Final answer:
E2 reactions are stereospecific because the beta proton that is abstracted and the leaving group need to be anti-periplanar to each other, which leads to the formation of a particular stereoisomer. This specificity is especially pronounced when there is only one proton at the beta position, as it must have the correct stereochemical orientation relative to the leaving group.
Step-by-step explanation:
E2 Reaction Stereospecificity
During an E2 reaction, a base abstracts a proton from the beta position, and a leaving group departs from the adjacent alpha position, forming a double bond. This type of reaction is stereospecific due to the geometrical orientation of the atoms involved. It is crucial for the base to abstract the beta proton that is anti-periplanar (180 degrees opposite) to the leaving group to achieve optimal orbital overlap and allow for the double bond to form correctly.
When the beta position carries only one proton, this single proton must be in the correct stereochemical orientation relative to the leaving group. Essentially, this must occur in such a way that the abstracted proton and the leaving group are on opposite sides of the molecule, which determines that one specific stereoisomer will be formed.
Since the substrate structure also influences the prevalence of an E2 reaction, steric hindrance can promote E2 when the substrate is tertiary, and in secondary substrates, E2 competes with SN2 mechanisms. This contrast highlights the reaction's sensitivity to the configuration of substrate atoms, affirming that E2 reactions are stereospecific and, in certain conditions, can be stereoselective.