Abstract:

Reaction of bromine and the C＝C double bond is one of common elementary reactions in organic chemistry. Bromine and alkenes undergo an electrophilic addition. Aliphatic alkenes undergo formation of three-membered cyclic bromonium intermediates and subsequent ring-opening mechanism, affording stereospecific trans-dibromoadducts, while the three-membered cyclic bromonium intermediates of arylolefins favor to dissociate their benzylic C―Br bond due to aryl stabilizing the generated carbocations, producing a mixture of cis- and trans-dibromoadducts. However, bromine and enols or enolates generated from ketones or carboxylic halides undergo electrophilic substitutions. Bromine does not generate the corresponding three-membered cyclic bromonium intermediates with their C＝C bond in the reactions. Similarly, the reactions of viny ethers and enamines with bromine do not undergo the three-membered cyclic bromonium intermediates, either, generating nonstereospecific adducts and substituted products, respectively. This article rationally explains the mechanistic and stereoselective differentiations between the reactions of bromine and these two classes of reactants with reactive molecular orbitals in the elementary reaction, and summarizes the identification method on the reaction mechanism and stereoselectivity of the reactions between bromine and different C＝C bonds, hoping to be convenient for teaching and understanding.

Key words: Reaction, Bromine, C=C bond, Mechanism, Molecular orbital