Advices For Studying Organic Chemistry

2) The transition state resembles the less stable 1° carbocation ⇒ the reaction

leading to the 1° carbocation (and ultimately to 1-bromopropane) has a higher

free energy of activation.

3) The second reaction is much slower and does not compete with the first one.

4. The reaction of HBr with 2-methylpropene produces only tert-butyl bromide.
   
   
   1. The difference between a 3° and a 1° carbocation.
   
   
   2. The formation of a 1° carbocation is required ⇒ isobutyl bromide is not
      obtained as a product of the reaction.
   
   
   3. The reaction would have a much higher free energy of activation than that
      leading to a 3° carbocation.
   
   
   
   

A Mechanism for the Reaction

Addition of HBr to 2-Methylpropene
This reaction takes place:

H 3 CC

CH 3

CH 2

H Br

CCH 2

H 3 C

+ H H 3 C C

CH 3

CH 3

H 3 C −

3 o Carbocation

Br

tert-Butyl bromide

Br

(more stable carbocation) Actual product

This reaction does not occur appreciably:

2 X H 3 C 2 3 2

−

C CH

H

X H CCHCH Br

Isobutyl bromide

Not formed

H 3 CCCH

Br

CH 3

+

CH 3 CH 3

H Br

1 o Carbocation

(less stable carbocation)

(^)

5. When the carbocation initially formed in the addition of HX to an alkene can
   rearrange to a more stable one ⇒ rearrangements invariably occur.