74
STEREOCHEMISTRY
two hands. You will see that they appear identical
(allowing for minor blemishes or broken fingernails).
However, do what you will, it is not possible
to superimpose them, and you should be able to
appreciate the mirror image relationship. The two
different arrangements – non-superimposable mirror
images – are called enantiomers (Greek: enantios =
opposite), and we say that enantiomers have different
configurations. The configuration is thus the spatial
sequence about a chiral centre. It is also apparent that
enantiomers are not going to interconvert readily, and
to achieve interconversion we would have to break
one of the bonds then remake it so as to get the other
configuration.
Note that the enantiomer of a particular compound
can be drawn by reversing two of the substituents;
this is actually much easier than drawing the mirror image compound, especially in more complicated
structures. As an alternative, the wedge–dot relationship could be reversed.
A
B
D
E
A
B
D
E
mirror
A
B
E
D
≡
enantiomer is obtained by
reversing two substituents
A
B
D
E
≡
enantiomer can also be
obtained by reversing the
wedge−dot relationship
rotation about
C−A axis
Molecules that are superimposable on their mirror
images are said to be achiral. With tetrahedral carbon,
this is typically the case when two or more of the
attached groups are the same. This introduces a plane of
symmetry into the molecule; molecules with a plane of
symmetry can be superimposed on their mirror images.
A
B
D
A
A
B
D
A
molecules that are superimposable
on their mirror images are achiral
mirror
A
A
B
D
molecule with a plane of
symmetry is achiral
A
A
B
D
plane of
symmetry
Note that chirality is not restricted to tetrahedral carbon; it can also be associated with other
tetrahedral systems, such as quaternary nitrogen
compounds.
A
N
B
D
E
A
N
B
D
E
quaternary N can also be chiral
mirror
N
A
C
B
N
A
C
B
rapid nitrogen inversion
means individual enantiomers
are not isolated
However, non-quaternary nitrogen, although tetrahedral, is not chiral. There is a rapid inversion that converts one enantiomer into the other;
effectively, the lone pair does not maintain its position. The energy barrier to interconversion is about
25 kJ mol
−1 , which is sufficiently low that inversion
occurs readily at room temperature. This usually
makes it impossible to obtain neutral amines in optically active form; quaternization stops this inversion.
We shall later need to introduce a related term,
prochiral. The concept of prochirality is discussed
in Section 3.7.
STEREOCHEMISTRY
two hands. You will see that they appear identical
(allowing for minor blemishes or broken fingernails).
However, do what you will, it is not possible
to superimpose them, and you should be able to
appreciate the mirror image relationship. The two
different arrangements – non-superimposable mirror
images – are called enantiomers (Greek: enantios =
opposite), and we say that enantiomers have different
configurations. The configuration is thus the spatial
sequence about a chiral centre. It is also apparent that
enantiomers are not going to interconvert readily, and
to achieve interconversion we would have to break
one of the bonds then remake it so as to get the other
configuration.
Note that the enantiomer of a particular compound
can be drawn by reversing two of the substituents;
this is actually much easier than drawing the mirror image compound, especially in more complicated
structures. As an alternative, the wedge–dot relationship could be reversed.
A
B
D
E
A
B
D
E
mirror
A
B
E
D
≡
enantiomer is obtained by
reversing two substituents
A
B
D
E
≡
enantiomer can also be
obtained by reversing the
wedge−dot relationship
rotation about
C−A axis
Molecules that are superimposable on their mirror
images are said to be achiral. With tetrahedral carbon,
this is typically the case when two or more of the
attached groups are the same. This introduces a plane of
symmetry into the molecule; molecules with a plane of
symmetry can be superimposed on their mirror images.
A
B
D
A
A
B
D
A
molecules that are superimposable
on their mirror images are achiral
mirror
A
A
B
D
molecule with a plane of
symmetry is achiral
A
A
B
D
plane of
symmetry
Note that chirality is not restricted to tetrahedral carbon; it can also be associated with other
tetrahedral systems, such as quaternary nitrogen
compounds.
A
N
B
D
E
A
N
B
D
E
quaternary N can also be chiral
mirror
N
A
C
B
N
A
C
B
rapid nitrogen inversion
means individual enantiomers
are not isolated
However, non-quaternary nitrogen, although tetrahedral, is not chiral. There is a rapid inversion that converts one enantiomer into the other;
effectively, the lone pair does not maintain its position. The energy barrier to interconversion is about
25 kJ mol
−1 , which is sufficiently low that inversion
occurs readily at room temperature. This usually
makes it impossible to obtain neutral amines in optically active form; quaternization stops this inversion.
We shall later need to introduce a related term,
prochiral. The concept of prochirality is discussed
in Section 3.7.