Specification of configuration-The R or S system

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 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.

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 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 ([7]) cumulene) polymerize with  extreme ease; nothing is known about the cis-trans isomerism in these compound.

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