Advices For Studying Organic Chemistry

3. 1,3-Butadiene shows 1,4-addition reactions with electrophilic reagents other than
   HCl.
   CH 2 =CHCH=CH 2 40 HBroC CH 3 CHBrCH=CH 2 + CH 3 CH=CHCH 2 Br

(20%) (80%)

CH 2 =CHCH=CH (^2) −Br 15 o^2 C CH 2 BrCHBrCH=CH 2 + CH 2 BrCH=CHCH 2 Br
(54%) (46%)

4. Reactions of this type are quite general with other conjugated dienes.
   
   
   1. Conjugated trienes often show 1,6-addition.
   
   
   
   

Br

Br

Br 2

CHCl 3

(>68%)

13.10A KINETIC CONTROL VERSUS THERMODYNAMIC CONTROL OF

A CHEMICAL REACTION

1. The relative amounts of 1,2- and 1,4-addition products obtained from the addition
   of HBr to 1,3-butadiene are dependent on the reaction temperature.

1. When 1,3-butadiene and HBr react at a low temperature (–80 °C) in the absence
   of peroxides, the major reaction is 1,2-addition ⇒ 80% of the 1,2-product and
   only 20% of the 1,4-product.


2. At higher temperature (40 °C) the result is reversed: the major reaction is
   1,4-addition ⇒ 80% of the 1,4-product and only about 20% of the 1,2-product.


3. When the mixture formed at the lower temperature is brought to higher
   temperature, the relative amounts of the two products change.
   i) This new reaction mixture eventually contains the same proportion of products
   given by the reaction carried out at the higher temperature.