Geometric isomerism
In chemistry, geometric isomerism isformstereoisomerismdescribesorientationfunctional groups atends ofbond around which no rotationpossible. Such bondstypically double bonds, butcan also be part ofring structure which prevents rotation.Sometimesterm "geometric isomerism"used assynonymstereoisomerism, i.e. optical isomersconsideredbe geometric isomers. The exact termstereoisomers thatnot optical isomersdiastereomers.
Theretwo forms ofgeometric isomer,cistrans versions. The formwhichsubstituent groupsonsame side ofbond that doesn't allow rotationcalled cis,formwhichsubstituentson opposite sides ofbondcalled trans:
Cistrans isomers ofsubstance have different physical properties. Trans isomers generally have higher boiling pointslower densities. Thisbecausetrans isomers molecules can line upfit together better thancis form.
As an example ofgeometric isomer due toring structure, consider 1,2-dichlorocyclohexane:
| image:trnC6H10Cl2.png | image:cisC6H10Cl2.png |
| trans-1,2-dichlorocyclohexane | cis-1,2-dichlorocyclohexane |
Another notation can be usedcistrans molecules. Z fromGerman Zusammen, which means together can replaceterm cis;E fromGerman Entgegen who means in opposite of can replaceterm trans. This alternate systemdesignationnecessary when theremore than two different substituents ondouble bond. Whethermolecular configurationZ or Edetermined byCahn Ingold Prelog priority rules ofsubstituents. For each oftwo atoms indouble bond, individually determine which oftwo substituentsofhigher priority. If both ofsubstituentshigher priorityonsame sidearrangementZ, if theyon opposite sidesarrangementE.
