Specification of configuration-The R or S system

Specification of configuration-The R or S system can be explained by the Cahn, Ingold, Prelog sequence rules.

Cahn, Ingold, Prelog sequence rules:

Cahn-Ingold-Prelog Priority a>b and c>d

Cis-trans isomerism ( also called geometric isomerism) is a type of diastereomerism.
They are not (rare exception) mirror images.
The necessary & sufficient condition for the existence of cis-trans isomerism is a # b and c # d. No restriction as to the identity of a, b, c, d
Other conditions are implicit)torsion angles a-C-C-c; a-C-C-d, b-C-C-c, b-C-C-d be near O0 ( planar) or near 180o ( near planar)

b) barrier for interconversion of the cis-trans isomers is high enough for these isomers to be distinguishable entities.

Nomenclature

Higher priority on the same side is called zusammen, that’s means together, it’s denoted by Z

e.g. (Z)-1-bromo-1,2-dichloroethene.

When higher priority is on the opposite side then it is called entgegen, that’s means opposite, it’s denoted by E.

What happens when the torsion angle is not 0^o or 180^o? i.e. non-planar.

Still, there is an isomerism of a sizeable perpendicular (90⁰) barrier is there.

The torsion angle w<90⁰) between the highest priority groups is called Z; w>90⁰ E.

Z does not always correspond to cis or syn as well as E does not always correspond to trans or anti.

Interconversion of cis-trans isomers

Cumulenes ( polyenes with odd no of C=C bonds) [ ab(C=)nC cd]

cis-trans isomerism is possible due to the fact that successive planes of π–bonds are orthogonal to each other.
In butatriene the barrier is 31.0 kcal/mol forCH3CH = C = C = CHCH3 27.0
The low barriers are presumably due to Zwitterionic or biradical resonance

The higher cumulenes e.g. octaheptaenes polymerize with extreme ease; nothing is known about the cis-trans isomerism in these compounds.