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1. Explain the following with examples.
(1) 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. Example.,
This isomerism arises out of the hindrance to rotation about the C=C bond in such molecules. The cis-trans isomers do not differ much in chemical properties. They differ in physical properties like boiling point, melting point, crystal structure, solubility and refractive index. Highly substituted olefin is more stable than less substituted olefin. Among substituted olefins, trans olefin is more stable than cis olefin. In the cis isomer because similar groups are very near each other, Vander Waals repulsion and steric hindrance make the molecule much unstable. In the trans isomer, similar groups are diagonally opposite to each other. Hence there is no such steric interaction. Generally trans isomer is more stable than cis isomer. Hence reactivity of cis isomer may be little higher than the trans isomer. The energy of the cis isomer is greater than that of trans isomer.
Though at room temperature, cis and trans isomers are stable and are not interconvertible, on heating to a certain temperature, trans isomer can be converted to cis isomer and vice-versa. ‘Breaking of carbon-carbon bond and its reformation is responsible for the interconversion
(2) Optical isomerism may be defined as the isomerism of chemical compounds in which the two isomers differ only in that their molecules are mirror images of each other . Consider ethanol and deuterated ethanol
The isomers-called optical isomers have configurations which are non super imposable.
Hence the criteria for a molecule to be asymmetric (hence optically active) is that its configuration is non superimposable on its mirror image. Such molecules are also called ‘chiral’ molecules.
2. Distinguish enantiomers and diasteromers ?
3. What is a racemic mixture ? Explain with suitable example.
When equal amounts of d-isomer and l-isomer are mixed one gets a ‘‘racemic mixture’’ and this process is called racemisation. A racemic mixture becomes optically inactive. Because, in this mixture rotation towards clockwise direction by the dextro isomers is compensated by the rotation towards the anticlockwise direction by the laevo isomers. The optical inactivity of a racemic mixture is said to be due to ‘external compensation’. Any how an optically inactive racemic form can be separated into two active forms. This process of separation is called ‘‘resolution of the racemic mixture’’.
Example:
4. Mesotartaric acid is an optically inactive compound with chiral carbon atoms. Justify.
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.
Though the Meso isomer 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. Mesoform cannot be separated into optically active enantiomeric pairs. This form is a single substance and not a mixture.
5. Distinguish racemic form from Mesoform
6. Describe the D, L-system of designation of configurations
In order to indicate the exact spatial arrangement of atoms or groups in a molecule having asymmetric carbon atom, Fischer proposed DL-system of nomenclature. Glyceraldehyde was taken as the standard. The dextro isomer and laevo isomer of glyceraldehyde are designated as D and L as follows. These are Fischer’s projection formulae.
In general more oxidised group is shown at the top and the reduced group at the bottom. The chiral molecule is viewed in such a way that H, Xlie above the plane of the paper and R1, R2-lie below the plane of the paper. The D, L-configurations are shown as :
The above projection formulae can be understood from the following diagram
7. Explain the energy level diagram for axial-equitorial cyclo hexanol conformational changes.
Cyclohexanol exists in the two chair forms. These two forms are interconvertible and exist in equilibrium
In one form (I) the –OH group is axially oriented. In the other form (II) the –OH group is equatorially oriented. The energy of the axial conformer is little higher than that of the equatorial conformer. 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. 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%.
Energy level diagram for axial and equatorial alcohols
8. Describe the conformations of cyclohexanol. Comment on their stability.
Cyclohexanol exists in the two chair forms. These two forms are interconvertible and exist in equilibrium.
In one form (I) the –OH group is axially oriented. In the other form (II) the –OH group is equatorially oriented. The energy of the axial conformer is little higher than that of the equatorial conformer. 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. 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%.
9. Define isomerism and different types of isomerism.
Isomers are compounds having same molecular formula but differ in physical or chemical or both physical and chemical properties. This phenomenon is known as isomerism.
Isomerism is of two types
Structural isomerism arises out of the difference in the arrangement of atoms in a molecule, (without referring to space), type of linkage and the atoms which are linked to each other in the moelcule. Stereoisomerism arises out of the difference in the arrangement of atoms in the molecule, with reference to each other in space
10. Write a note on optical activity.
Light is propagated in the form of waves. Ordinary monochromatic light is supposed to consist of waves vibrating in all planes. If such a light is passed through a Nicol prism, the emergent light is found to consist of waves vibrating in only one plane. Such a light is called plane polarised light.
When the plane polarised light is passed through certain substances or solutions (such as lactic acid), the emergent light is found to be vibrating in a different plane. This is called optical rotation.
Such substances are called optically active substances. This property of such substances is called ‘Optical activity’.
If the rotation is towards the right side [clockwise] it is called dextrorotation and the substance is said to be dextrorotatory. Dextrorotation is indicated by the symbol (+) or ‘d’. If the rotation is towards the left side [Anticlockwise] it is called laevorotation and the substance is said to be laevo rotatory. Laevorotation is indicated by the symbol (–) or ‘l’.
11.What are the conditions of optical activity?
All substances are optically active. For example substances like CHCl3, CH3CH2OH, CH3CH2CH=CH2, CH3CH2COCH3 are not optically active. On the other hand substances like CHIBrCl, CH3CHBr–CH=CH2,
CH3CH(OH)COCH3, CH3CHDOH are found to be optically active. When the structures of the compounds which are optically inactive and optically active, are compared, one thing is clear. All the optically active compounds, mentioned above have atleast one carbon atom that is bonded to four different atoms or groups. A carbon atom attached to four different atoms or groups is called ‘‘asymmetric carbon atom’’.
Pasteur suggested that optical activity arises from molecular dissymmetry which is supported by Vant-Hoff and LeBel. According to them, a carbon atom has its four valencies directed towards the four corners of a regular
tetrahedron. In this situation a molecule with an ‘asymmetric carbon atom’ becomes asymmetric. It so happens that an asymmetric molecule is not superimposable on its mirror image. That means an optically active molecule has more than [atleast two] one configuration. [configuration refer to three dimensional arrangement of atoms in a molecule] that is, an optically active molecule exhibits isomerism which is called optical isomerism
12.What are enantiomers?
Chiral molecules can have two different configurations. Each configuration stands for one optical isomer. Thus dextrorotatory isomer has configuration which is the mirror image of the laevorotatory isomer. Such optical isomers which differ only in the sign of (or direction of) optical rotation are called ‘‘enantiomers’’.
13.What is a racemic mixture?
When equal amounts of d-isomer and l-isomer are mixed one gets a ‘‘racemic mixture’’ and this process is called racemisation.
14. Write a note on recemic mixture?
When equal amounts of d-isomer and l-isomer are mixed one gets a ‘‘racemic mixture’’ and this process is called racemisation. A racemic mixture becomes optically inactive. Because, in this mixture rotation towards clockwise direction by the dextro isomers is compensated by the rotation towards the anticlockwise direction by the laevo isomers. The optical inactivity of a racemic mixture is said to be due to ‘external compensation’. Any how an optically inactive racemic form can be separated into two active forms. This process of separation is called ‘‘resolution of the racemic mixture’’.
15. Briefly describe the isomerism 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. 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. In the same way in the l - isomer, the overall rotation is laevo. 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.
Though the Meso isomer 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. Mesoform cannot be separated into optically active enantiomeric pairs. This form is a single substance and not a mixture.
16. Describe the configuration of disubstitutedbenzene.
When any two hydrogen atoms of the benzene ring are replaced by any other atoms or groups disubstituted benzene is obtained.
Example: C6H4Cl2, C6H4 (OH)2, CH3 C6H4Br, HOC6H5NO2. The disubstituted benzenes can exist in three isomeric forms
X = any substituent
These are three distinct forms differing in many properties though they have the same formula. Hence they are said to exhibit isomerism. In these three structures the isomers differ in the relative position of the substitutents. Hence they are called position isomers. When the substituents are in adjacent positions in the benzene ring, it is
called ortho isomer. When the two substituents are exactly opposite to each other, it is a para isomer. When the angle between two substituents is 120o. It is a meta isomer.
Example:
17. Note on types of c-h bonds.
