Halogenation of Alkynes
Notes: The halogen (X2) here can be Cl2, Br2, or I2. The solvent is generally an inert halogenated solvent like CH2Cl2 or CCl4. Note that the stereochemistry of addition is anti. Addition of a second equivalent of halogen leads to formation of the tetra-halide product.
Notes: In the first example, Cl2 adds to the alkyne to give the trans product. The solvent is CH2Cl2 (dichloromethane). The second example shows halogenation by bromine (Br2) to give the trans dibromide. The third example shows halogenation by iodine to give the trans product. The fourth example shows addition of “excess” Br2 (more than 2 equivalents) which is sufficient to produce the tetrahalogenated product. If one is careful with the number of equivalents, one can even perform two successive additions, as is done in the fifth example (chlorination, then bromination).
Mechanism: The mechanism is essentially the same as halogenation of alkenes. In the first step, a bromonium ion is formed from the alkene (Step 1, arrows A, B, and C). In the second step, this bromonium ion is attacked by bromide, resulting in formation of the trans dibromide.
The alkenyl dihalide can be halogenated if a second equivalent of halogen is added. For example, a second equivalent of Br2 leads to the formation of the tetrabromo product shown below.
Notes: Alkynes are less reactive than alkenes towards halogenation because the intermediates have some “anti aromatic” character, a concept that doesn’t generally come up until second-semester organic chemistry.