Question 7

First, identify the halogen atom $X$ in each compound:

1. Reaction of cyclohexene with $HBr$ yields bromocyclohexane ($P$). Thus, $X = Br$ in $P$.
2. The Finkelstein reaction typically involves the exchange of a halide (like $Br$) with $I$ (using $NaI$ in acetone). Thus, $Q$ is iodocyclohexane, and $X = I$ in $Q$.
3. The Swarts reaction involves the exchange of a halide with $F$ (using metallic fluorides like $AgF$). Thus, $R$ is fluorocyclohexane, and $X = F$ in $R$.

Now, evaluate the statements based on the halogen properties ($F, Br, I$):

• (A) $C-X$ Bond Length: Atomic size follows the order $I > Br > F$. Consequently, $C-I > C-Br > C-F$, which means the order is $Q > P > R$. Statement (A) is incorrect.
• (B) $C-X$ Bond Enthalpy: Bond enthalpy is generally inversely proportional to bond length. The order of bond strength is $C-F > C-Br > C-I$, which means $R > P > Q$. Statement (B) is correct.
• (C) $S_N2$ Reactivity: Reactivity depends on leaving group ability, which follows the order $I^- > Br^- > F^-$. Thus, the reactivity order is $Q > P > R$. Statement (C) is incorrect.
• (D) $pK_a$ of Conjugate Acids: The conjugate acids of the leaving groups are $HI$, $HBr$, and $HF$. Their $pK_a$ values are approximately: $HF (3.2)$, $HBr (-9)$, and $HI (-10)$. The order of $pK_a$ is $HF > HBr > HI$, which translates to $R > P > Q$. Statement (D) is incorrect.