Rotation about a single bond

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... 

The conversion of 87a into 87b by a rotation about a single bond needs a lot of space which might be available in solution but not in the crystalline complex. This rotation might be severely hindered especially in the complex with 4. 

For a long time the idea was prevailing that rotation about a single bond was completely free. It was only in 1936 that it was suggested that this rotation was hindered and this hindrance was different for different saturated compounds (Kemp and Pitzer). 

The word atropisomerism was coined by Kuhn (1) to cover isomerism caused by freezing the internal rotation about a single bond in a molecule. Indeed,... 

Isomers that lack internal planes of symmetry or points of symmetry (hence, are chiral) due to a prevention (or significant hindrance) to rotation about a single bond. 

A conformation is the different special arrangement of a ligand s atoms that result from rotation about a single bond a specific conformation is a conformer. Unlike constitutional isomers, different conformations cannot be isolated because they quickly interconvert, yet there are several computational methods to determine the possible conformations a molecule may possess. The energy of... 

Special cases of these involving transition states for rotation about single bonds, inversion of pyramidal nitrogen and phosphorus centers and ring inversion in cyclohexane, have been discussed in the previous chapter. The only difference is that these conformational processes are typically well described in terms of a simple motion, e.g., rotation about a single bond, whereas the motion involved in a chemical reaction is likely to be more complex. 

The energy of rotation about a single bond is a periodic function of the torsion angle and is, therefore, appropriately described in terms of a truncated Fourier series, the simplest acceptable form of which is given by. 

Conformalions can be interconverted by rotation about a single bond, while configurations are not interconvertible by rotation. 

The chirality of biphenyls results from restricted rotation about a single bond imposed by the bulky nature of ortho substituents. Models will help you. visualize the degree of difficulty of having the substituents pass by one another. If X = H and Y = F (Table 13-3), the enantiomers are not stable at room temperature if X = H and Y = Br, they are marginally stable if X = H and... 

A conformation is any three-dimensional arrangement of atoms in a molecule that results from rotation about a single bond. In this chapter we are concerned only with conformations that result from rotation about carbon-carbon single bonds. 

High-performance liquid chromatography can sometimes separate atropi-somers, i.e., a compound with highly hindered rotation about a single bond, producing effectively two molecular species. 

In Bo-butane [Fig. 9-46(c) and (d) one isomer (c) has the Cl on the central carbon and there is only one other isomer (d) because the three terminal carbons are identical. A point worth noting is that all of the C to C bonds are single bonds this means that, since there is free rotation about a single bond, all of the hydrogens bonded to a particular carbon are identical. Because of this factor, replacing one H on a carbon with a Cl is the same as replacing any other. 

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. 

Conformation (Section 6.3) A shape that a molecule can assume by rotation about a single bond. Conformational analysis (Section 6.3) Analysis of the energies of the various conformations of a compound. 

We can draw many structures for ethane, differing only in how one methyl group is twisted in relation to the other one. Such structures, differing only in rotations about a single bond, are called conformations. Two of the infinite number of conformations of ethane are shown in Figure 2-20. Construct a molecular model of ethane, and twist the model into these two conformations. 

Why should there be an energy barrier in the rotation about a single bond In order to answer this question, we should start with the simplest C-C bond possible—the one in ethane. Ethane has two extreme conformations called the staggered and eclipsed conformations. Three different views of these arc shown below. 

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