Rotation barrier single bond

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. 

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... 

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. 

The energetical description of rotations around bonds with high torsional barriers (e.g. the C=C double bond) demands the evaluation of the influence of higher cosine terms. Rotations around single bonds with sixfold symmetric torsional potentials have very low barriers (18) they occur in alkylsubstituted aromatic compounds (e.g. toluene), in nitro-alkanes and in radicals, for example. 

With a double bond, rotation would destroy the tt bond that arises from overlap of p orbitals consequently, there is a very large barrier to rotation. It is of the order of 263 kJmol , which is very much higher than any of the barriers to rotation about single bonds that we have seen for conformational isomerism. Accordingly, cis and trans isomers do not interconvert under normal conditions. Ring systems can also lead to geometric isomerism, and cis and trans isomers... 

This is originated by the low values of the energy barriers (<5 Kcal/mole) hindering the free rotation around single bonds. 

We saw in Chapter 7 that rotation about the C-N bond in an amide is relatively slow at room temperature—the NMR spectrum of DMF clearly shows two methyl signals (p. 165). In Chapter 13 you learned that the rate of a chemical process is associated with an energy barrier (this holds both for reactions and simple bond rotations) the lower the rate, the higher the barrier. The energy barrier to the rotation about the C-N bond in an amide is usually about 80 kj mol-1, translating into a rate of about 0,1 s-1 at 20 °C. Rotation about single bonds is much faster than this at room temperature, but there is nonetheless a barrier to rotation in ethane, for example, of about 12 kj mol-1. 

Atropisomers - Atropisomers are stereoisomers resulting from hindered rotation about single bonds where the steric barrier to rotation is high enough to allow for the isolation of the conformers <2004T4335>. For examples see Section 2.3.43.2. 

Brunck TK, Weinhold F (1979) Quantum-mechanical studies on the origin of barriers to internal rotation about single bonds. J Am Chem Soc 101 1700-1709... 

While many chemists are familiar with the problems posed by abnormally large barriers to rotation about single bonds when it comes to interpreting NMR spectra, few have sought to make use of these barriers as tools for stereoselective synthesis. A renewal of interest in this prospect followed Fuji s remarkable observation, published in 1991, of stereospecific alkylation of ketone 1. The stereochemistry of the starting material was retained in the product 2 despite the intermediacy of an apparently achiral enolate (Scheme 1) [1]. 

Reactions like these, in which stereoselectivity is the consequence of steric hindrance to bond rotation, are most well known among the biaryls, and derivatives of binaphthyl have provided chemists with a valuable range of chiral ligands [4-6]. But the biaryls are only a small subset of axially chiral compounds containing two trigonal centres linked by a rotationally restricted single bond. Many others are known, some with much greater barriers to rotation than Fuji s enol ether [7]. Yet until quite recently there were no reports of reactions in which nonbiaryl atropisomers were the source, conveyor, or product of asymmetric induction. 

It is difficult not to conclude however that some result do reflect higher barriers to rotation about single bonds in acyclic compounds. The barriers to rotation in hydrogen peroxide and hydrogen persulphide are 7.0 and 6.8 kcal/mol compared with a value of 2.9 kcal/mol for ethane 8), and these larger barriers seem to be reflected in the ring inversion barriers for the compounds 24—26. 65,87-89)... 

Studies on the Origin of Barriers to Internal Rotation about Single Bonds. 

Normally, rotation around single bonds has a barrier below 5 kcal mol and occurs faster than the NMR time scale. Rotation around the double bond of alkenes, on the other hand, has a barrier that is normally above 50 kcal mol and is slow on the NMR time scale. There are numerous examples of intermediate bond orders, whose rotation occurs within the NMR time scale. Hindered rotation about the C —N bond in amides such as A, A-dimethylformamide (5-4) provides a classic example of site exchange. At room... 

Conformational isomers differ only in rotations about single bonds. The eclipsed and staggered conformational isomers of ethane, CH3CH3, are shown in Figure 1.11. Although single bonds are drawn as if they were frozen in space, rotation usually has no significant barrier at room temperature (free rotation). Ethane s rotational barrier is tiny, only 3 kcal/mol (13 kJ/mol). One kilocalorie/mol is equal to 4.184 kilojoules/mol. 

Rotation about single bonds can occur freely at room temperature in particular cases. For example, a rotational barrier... 

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