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Basic Principles of Organic Chemistry

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Stereoisomerism

Isomers which have the same structures but differ in the relative arrangements of atoms or groups in space are called stereoisomers. It is of three types, i.e. conformational isomerism, optical isomerism and geometrical isomerism.

Geometrical Isomers: Stereoisomers which have the same structural formula but differ in the spatial arrangement of atoms around the double bond are called geometrical isomers. The isomer in which the two similar atoms/groups lie on the same side of the double bond is called the cis-isomer while that isomer in which the two similar atoms/groups lie on the opposite side of the double bond is called the trans-isomer.

• The necessary and sufficient condition for a molecule to exhibit geometrical isomerism is that each ot the two carbon atoms of the double bond must have different atoms/groups which may be same of different. Gor example , alkenes of the types abC=Cab and abC=Cde show geometrical isomerism

• Compounds containing C=N and N=N also show geometrical isomerism

• 1,2-1,3 and 1,4-disubstituted cycloalkanes also show geometrical isomerism

• The dipole moment of a cis-isomer is higher than that of a trans-isomer

• E and Z configuration of geometrical isomers

  • Give priority to the four groups attached to the double bonded C-atoms. Higher the atomic number of the atom of the group attached to the double bonded carbon atom higher will be its priority. If priority cannot be decided the atoms directly attached to double bonded C-atoms then next atoms of the group are compared

  • If two higher priority groups are on the same side of the double bond then it is called a Z-isomer otherwise it is called an E-isomer

Conformational Isomerism: The infinite number of momentary arrangement of the atoms in space which result through rotation about a single bond are called conformations or rotational isomers

• Ethane has infinite number of conformations, of which, only two, i.e. staggered and eclipsed are important. The staggered conformation of ethane is more stable than the eclipsed conformation by about 3.0 kcal or 12.55 kJ mol^-1

• Important conformations of n-butane are staggered (or anti), gauche (or skew), partially eclipsed and fully eclipsed. Their relative stability is in the order: staggered>gauche>partially eclipsed>fully eclipsed

• Cyclohexane exists in two non-planer conformations, i.e., chair and the boat form. Both these conformations are free from angle strain (i.e., have tetrahedral angles)

• The boat form of cyclohexane is less stable than the chair form by about 44 kJ mol^-1

• The two conformations of cyclohexane cannot be separated but ar readily interconvertible