Alkyne Reactions: Alkyne Halogenation using Br2/Cl2 and CCl4

Bromine (Br2) or Chlorine (Cl2) in the presence of alkynes reacts to form dihalides and tetrahalides:

Depending on how many equivalents of Br2/CCl4 or Cl2/CCl4 are added to the reaction, an alkene dihalide or alkane tetrahalide may form. The alkyne is more reactive than the resulting alkene bond, which is why the amount of reagent plays a role in determining the final product:

One Equivalent

Two (excess) Equivalents

The reaction mechanism is depicted below:

In the first step, electrons from the alkyne bond react with the first chlorine (halogen) to make a bond, and electrons from the first chlorine (halogen) atom react with the other carbon to form another bond, forming a ring structure as an intermediate.

In the second step, the free, negatively charged chlorine (halogen), attacks the backside of the intermediate structure, causing one chlorine (halogen) to bond to one carbon and the other chlorine (halogen) to bond to the other carbon. This results in a sort of “vinylic” dihalide.

In the third step, electrons from the alkene bond react with the first chlorine (halogen) to make a bond, and electrons from the first chlorine (halogen) atom react with the other carbon to form another bond, forming another ring structure as an intermediate.

In the fourth step, the free, negatively charged chlorine (halogen), attacks the backside of the intermediate structure, again causing one chlorine (halogen) to go to one carbon and the other chlorine (halogen) to go to the other carbon.

The reagents used for this reaction are Br2/CCl4, Cl2/CCl4, and I2/CCl4. If a reaction uses an equivalent of one halogen first (ex. Br2) and uses a second equivalent of a different halogen (ex. Cl2), each carbon will have 1 Br and 1 Cl (see mechanism for visualization why).