Question

The correct statement of $\alpha$ elimination is?
A) It forms cyclic compounds.
B) It forms carbene or substituted carbene.
C) Two atoms are removed from $\alpha$ and $\beta$ positions.
D) In $CHC{l_3}$,$\alpha$ elimination is not possible.

Answer 1

Alpha$(\alpha )$elimination is the elimination in which both the proton and the leaving group are located on the same atom ($\alpha$atom). The proton is eliminated first followed by the elimination of the leaving group.

Complete answer:
Elimination reactions are the type of chemical reactions in which two substituents are removed from a molecule in either a one-step or a two-step mechanism. Alpha elimination is a category of elimination reactions in which the proton and the leaving group both are eliminated from the same$(\alpha )$ atom. A good example of alpha elimination is the reaction of chloroform ($CHC{l_3}$) with a base like hydroxide ion. The mechanism for the following reaction is:

The base ($O{H^ - }$ in this case) firstly removes the hydrogen as a proton (${H^ + }$) to form a water molecule and $CC{l_3}^ -$. Then $CC{l_3}^ -$ eliminates a chlorine ion which then forms a reactive intermediate known as a carbene. The carbon atom has only three lone pairs of electrons around it (Two pairs in the C-Cl bond and other two electrons). There are two possibilities for how the two nonbonding electrons are distributed and thus two types of carbenes-
Triplet carbene- It has two unpaired electrons, one in an ‘$s{p^2}$’ orbital and the other in a ‘p’ orbital.
Singlet carbene: It has a lone pair of electrons in a nonbonding ‘$s{p^2}$’ orbital, a vacant ‘p’ orbital.

Hence, the correct answer is (B).

Note:
Remember that carbenes are very reactive and generally react with other compounds to form a cyclic compound. The reactions of singlet carbene are stereospecific whereas those of triplet carbene are stereoselective.