Cis–trans isomerism

Cis–trans isomerism, also known as geometric isomerism or configurational isomerism, is a term used in organic chemistry. 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 are on the same side of the carbon chain while trans conveys that functional groups are on opposing sides of the carbon chain. Cis-trans isomers are stereoisomers, that is, pairs of molecules which have the same formula but whose functional groups are rotated into a different orientation in three-dimensional space. It is not to be confused with E–Z isomerism, which is an absolute stereochemical description. In general, stereoisomers contain double bonds that do not rotate, or they may contain ring structures, where the rotation of bonds is restricted or prevented. 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. Cis–trans isomerism, also known as geometric isomerism or configurational isomerism, is a term used in organic chemistry. 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 are on the same side of the carbon chain while trans conveys that functional groups are on opposing sides of the carbon chain. Cis-trans isomers are stereoisomers, that is, pairs of molecules which have the same formula but whose functional groups are rotated into a different orientation in three-dimensional space. It is not to be confused with E–Z isomerism, which is an absolute stereochemical description. In general, stereoisomers contain double bonds that do not rotate, or they may contain ring structures, where the rotation of bonds is restricted or prevented. 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. The term 'geometric isomerism' is considered by IUPAC to be an obsolete synonym of 'cis–trans isomerism'. 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. Alicyclic compounds can also display cis–trans isomerism. As an example of a geometric isomer due to a ring structure, consider 1,2-dichlorocyclohexane: Cis 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-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 (it does however have a non-zero quadrupole). The two isomers of 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 isomer 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, even if it does not alter the polarity of the molecule. One 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.