Question

(a) Draw the structure of major monohalo products in each of the reaction.(refer diagram).
(b) Which halogen compound in each of the following pairs will react faster in ${S_N}2$ reaction:
(i) $C{H_3}Br$ or $C{H_3}I$
(ii) ${(C{H_3})_3}C - Cl$ or $C{H_3} - Cl$

Answer 1

We will draw the diagram of a monhalo compound, based on their placement in phenyl. For the second part we will use the mechanism of ${S_N}2$ reaction and electron affinity to find the faster reacting compound.

Complete step by step answer:
a. A compound is classified as a monohalo compound when the hydrogen atom on carbon atom is replaced by the halogen. Depending upon how the carbon hybridized the compound will form a monohalo structure. Similarly in the given reaction we can see that hydrogen atom is replaced by chlorine from $PC{l_5}$ and resulted in hydrochloric acid production as well as the oxygen atom is replaced which resulted in $POC{l_3}$.

In this reaction the double bond is broken, and halogen is added on carbon.

b. ${S_N}2$ reaction is a nucleophilic substitution reaction in which the reactions are bimolecular with simultaneous bond formation and breakage. It doesn’t require an intermediate to facilitate the reaction. The final product produced after ${S_N}2$ reactions are stereochemistry inversion at their centre and have steric effect.
i. Between $C{H_3}Br$ or $C{H_3}I$, $C{H_3}I$ will react faster to the ${S_N}2$ reaction because Iodine present in the compound will leave easily due to large atomic size compared to atomic size of bromine.
ii. Between ${(C{H_3})_3}C - Cl$ or $C{H_3} - Cl$, $C{H_3} - Cl$ will react faster to the ${S_N}2$ reaction because $C{H_3} - Cl$ is a primary halide and ${(C{H_3})_3}C - Cl$ is tertiary halide and we know that primary halide reacts faster to ${S_N}2$ reaction.

Note:
Student will make mistakes while making the monohalo diagrams of the compounds and they will also make mistakes in selecting the properties for elements which react faster in the ${S_N}2$ reaction.