<P> Usually for acyclic systems trans isomers are more stable than cis isomers . This is typically due to the increased unfavourable steric interaction of the substituents in the cis isomer . Therefore, trans isomers have a less exothermic 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, such as 1, 2 - difluoroethylene, 1, 2 - difluorodiazene (FN = NF), and several other halogen - and oxygen - substituted ethylenes . In these cases, the cis isomer is more stable than the trans isomer . This phenomenon is called the cis effect . </P> <P> The cis--trans system for naming alkene isomers should generally only be used when there are only two different substituents on the double bond, so there is no confusion about which substituents are being described relative to each other . For more complex cases, the cis / trans designation is generally based on the longest carbon chain as reflected in the root name of the molecule (i.e. an extension of standard organic nomenclature for the parent structure). The IUPAC standard designations E--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 . </P> <P> 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 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 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 . </P> <P> 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). </P>

Why does 2 chlorobut 2 ene exist as a pair of stereoisomers