Question

Which of the following is an example of \[1,2 - \] elimination?
A. $C{H_3} - CH(Br) - C{H_3}\xrightarrow{{\operatorname{Re} agent}}C{H_3} - CH = C{H_2}$
B. $X - C{H_2} - C{H_2} - C{H_2} - Y\xrightarrow{{\operatorname{Re} agent}}\Delta $
C. \[X - C{H_2} - C{H_2} - C{H_2} - C{H_2} - Y\xrightarrow{{\operatorname{Re} agent}}C{H_3} - CH = CH - C{H_3}\]
D. All of these.

Answer 1

Hint:An elimination reaction is one of the types of organic reaction in which two substituents are removed from a molecule in either a one or a two-step mechanism. The one-step mechanism is known as the \[{E_2}\] reaction, and the two-step mechanism is known as the \[{E_1}\] reaction. The numbers do not refer to the number of steps in the mechanism, but rather to the kinetics of the reaction. \[{E_2}\] is a bimolecular or a second-order reaction while \[{E_1}\] is unimolecular or a first-order reaction.

Complete step by step answer:
\[1,2 - \] elimination can be defined as the type of elimination reaction in which there is a removal of two substituents, one from each adjacent carbon atom and thus, helps in the formation of a multiple bond between the two adjacent carbon atoms. In the first option, we find that the reactant has a bromine atom at the middle carbon and a hydrogen atom at the adjacent carbon atom. On using a suitable reagent and heating the reactants together, there is a loss of the bromine atom from the central carbon and there is a formation of a stable intermediate secondary carbocation. Now, this bromide ion $B{r^ - }$ is free in the solution and acts as a Lewis base and attacks the hydrogen atom on the adjacent carbon atom and extracts it in order to form $HBr$ . The electrons of the $C - H$ bond, shift to the $C - C$ bond and neutralize the carbocation, thus forming a double bond and showing a \[1,2 - \] elimination.
In the case of the second and third option, they do not undergo a \[1,2 - \] elimination.
Thus, the correct option is A. $C{H_3} - CH(Br) - C{H_3}\xrightarrow{{\operatorname{Re} agent}}C{H_3} - CH = C{H_2}$ .

Note: An important class of elimination reactions is those which involve alkyl halides along with good leaving groups and which react with a Lewis base in order to form an alkene. Elimination may be considered the reverse of an addition reaction and there is a simple removal of substituents to form multiple bonds in the organic compound.