Rotation in Substituted Ethanes

How does the potential-energy diagram change when a substituent is added to ethane Consider, for example, propane, whose structure is similar to that of ethane, except that a methyl group replaces one of ethane s hydrogen atoms. 

Sterk hindrance raises the energy barrier to rotation 

There can be more than one staggered and one eclipsed conformation conformational analysis of butane 

Solved Exercise 2-22 Working with the Concepts Conformations 

E raw a qualitative potential-energy diagram for rotation about the C3-C4 bond in 2-methylpentane. Show Newman projections for all conformations located at the maximum and minimum points on your graph. Describe similarities and differences with other molecules discussed in this section. 

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Sheppard, N. Turner, J. J. High-resolution nuclear magnetic resonance (nmr) spectra of hydrocarbon groupings. II. Internal rotation in substituted ethanes and cyclic ethers, Proc. Roy. Soc. (London) 1959, A252, 506-519. 

The Energy Profile for Rotation about the C-C Bond in Substituted Ethanes 95... 

It has long been known that molecules do not rotate freely. In 1891, Bischoff proposed that ethane preferred a staggered conformation and that restricted rotation occurred in substituted ethanes.l" Christie and Kenner first demonstrated restricted rotation in 1922 by resolving 2,2 -dinitrophenyl-6,6 -dicarboxylic acid into optically active isomers.l" Pitzer showed that the calculated and observed entropies for ethane were identical if restricted rotation was considered.l" " The phenomenon of restricted rotation appears to be ubiquitous and has stimulated intense interest and research. 

Historically too, ethane has a central positon. Van t Hoff was of the opinion that free rotation could occur around carbon-carbon single bonds, owing to the lack of rotational isomers in compounds such as CICH2CH2CI. The first chemist to propose restricted rotation was Bischoff, who in the 1890s suggested (i) that the rotation was not free in substituted ethanes, and (ii) that ethane assumed a staggered conformation (Bischoff, 1890, 1891a,b). 

With accurate calculated barriers in hand, we return to the question of the underlying causes of methyl barriers in substituted toluenes. For simpler acyclic cases such as ethane and methanol, ab initio quantum mechanics yields the correct ground state conformer and remarkably accurate barrier heights as well.34-36 Analysis of the wavefunctions in terms of natural bond orbitals (NBOs)33 explains barriers to internal rotation in terms of attractive donor-acceptor (hyperconjuga-tive) interactions between doubly occupied aCH-bond orbitals or lone pairs and unoccupied vicinal antibonding orbitals. 

The hindered rotation about the C—C bond in ethane derivatives is a periodic function of the vicinal angle specified to describe the rotation. The symmetry of the periodic potential function is dependent upon the symmetry of the substitution in the ethane derivative. In most ethane derivatives the potential energy barriers to rotation are low (2-6 kcal mole-1) and rates of inter conversion between rotational isomers are too... 

Pople (1958a) has classified the symmetry of the nuclear spin systems of substituted ethanes which are either locked or rapidly rotating in terms of the usual rotation (Pople et al., 1957) for N.M.R. spectra, as shown in Table 2. Use of this table can be illustrated by the specific example of the molecule CH2X.CH2X. This has three rotational isomers... 

If two conformational i.somers arc cooled to a temperature low enough to prevent their interconversion by bond rotation or other motion, then they can be described us stereoisomers structures with the same connectivity but with different atomic arrangements in three dimensions. For example, the conformers in (d) are nonsuperimposable mirror images (enantiomers) if bond rotation is not allowed to occur (make a model ). Those in (g) are not enantiomers, but they are still stereoisomers. The temperatures required to "freeze out" confor-inaiional interconversion are very low on the order of -200 C for substituted ethanes and -I00 C for cyclohexanes. Conformers of these types are occasionally referred to as iiuoivonvertihle stereoisomers,... 

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