Question

Which of the following stereoisomers of $t-$butylcyclohexyl iodide $\left({}^{127}{I}^{-}\right)$ will undergo $S_{N^2}$ substitution with ${}^{128}{I}^{-}$ faster, and why ?

• A. $A$ will react faster because it is the more stable of the two isomers
• B. $A$ will react faster because it will yield more stable product, and the transition state for both reactions is of the same energy
• C. $A$ will react faster because the approach of ${}^{128}{I}^{-}$ can depart unhindered.
• D. $B$ will react faster because it is less stable than $A$, and the transition state for both reactions is of the same energy.

Answer 1

The correct option is D $B$ will react faster because it is less stable than $A$, and the transition state for both reactions is of the same energy.

In this reaction, $B$ will react faster than $A$. This because $B$ is less stable than $A$ and also the transition state of both the reactions is of same energy. Also, $B$ is more stearically hindered and reacts at as lesser speed for substitution of $I^{-128}$.