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)

What is the product formed when 2-Bromo-2-methylpropane reacts hydroxide ion?

2-Methylpropan-2-ol

The correct answer is option 1. 2-Methylpropan-2-ol.

The reaction between tert-butyl bromide (i.e., 2-Methylpropan-2-ol) and hydroxide ion to form tert-butyl alcohol follows first-order kinetics i.e., the rate depends on the concentration of one reactant only. It has been noted that the rate of reaction depends entirely on the alkyl halide concentration.

This reaction is called substitution nucleophilic unimolecular because only one molecule is involved in the rate-determining step of the reaction and is written as an S_N1 reaction.

Step 1. Formation of a carbocation: The polarised C — halogen bond (C— Br) undergoes slow cleavage to form a carbocation and a halide ion

Step 2. Nucleophilic attack on carbocation:The carbocation is very reactive species. Therefore, it readily reacts with a nucleophile, OH– to form the product.

The first step is slow and can be reversed. It involves the cleavage of the C—Br bond for which energy is obtained through the solvation of bromide (halide) ion with the proton of the protic solvent. Since the slowest step is the rate determining step, the rate of reaction depends only on the concentration of tert-butyl bromide (alkyl halide). It is a first-order or unimolecular reaction as a result.