Alkyne halogenation
Abstract
Halogenation of alkynes is an electrophilic addition reaction in which molecular bromine (Br2) or chlorine (Cl2) adds across the carbon-carbon triple bond. Depending on the amount of halogen employed, the reaction can stop at the dihaloalkene stage or proceed to the fully saturated tetrahalide.
Keywords
Alkyne, Dihaloalkene, Tetrahaloalkane
Category
Alkyne
Mechanism
Mechanistically, 𝜋 electrons of the alkyne act as nucleophile and attack one of the bromine atoms of the dibromine, polarizing the Br-Br bond and resulting in the departure of a bromide ion. The first bromine atom bridges the two carbons of the former double bond, forming a cyclic bromonium ion. Subsequently, the bromonium ion is then opened by backside attack of the bromide ion, resulting in anti addition across the triple bond and formation of a trans (E)-dihaloalkene.
When one equivalent of bromine is used, the reaction typically stops at this stage, yielding the trans dibromoalkene as the major product. However, because the resulting alkene remains reactive toward electrophilic addition, treatment with a second equivalent of bromine leads to a second halogenation. This proceeds through the same bromonium ion mechanism and ultimately affords the corresponding tetrabromide.
