B − B −
Anti periplanar transition state
(preferred)
Syn periplanar transition state
(only with certain rigid molecules)
C C
H
L
H
CC
L
3) Syn periplannar conformation:
i) The syn periplannar transition state is eclipsed and occurs only with rigid
molecules that are unable to assume the anti arrangement.
- Neomenthyl chloride and menthyl chloride:
H 3 C
Cl
CH(CH 3 ) 2
Neomenthyl chloride Cl
H 3 C CH(CH 3 ) 2
H 3 C
Cl
CH(CH 3 ) 2
Menthyl chloride
H 3 C ClCH(CH 3 ) 2
1) The β-hydrogen and the leaving group on a cyclohexane ring can assume an anti
periplannar conformation only when they are both axial:
H H
HH
B −
Here the β-hydrogen and the
chlorine are both axial. This allow
an antiperiplanar transition state.
HH
Cl H
A Newman projection formula shows
that the β-hydrogen and the chlorine are
anti periplanar when they are both axial.
H
Cl
H
H
2) The more stable conformation of neomenthyl chloride:
i) The alkyl groups are both equatorial and the chlorine is axial.
ii) There also axial hydrogen atoms on both C1 and C3.
