucleophilic substitution reaction is a reaction in which the electron rich nucleophile attacks the electrophile which is positively charged to replace a group from the compound. There are two types of Nucleophilic substitution reactions which are known as SN1 and SN2.
SN1 is a nucleophilic substitution reaction which is unimolecular and the rate of the reaction depends on the concentration of one reactant. The removal of one group and the addition of the other occur in two different steps. They describe a bond breaking process. SN2 is a nucleophilic substitution reaction in the removal of a group and the addition of the other group occurs simultaneously as one step. A bond is formed and one bond is broken. It is a bimolecular reaction which describes the bond making process. It is a faster process as it involves one step as compared to the two steps in SN1 reaction.
For the above given question we consider that,
$C{H}_2=CH-C{H}_2-Cl$ is allyl chloride and
$C{H}_{{}_3}-C{H}_2-C{H}_2-Cl$ is n-propyl chloride
As the nucleophilic substitution reaction takes place, we see that the chloride bond is broken resulting in a positive charge on carbon atom
For allyl chloride the structure becomes
$$\begin{gathered} {}_{+} \\ C{H}_2=CH-C{H}_2 \end{gathered}$$
And for n-propyl chloride the structure becomes
$$\begin{gathered} + \\ C{H}_3-C{H}_2-C{H}_2 \end{gathered}$$
We notice that a resonance structure of allyl chloride is possible and this resonance structure makes the carbocation stable.
$$\begin{gathered} {}_{+} \\ C{H}_2-CH=C{H}_2 \end{gathered}$$
Whereas there is no resonance structure for n-propyl chloride and thus the carbocation is not stable and due to the stable carbocation of allyl chloride they undergo nucleophilic substitution reaction faster and are thus, more reactive.

Note:
Allyl chloride is used in various industries for preparation of plastics, polymers, adhesives and preparation of catalysts and pesticides and is used in various other industries. It is obtained by the chlorination of propylene. Allyl chloride is more reactive than vinyl chloride.