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.
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.
Show Hint
- Both (A) and (R) are correct statements but (R) is not the correct explanation of (A)
- Both (A) and (R) are correct statements and (R) is the correct explanation of (A)
- (A) is correct statement but (R) is a wrong statement
- Both (A) and (R) are wrong statements.
The Correct Option is C
Solution and Explanation
Reason: In a vinyl halide the lone pair of X is in resonance with the C=C (CH2=CH-X ↔ -CH2-CH=X+), giving the C-X bond partial double bond character, so it is stronger and shorter. Also the carbon is sp2 hybridised (more electronegative, holds the bond tighter). Cleavage is therefore difficult because of strong bonding, which is the true cause.
However, the vinylic carbocation (CH2=CH+) that would form in an SN1 path is NOT stabilised by the loosely held p-electrons; it is highly unstable (its empty orbital is sp hybrid and orthogonal to the pi system). So the statement in (R) about carbocation stabilisation is wrong.
Hence (A) is correct but (R) is wrong.
Answer: (3) (A) is correct statement but (R) is a wrong statement
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