Question

Which of the following statements are correct about the reactivities of (I) n-propyl chloride and (II) allyl chloride?
(A) Rate if ${S_N}1$ reaction of (I) $>$(II)
(B) Rate if ${S_N}1$ reaction of (II) $>$(I)
(C) Rate if ${S_N}2$ reaction of (I) $>$(II)
(D) Rate if ${S_N}2$ reaction of (II) $>$(I)

Answer 1

Tertiary alkyl halides are most reactive for ${S_N}1$ reactions and similarly, primary halides for ${S_N}2$ reactions.
The rate of these reactions depends upon the types of compounds.
Also, which mechanism will be more favored is dependent on it.

Complete step by step answer:
The IUPAC name of n-propyl chloride is given as
1-Chloropropane
And that of allyl chloride is given as
3-Chloroprop-1-ene
The structure of n-propyl chloride is
$C{H_3} - C{H_2} - C{H_2} - Cl$
The structure of allyl chloride is
$C{H_2} = CH - C{H_2} - Cl$
Here, both the chlorine atoms are attached to primary carbon but in allyl chloride, there is a double bond present.
The reactivity of ${S_N}1$ reaction is given as
Tertiary alkyl halide $>$ secondary alkyl halide $>$ primary alkyl halide
And the reactivity of ${S_N}2$ reaction is given as
Primary alkyl halide $>$ secondary alkyl halide $>$ Tertiary alkyl halide
The allyl chloride molecule is stabilized by resonance.
Therefore, it’s more reactive. It is reactive in both ${S_N}1$ and ${S_N}2$ mechanisms but more in ${S_N}1$ mechanism.
Both the molecules are similar except for the double bond in allyl chloride.
So, they both will undergo ${S_N}2$ mechanisms but as allyl chloride is more reactive than n-propyl chloride, the rate of nucleophilic substitution of allyl chloride will be more in both ${S_N}1$ and ${S_N}2$ mechanisms.
Therefore, the correct options are
Option (B) Rate if ${S_N}1$ reaction of (II) $>$ (I) and
Option (D) Rate if ${S_N}2$ reaction of (II) $>$ (I)

So, the correct answer is Option B,D.

Note: ${S_N}1$ and ${S_N}2$ reactions are nucleophilic substitution reactions .
For ${S_N}1$, there is one rate-determining step and is unimolecular.
And for ${S_N}2$ reaction there are two rate-determining steps and is a bimolecular reaction.
There is a difference in energy levels of the two reactions as well.