Ethane molecule, free rotation

One of the simplest organic molecules showing a variety of conformations is ethane. The free rotation of such a molecule (CH3-CH3) about... 

Before considering the special case of rotation about bonds in polymers it is useful to consider such rotations in simple molecules. Although reference is often made to the free rotation about a single bond, in fact rotational energies of the order of 2kcal/mole are required to overcome certain energy barriers in such simple hydrocarbons as ethane. During rotation of one part of a molecule about... 

Despite what we ve just said, we actually don t observe perfectly free rotation in ethane. Experiments show that there is a small (12 kj/mol 2.9 kcal/mol) barrier to rotation and that some conformers are more stable than others. The lowest-energy, most stable conformer is the one in which all six C-H bonds are as far away from one another as possible—staggered when viewed end-on in a Newman projection. The highest-energy, least stable conformer is the one in which the six C-H bonds are as close as possible—eclipsed in a Newman projection. At any given instant, about 99% of ethane molecules have an approximately staggered conformation... 

Actually this energy barrier creates hindrance in free rotation in the molecule. Therefore, strictly speaking there is not free rotation in ethane. But since this value is small we, may neglect it and regard that there is free rotation about C—C single bond in ethane. 

The calculated value incorrectly assumes unrestricted free rotation so that all conformations are equally probable. Since most molecules of ethane have the staggered conformation, the structural randomness is less than calculated, and the actual observed entropy is less. This discrepancy led to the concept of conformations with different energies. 

The pre-exponential factor A3 has been discussed (for RH == methane and cyclohexane) previously (22). For 2-methylpentane at 550°K., As 10 12 9 cc. molecule"1 sec."1, assuming free rotation in the transition state. Corresponding values for 2-methylpropane and ethane are 10 12 6 and 10"12 2 cc. molecule 1 sec. 1. The corresponding rate constants are given in Table II. 

Problem 4.3 (a) Are the staggered and eclipsed conformations the only ones possible for ethane (b) Indicate the preferential conformation of ethane molecules at room temperature, (r) What conformational changes occur as the temperature rises (d) Is the rotation about the a C—C bond, as in ethane, really free ... 

The picture-is not yet complete. Certain physical properties show that rotation is not quite free there is an energy barrier of about 3 kcal/mole. The potential energy of the molecule is at a minimum for the staggered conformation, increases with rotation, and reaches a maximum at the eclipsed conformation (Fig. 3.3). Most ethane molecules, naturally, exist in the most stable, staggered conformation or, put differently, any molecule spends most of its lime in the most stable conformation. 

Depending on the relative orientation of the two methyl groups, the total electronic energy is different. This means that rotation about the carbon-carbon axis is not free , but involves an energy barrier. The maximum occurs for the eclipsed conformation. For a simple m.o. treatment of the barrier to internal rotation in the ethane molecule, see ref. 80. 

Molecules which are roughly spherical have values of AfS which are approximately the same as for the elements, as is shown in table 14.5, while for straight chain molecules the entropy of fusion increases steadily with the chain length. Thus, in table 14.6, we see that for methane the entropy of fusion corresponds to its spherical shape, while for ethane which is a long ellipsoid the value is over twice as great. Similarly for 72.-octane which melts at 216 °K, Afh is 4,930 cal./mole so that AfS = 22-8 cal./deg. mole. while its isomer hexamethylethane which is nearly spherical melts at 377 °K with Afh —1,700 cal./mole, and an entropy of fusion of only 4 5 cal./deg. mole. There are many other examples of this kind of behaviour. The interpretation of the small entropy of fusion of spherical molecules is related to the fact that these compounds acquire free rotation while still in the solid state.f... 

The mode of overlap also influences molecular rotation, the ability of one part of a molecule to rotate relative to another part. A a bond allows free rotation of the parts of the molecule with respect to each other because the extent of overlap is not affected. If you could hold one CH3 group of the ethane molecule, the other CH3 group could spin like a pinwheel without affecting the C—C a-bond overlap (see Figure 11.9). 

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