Correct option is A
The nucleophilic substitution of RR'R"SiX (R, R', R" = alkyl groups) by a nucleophile Y typically proceeds through a mechanism known as the SN2 reaction. In this mechanism, a nucleophile (Y) attacks the silicon atom, leading to the formation of a new Si-Y bond and the cleavage of the Si-X bond.
A. Silylium cation is formed during the reaction.
This statement is not correct. In SN2 reactions involving silicon compounds, silylium cations (SiR3+) are not typically formed. The reaction proceeds via a concerted mechanism in which the nucleophile Y attacks the silicon atom while the leaving group X is displaced, without the formation of a stable silylium cation.
B. It is a second-order reaction.
This statement is correct. The SN2 reaction is indeed a second-order reaction because the rate of the reaction depends on the concentration of both the nucleophile (Y) and the silicon compound (RR'R"SiX).
C. The cleavage of the Si-X bond is not the rate-determining step.
This statement is correct. In SN2 reactions, the rate-determining step is the formation of the transition state in which the nucleophile Y attacks the silicon atom while the leaving group X is being displaced. This step involves the simultaneous breaking of the Si-X bond and the formation of the Si-Y bond. The transition state has a high energy barrier, making it the slowest step in the reaction, and therefore, it is the rate-determining step.
D. The product always shows an inversion of configuration.
This statement is not necessarily correct. In SN2 reactions, the stereochemistry of the product depends on the configuration of the starting material. Specifically, the nucleophile Y attacks from the side opposite to the leaving group X (anti-addition). This inversion of configuration is observed when the starting material has a chiral center at the silicon atom (i.e., RR'R"SiX is chiral). However, if the silicon compound is not chiral (achiral), there may not be a clear inversion of configuration.
