Chemistry Reference
In-Depth Information
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
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 mir-
ror image compound, especially in more complicated
structures. As an alternative, the wedge - dot relation-
ship could be reversed.
=
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
rotation about
C
enantiomer can also be
obtained by reversing the
wedge
mirror
−
A axis
A
A
A
A
−
dot relationship
≡
≡
E
D
E
D
E
B
B
B
B
enantiomer is obtained by
reversing two substituents
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.
mirror
A
A
D
B
D
B
plane of
symmetry
molecule with a plane of
symmetry is achiral
A
D
A
A
A
A
A
B
B
molecules that are superimposable
on their mirror images are
achiral
Note that chirality is not restricted to tetrahe-
dral carbon; it can also be associated with other
tetrahedral
systems,
such
as
quaternary
nitrogen
compounds.
mirror
A
N
A
N
C
B
A
rapid nitrogen inversion
means individual enantiomers
are not isolated
N
N
C
B
E
D
E
A
B
B
quaternary N can also be chiral
However, non-quaternary nitrogen, although tetra-
hedral, is not chiral. There is a rapid inver-
sion that converts one enantiomer into the other;
effectively, the lone pair does not maintain its posi-
tion. 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 opti-
cally 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.
