The nucleophilic substitution of RR'R''SiX (R, R', R" = alkyl groups) by a nucleophile Y gives the product RR'R"SiY. Among the following,

A. Silylium cation is formed during the reaction.

B. It is a second-order reaction.

C. The cleavage of the Si-X bond is not the rate-determining step.

D. The product always shows an inversion of configuration.

Identify the correct statements.

Explanation:-

• The nucleophilic substitution of RR'R''SiX (R, R', R'' = alkyl groups) by a nucleophile Y typically proceeds through a mechanism known as the S_N2 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. Let's analyze each statement to determine which ones are correct:

A. Silylium cation is formed during the reaction.

• This statement is not correct. In S_N2 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 S_N2 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 S_N2 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 S_N2 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.

In summary, statements B and C are correct. The S_N2 reaction is second-order, and the rate-determining step involves the formation of the transition state where the Si-X bond cleavage and Si-Y bond formation occur simultaneously.

Statement D is not always correct, as the inversion of configuration depends on the chirality of the starting material. Statement A is not correct because silylium cations are not typically formed in S_N2 reactions involving silicon compounds.

Conclusion:-

Hence, the correct statements are B and C only.