Specification of configuration-The R or S system
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 image.
- 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 implicita)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 inter conversion of the cis-trans isomers be high enough for these isomers to be distinguishable entities.
Higher priority on the same side is called zusammen, that’s mean together, it’s denoted by Z
When higher priority on the opposite side then it is called entgegen, that’s means opposite, it’s denoted by E.
What happens when torsion angle is not 00 or 1800 ? i.e. non planar.
Still there is isomerism of sizeable perpendicular (900) barrier is there.
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.
Cumulenes ( polyenes with odd no of C=C bonds) [ ab(C=)nC cd]
- cis – trans isomerism 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 () cumulene) polymerize with extreme ease; nothing is known about the cis-trans isomerism in these compound.