CONFIGURATIONAL ISOMERS
75
Box 3.5
Manipulating stereostructures
It is not always easy to look at stereostructures – two-dimensional representations of three-dimensional
molecules – and decide whether two separate representations are the same or different. To compare structures, it is
usually necessary to manipulate one or both so that they can be compared directly. Here are a few demonstrations
of how to approach the problem on paper. Of course, constructing models for comparison is the easiest method,
but there will always be occasions when we have to figure it out on paper.
Answer: A and B are same as original; C and D are enantiomers of original
OH
H
H
OH
CH 3
H
HO
OH
H
H
Me
H
turn sideways
OH
HO
H
H
CH 3
H
not a chiral
centre
OH
HO
CH 3
H
H
OH
CH 3 H
OH
H
turn sideways
H
H
OH
H 3 C
OH
H
H
OH
H 3 C
OH
look towards CH 2 OH
sequence Me → H → OH is clockwise
look towards CH 2 OH
sequence Me → H → OH is clockwise;
this is the same as original
HO
OH
H
Me
OH
H
look towards CH 2 OH
sequence Me → H → OH is anticlockwise;
this is enantiomer of original
HO
look towards CH 2 OH
sequence Me → H → OH is anticlockwise;
this is enantiomer of original
Note: use any sequence. For this purpose we
do not need to obey any priority rules
turn round
180°
HO
OH
H CH 3
rotate 60° about
central bond
Question: A molecule is represented by the Newman projection:
Which of the following are equivalent to the above Newman projection, or to its enantiomer?
OH
H
H
OH
CH 3
H
H
OH
CH 3 H
OH
H
OH
HO
HO
OH
H
H
Me
H
D
H 3 C
H
CH 2 OH and OH still in plane of paper;
methyl was at front, now at rear;
hydrogen was at rear, now at front
OH
HO
OH
HO
OH
HO
HO
OH
look towards CH 2 OH
sequence Me → H → OH is clockwise;
this is the same as original
turn 30°
turn 30°
≡
≡
≡
≡
≡
A
B
C
D
A
B
C