Alkyl halides undergoing nucleophilic bimolecular substitution involve:

Inversion of configuration

The correct answer is option 3. Inversion of configuration.

Let us consider an example to understand this process:

The reaction between methyl chloride \((CH_3Cl)\) and hydroxide ion to yield methanol \((CH_3OH)\) and chloride ion follows a second order kinetics, i.e., the rate depends upon the concentration of both the reactants.

The rate of the reaction is expressed as:

\(\text{Rate }= k [CH_3Cl][OH^—]\)

In fact, methyl or primary alkyl halides follow this type of mechanism and it may, in general, be written as

\(\text{Rate }= k [RX][OH^—]\)

This reaction is also called nucleophilic substitution bimolecularbecause two molecules take part in determining the rate of the reaction. It is written as S_N2, short form for substitution nucleophilic bimolecular.

This type of reaction occurs in single (concerted) step through the formation of a transition state. In this mechanism, the nucleophile, \(OH^—^\) attacks the partially positively charged carbon atom of carbon halogen bond from the direction 180° away from the halogen atom i.e., from the backside. This leads to a transition state with a partially formed \(C—OH\) bond and a partially broken \(C—Br\) bond. This process is a one-step reaction. In the transition state, the negative charge is shared by both the incoming nucleophile as well as outgoing chloride. Hydroxide has a diminished negative charge because it has begun to share its electrons with carbon while chlorine has developed a partial negative charge because it has partly removed a pair of electrons from carbon. Therefore, both \(OH\) and \(Br\) have a partial negative charge i.e., δ^–. The remaining three bonds to carbon in the transition state adopt a planar arrangement.

This arrangement may be described as the \(C—H\) bonds being arranged like the spokes of a wheel with \(C—OH\) and \(C—Br\) bonds lying along the axle.

It is clear from the above one-step mechanism, that the formation of the transition state is a rate-determining step, and therefore, the rate of the reaction depends upon the concentration of both alkyl halide and \(OH^—\). Hence, it is a second order reaction or a bimolecular reaction.

It is interesting to note that in S_N2 reactions, the attack of the nucleophile occurs from the back side and the leaving group leaves from the front side. As a result, S_N2 reactions are always accompanied by inversion of configuration. This inversion of configuration is also called Walden inversion.