Single bonds rotation about

Structures A B and C represent different conformations of hydrogen peroxide Conformations are different spatial arrangements of a molecule that are generated by rotation about single bonds Although we can t tell from simply looking at these struc tures we now know from experimental studies that C is the most stable conformation... 

Determine whether the two structures in each of the following pairs represent constitutional isomers different conformations of the same compound or stereoisomers that cannot be inter converted by rotation about single bonds... 

Conformational analysis (Section 3 1) Study of the conforma tions available to a molecule their relative stability and the role they play in defining the properties of the molecule Conformations (Section 3 1) Nonidentical representations of a molecule generated by rotation about single bonds Conformers (Section 3 1) Different conformations of a single molecule... 

Conformational Isomers. A molecule in a conformation into which its atoms return spontaneously after small displacements is termed a conformer. Different arrangements of atoms that can be converted into one another by rotation about single bonds are called conformational isomers (see Fig. 1.1). A pair of conformational isomers can be but do not have to be mirror images of each other. When they are not mirror images, they are called diastereomers. 

Torsional Asymmetry. Rotation about single bonds of most acyclic compounds is relatively free at ordinary temperatures. There are, however, some examples of compounds in which nonbonded... 

To understand the function of a protein at the molecular level, it is important to know its three-dimensional stmcture. The diversity in protein stmcture, as in many other macromolecules, results from the flexibiUty of rotation about single bonds between atoms. Each peptide unit is planar, ie, oJ = 180°, and has two rotational degrees of freedom, specified by the torsion angles ( ) and /, along the polypeptide backbone. The number of torsion angles associated with the side chains, R, varies from residue to residue. The allowed conformations of a protein are those that avoid atomic coUisions between nonbonded atoms. 

There are a number of structural features which have a bearing on the value of the glass transition temperature. Since this temperature is that at which molecular rotation about single bonds becomes restricted, it is obvious that these features are ones which influence the ease of rotation. These can be divided into two groups ... 

One of the frmdamental structural facets of organic chemistry, which has been explained most satisfactorily in MO terms, is the existence of a small barrier to rotation about single bonds. In ethane, for example, it is known that the staggered conformation is about 3kcal/mol more stable than the ecl sed conformation so that the eclipsed conformation represents a transition state for transformation of one staggered conformation into another by rotation. 

In addition to constitution and configuration, there is a third important level of structure, that of conformation. Conformations are discrete molecular arrangements that differ in spatial arrangement as a result of facile rotations about single bonds. Usually, conformers are in thermal equilibrium and cannot be separated. The subject of conformational interconversion will be discussed in detail in Chapter 3. A special case of stereoisomerism arises when rotation about single bonds is sufficiently restricted by steric or other factors that- the different conformations can be separated. The term atropisomer is applied to stereoisomers that result fk m restricted bond rotation. ... 

The torsional strain is a sinusoidal function of the torsion angle. Torsional strain results from the barrier to rotation about single bonds as described for ethane on p. 56. For molecules with a threefold barrier such as ethane, the form of the torsional barrier is... 

Although these reactions are thus closely related to the acyl-alkyl diradical disproportionation to ketenes, the stereospecificity of (55) -> (56) and (57) -> (58) shows that these hydroxyketones cannot proceed through free radicals capable of rotating about single bonds prior to the intramolecular hydrogen... 

Conformations (Section 3.1) Nonidentical representations of a molecule generated by rotation about single bonds. 

Rotation about single bonds and conformational changes can be studied. Amides constitute a classic example. Because of the partial double bond character of the carbon-nitrogen bond as a consequence of the contribution of 2 to the electronic structure, there is an energy barrier to rotation about this bond. 

K. S. Pitzer, Potential Energies for Rotation about Single Bonds . Discuss. Faradav Soc., 10,66-73 (1951). 

The conformations of H2NS02F and HC(S02F)3 as a result of rotation about single bonds are shown in Figure 8 by projection formulas. The S—C bond length in the latter is 1.831(5) A. 

There is no plane of symmetry and the molecule is chiral many such compounds have been resolved. Note that groups in the para position cannot cause lack of symmetry. Isomers that can be separated only because rotation about single bonds is prevented or greatly slowed are called atropisomers. 9,9 -Bianthryls also show hindered rotation and exhibit atropisomers. °... 

Because of the atoms freedom to rotate about single bonds, a chain of carbon atoms can achieve various positions in space. On one extreme is the zig-zag extended chain and on the other is a coil. Such spatial structures become particularly important in determining properties of very long chained compounds known as polymers (Chapter 5). 

Addition reactions with alkenes to form cyclopropanes are the most studied reactions of carbenes, both from the point of view of understanding mechanisms and for synthetic applications. A concerted mechanism is possible for singlet carbenes. As a result, the stereochemistry present in the alkene is retained in the cyclopropane. With triplet carbenes, an intermediate 1,3-diradical is involved. Closure to cyclopropane requires spin inversion. The rate of spin inversion is slow relative to rotation about single bonds, so mixtures of the two possible stereoisomers are obtained from either alkene stereoisomer. 

Conformational analysis the analysis of the energy changes that a molecule undergoes as groups rotate about single bonds. 

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