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

Question

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?

Answer 1

Remember the basic difference between alkenes and alkynes. Alkenes contain double bonds. Alkynes usually contain one triple bond. A triple bond is sterically-hindering bc of all the non-bonded electron pi orbitals and the sp bond between the triple bonded atoms.

Anti addition requires addition on opposite sides or faces of a molecule (double or triple bond) resulting in a trans molecule. Usually cycloalkenes are used as an example.

So why not an alkyne?? Take a simple alkyne: acetonitrile: CH3-C(triple bond)N:

The methyl C cannot accept Br2 in an electrophilic rxn; how could there be a leaving group; this would have to be an electrophilic addition rxn. So Br+ would have to add to N::- move one bond of triple bond to nonbonded N orbitals. to form required bonding, Not going to happen. Mother nature is lazy. Thermodynamically impossible and structurally blocked by non-friendly steric hindrance. Hope this helps! Alison D.