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Question: Which of the following is an example of \[1,2 - \] elimination? A. \(C{H_3} - CH(Br) - C{H_3}\xrig...

Which of the following is an example of 1,2−1,2 - elimination?
A. CH3−CH(Br)−CH3→Re⁡agentCH3−CH=CH2C{H_3} - CH(Br) - C{H_3}\xrightarrow{{\operatorname{Re} agent}}C{H_3} - CH = C{H_2}
B. X−CH2−CH2−CH2−Y→Re⁡agentΔX - C{H_2} - C{H_2} - C{H_2} - Y\xrightarrow{{\operatorname{Re} agent}}\Delta
C. X−CH2−CH2−CH2−CH2−Y→Re⁡agentCH3−CH=CH−CH3X - 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.

Explanation

Solution

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 E2{E_2} reaction, and the two-step mechanism is known as the E1{E_1} reaction. The numbers do not refer to the number of steps in the mechanism, but rather to the kinetics of the reaction. E2{E_2} is a bimolecular or a second-order reaction while E1{E_1} is unimolecular or a first-order reaction.

Complete step by step answer:
1,2−1,2 - 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 Br−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 HBrHBr . The electrons of the C−HC - H bond, shift to the C−CC - C bond and neutralize the carbocation, thus forming a double bond and showing a 1,2−1,2 - elimination.
In the case of the second and third option, they do not undergo a 1,2−1,2 - elimination.
Thus, the correct option is A. CH3−CH(Br)−CH3→Re⁡agentCH3−CH=CH2C{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.