Geometric isomerism 1.

Geometric isomerism results from the restricted rotation about bounds, usually about double bonds or when the single bond is incorprated in a cyclic structure. Most commonly there are two different substituents at the ends of the double bond, for example:

In the classical nomenclature the cis and trans prefixes are used to distinguish between the pair of isomers. A cis/trans pair are two different compounds having different physical and chemical properties. For example, maleic acid yields to maleic anhydride at about 140 deg, while the isomeric fumaric acid must be heated to higher temperature, because it has to isomerize prior to the anhydride formation, and this requires the momentary break-up of the double bond:


If several double bonds are present in the molecule the number of possible geometric isomers is 2n, for example in the case of 1-phenyl-1,3-pentadiene it is 22 = 4:

The terms cis and trans are unambigous only in disubstituted alkenes. If the alkene is tri- or tetrasubstituted this notation cannot be applied. Therefore the E-Z system is used to designate the stereochemistry nowadays, which is based on the Cahn-Ingold-Prelog (CIP) convention (similarly to the optical isomers).

In the E-Z system the two substituents attached to each carbon atom of the double bond are arranged in order of priority:

  1. The atom of lower atomic number is assigned lower priority;
  2. If the relative priorities cannot be determined as above, the secondary (or beta) atoms are to be compared, thus e.g.: ‑CH3 < ‑CH2CH3 < ‑CH(CH3)2 < ‑CH2OCH3 < ‑Cl < ‑Br stb.;
  3. In the case of double or triple bond, the atom is considered to be duplicated or triplicated (but these phantom atoms have lower priority than that of two real ones): ‑CH2OH < ‑CHO [= ‑CH(O‑)2] < ‑CH(OR)2

If the two groups of higher priority are located on the same side of the double bond the alkene is designated Z isomer (German zusammen – together), otherwise it is assigned to the E isomer (entgegen – opposite):


Geometric isomerism arises in the case C=N or N=N double bonds, too. The older nomenclature commonly used the syn and anti prefixes in these cases, for example oximes. In the syn oxime the hydroxyl group on nitrogen and hydrogen (or the first named group) on carbon are on the same side, whereas in the anti isomer these are on the opposite side. Because of ambiguity the use of E/Z notation is more convenient. The lone electron pair of the nitrogen is regarded as the lowest priority group:


The E/Z system is an absolute designation mode, whereas syn/anti is a relative one, it is necessary to state which substituents are syn or anti to each other.

The cis/trans relative designation can also be used in the case of cycloalkane rings with no possibility of rotation about any of the C-C bonds. The two substituents can both lie on the same side of the ring, or one can be below the ring and the other above. The cis/trans prefix indicates also the mode of junction in molecules with two or more fused multiple rings:

Excercises

Which alkene can exist as more than one stereoisomer?


 A
 B
 C
 D
 E

Identify the configuration of the molecules:

 cis,trans
 cis,cis

 cis,trans
 Z,Z
 E,Z


 E
 Z

 E
 Z


 Z
 E

 Z
 E

These last two questions are good examples that...