Isomerism in Organic Chemistry Five Marks

1. Describe / Write a note on the conformations of cyclohexanol. Comment on their stability.

1. A mono substituted cyclohexane like cyclohexanol exists in the two chair forms. These two forms are interconvertible and exist in equilibrium.
2.

In one form (I) the –OH group is axially oriented. In the other form (II) the –OH group is equatorially oriented.

3. The energy of the axial conformer is little higher than that of the equatorial conformer.

4. Because the axial substituent experiences steric interaction with the axial H–atoms present at the third carbon atoms. This decreases the stability of the axial conformer. This is called 1:3-diaxial interaction.

5. This interaction is absent in the equatorial conformer. Hence equatorial cyclohexanol is present to an extent of about 90% in the equilibrium mixture. The axial isomer is present only to 10%. 

2. Discuss / Write in detail about the optical isomerisim in tartaric acid. Or Discuss / Explain the optical activity of tartaric acid.

Tartaric acid is dihydroxy dioic acid, having two identical chiral carbon atoms.

The enantiomers of tartaric acid, have the same magnitude but different sign of optical rotation. They have object-mirror image relationship.

1. (+) or Dextrorotatory or d-isomer:

In the d-isomer, each of the two asymmetric carbon atoms rotate the plane of the polarised light towards right leading to overall dextro rotation.

2. (–) or Laevorotatory or l-isomer:

In the l - isomer, each of the two asymmetric carbon atoms rotate the plane of the polarised light towards left leading to overall laevo rotation.

3. Meso isomer:

There is another optical isomer for tartaric acid in which one asymmetric carbon atom is dextrorotatory and the other laevorotatory-both rotating to the same extent in opposite directions. The net result is, that this isomer becomes optically inactive and is called the ‘‘Meso’’ isomer.

The optical inactivity of the ‘Meso’ isomer is due to the internal compensation. It is due to the inherent symmetry in the molecule. Mesoform cannot be separated into optically active enantiomeric pairs. This form is a single substance and not a mixture.

4. (±) – mixture or Racemic mixture or dl – mixture:

When equal amounts of d-tartaric acid and l-tartaric acid are mixed, we get racemic tartaric acid which is an optically inactive mixture. This can be separated into two optically active forms.

d-tartaric acid and l-tartaric acid are called Enantiomers.

d-tartaric acid and Meso tartaric acid or l-tartaric acid and Meso tartaric acid are called Diastereomers.

3. Discuss cis-trans isomerism with a suitable example. Or Explain geometrical isomerism with example.

Isomerism that arises out of difference in the spatial arrangement of atoms or groups about the doubly bonded carbon atoms is called Geometrical isomerism. These isomers are not mirror images of each other. Rotation about C = C is not possible at normal conditions and hence the isomers are isolable.

If different atoms or groups are bonded to the ‘C = C’ bond in a molecule, more than one spatial arrangement is possible. For example, 2-butene exists in two isomeric forms.

The isomer in which similar groups lie on the same side is called ‘cis isomer’ (I). The other in which similar groups lie in opposite direction is called ‘Trans isomer’ (II). This isomerism is called ‘Cis-Trans’ isomerism.

The two groups attached to the carbon atoms need not be same, it may be different also. e.g.,

This isomerism arises out of the hindrance to rotation about the C=C bond in such molecules.
Or

‘Cis-trans’ system of nomenclature may not be suitable for many substituted olefins.

For example,

 A newer system based on the priority of groups is the Cahn-Ingold-Prelog convention. This system is called the (E - Z) system, applies to alkene diastereomers of all type.

If the two groups of higher priority are on the same side of the double bond, the alkene is designated ‘Z’ (from the German word Zusammen-meaning together).

If the two groups of high priority are on opposite sides of the double bond, the alkene is designated ‘E’ (from the German, entgegen, meaning opposite)

The priorities follow the order of decreasing atomic number of the atom directly bonded to the carbon.

For some compounds the priority is shown by numbers as

 

4. Explain the following isomerism with examples.

 i) cis–trans isomerism in organic compounds

ii) optical isomerism in organic compounds

Answer: Refer Qs 2. 3

 5. Distinguish enantiomers from diastereomers. Give an example each.

S.No	Enantiomer	Diastereomer
1	Optical isomers having the same magnitude but different sign of optical rotation.	Differ in the magnitude of optical rotation.
2	They have configuration with non-super imposable object mirror image relationship.	They are never mirror images.
3	Identical in all properties except the sign of optical rotation.	Differ in all physical propeties.
4	Separation of enantiomers is a tedious process.	Separation from the other pairs of enantiomers is easy.
5	Example: d-tartaric acid and l-tartaric acid	Examples: d-tartaric acid and Meso tartaric acid or l-tartaric acid and Meso tartaric acid

6. Distinguish racemic mixture meso-form with suitable example.

S.No	Racemic mixture	Meso form
1	It is a mixture that can be separated into two optically active forms.	It is a single compound and hence cannot be separated.
2	Optically inactive due to external compensation.	Optically inactive due to internal compensation.
3	Molecules of isomers present are chiral.	Molecules are achiral.
4	Examples: 1. Equal amounts of d-Tartaric acid and l-Tartaric acid 2. Equal amounts of d-Lactic acid and l- Lactic acid	Example: Meso tartaric acid

 7. Explain internal and external compensation with suitable example.

Internal compensation

Though the Meso tartaric acid has two asymmetric carbon atoms the configuration of one carbon is the mirror image of the other, the net result being the molecule as a whole becomes symmetric. This molecule is said to have a symmetric plane, which divides the molecule into two equal halves. The molecule becomes ‘‘achiral’’. It has configuration which is superimposable on its mirror image.

The optical inactivity of the ‘Meso’ isomer is due to the internal compensation. It is due to the inherent symmetry in the molecule. Meso form cannot be separated into optically active enantiomeric pairs. This form is a single substance and not a mixture.

Example:

Meso tartaric acid

External compensation

1 : 1 mixture of d-isomer and l-isomer is called ‘‘racemic mixture’’. A racemic mixture becomes optically inactive. Because in this mixture rotation towards clockwise direction by the dextro isomer is compensated by the rotation towards the anticlockwise direction by the laevo isomer. The optical inactivity of a racemic mixture is said to be due to ‘external compensation’.

Example:

1. When equal amounts of d-Tartaric acid and l-Tartaric acid are mixed, we get racemic tartaric acid which is an optically inactive mixture. This can be separated into two optically active forms.

2. Equal amounts of d-Lactic acid and l- Lactic acid

8. Write the possible isomers of dichlorobenzene and their dipole moment order

  Isomers can also be named using the number of the carbon atom thus

Ortho isomer is 1,2-dichloro benzene

Meta isomer is 1,3-dichloro benzene

Para isomer is 1,4-dichloro benzene

Their Dipole moment (DPM), is in the order.

Ortho isomer > Meta isomer > Para isomer

for para dichloro benzene Dipole moment, DPM = 0.

9. Discuss cis - trans isomerism with a suitable example.
10. Write a note on the different types of hydrogen in cyclohexane

There are two types of C–H bonds.

 

THREE MARKS