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

Hint: 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.