Bond rotation, alkanes ethane

Rotation around Carbon-Carbon Bonds in Alkanes Ethane... 

Ethane is the prototype molecule for rotation around carbon-carbon single bonds in alkanes, and has therefore probably been the most frequently studied molecule with respect to hindered rotation. 

Propane, the next higher member in the alkane series., also has a torsional barrier that results in hindered rotation around the carbon-carbon bonds. The barrier is slightly higher in propane than in ethane—a total of 14 kj/mol (3.4 kcal/mol) versus 12 kj/mol. 

Conformational isomerism in propane Propane is a three-carbon- (sp -hybridized) atom-containing linear alkane. All are tetrahedrally arranged. When a hydrogen atom of ethane is replaced by a methyl (CH3) group, we have propane. There is rotation about two C-C cr bonds. 

The results of a valence bond treatment of the rotational barrier in ethane lie between the extremes of the NBO and EDA analyses and seem to reconcile this dispute by suggesting that both Pauli repulsion and hyperconjugation are important. This is probably closest to the truth (remember that Pauli repulsion dominates in the higher alkanes) but the VB approach is still imperfect and also is mostly a very powerful expert method [43]. VB methods construct the total wave function from linear combinations of covalent resonance and an array of ionic structures as the covalent structure is typically much lower in energy, the ionic contributions are included by using highly delocalised (and polarisable) so-called Coulson-Fischer orbitals. Needless to say, this is not error free and the brief description of this rather old but valuable approach indicates the expert nature of this type of analysis. 

Conformational isomers of alkanes arise from the rotation of C-C single bonds. Different shapes of alkanes can be adopted by a molecule like ethane by rotation around the C-C bond. The most distinctive ones (Fig.H) are I and II are known as staggered and eclipsed respectively. In conformation I, the C-H bonds on carbon... 

The presence of the k bond confers properties on an alkene that mark it out as different from an alkane. In particular, the n bond, by the nature of its sideways overlap of the constituent p orbitals, is weaker than a a bond. Moreover, the electrons of the n bond are relatively exposed, above and below the plane of the alkene. These electrons are the source of reactivity of the alkene toward electrophiles, as in, say, electrophilic addition of bromine (Chapter 4). The n bond in ethene (and other alkenes) is, however, sufficiently strong that it prevents rotation around the carbon-carbon a bond, which is a well-documented property of the carbon-carbon bond in ethane (Section 1.6). The bonding between sp2... 

Because there is free rotation aroimd a carbon-carbon single bond, even a very simple alkane, like ethane, can exist in an unlimited number of forms. These different arrangements are called conformations, or conformers. 

Alkanes can have different conformations. By analyzing the structure of ethane, we can define certain aspects regarding its conformations. Conformations are different arrangements of the atoms in a molecule, as a result of rotation around a single bond. 

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