Question

In ${{S}_{N}}2$ reactions, the correct order of reactivity for the following compounds
$C{{H}_{3}}Cl$, $C{{H}_{3}}C{{H}_{2}}Cl$, ${{(C{{H}_{3}})}_{2}}CHCl$, ${{(C{{H}_{3}})}_{3}}CCl$ is:
(A) $C{{H}_{3}}C{{H}_{2}}Cl$ > $C{{H}_{3}}Cl$ > ${{(C{{H}_{3}})}_{2}}CHCl$ > ${{(C{{H}_{3}})}_{3}}CCl$
(B) ${{(C{{H}_{3}})}_{2}}CHCl$ > $C{{H}_{3}}C{{H}_{2}}Cl$ > $C{{H}_{3}}Cl$ > ${{(C{{H}_{3}})}_{3}}CCl$
(C) $C{{H}_{3}}Cl$ > ${{(C{{H}_{3}})}_{2}}CHCl$ > $C{{H}_{3}}C{{H}_{2}}Cl$ > ${{(C{{H}_{3}})}_{3}}CCl$
(D) $C{{H}_{3}}Cl$ > $C{{H}_{3}}C{{H}_{2}}Cl$ > ${{(C{{H}_{3}})}_{2}}CHCl$ > ${{(C{{H}_{3}})}_{3}}CCl$

Answer 1

The order of reactivity depends upon the steric hindrance caused in the molecule and also the stability of the formed molecule.

Complete Solution :
For solving the above question, we should firstly know the concept of ${{S}_{N}}1$ and ${{S}_{N}}2$ reactions. Here, we will concentrate on the ${{S}_{N}}2$ mechanism.
${{S}_{N}}2$ reaction-
The ${{S}_{N}}2$ reaction is the nucleophilic substitution reaction where a bond is broken and another is formed synchronously. Two reacting species are involved in the rate determining step of the reaction.
The term ${{S}_{N}}2$ stands for- Substitution Nucleophilic Bimolecular.

This can also be referred to as associative substitution and interchange mechanism.
${{S}_{N}}2$ reaction mechanism-
This reaction proceeds through a backside attack by the nucleophile on the substrate. The nucleophile approaches the given substrate at an angle of $180{}^\circ$ angle to the carbon leaving group bond.
The carbon nucleophile bond forms and carbon leaving group bond breaks simultaneously through a transition state.
Now, the leaving group is pushed out of the transition state on the opposite side of the carbon nucleophile bond, forming the required product.

Steric hindrance-
Inhibition by steric hindrance ${{S}_{N}}2$ reactions are particularly sensitive to steric factors, since they are greatly retarded by steric hindrance (crowding) at the site of reaction.
Effects of steric hindrance on ${{S}_{N}}2$ reaction mechanism-
As each hydrogen is replaced by an R group, the rate of reaction is significantly diminished. This is because the addition of one or two R groups shields the backside of the electrophilic carbon, looming the nucleophilic attack.

Illustration-
Steric hindrance (crowding) is the basis of ${{S}_{N}}2$ reaction, by which we can arrange the reactant in correct order of their reactivity towards ${{S}_{N}}2$ reaction.
As steric hindrance increases, the rate of ${{S}_{N}}2$ reaction decreases.
i.e. Rate of ${{S}_{N}}2$ reaction $\propto$ $\dfrac{1}{Crowding}$
The order of reactivity towards ${{S}_{N}}2$ reaction for alkyl halides is
Primary halides > Secondary halides > Tertiary halides

Thus, For the given molecules:
$C{{H}_{3}}Cl$ > $C{{H}_{3}}C{{H}_{2}}Cl$ > ${{(C{{H}_{3}})}_{2}}CHCl$ > ${{(C{{H}_{3}})}_{3}}CCl$
is the correct order of reactivity in ${{S}_{N}}2$ reaction.
So, the correct answer is “Option D”.

Note: Do note that the product of ${{S}_{N}}2$ reaction is formed with an inversion of tetrahedral geometry at the atom in the centre.
- Steric hindrance increases as the R group replaces the H atom in the molecule, so option with consecutive molecules with the replacement of H atom will always be a priority while answering such questions.