How does the formation of abromonium ion account for anti steriochemistry of addition to cyclopentene? If a bromonium ion is formed as an intermediate,we can image that the large bromine atom might shield one side of the molecule.Attack by Br- ion in the second step could then occur only from the opposite,unshielded side to give trans product.
The bromonium ion postulate,made more than60year ago to explain the steriochemistry of haiogen addition to alkenes,is a remarkable example of deductive logic in chemistry.Arguing from experimental result,