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HOW TO APPROACH EXAMINATION QUESTIONS: SELECTED PROBLEMS AND ANSWERS
Problem 16.06 (level 1; chapters 3 and 6)
Treatment of (2R,3R)-2-bromo-3-methylpentane with sodium ethoxide in ethanol gives (E)-3-methylpent-2-ene
as the major product. What would be the major product from similar base treatment of (2S,3S)- and (2R,3S)-2bromo-3-methylpentane? Hint: Newman projections will be useful here.
Answer 16.06
(2R,3R)-2-bromo-3-methylpentane
NaOEt
(E)-3-methylpent-2-ene
(major product)
Draw out the structures with stereochemistry. Since it is not always easy to visualize a stereoisomer with the
required configuration, draw one tentative arrangement, work out the chirality, then adjust if necessary. It is
always easiest to put the group of lowest priority (usually H) away from you, i.e. dotted bond.
Br
NaOEt
E
2R,3R
H
H
E2
2
3
EtO
Br
H
H
≡
priority 1
priority 2
priority 3
priority 4
Br
CHMeEt
H
Me
H
Me
Et
CHBrMe
2
3
priority 1
priority 2
Me
H Me
Et
E2 elimination
for elimination,
H and Br need to be anti
rotate 60˚
about 2−3 bond
Br
H
e.g.
priority 1
priority 2
priority 3
priority 4
R
Br
H
S
or
Br
H
S
Br
2-bromo-3-methylpentane
changing 2 groups gives
alternative configuration
2
3
C-2
C-3
C-2 C-3
Draw the starting material as a Newman projection
for elimination,
H and Br need to be anti
Me
Et
Me
H
Me
Et
H
Br
Me
Me
Et
H
Me
Br
H
H
2
3
look along
2−3 bond
H
Me
Et
H
Br
Me
rotate 60˚ to locate H
and Br on verticals
2R,3R
E
H
Me
Et
H
Br
Me