Animated Solution for Chemistry - Haloalkanes & Haloarenes: An Assertion and a Reason are given below. Choose the correct answer from the following options. Assertion (A) Vinyl halides do not undergo nucleophilic substitution easily. Reason (R) Even though the intermediate carbocation is stabilised by loosely held $\pi$-electrons, the cleavage is difficult because of strong bonding.

Let's dive into the fascinating world of nucleophilic substitution and see why vinyl halides are so notoriously unreactive. The problem presents us with an Assertion and a Reason, and we need to evaluate the truth of each statement independently before checking if the reason correctly explains the assertion.

Assertion (A): Vinyl halides do not undergo nucleophilic substitution easily.

Is this true? Yes, it is a well-established fact in organic chemistry. Vinyl halides, such as vinyl chloride ($C{H}_2=CH-Cl$), are extremely inert towards nucleophilic substitution reactions (both $S_{N}1$ and $S_{N}2$). But to understand why, we need to look at the molecular structure.

Resonance

The secret lies in the phenomenon of resonance. Look closely at the halogen atom (let's say, chlorine) attached to the $s{p}^2$-hybridized carbon of the double bond. The chlorine atom possesses lone pairs of electrons. These lone pairs are in direct conjugation with the adjacent $\pi$-bond of the alkene.

Because of this conjugation, the lone pair electrons delocalize into the $\pi$-system. We can draw resonance structures to represent this:

Due to this electron delocalization, the carbon-chlorine bond is no longer just a simple single bond. It acquires a partial double bond character. This makes the $C-Cl$ bond significantly shorter and much stronger than a typical alkyl halide $C-Cl$ bond. Breaking this strong bond requires a lot of energy, which is the primary reason why nucleophilic substitution is so difficult. Therefore, the Assertion (A) is absolutely correct.

Now, let's evaluate the Reason statement.

Reason (R): Even though the intermediate carbocation is stabilised by loosely held $\pi$-electrons, the cleavage is difficult because of strong bonding.

Let's imagine a hypothetical scenario where the $C-Cl$ bond actually manages to break heterolytically (as it would in an $S_{N}1$ mechanism). The departure of the chloride ion would leave behind a vinyl carbocation ($C{H}_2=C{H}^{+}$).

Is this carbocation stable? Absolutely not! There is a major catch here. The positive charge in a vinyl carbocation resides on an $sp$-hybridized carbon atom. We know that $sp$-hybridized carbons have $50\%$ s-character, making them highly electronegative. An electronegative atom strongly resists bearing a positive charge.

Furthermore, the empty p-orbital of the carbocation is orthogonal (perpendicular) to the $\pi$-bond, meaning the $\pi$-electrons cannot delocalize to stabilize the positive charge. The vinyl carbocation is one of the most unstable carbocations encountered in organic chemistry.

Therefore, the claim in Reason (R) that "the intermediate carbocation is stabilised by loosely held $\pi$-electrons" is completely false.

Since Assertion (A) is a correct statement but Reason (R) is a wrong statement, the correct option is (c).