Advices For Studying Organic Chemistry

H 2 CCHCH 2 H + Br H 2 CCHCH 2 + H Br

H 2 CCHCH 2 + Br Br H 2 CCHCH 2 Br + Br

3) NBS is nearly insoluble in CCl 4 and provides a constant but very low

concentration of bromine in the reaction mixture.

i) NBS reacts very rapidly with the HBr formed in the substitution reaction ⇒

each molecule of HBr is replaced by one molecule of Br 2.

N

O

O

O

O

Br + HBr N H + Br 2

ii) In a nonpolar solvent and with a very low concentration of bromine, very

little bromine adds to the double bond; it reacts by substitution and replaces an

allylic hydrogen atom.

2. Why does a low concentration of bromine favor allylic substitution over
   addition?

1. The mechanism for addition of Br 2 to a double bond:

Br Br Br− Br

Br

C

C

C

C

+++Br C

C

i) In the first step only one atom of the bromine molecule becomes attached to the

alkene in a reversible step.

ii) The other atom (now the bromide ion) becomes attached in the second step.

iii) If the concentration of bromine is low, the equilibrium for the first step will lie

far to the left. Even when the bromonium ion forms, the probability of its