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Organic Chemistry

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Stereo-isomerism

Stereoisomerism

Compounds having the same molecular formula, the same connectivity but have a difference in spatial arrangements of atoms are called stereoisomers, and this phenomenon is called stereoisomerism. They are further classified into:

  1. Configuration isomerism
  2. Conformational isomerism

Configuration Isomerism: The isomers which have a difference in spatial arrangements but can’t be converted to another form at room temperature are called configuration isomers.

Configuration isomerism is further classified into:

  1. Geometrical Isomerism
  2. Optical Isomerism

Geometrical Isomerism: This type of isomerism arises due to the restricted rotation of atoms of a cyclic system.

Geometrical isomers are named differently as cis-trans, E – Z, or syn-anti isomers.

Cis-trans isomers

When the same atoms or groups of atoms are attached at the same side of the double-bonded carbon atom are called cis. Isomers.

When the same atoms of a group of atoms are attached at the opposite side of doubly bonded carbon, then it is called trans-isomers.

Example:

b) E and Z isomers

(German Keywords)

E = Entegen

Z= Zussammen

  • The isomers of alkenes in which all the four substituents are different

  • It is E form isomer if the two groups of higher priority are on the opposite sides of the double bond.
  • It is Z form isomer if the two groups of higher priority are on the same sides of the double bond.

(CIP Rule: Cahn, Ingold and Prelog Rule)

  1. The group of atoms that has a higher atomic number will have higher priority.

When an atomic number is the same, then the priority is given to the group with a higher atomic weight.

  1. The next joining atom is considered to be a high priority if atomic weight and atomic number are the same.
  2. If multiple bonded groups attach to the double-bonded carbon, then they are considered in the following manner.

c) Syn-anti isomers: Oxime and Azzo compound exhibit syn-anti isomers. Oxime is the organic compounds formed by the reaction of ammonia derivatives(hydroxylamine) with aldehyde and ketone. Azzo is the compounds that consist of (-N=N-) bond. They exhibit all the necessary properties for geometrical isomers.

(Oxmine of aldehyde and oxime of unsymmetrical ketone exhibit geometrical isomerism)

  • When –OH group and H atom exists on the same side, it is said to be syn form else wise; it is called anti form.
  • For unsymmetrical ketoxime, if –OH and the alphabetical alkyl present on the same side of the double bond, then it is termed as a syn form whereas the other isomer is anti-form.

 

 

 

 

 

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