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Conformational isomerism

Free rotation is possible about the carbon-carbon single bond axis.

This rotation makes the atoms in a molecule assume diffeent spatial positions with respect to each other.

The different spatial positions are called "conformations or conformers ".

A molecule as simple as ethane can have infinite number of such spatial positions. The conformation in which the atoms are closest to each other is called the eclipsed, and the one in which they are farthest apart is known as the staggard conformation. In between these two extremes there are infinite number of forms called the "skew" forms.

Inter atomic repulsions are maximum in the eclipsed and minimum in the staggard form thereby making the former less stable than the later.

Very little energy is needed for conformational changes, so they will be taking place at all temperatures, less at lower temperatures. For instance the eclipsed form will be converting to the staggard form and vice-versa. So the two forms are in equilibrium with each other at any given temperature with the more stable one predominating.

For better understanding please refer to the following books,

Organic Chemistry:

Graham Solomon or Wade.

At a higher level please consult Stereochemistry by Eliel or Nasipuri.

Test Your Knowledge

1. Which of the following molecules can exist in different conformations?

a) Ethene b) Benzene c) 2-Butene d) Ethane

2. Which of the following statements is true concerning the fact that ethene cannot exist in different conformations?

a) there is no free rotation possible about the bond axis of any two atoms.
b) Ethene has no sigma bond between C-C bond axis.
c) Ethene has one C=C double bond about which free rotation is not possible.
d) Free rotation is not possible about the C-H single bond

3. The staggered form of ethane is most stable because:

a) The atoms in the molecule are as far away from each other as possible
b) Inter atomic repulsions are minimum.
c) The staggard conformation is associated with the lowest energy
d) All of the above are true.

4. In butane the most stable conformation is:

a) when a methyl is eclipsed with a hydrogen
b) when two hydrogen atoms are eclipsed
c) when two methyl groups are eclipsed.
d) when one methyl group is eclipsed with one hydrogen and the other methyl is eclipsed with another hydrogen.

5. In propane CH3CH2CH3 the most stable conformation is:

a) When the two-methyl groups are staggered.
b) When the two-methyl groups are eclipsed.
c) When two hydrogen atoms are eclipsed.
d) None of these.

6. The number of conformational isomers in propane and butane in which a methyl group is eclipsed with a hydrogen are:

a) the same b) more in butane c) more in propane d) none of these.

7. The eclipsed and staggered forms in ethane are in equilibrium.

True or False

8. With respect to a specific hydrogen atom in ethane, there are three eclipsed conformations of equal energy possible.

True or False

9. In butane the conformation with two methyl groups eclipsed is less stable than when a methyl is eclipsed with a hydrogen

True or False

10. At any given temperature the staggered and eclipsed forms are in equilibrium. But the % of each form remains unchanged. This can be calculated by using the Gibbs equation ΔG0 = -2.303 nRT log K
(n = number of moles, R = universal gas constant, T = temp in k, K = equlibrium constant and ΔG0 = free energy difference between the two conformers.)
Thus if the energy difference is say 5Kcals at a temperature of 270C, K can be calculated and K = x/(100-x) where x is the % of one conformation, solving for x gives the % of each form. From the data given calculate the % of each conformer in one mole of the sample.

11. In cis-1,3-dimethylcyclohexane the two methyl groups are:

a) both can be axial b) both can be equatorial c) one is axial and one equatorial d) both a) and b) are true.

12. In trans-1,2-dimethylcyclohexane the two methyl groups are:

a) both can be axial b) both can be equatorial c) one is axial and one equatorial d) both a) and b) are true.

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