Stereoisomers have the same atoms or isotopes connected by bonds of the same type, but differ in the relative positions of those atoms in space. Two broad types of stereoisomers exist, enantiomers and diastereomers. Enantiomers have identical physical properties but diastereomers do not.
Enantiomers Two compounds are said to be
enantiomers if their molecules are mirror images of each other and cannot be made to coincide only by rotations or translations – like a left hand and a right hand. The two shapes are said to be
chiral. A classic example is
bromochlorofluoromethane (CHFClBr). The two enantiomers can be distinguished, for example, by whether the path F->Cl->Br turns clockwise or counterclockwise as seen from the hydrogen atom. In order to change one conformation to the other, at some point those four atoms would have to lie on the same plane – which would require severely straining or breaking their bonds to the carbon atom. The corresponding energy barrier between the two conformations is so high that there is practically no conversion between them at room temperature, and they can be regarded as different configurations. The compound
chlorofluoromethane CH2ClF, in contrast, is not chiral; the mirror image of its molecule is also obtained by a half-turn about a suitable axis. Another example of a chiral compound is
2,3-pentadiene H3C-CH=C=CH-CH3, a hydrocarbon that contains two overlapping double bonds. The double bonds are such that the three middle carbons are in a straight line, while the first three and last three lie on perpendicular planes. The molecule and its mirror image are not superimposable, even though the molecule has an axis of symmetry. The two enantiomers can be distinguished, for example, by the
right-hand rule. This type of isomerism is called
axial isomerism. Enantiomers behave identically in chemical reactions, except when reacting with chiral compounds or in the presence of chiral
catalysts, such as most
enzymes. For this latter reason, the two enantiomers of most chiral compounds usually have markedly different effects and roles in living organisms. In
biochemistry and
food science, the two enantiomers of a chiral molecule – such as
glucose – are usually identified and treated as very different substances. Each enantiomer of a chiral compound typically rotates the plane of
polarized light that passes through it. The rotation has the same magnitude but opposite senses for the two isomers, and can be a useful way of distinguishing and measuring their concentration in a solution. For this reason, enantiomers were formerly called "optical isomers". However, this term is ambiguous and is discouraged by the
IUPAC. Some enantiomer pairs (such as those of
trans-cyclooctene) can be interconverted by internal motions that change bond lengths and angles only slightly. Other pairs (such as CHFClBr) cannot be interconverted without breaking bonds, and therefore are different configurations.
Diastereomers Stereoisomers that are not enantiomers are called
diastereomers. Some diastereomers may contain
chiral centers, and some may not.
Cis–trans isomerism A double bond between two carbon atoms forces the remaining four bonds (if they are single) to lie on the same plane, perpendicular to the plane of the bond as defined by its
π orbital. If the two bonds on each carbon connect to different atoms, two distinct conformations are possible that differ from each other by a twist of 180 degrees of one of the carbons about the double bond. The classical example is dichloroethene C2H2Cl2, specifically the structural isomer Cl-HC=CH-Cl that has one chlorine bonded to each carbon. It has two conformational isomers, with the two chlorines on the same side or on opposite sides of the double bond's plane. They are traditionally called
cis (from Latin meaning "on this side of") and
trans ("on the other side of"), respectively, or
Z and
E in the
IUPAC recommended nomenclature. Conversion between these two forms usually requires temporarily breaking bonds (or turning the double bond into a single bond), so the two are considered different configurations of the molecule. More generally,
cis–trans isomerism (formerly called "geometric isomerism") occurs in molecules where the relative orientation of two distinguishable functional groups is restricted by a somewhat rigid framework of other atoms. For example, in the cyclic alcohol
inositol (CHOH)6 (a six-fold alcohol of cyclohexane), the six-carbon cyclic backbone largely prevents the hydroxyl -OH and the hydrogen -H on each carbon from switching places. Therefore, one has different configurational isomers depending on whether each hydroxyl is on "this side" or "the other side" of the ring's mean plane. Discounting isomers that are equivalent under rotations, there are nine isomers that differ by this criterion, and behave as different stable substances (two of them being enantiomers of each other). The most common one in nature (
myo-inositol) has the hydroxyls on carbons 1, 2, 3 and 5 on the same side of that plane, and can therefore be called
cis-1,2,3,5–
trans-4,6-cyclohexanehexol. And each of these
cis–
trans isomers can possibly have stable "chair" or "boat" conformations (although the barriers between these are significantly lower than those between different
cis–
trans isomers). and
transplatin, are examples of square planar MX2Y2 molecules with M = Pt.
Cis and trans isomers also occur in inorganic
coordination compounds, such as
square planar MX2Y2 complexes and
octahedral MX4Y2 complexes. For more complex organic molecules, the
cis and
trans labels can be ambiguous. In such cases, a more precise labeling scheme is employed based on the
Cahn-Ingold-Prelog priority rules. The situation for
butane is similar, but with sightly lower
gauche energies and barriers. This
steric hindrance effect is more pronounced when those four hydrogens are replaced by larger atoms or groups, like chlorines or
carboxyls. If the barrier is high enough for the two rotamers to be separated as stable compounds at room temperature, they are called
atropisomers.
Topoisomers Large molecules may have isomers that differ by the
topology of their overall arrangement in space, even if there is no specific geometric constraint that separate them. For example, long chains may be twisted to form topologically distinct
knots, with interconversion prevented by bulky substituents or
cycle closing (as in circular
DNA and
RNA plasmids). Some knots may come in mirror-image enantiomer pairs. Such forms are called topological isomers or
topoisomers. Also, two or more such molecules may be bound together in a
catenane by such topological linkages, even if there is no chemical bond between them. If the molecules are large enough, the linking may occur in multiple topologically distinct ways, constituting different isomers.
Cage compounds, such as
helium enclosed in
dodecahedrane (He@) and
carbon peapods, are a similar type of topological isomerism involving molecules with large internal voids with restricted or no openings.--> ==Isotopes and spin==