Specification of configuration-The R or S system

Specification of configuration-The R or S system

Cahn, Ingold, Prelog sequence rules:

Specification of configuration-The R or S system

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

  1.  Cis-trans isomerism ( also called geometric isomerism) is a type of diastereomerism .
  2. They are not  (rare exception) mirror image.
  3. 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
  4. 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

Specification of configuration-The R or S system

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

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

Specification of configuration-The R or S system

When higher priority 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 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

Specification of configuration-The R or S system

Specification of configuration-The R or S system

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

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