The halogen atom in haloalkanes is more electronegative than the carbon atom attached to it. As a result, the atom acquires a partial positive charge and the halogen atom a partial negative charge.

The presence of this small positive charge on the carbon atom makes it susceptible to attack by nucleophilic reagents. Thus, when a nucleophile stronger than the halide ion approaches the positively charged carbon atom of an alkylhalide, the halogen atom with its bonding pair of electrons is displaced and a new bond between the carbon atom and the incoming nucleophile is formed:

Such reactions in which a stronger nucleophile displaces a weaker nucleophile are called nucleophilic substitution reactions and the atom or group which departs with its bonding pair of electrons is called leaving group. The better the leaving group, the more facile the nucleophilic substitution reaction.

Amongst the halide ions, the order in which the leaving group departs follows the sequence:

\[I^− > Br^− > Cl^− > F^−\]

It is because of this reason that the order of reactivity of haloalkanes follows the sequence:

\[iodoalkanes > bromoalkanes > chloroalkanes > fluoroalkanes\]

There are two types of nucleophilic substitution reactions. These are:

(a)   Substitution Nucleophilic Bimolecular (S_N2)

(b)   Substitution Nucleophilic Unimolecular (S_N1)

In the given reaction:

Arrange the steps involved in the mechanism in the serial order:

(i) Deprotonation

(ii) Loss of Leaving group

(iii)  Rearrangement

(iv) Addition of nucleophile to carbocation

(ii), (iii), (iv), (i)

The correct answer is option 2. (ii), (iii), (iv), (i).

The S_N1 Reaction With Hydride Shift: Arrow Pushing Mechanism

In the first step in the S_N1 is that the leaving group leaves to give a carbonation.

In the case below, the carbocation that is formed is secondary, and there’s a tertiary carbon next door. Therefore, a rearrangement can occur to give the more stable tertiary carbocation, which is then attacked by the nucleophile (water in this case).

Finally, the water is deprotonated to give the neutral alcohol. So this is an example of an S_N1 reaction with rearrangement.