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
JHH), 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. This phenomenon is called the
cis effect.
E–Z notation than chlorine, so this alkene is the
Z isomer 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 ==