In the given reaction

(A) B is intermediate

(B) B is transition state

(C) B is formed in a single step

(D) B is stable

(E) B is unstable

Choose the correct answer from the options given below:

B, C and E only

The correct answer is option 3. B, C, and E only.

The given step is of the \(S_N2\) reaction. Let us look at this in detail:

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:

Rate = k[CH₃Cl][OH^–]

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

Rate = k[RX] [OH^–]

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

This type of reaction occurs in a 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\((B)\) 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