WBJEE 2024 Physics & Chemistry Paper

The correct statements about the $\mathrm{S}_{\mathrm{N}} 2$ reaction mechanism are :Option A : The rate of reaction increases with increasing nucleophilicity. This statement is true. In an $\mathrm{S}_{\mathrm{N}} 2$ reaction, the nucleophile attacks the electrophilic carbon in a single, concerted step. Therefore, a stronger (more nucleophilic) nucleophile will generally result in a faster reaction because it can more readily attack the carbon center. Option B : The number 2 denotes a second order reaction.This statement is also true. The term $\mathrm{S}_{\mathrm{N}} 2$ stands for bimolecular nucleophilic substitution and indicates that the rate-determining step involves two reacting species: the nucleophile and the electrophilic substrate. The rate law for an $\mathrm{S}_{\mathrm{N}} 2$ reaction is given by:Rate $=k[$ Nucleophile $][$ Electrophile $]$This indicates that the reaction is second order overall. Option C: Tertiary butyl substrates do not follow this mechanism.This statement is true. Tertiary butyl substrates are typically very sterically hindered, making it difficult for the nucleophile to approach the electrophilic carbon from the backside, which is required for the $\mathrm{S}_{\mathrm{N}} 2$ mechanism. As a result, tertiary substrates usually do not undergo $\mathrm{S}_{\mathrm{N}} 2$ reactions. Option D : The optical rotation of substrates always changes from $(+)$ to $(-)$ or from $(-)$ to $(+)$ in the products.This statement is incorrect because in as $\mathrm{S}_{\mathrm{N}} 2$ reaction inversion does take place but we can never say a dextro substrate will produced a levo product or vice-versa. Because the dextro or levo nature of the reactant or product are purely experimental.