Ethane, energy barrier

The preferred conformations of carbonyl compounds, like 1-alkenes, are eclipsed rather than bisected, as shown below for ethanal and propanal. The barrier for methyl group rotation in ethanal is 1.17kcal/mol. Detailed analysis has indicated that small adjustments in molecular geometry, including a-bond lengthening, must be taken into account to quantitatively analyze the barrier. The total barrier can be dissected into nuclear-nuclear, electron-electron, nuclear-electron, and kinetic energy (At), as described in Topic 1.3 for ethane. MP2/6-311+G (Mf,2p) calculations lead to the contributions tabulated below. The total barrier found by this computational approach is very close to the experimental value. Contributions to the ethanal energy barrier in kcal/mol are shown below. 

Potential energy barriers which are equal in height, on the order of 12 kJ mol , for ethane. 

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... 

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...

Fig. 14. Potential energy barrier heights as calculated using ab initio theory for insertion of transition metal atoms into a C-H or C-C bond of (a) ethane and (b) cyclopropane. Values taken from Ref. 22.

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. 

The pulsed electron beam MS technique was also used by Hiraoka and Kebarle842 to study the C4H + cations. In the ion-molecule reaction of ethane and the ethyl cation, two species were observed and identified as the 2-//-n-butoniu m cation 469 and the 2-C-w-butonium cation 470. C—C protonated ion 470 formed first rearranges to C—H protonated ion 469 (energy barrier = 9.6 kcal mol-1) and then dissociation to sec-C4H9+ + H2 takes place. 

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