Rotational energy barriers ethane

Figure 4.2. Rotational-energy barriers as a function of substitution. Tbe small barrier ( 2kcal) in ethane (a) is lowered even further ( O.Skcal) if three bonds are tied back by replacing three hydrogen atoms of a methyl group by a triple-bonded carbon, as in methylacetylene (b). The barrier is raised 4.2 kcal) when methyl groups replace the smaller hydrogen atoms, as in neopentane (c). Dipole forces raise the barrier further ( 15 kcal) in methylsuccinic acid (d) (cf. Figure 4.3). Steric hindrance is responsible for the high barrier (> 15 kcal) in the diphenyl derivative (e). (After...

RNA, mRNA, rRNA, and tRNA. See Ribonucleic acid Roberts, John D., 928 Robinson, Sir Robert, 4, 402, 724 Robinson annulation, 724, 728 Rotamer, 90. See also Conformation Rotational energy barrier alkenes, 172—173 amides, 779 butane, 94—95 conjugated dienes, 31(r-371 ethane, 93—94... 

Threefold rotational potential energy barriers such as in ethane and butane are not always encountered. Twofold potential rotational energy barriers are produced by 1,4-phenylene groups in the main chain, for example. Catena-po y(su uv) also has a twofold rotational potential energy barrier. 

Several of the more common MO methods were mentioned in Chapter 1, where it was noted that most of the methods were very good at predicting minimum-energy geometries. As a result, there have been numerous calculations of the rotational energy barrier in ethane and related molecules that have matched the experimentally determined barriers quite well. Rather than elaborating on all the conformational equilibria that have been treated by MO methods, the emphasis in this section will be on the unique information available through MO calculations that is absent in molecular mechanics calculations. 

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... 

Methyl rotors pose relatively simple, fundamental questions about the nature of noncovalent interactions within molecules. The discovery in the late 1930s1 of the 1025 cm-1 potential energy barrier to internal rotation in ethane was surprising, since no covalent chemical bonds are formed or broken as methyl rotates. By now it is clear that the methyl torsional potential depends sensitively on the local chemical environment. The barrier is 690 cm-1 in propene,2 comparable to ethane,... 

Certain physical properties show that rotation about the single bond is not quite free. For ethane there is an energy barrier of about 3 kcal mol-1 (12 kJ mol-1). 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. The energy required to rotate the atoms or groups about the carbon-carbon bond is called torsional energy. Torsional strain is the cause of the relative instability of the eclipsed conformation or any intermediate skew conformations. 

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 two representations shown here are actually two different conformers of ethane there will be an infinite number of such conformers, depending upon the amount of rotation about the C-C bond. Although there is fairly free rotation about this bond, there does exist a small energy barrier to rotation of about 12kJmol due to repulsion of the electrons in the C-H bonds. By inspecting the Newman projections, it can be predicted that this repulsion will be a minimum when the C-H bonds are positioned as far away from each other... 

It follows that the preferred conformation of ethane is a staggered one but, since the energy barrier to rotation is relatively small, at room temperature there will be free rotation about the C-C bond. 

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