April 04, 2012

Isomerism in Organic Chemistry Three Marks

1. Differentiate diastereomer from enantiomer. Or Distinguish enantiomers and diastereomers.
Optical isomers having the same magnitude but different sign of optical rotation.
Differ in the magnitude of
optical rotation
They have configuration with
non-super imposable object mirror image relationship.
They are never mirror images.
Identical in all properties except the sign of optical rotation.
Differ in all physical propeties.
Separation of enantiomers is a
tedious process
Separation from the other
pairs of enantiomers is easy.
2.  Give the conditions required for a compound to exhibit opticalisomerism. Or What are the conditions for a compound to be optically active?
1. All the optically active compounds have at least one carbon atom that is bonded to four different atoms or groups (asymmetric or chiral carbon atom).
2. An asymmetric molecule is not super imposable on its mirror image.Such molecules are also called ‘chiral’ molecules
Chirality’ is ‘the essential and the sufficient’ condition for a molecule to be optically active.
3.   Give the structure of cis-trans isomers of 2-pentene.
4.  Identify cis or trans isomers from the following:
5. Label the following as, E, Z isomers.

In compound 1, the two groups of high priority are on opposite sides of the double bond, the alkene is designated ‘E’.
In compound 2, the two groups of higher priority are on the same side of the double bond, the alkene is designated ‘Z’.
 6.  Mesotartaric acid is an optically inactive (compound with asymmetric atoms / chiral carbon atom.) Justify.
     Though the Meso isomer has two asymmetric carbon atoms the configuration of one carbon is the mirror image of the other, making 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.
7. Trans-isomer is more stable than cis-isomer. Why?
     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.
8. What are optical isomers? Give example.
The compounds having same molecular formula and almost having the same physical and chemical properties but differ in the rotation of plane polarized light are called optical isomers and the phenomenon is called optical isomerism.
Or The compounds having same molecular formula but different three-dimensional arrangements of atoms around one or more chiral carbon atoms
Lactic acid, Tartaric acid, Glucose, Fructose, All the a-amino acids (except glycine)
9. What is racemic mixture? Give an example. Or Write briefly on 'Racemic mixture' with an example.
When equal amounts of (+) or dextrorotatory or d-isomer and (–) or laevorotatory or l-isomer are mixed we get a ‘‘racemic mixture’’ or (±) – mixture or dl – mixture and this process is called racemisation.
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’.
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’’.

When equal amounts / 1:1 ratio 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. 
10. Draw the structure of S – cis and S – trans form of 1, 3 - butadiene
11. Give the structure of Z and E forms of Cinnamic acid 

 12. Distinguish Meso form from Racemic mixture
S. No
Racemic mixture
Meso form
It is a mixture that can be separated into two optically active forms.
It is a single compound and hence cannot be separated.
Optically inactive due to external compensation.
Optically inactive due to internal compensation.
Molecules of isomers present are
Molecules are achiral.
1. Equal amounts of d-Tartaric acid and l-Tartaric acid
2. Equal amounts of d-Lactic acid and l- Lactic acid
Meso tartaric acid
Write the formula (configuration) for d, l and meso tartaric acid.


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