Question

The order of reactivities of the following alkyl halides for an ${\rm{S}}{{\rm{N}}^2}$reaction is:
(A) ${\rm{RF}}$>${\rm{RCl}}$>${\rm{RBr}}$>${\rm{RI}}$
(B) ${\rm{RF}}$>${\rm{RBr}}$>${\rm{RCl}}$>${\rm{RI}}$
(C) ${\rm{RCl}}$>${\rm{RBr}}$>${\rm{RF}}$>${\rm{RI}}$
(D) ${\rm{RI}}$>${\rm{RBr}}$>${\rm{RCl}}$>${\rm{RF}}$

Answer 1

As we know that, as we go down the group in periodic table, the size of atoms increases because the principal quantum number increases. The reactivity of any compound depends upon the leaving ability of the atoms or molecules present in it.

Complete step by step answer:
${\rm{S}}{{\rm{N}}^2}$is known as a nucleophilic substitution reaction of order two which means that it depends upon two factors as:
(a) the order of alkyl group for ${\rm{S}}{{\rm{N}}^2}$is ${{\rm{1}}^{\rm{0}}}{\rm{ > }}{{\rm{2}}^{\rm{0}}}{\rm{ > }}{{\rm{3}}^{\rm{0}}}$.
(b) the concentration of the nucleophile should be high or the nucleophile should be strong.
In this reaction, substrate is alkyl halide thus, for greater reactivity leaving the group must be good as the size of halide increases, the strength of bonding between halides and carbon decreases. Because the orbitals of carbon and orbitals of halogen do not overlap effectively. Hence, there is weak bonding.
Therefore, from the above discussion we conclude that the order of reactivities of the alkyl halides is ${\rm{RI}}$>${\rm{RBr}}$>${\rm{RCl}}$>${\rm{RF}}$.

So, the correct answer is “Option D”.

Note:
The unimolecular nucleophilic substitution reaction depends only on the stability of carbocation. The above order is reversed for the substrate because tertiary carbocation is more stable than secondary followed by primary carbocation.