Cis–trans isomerism, also known asgeometric 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.[1] In the context of chemistry,cis indicates that thefunctional groups (substituents) are on the same side of some plane, whiletrans conveys that they are on opposing (transverse) sides.Cis–trans isomers arestereoisomers, that is, pairs of molecules which have the same formula but whose functional groups are in different orientations in three-dimensional space.Cis andtransisomers occur both in organic molecules and in inorganic coordination complexes.Cis andtransdescriptors are not used for cases ofconformational 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 toIUPAC, "geometric isomerism" is an obsolete synonym of "cis–trans isomerism".[2]
Cis–trans or geometric isomerism is classified as one type ofconfigurational isomerism.[3]
Very often,cis–trans stereoisomers containdouble bonds or ring structures. In both cases the rotation of bonds is restricted or prevented.[4] When thesubstituent groups are oriented in the same direction, thediastereomer is referred to ascis, whereas when the substituents are oriented in opposing directions, the diastereomer is referred to astrans. An example of a small hydrocarbon displayingcis–trans isomerism isbut-2-ene. 1,2-Dichlorocyclohexane is another example.
  |   |
| trans-1,2-dichlorocyclohexane | cis-1,2-dichlorocyclohexane |
Cis andtrans isomers have distinct physical properties. Their differing shapes influences thedipole moments, boiling, and especially melting points.
 |  |
| cis-2-pentene | trans-2-pentene |
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| cis-1,2-dichloroethene | trans-1,2-dichloroethene |
These differences can be very small, as in the case of the boiling point of straight-chain alkenes, such aspent-2-ene, which is 37 °C in thecis isomer and 36 °C in thetrans isomer.[5] The differences betweencis andtrans isomers can be larger if polar bonds are present, as in the1,2-dichloroethenes. Thecis isomer in this case has a boiling point of 60.3 °C, while thetrans isomer has a boiling point of 47.5 °C.[6] In thecis isomer the two polar C–Clbond dipole moments combine to give an overall molecular dipole, so that there are intermoleculardipole–dipole forces (or Keesom forces), which add to theLondon dispersion forces and raise the boiling point. In thetrans 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-zeroquadrupole moment).
 |  |
| cis-butenedioic acid (maleic acid) | trans-butenedioic acid (fumaric acid) |
 |  |
| cis-9-octadecenoic acid (oleic acid) | trans-9-octadecenoic acid (elaidic acid) |
The differing properties of the two isomers of butenedioic acid are often very different.
| maleic acid | fumaric acid | |
|---|---|---|
| color | white | white |
| melting point, °C | 130 | 286 |
| water solubility, g/L | 788 | 7 |
| Acid dissociation constant, pKa1 | 1.90 | 3.03 |
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 betweenoleic acid andelaidic acid; oleic acid, thecis isomer, has a melting point of 13.4 °C, making it a liquid at room temperature, while thetrans isomer, elaidic acid, has the much higher melting point of 43 °C, due to the straightertrans 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, andcis 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 acis isomer will add up to give an overall molecular dipole. In atrans isomer, the dipoles of the substituents will cancel out[7] due to being on opposite sides of the molecule.Trans isomers also tend to have lower densities than theircis counterparts.[citation needed]
As a general trend,trans alkenes tend to have highermelting points and lowersolubility in inert solvents, astrans alkenes, in general, are more symmetrical thancis alkenes.[8]
Vicinalcoupling constants (3JHH), measured byNMR spectroscopy, are larger fortrans (range: 12–18 Hz; typical: 15 Hz) than forcis (range: 0–12 Hz; typical: 8 Hz) isomers.[9]
Usually for acyclic systemstrans isomers are more stable thancis isomers. This difference is attributed to the unfavorablesteric interaction of the substituents in thecis isomer. Therefore,trans isomers have a less-exothermicheat of combustion, indicating higherthermochemical stability.[8] In the Bensonheat of formation group additivity dataset,cis isomers suffer a 1.10 kcal/mol stability penalty. Exceptions to this rule exist, such as1,2-difluoroethylene,1,2-difluorodiazene (FN=NF), and several other halogen- and oxygen-substituted ethylenes. In these cases, thecis isomer is more stable than thetrans isomer.[10] This phenomenon is called thecis effect.[11]
-1-Bromo-1%2c2-dichloroethene.svg)
In principle,cis–trans notation should not be used for alkenes with two or more different substituents. Instead theE–Z notation is used based on the priority of the substituents using theCahn–Ingold–Prelog (CIP) priority rules for absolute configuration. The IUPAC standard designationsE andZ 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 ascis ortrans to each other.
Z (from the Germanzusammen) means "together".E (from the Germanentgegen) means "opposed" in the sense of "opposite". That is,Z has the higher-priority groupscis to each other andE has the higher-priority groupstrans to each other. Whether a molecular configuration is designatedE orZ 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 isZ; if on opposite sides, the arrangement isE.
Because thecis–trans andE–Z systems compare different groups on the alkene, it is not strictly true thatZ corresponds tocis andE corresponds totrans. For example,trans-2-chlorobut-2-ene (the two methyl groups, C1 and C4, on thebut-2-ene backbone aretrans to each other) is (Z)-2-chlorobut-2-ene (the chlorine and C4 are together because C1 and C4 are opposite).
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 byIUPAC but may still be required by computer software.[12]
Cis–trans isomerism can also occur in inorganic compounds.
Diazenes (and the relateddiphosphenes) can also exhibitcis–trans isomerism. As with organic compounds, thecis isomer is generally the more reactive of the two, being the only isomer that can reducealkenes andalkynes toalkanes, but for a different reason: thetrans isomer cannot line its hydrogens up suitably to reduce the alkene, but thecis isomer, being shaped differently, can.
  |   |
| trans-diazene | cis-diazene |
Coordination complexes with octahedral or square planar geometries can also exhibitcis-trans isomerism.
For example, there are two isomers ofsquare planar Pt(NH3)2Cl2, as explained byAlfred Werner in 1893. Thecis isomer, whose full name iscis-diamminedichloroplatinum(II), was shown in 1969 byBarnett Rosenberg to have antitumor activity, and is now a chemotherapy drug known by the short namecisplatin. In contrast, thetrans isomer (transplatin) has no useful anticancer activity. Each isomer can be synthesized using thetrans effect to control which isomer is produced.
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Foroctahedral complexes of formula MX4Y2, two isomers also exist. (Here M is a metal atom, and X and Y are two different types ofligands.) In thecis isomer, the two Y ligands are adjacent to each other at 90°, as is true for the two chlorine atoms shown in green incis-[Co(NH3)4Cl2]+, at left. In thetrans 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 isfacial–meridional (orfac–mer) isomerism, in which different numbers of ligands arecis ortrans to each other. Metal carbonyl compounds can be characterized asfac ormer usinginfrared spectroscopy.
Authority control databases: National |
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