Advices For Studying Organic Chemistry

7.6B AN EXCEPTION TO ZAITSEV’S RULE

1. A bulky base such as potassium tert-butoxide in tert-butyl alcohol favors the
   formation of the less substituted alkene in dehydrohalgenation reactions.

H 3 C C

CH 3

O−

CH 3

CH 3 CH 2 C

CH 3

Br

CH 3

CH 3 CH C

CH 3

CH 3

CH 3 CH 2 C

CH 2

CH 3

(^75) +
oC
(CH 3 ) 3 COH
2-Methyl-2-butene
(27.5%)
(more substituted)
2-Methyl-1-butene
(72.5%)
(less substituted)

1. The reason for leading to Hofmann’s product:
   i) The steric bulk of the base.
   ii) The association of the base with the solvent molecules make it even larger.
   iii) tert-Butoxide removes one of the more exposed (1°) hydrogen atoms instead of
   the internal (2°) hydrogen atoms due to its greater crowding in the transition
   state.

7.6C THE STEREOCHEMISTRY OF E2 REACTIONS: THE ORIENTATION

OF GROUPS IN THE TRANSITION STATE

1. Periplannar:
   
   
   1. The requirement for coplanarity of the H–C–C–L unit arises from a need for
      proper overlap of orbitals in the developing π bond of the alkene that is being
      formed.
   
   
   2. Anti periplannar conformation:
      i) The anti periplannar transition state is staggered (and therefore of lower energy)
      and thus is the preferred one.