Interconversion around the single bonds of chains

The staggered rotamers of l-bromo-2-chloroethane (Fig. 4.40b) are distinguishable. However, in the anti rotamer, Ha and Hfc are chemical-shift equivalent (en- 

In the same way the three rotating methyl groups of a f-butyl group are chemical shift equivalent except for rare steric hindrance. Both the methyl group and the f-butyl group are described as symmetry tops.  

FIGURE 3.46 (A) Newman projection of the staggered rotamers of a molecule with a methyl group attached to a chiral sp3 carbon atom. (B) l-Bromo-l,2-dichloroethane. (C) l-bromo-2-chloroethane. 

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The three-dimensional character of molecules is expressed by its stereochemistry. By looking at the stereochemistry of a molecule, the spatial relationships between atoms on one carbon and the atoms on an adjacent carbon can be examined. Since rotation can occur around carbon-carbon single bonds in open chain molecules, the atoms on adjacent carbons can assume different spatial relationships with respect to each other. The different arrangements that atoms can assume as a result of a rotation about a single bond are called conformations. A specific conformation is called a conformer. While individual isomers can be isolated, conformers cannot since interconversion, by rotation, is too rapid. 

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