Advices For Studying Organic Chemistry

CH^12 CH^23 CH CH^42

+

H Cl

1,2-addition CH

2 CH CH CH^2

Cl

3-Chloro-1-butene

H

2) 1,4-Addition:

CH 2 CH CH CH 2

1 234

+

H Cl

1,4-addition CH

2 CH CH CH^2

Cl

1-Chloro-2-butene

H

2. 1,4-Addition can be attributed directly to the stability and delocalized nature of
   allylic cation.

1. The mechanism for the addition of HCl:
   Step 1

CH 2 CH CH CH 3 CH CH CH 3 CH CH CH 2

H Cl

+ Cl −

An allylic cation equivalent to

CH (^2) ++CH 2
CH 3 CH CH CH 2
δ+ δ+
Step 2
CH 2 CH CH
H Cl
CH 2 CH CH CH 2
H Cl
CH 3 CH CH CH 2
δ+ δ+

• Cl^ −
  (a) (b)
  (a) 1,2-Addition
  (b) 1,4-Addition
  CH 2

2. In step 1, a proton adds to one of the terminal carbon atoms of 1,3-butadiene to
   form the more stable carbocation ⇒ a resonance stabilized allylic cation.
   i) Addition to one of the inner carbon atoms would have produced a much less 1°
   cation, one that could not be stabilized by resonance.
   CH 2 CH CH
   H Cl

X CH+ 2 CH 2 CH + Cl −

A 1o carbocation

CH 2 CH 2