Молекулалардың кеңістіктік изомерлері: Цис-транс және E-Z номенклатурасы

Pairs of molecules with same chemical formula showing different spatial orientations

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.

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.

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 zusammen) means "together". E (from the German entgegen) 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).

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.

Cis–trans isomerism can also occur in inorganic compounds.