Question

ASSERTION: $E1cB$ reaction is favoured by stabilization of carbanions and poor leaving groups.
REASON: The reaction is kinetically of the second-order and unimolecular.
A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
C. Assertion is correct but Reason is incorrect
D. Assertion is incorrect but Reason is correct
E. Both Assertion and Reason are incorrect

Answer 1

$E1cB$ reaction involves two steps. In the 1st step, an acidic hydrogen atom from the beta carbon atom is attracted to the base to form a carbanion transition state, which is stabilized by resonance. In the 2nd step, the lone pair of electrons moves to the neighbouring atom to form a pi bond with elimination product of the leaving group.

Complete step by step answer: $E1cB$ Reaction is a type of elimination reaction in which under basic conditions, the hydrogen is removed which is relatively acidic, while the other leaving group such as $- OH$ or $- OR$ is poor one. Commonly a moderate to strong base is present in $E1cB$ reaction. This reaction is a two-step process, in which the 1st step may be or may not be reversible. Initially, a base takes out a relatively acidic proton to generate a stabilized anion. After that the lone pair of electrons on the anion moves to the neighbouring atoms, thus removing the leaving group and forming a double or triple bond.
$E1cB$ - Stands for Elimination Unimolecular Conjugate Base. Elimination refers to the mechanism of an elimination reaction and will lose two substituents. Unimolecular refers to the rate-determining step of this reaction which involves only one molecular entity. Finally, conjugate base refers to the formation of the carbanion intermediate, which is the conjugate base of the starting material.
In the given question the assertion states that $E1cB$reaction is favoured by stabilization of carbanions and poor leaving group and it’s true because in $E1cB$reaction formation of carbanions takes place. More is the stability of carbanion, more favourable the reaction will be. And the reason also stands true as it is a 2nd order kinetic and is unimolecular. But it doesn’t give explanation about the assertion
Therefore, Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion, so the correct option is B.

Note: The rate law for this reaction can be given as:
Rate = k$\times$[concentration of substrate][concentration of base]
Therefore, higher the stability of the carbanion intermediate faster will be the reaction.