Geometric isomer

Cis-trans isomerism

In chemistry, cis-trans isomerism or geometric isomerism or configuration isomerism is a form of stereoisomerism describing the orientation of functional groups within a molecule. In general, such isomers contain double bonds, which cannot rotate, but they can also arise from ring structures, wherein the rotation of bonds is greatly restricted.

The term "geometric isomerism" is considered an obsolete synonym of "cis-trans isomerism" by IUPAC. It is sometimes used as a synonym for general stereoisomerism (e.g., optical isomerism being called geometric isomerism); the correct term for non-optical stereoisomerism is diastereomerism.

There are two forms of a cis-trans isomer, the cis and trans versions. 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 2-butene.

Alicyclic compounds can also display cis-trans isomerism. As an example of a geometric isomer due to a ring structure, consider 1,2-dichlorocyclohexane:

trans-1,2-dichlorocyclohexane cis-1,2-dichlorocyclohexane

Cis vs trans physical properties

Cis isomers and trans isomers often have different physical properties. Differences between isomers, in general, arise from the differences in the shape of the molecule or the overall dipole moment. These differences can be very small, as in the case of the boiling point of straight-chained alkenes, such as pent-2-ene, wherein the trans isomer has a boiling point of 36°C and cis isomer is 37°C . The differences between cis and trans isomers can also be quite large, as in the case of cyclooctene. The cis isomer in this case has a boiling point of 145°C , while the trans isomer has a boiling point of 75°C. The large difference between the two isomers of cyclooctene arises from the large ring strain for trans-cyclooctene, which also makes it significantly less stable than the cis isomer. In fact, the two isomers of 2-butenedioic acid have such large differences in properties and reactivities that they were actually given completely different names. The cis isomer is called maleic acid and the trans is named fumaric acid. Polarity is key in determining relative boiling point as it causes increased intermolecular forces, thereby raising 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. 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 their being on opposite site of the molecule. Trans isomers also tend to have lower densities than their cis counterparts.

March observes that, as trans alkenes, in general, have more symmetry than cis alkenes, the trans alkenes also tend to have higher melting points and lower solubility in inert solvents.

Vicinal coupling constants (3JHH), 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, trans isomers are more stable than the cis isomers. This is partly due to their shape, the straighter shape of the trans isomer leads to hydrogen intermolecular forces that make the isomer more stable . According to March, trans isomers also have a lower 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. For instance, for 1,2-difluoroethylene, 1,2-difluorodiazene (FN=NF), and several other halogen- and oxygen-substituted ethylenes. In this case, the cis isomer is more stable than the trans isomer. This phenomenon is called the cis effect.

E/Z notation

The cis/trans system for naming isomers is not effective when there are more than two different substituents on a double bond. The E/Z notation should then be used. Z (from the German zusammen) means together and corresponds to the term cis; E (from the German entgegen) means opposite and corresponds to the term trans.

Whether a molecular configuration is designated E or Z is determined by the Cahn-Ingold-Prelog priority rules (higher atomic numbers are given higher priority). For each of the two atoms in the double bond, it is necessary to determine which of the two substituents is of a higher priority. If both of the substituents of higher priority are on the same side, the arrangement is Z; if they are on opposite sides, the arrangement is E.

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