Stereochemical considerations in the E2 reactions The E2 follows a
concerted mechanism, where removal of the proton and formation of the
double bond occur at the same time. The partial p bond in the transition state
requires the parallel alignment or coplanar arrangement of the p orbitals.
When the hydrogen and leaving group eclipse each other (0
), this is known
as the syn-coplanar conformation.
H
X
H
X
H X
H
X
syn-Elimination
anti-Elimination
B: −
syn-Coplanar (0 o )
anti-Coplanar (180 o )
B: −
When the leaving group and hydrogen atom are anti to each other (180
),
this is called the anti-coplanar conformation. The anti-coplanar conformation is of lower energy, and is by far the most common. In the
anti-coplanar conformation, the base and leaving group are well separated, thus removing electron repulsions. The syn-coplanar conformation requires the base to approach much closer to the leaving group,
which is energetically unfavourable.
The E2 reaction is a stereospecific reaction, i.e. a particular stereoisomer reacts to give one specific stereoisomer. It is stereospecific, since
it prefers the anti-coplanar transition state for elimination. The (R,R)
diastereomer gives a cis-alkene, and the (S,R) diastereomer gives a
trans-alkene.
H
C C
Br
C
H 3
C 2 H 5
H 5 C 2
H
H
C C
Br
C
H 3
H
H 5 C 2
C 2 H 5
C 2 H 5
C 2 H 5
C
H 3
H
C 2 H 5
H
C
H 3
C 2 H 5
(S,R)
trans-configuration
(R,R)
cis-configuration
+ B-H
+ B-H
B: −
B: −
+ Br: −
+ Br: −
E2 elimination of HX in the cyclohexane system
Almost all cyclohexane systems are most stable in the chair conformations.
In a chair, adjacent axial positions are in an anti-coplanar arrangement, ideal
for E2 eliminations. Adjacent axial positions are said to be in a trans-diaxial
arrangement. E2 reactions only proceed in chair conformations from
230
CH5 ORGANIC REACTIONS