Molecules that are not superimposahle on their mirror images are chiral.
Chirality is the necessary and sufficient condition for the existence^oT enantiomers. That is to say: a compound \\hose molecules are chiral can exist as enantiomers; a compound whose molecules are achiral (vMthout chirality) cannot exist as enantiomers.
When we say that a molecule and its mirror image are superimposable, we
mean that if in our mind s eye we were to bring the image from behind the
mirror where it seems to be, it could be made to coincide in all its parts with the molecule. To decide whether or not a molecule is chiral, therefore, we
make a model of it and a model of its mirror image, and see if we can superimpose them. This is the safest way, since properly handled it must give us the right answer. It is the method that we should use until we have become quite familiar with the ideas involved; even then, it is the method we should use when we encounter a new type of compound.
After we have become familiar with the models themselves, we can draw
pictures of the models, and mentally try to superimpose them. Some, we find, are not superimposablc, like these: These molecules are chiral, and we know that chloroiodomethanesulfonic acid can exist as enantiomers, which have the structures we have just made Or drawn. Others, we find, are superimposable, like these : These molecules are achiral, and so we know that isopropyl chloride cannot exist as enantiomers.
"I call any geometrical figure, or any group of points, chiral, and say it has
chirality, if its image in a plane mirror, ideally realized, cannot be brought to coincide with itself."Lord Kelvin, 1893.
In 1964, Cahn, Ingold, and Prelog(see p. 130) proposed that chemists use the terms "chiral" and "chirality" as defined by Kelvin. Based on the Greek word for "hand" (cheir\ chirality means "
handedness," in reference to that pair of non-superimposable
mirror images \ve constantly -have before us: our two hands. There has been wide-spread acceptance of Kelvin s terms, and they have largely displaced the earlier "dissymmetric" and "dissymmetry" (and the still earlier and less accurate "asymmetric" and "asymmetry"),
although one must expect to encounter the older terms in the older chemical
literature. Whatever one calls it,
it is non-superimposability-on-mirror-image that is the necessary
and sufficient condition for enantiomerism ; it is also a necessary- but not sufficient condition for optical activity (see Sec. 4.13).
4.10 The chiral center So far, all the chiral molecules we have talked about happen to be of the kind
CWXYZ; that is, in each molecule there is a carbon (C*) that holds four different groups. A carbon atom to which four different groups are attached is a chiral center. (Sometimes it is called chiral carbon, when it is necessary to distinguish it from chiral nitrogen, chiral phosphorus, etc.)
Many but not all molecules that contain a chiral center are chiral. Many
but not all chiral molecules contain a chiral center. There are molecules that
contain chiral centers and yet are achiral (Sec. 4.18). There are chiral molecules that contain no chiral centers (see, for example, Problem 6, p. 315). The presence or absence of a chiral center is thus no criterion of chirality.
However, most of the chiral molecules that we shall take up do containchiral
centers, and it will be useful for us to look for such centers; if we find a chiral center, then we should consider the possibility that the molecule is chiral, and hence can exist in enantiomeric forms. We shall later (Sec. 4.18) learn to recognize the kind of molecule that may be achiral in spite of the presence of chiral centers; such molecules contain more than one chiral center.
After becoming familiar with the use of models and of pictures of models,
the student can make use of even simpler representations of molecules containing chiral centers, which can be drawn much faster. This is a more dangerous method, however, and must be used properly to give the right answers. We simply draw a cross and attach to the four ends the four groups that are attached to the chiral center. The chiral center is understood to be located where the lines cross. Chemists have agreed that such a diagram stands for a particular structure: the horizontal lines represent bonds coming toward us out of the plane of the paper, whereas the vertical lines represent bonds going away from us behind the plane of the paper.