Question

:a. How does the first step in the reaction of propene with Br₂ differ from the first step in the reaction of propene with HBr?

b. To understand why Br^- adds to a carbon of the bromonium ion rather than to the positively charged bromine, draw the product that would be obtained if Br^- did add to bromine.

Short answer

(a). In the reaction of propene with Br₂, the first step produces a cyclic bromonium ion, whereas in the reaction of propene with HBr, the first step produces a carbonation.

(b). A very unstable molecule (with a negative charge on the carbon atom and a positive charge on bromine) would be created if the bromide ion attacked the positively charged bromine.

Step-by-step solution

 Reaction of propene with Br2-

One of the bromine atoms becomes linked to the carbon atoms of propene in the initial stage of the process, with the positive charge on the bromine atom. An ion of bromine is produced.

Reaction of propene with HBr-

whereas in the reaction of propene with HBr, the first step produces a carbocation.

Reaction when Br- adds to a carbon of the bromonium ion-

If the bromide ion attacked the positively charged bromine, a very unstable molecule (with a negative charge on the carbon atom and a positive charge on bromine) would be formed.