Question

How many products are formed if the bromination reaction proceeds through the bromonium ion intermediate? How many products are formed if the reaction proceeds through a resonance stabilized cationic intermediate instead? Can you draw the mechanisms and products? What is the stereochemical relationship between the products formed in the reaction?

Answer 1

1) If the bromination reaction proceeds through the bromonium ion intermediate, then the reaction proceeds in two steps .

In step 1 , Br+ electrophile attacks the alkene double bond(electron rich so acts as nucleophile).Then in step 2 , the bromonium ion intermediate acts as nucleophile and Br- attacks from backside giving anti or trans product (single product)

2) If the reaction proceeds through a resonance stabilized cationic intermediate instead , ( eg in aromatic compounds ) ,then the reaction occurs in two steps again, addition of Br+ as electrophile to the nucleophillic compound.Then elimination occurs to eliminate the –H from the cationic intermediate.

Both cis and trans product can be formed as Br+ can attack from both sides on planar benzene ring (in the example) if benzene ring is substituted with a bulky group (like t-butyl - (CH3)3-CH-) then , Br + attacks on the ring from less hindered side .(single product) .If ring is substituted by a less bulky group then both side attack by Br+ takes place giving two product .(cis as well as trans)