Advices For Studying Organic Chemistry

CH 2 CHCH +HBr

− 80 oC

40 oC

CH 3 CHCH CH 2

Br

(80%)

CH 3 CHCH CH 2

Br

(20%)

CH 3 CH CHCH 2

(20%)

Br

+

CH 3 CH CHCH 2

(80%)

Br

+

CH 2 40 oC

ii) At the higher temperature and in the presence of HBr, the 1,2-adduct

rearranges to 1,4-product and that an equilibrium exists between them.

CH 3 CHCH CH 2

Br

1,2-Addition product

CH 3 CH CHCH 2

Br

1,4-Addition product

40 oC, HBr

iii) The equilibrium favors the 1,4-addition product ⇒ 1,4-adduct must be more

stable.

2. The outcome of a chemical reaction can be determined by relative rates of
   competing reactions and by relative stabilities of the final products.

1. At lower temperature, the relative amounts of the products of the addition are
   determined by the relative rates at which the two additions occur; 1,2-addition
   occurs faster so the 1,2-addition product is the major product.


2. At higher temperature, the relative amounts of the products of the addition are
   determined by the position of an equilibrium; 1,4-addition product is the more
   stable, so it is the major product.


3. The step that determines the overall outcome of the reaction is the step in which
   the bybrid allylic cation combines with a bromide ion:

CH 2 CHCH CH 2

HBr

CH 3 CH CH CH 2

δ+ δ+

Br−

Br−

CH 3 CHCH CH 2

Br

CH 3 CH CHCH 2

Br

1,2 Product

1,4 Product

This step determines

the regioselectivity

of the reaction.