The reaction of acetylene (C₂H₂) with bromine (Br₂) is a halogenation reaction. The outcome depends on the reaction conditions, specifically the amount of bromine used.

1. Addition Reaction (with excess bromine):

When acetylene reacts with excess bromine, a dibromination reaction occurs. Two bromine molecules add across the triple bond, forming 1,1,2,2-tetrabromoethane.

C₂H₂ + 2Br₂ → Br₂CHCHBr₂

2. Partial Bromination (with limited bromine):

If the reaction is carried out with a limited amount of bromine, only one bromine molecule adds across the triple bond, resulting in the formation of 1,2-dibromoethene (also known as acetylene dibromide).

C₂H₂ + Br₂ → BrCH=CHBr

Reaction Mechanism:

The reaction proceeds through an electrophilic addition mechanism. The bromine molecule is polarized by the triple bond of acetylene, leading to an electrophilic attack of bromine on the carbon atom. This forms a cyclic bromonium ion intermediate, which is then attacked by another bromide ion, leading to the final product.

Note:

* The reaction is usually carried out in an inert solvent like carbon tetrachloride (CCl₄).

* The reaction is exothermic and can be violent if not carefully controlled.

* The products of the reaction are useful intermediates in organic synthesis.