Cis–trans isomerism

''Cis''–''trans'' isomerism, also known as geometric isomerism, describes certain arrangements of atoms within molecules. The prefixes "''cis''" and "''trans''" are from Latin: "this side of" and "the other side of", respectively. In the context of chemistry, ''cis'' indicates that the functional groups (substituents) are on the same side of some plane, while ''trans'' conveys that they are on opposing (transverse) sides. ''Cis''–''trans'' isomers are stereoisomers, that is, pairs of molecules which have the same formula but whose functional groups are in different orientations in three-dimensional space. ''Cis'' and ''trans'' isomers occur both in organic molecules and in inorganic coordination complexes. ''Cis'' and ''trans'' descriptors are not used for cases of conformational isomerism where the two geometric forms easily interconvert, such as most open-chain single-bonded structures; instead, the terms "''syn''" and "''anti''" are used.

According to IUPAC, "geometric isomerism" is an obsolete synonym of "''cis''–''trans'' isomerism".

''Cis–trans'' or geometric isomerism is classified as one type of configurational isomerism.

Organic chemistry

Very often, ''cis''–''trans'' stereoisomers contain double bonds or ring structures. In both cases the rotation of bonds is restricted or prevented. When the substituent groups are oriented in the same direction, the diastereomer is referred to as ''cis'', whereas when the substituents are oriented in opposing directions, the diastereomer is referred to as ''trans''. An example of a small hydrocarbon displaying ''cis''–''trans'' isomerism is but-2-ene. 1,2-Dichlorocyclohexane is another example.

Comparison of physical properties

''Cis'' and ''trans'' isomers have distinct physical properties. Their differing shapes influences the dipole moments, boiling, and especially melting points.

These differences can be very small, as in the case of the boiling point of straight-chain alkenes, such as pent-2-ene, which is 37 °C in the ''cis'' isomer and 36 °C in the ''trans'' isomer. The differences between ''cis'' and ''trans'' isomers can be larger if polar bonds are present, as in the 1,2-dichloroethenes. The ''cis'' isomer in this case has a boiling point of 60.3 °C, while the ''trans'' isomer has a boiling point of 47.5 °C. In the ''cis'' isomer the two polar C–Cl bond dipole moments combine to give an overall molecular dipole, so that there are intermolecular dipole–dipole forces (or Keesom forces), which add to the London dispersion forces and raise the boiling point. In the ''trans'' isomer on the other hand, this does not occur because the two C−Cl bond moments cancel and the molecule has a net zero dipole moment (it does however have a non-zero quadrupole moment).

The differing properties of the two isomers of butenedioic acid are often very different.

Polarity is key in determining relative boiling point as strong intermolecular forces raise the boiling point. In the same manner, symmetry is key in determining relative melting point as it allows for better packing in the solid state, even if it does not alter the polarity of the molecule. Another example of this is the relationship between oleic acid and elaidic acid; oleic acid, the ''cis'' isomer, has a melting point of 13.4 °C, making it a liquid at room temperature, while the ''trans'' isomer, elaidic acid, has the much higher melting point of 43 °C, due to the straighter ''trans'' isomer being able to pack more tightly, and is solid at room temperature.

Thus, ''trans'' alkenes, which are less polar and more symmetrical, have lower boiling points and higher melting points, and ''cis'' alkenes, which are generally more polar and less symmetrical, have higher boiling points and lower melting points.

In the case of geometric isomers that are a consequence of double bonds, and, in particular, when both substituents are the same, some general trends usually hold. These trends can be attributed to the fact that the dipoles of the substituents in a ''cis'' isomer will add up to give an overall molecular dipole. In a ''trans'' isomer, the dipoles of the substituents will cancel out due to being on opposite sides of the molecule. ''Trans'' isomers also tend to have lower densities than their ''cis'' counterparts.

As a general trend, ''trans'' alkenes tend to have higher melting points and lower solubility in inert solvents, as ''trans'' alkenes, in general, are more symmetrical than ''cis'' alkenes.

Vicinal coupling constants (³''J''_HH), measured by NMR spectroscopy, are larger for ''trans'' (range: 12–18 Hz; typical: 15 Hz) than for ''cis'' (range: 0–12 Hz; typical: 8 Hz) isomers.

Stability

Usually for acyclic systems ''trans'' isomers are more stable than ''cis'' isomers. This difference is attributed to the unfavorable steric interaction of the substituents in the ''cis'' isomer. Therefore, ''trans'' isomers have a less-exothermic heat of combustion, indicating higher thermochemical stability. In the Benson heat of formation group additivity dataset, ''cis'' isomers suffer a 1.10 kcal/mol stability penalty. Exceptions to this rule exist, such as 1,2-difluoroethylene, 1,2-difluorodiazene (FN=NF), and several other halogen- and oxygen-substituted ethylenes. In these cases, the ''cis'' isomer is more stable than the ''trans'' isomer. This phenomenon is called the '' cis effect''.

''E''–''Z'' notation

In principle, ''cis''–''trans'' notation should not be used for alkenes with two or more different substituents. Instead the ''E''–''Z'' notation is used based on the priority of the substituents using the Cahn–Ingold–Prelog (CIP) priority rules for absolute configuration. The IUPAC standard designations ''E'' and ''Z'' are unambiguous in all cases, and therefore are especially useful for tri- and tetrasubstituted alkenes to avoid any confusion about which groups are being identified as ''cis'' or ''trans'' to each other.

''Z'' (from the German ) means "together". ''E'' (from the German ) means "opposed" in the sense of "opposite". That is, ''Z'' has the higher-priority groups ''cis'' to each other and ''E'' has the higher-priority groups ''trans'' to each other. Whether a molecular configuration is designated ''E'' or ''Z'' is determined by the CIP rules; higher atomic numbers are given higher priority. For each of the two atoms in the double bond, it is necessary to determine the priority of each substituent. If both the higher-priority substituents are on the same side, the arrangement is ''Z''; if on opposite sides, the arrangement is ''E''.

Because the ''cis''–''trans'' and ''E''–''Z'' systems compare different groups on the alkene, it is not strictly true that ''Z'' corresponds to ''cis'' and ''E'' corresponds to ''trans''. For example, ''trans''-2-chlorobut-2-ene (the two methyl groups, C1 and C4, on the but-2-ene backbone are ''trans'' to each other) is (''Z'')-2-chlorobut-2-ene (the chlorine and C4 are together because C1 and C4 are opposite).

Undefined alkene stereochemistry

Wavy single bonds are the standard way to represent unknown or unspecified stereochemistry or a mixture of isomers (as with tetrahedral stereocenters). A crossed double-bond has been used sometimes; it is no longer considered an acceptable style for general use by IUPAC but may still be required by computer software.

Inorganic chemistry

''Cis''–''trans'' isomerism can also occur in inorganic compounds.

Diazenes

Diazenes (and the related diphosphenes) can also exhibit ''cis''–''trans'' isomerism. As with organic compounds, the ''cis'' isomer is generally the more reactive of the two, being the only isomer that can reduce alkenes and alkynes to alkanes, but for a different reason: the ''trans'' isomer cannot line its hydrogens up suitably to reduce the alkene, but the ''cis'' isomer, being shaped differently, can.

Coordination complexes

Coordination complexes with octahedral or square planar geometries can also exhibit ''cis-trans'' isomerism.

For example, there are two isomers of square planar Pt(NH₃)₂Cl₂, as explained by Alfred Werner in 1893. The ''cis'' isomer, whose full name is ''cis''-diamminedichloroplatinum(II), was shown in 1969 by Barnett Rosenberg to have antitumor activity, and is now a chemotherapy drug known by the short name cisplatin. In contrast, the ''trans'' isomer ( transplatin) has no useful anticancer activity. Each isomer can be synthesized using the trans effect to control which isomer is produced.

For octahedral complexes of formula MX₄Y₂, two isomers also exist. (Here M is a metal atom, and X and Y are two different types of ligands.) In the ''cis'' isomer, the two Y ligands are adjacent to each other at 90°, as is true for the two chlorine atoms shown in green in ''cis''- o(NH₃)₄Cl₂sup>+, at left. In the ''trans'' isomer shown at right, the two Cl atoms are on opposite sides of the central Co atom.

A related type of isomerism in octahedral MX₃Y₃ complexes is
facial–meridional (or ''fac''–''mer'') isomerism, in which different numbers of ligands are ''cis'' or ''trans'' to each other. Metal carbonyl compounds can be characterized as ''fac'' or ''mer'' using infrared spectroscopy.

See also

* Chirality (chemistry)
* Descriptor (chemistry)
* ''E''–''Z'' notation
* Isomer
* Structural isomerism
* Trans fat

References

External links

IUPAC definition of "stereoisomerism"

{{DEFAULTSORT:Cis-trans isomerism
Stereochemistry
Isomerism
Orientation (geometry)