# Why don't alkynes undergo preferential anti bromination to the degree that alkenes do?

Alkenes, for the most part (unless there is a phenyl group or highly ionising solvent) undergo almost exclusively anti addition of bromine. However, under similar conditions alkynes, although undergo preferential anti-addition, it is not as exclusive as alkenes. Even with electron withdrawing substituents like two $\ce{COOH}$, only 70% is anti. (Peter Sykes, page 182) Why?

• I've checked two of my books but I didn't find anything. If I could make an assumption I would say that maybe the mechanism followed by alkynes differentiates from the one followed by alkenes? In alkenes' mechanism a three-membered ring is formed among one Br atom and the two double bonded carbons thus leading the other Br to bond with the carbons from the other side of the bond to create an anti-bromination – Αντώνιος Κελεσίδης Aug 13 '17 at 22:15

From what I know of the mechanism, Addition of Br2 takes place in the following mechanism

For an alkyne, the intermediate would not be stable due to the ring strain on 3 member ring with a double bond in it, thus reducing the rate of reaction.