Problem 58 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

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 stabilized by loosely held p-electrons, the cleavage is difficult because of strong bonding. (a) Both (A) and (R) are wrong statements. (b) Both (A) and \((\mathrm{R})\) are correct statements and (R) is the correct explanation of (A). (c) Both (A) and \((\mathrm{R})\) are correct statements but \((\mathrm{R})\) is not the correct explanation of (A). (d) (A) is a correct statement but \((\mathrm{R})\) is a wrong statement.

Step-by-Step Solution

Answer

(c) Both (A) and (R) are correct statements but (R) is not the correct explanation of (A).

Step 1: Understanding the Assertion

The assertion states that vinyl halides do not undergo nucleophilic substitution easily. Vinyl halides have a double bond directly attached to the halogen atom. The sp2 hybridized carbon makes the bond between carbon and halogen stronger than in alkyl halides, making it difficult for nucleophilic substitution to occur.

Step 2: Analyzing the Reason

The reason provided is that the intermediate carbocation is stabilized by loosely held pi electrons, but the cleavage is difficult due to strong bonding. Vinyl halides do not form stable carbocations under normal conditions due to the involvement of the double bond which makes the C-X bond strong. So, while the pi electrons can offer some stabilization during reactions, it is not typically by the formation of carbocations in these compounds.

Step 3: Assessing the Relationship Between Assertion and Reason

Both the assertion and the reason indicate difficulties in the substitution reaction but from different perspectives. The assertion focuses on the inherent characteristic of vinyl halides, whereas the reason talks about stabilization which is not directly applicable to vinyl halides. Thus, while both statements may be true individually, the reason does not correctly explain the assertion.

Step 4: Choosing the Correct Answer

Since both statements are correct individually, but the reason doesn't justify the assertion specifically in the context of vinyl halides, the correct answer is option (c): Both (A) and (R) are correct statements but (R) is not the correct explanation of (A).

Key Concepts

Vinyl Halides

Vinyl halides are a unique class of compounds where a halogen is bonded to a carbon atom that is part of a carbon-carbon double bond. This particular arrangement is what distinguishes them from other halides like alkyl halides.
The main reason vinyl halides do not undergo nucleophilic substitution easily lies in the nature of the double bond.

• In vinyl halides, the carbon attached to the halogen is sp² hybridized, making the bond to the halogen shorter and stronger.
• This difference in bonding makes it considerably more difficult for a nucleophile to displace the halogen atom.
• Additionally, the presence of the double bond adds rigidity to the molecular structure, resisting the typical pathways that facilitate substitution reactions.

Understanding this fundamental characteristic explains why these reactions in vinyl halides are not as common as in other haloalkanes.

Hybridization of Carbon

In the context of vinyl halides, the term 'hybridization' refers to the type of bond character the carbon atoms exhibit.

• The carbon atom bearing the halogen in vinyl halides is sp² hybridized.
• This hybridization means one of the (sp²) orbitals overlaps with the p orbital of the halogen atom, forming a distinct bond.
• This sp² hybridization leads to stronger carbon-halogen bonds compared to the sp³ hybridization found in alkyl halides.

The stronger bond is attributed to increased s-character in sp² bonds. Bonds with more s-character are shorter and possess more bond strength, further contributing to the resistance to nucleophilic substitution.

Carbocation Stability

Carbocations are intermediates in many organic reactions, notably in substitution reactions. Their stability often dictates the likelihood of a reaction pathway. However, for vinyl halides, this is slightly different.

• Vinyl halides do not form easily stabilized carbocations due to the involvement of the carbon-carbon double bond.
• This double bond does not offer the same type of electron stabilization as seen in other carbocation-forming scenarios.
• Pi electrons could offer stabilization in different compounds, but this isn't effective for carbocations in vinyl halides since the formation of a viable carbocation intermediate is very difficult.

Therefore, the lack of stable carbocation intermediates is another reason why substitution reactions for vinyl halides are rarer compared to their alkyl counterparts.

Chemical Bonding Strength

Chemical bonding strength is a crucial factor in determining how a molecule interacts in a reaction. For vinyl halides:

• The bond strength is notably high due to the carbon being sp² hybridized.
• The increased s-character of such bonds contributes to these stronger interactions.
• Strong bonding means more energy is required to break these bonds during reactions, hindering processes like nucleophilic substitution.

This not only influences reaction pathways but also the overall chemical reactivity, rendering vinyl halides less reactive compared to other halide types in substitution reactions. Understanding chemical bonding strength is vital in predicting and explaining the behavior of vinyl halides in different chemical environments.