Question

Which alkyl halide is more reactive?

Answer 1

The reactivity of alkyl halides depends on the type of reaction it is undergoing. It will be different for unimolecular and bimolecular reactions.

Complete answer:
Alkyl halides are those organic compounds which have a halide group attached to the carbon chain. The general formula of alkyl halides is $R-X$, where R is the alkyl group and X is the halogen group. The halogens of the periodic table are F, Cl, Br, I.
Alkyl halides are electronegative and they will attract a shared pair of electrons towards itself. Hence they can create polarity in the bond. This means that a slightly positive charge will be present on the carbon atom attached to the halide atom. Thus the carbon atom becomes susceptible to nucleophilic attacks.
For nucleophilic substitution reactions, there are two types. One is unimolecular and the other is bimolecular. The unimolecular nucleophilic substitution reaction $\left( S{{N}_{1}} \right)$ involves a single molecule in the rate-determining step and it undergoes the formation of carbocations.
On the other hand, the bimolecular nucleophilic substitution reaction $\left( S{{N}_{2}} \right)$ involves two molecules in the rate-determining step. The leaving group and the nucleophile both are attached to the carbon atom in this step. Thus, the steric factor will be important.
Now, we know that tertiary carbocation is much more stable than secondary and the secondary carbocation is much more stable than the primary due to the inductive effect of the alkyl groups. Hence for $S{{N}_{1}}$ reaction, the order of reactivity of the alkyl halides is tertiary > secondary > primary.
But, in the case of $S{{N}_{2}}$, the steric factor will determine the reactivity. As we know that tertiary alkyl halides will cause more steric hindrance than secondary and similarly the secondary alkyl halides will cause more steric hindrance than primary alkyl halides, the order of reactivity of the alkyl halides for $S{{N}_{2}}$ is primary > secondary > tertiary.
When the same type of alkyl halide (primary or secondary or tertiary) is considered, the order of reactivity depends on the ability to break the $R-X$ bond. The bond strength is inversely proportional to the bond length. Higher the bond length lower is the bond strength and that bond can be easily broken down and hence higher reactivity. Among the alkyl halides, the bond lengths are in the order R–I > R–Br > R–Cl > R–F.

Note:
For both the unimolecular and bimolecular nucleophilic substitution reaction, the allylic and benzylic halides are highly reactive and they are much more reactive than alkyl halides for the same reaction.