Dimethylformamide, restricted rotation

Interconversion Around a "Partial Double Bond" (Restricted Rotation) At room temperature, a neat sample of dimethylformamide shows two CH3 peaks because the rate of rotation around the hindered partial double bond is slow. At... 

Evidence for restricted rotation around amide CO-N bonds comes from NMR studies. At room temperature, the H NMR spectrum of A/,N-dimethylformamide shows three peaks 2.9 S (singlet, 3 H), 3.0 S (singlet. 3 H). 8.0 3 (singlet, 1 H). As the temperature is raised, however, the two singlets at 2.9 S and 3.0 5 slowly merge. At 180°C, the H NMR spectrum shows only two peaks 2.95 S (singlet. 6 H) and 8.0 S (singlet, 1 H). Explain this temperature-dependent behavior. 

Nonequivalence can also arise by restricted rotation imposed by steric or electronic factors. In such cases the affected protons will appear as separate peaks at lower temperatures, but as the temperature is raised the lines move closer together, and finally merge into a single peak. The methyl groups of dimethylformamide are nonequivalent at room temperature because of restriction of rotation about the N—CO bond due to the partial double... 

Many molecules show intramolecular mobility Rotations of groups about a bonds or inversions of cycloaliphatic rings are representative phenomena. A well-known example is N,N-dimethylformamide, which exists as an equilibrium mixture of ris and tram isomers due to the partial n character of the N — CO bond Rotation of the dimethyl-amino group is restricted at room temperature but occurs upon heating. 

However, there are many molecules where rotation around a particular single bond is restricted, that is, prevented from rotating freely. If rotation is much slower than the NMR time scale, the molecule is essentially locked in one conformation. One example of such a compound is N./V-dimethylformamide (DMF, 10-1), whose NMR data are given in Table 8.1 ... 

A simple example of exchange occurs in N,N-dimethylformamide, as illustrated in Fig. 2.15. The C—N bond in an amide has partial double bond character hence rotation about this bond is highly restricted but not entirely precluded. At room temperature the rotation rate is sufficiently slow that separate sharp lines are observed for the two methyl groups, which are in different environments because of proximity to the carbonyl group (see Chapter 4). With increasing temperature, however, the barrier to rotation is gradually surmounted, and the observed spectra follow the theoretically computed curves of Fig. 2.14. 

In polyacrylonitrile appreciable electrostatic forces occur between the dipoles of adjacent nitrile groups on the same polymer molecule. This restricts the bond rotation and leads to a stiff, rodKke structme of the polymer chain. As a result, polyacrylonitrile has a very high crystalline melting point (317°C) and is soluble in only a few solvents, such as dimethylformamide and dimethylacetamide, and in concentrated aqueous solutions of inorganic salts, such as calcium thiocyanate, sodium perchlorate, and zinc chloride. Polyacrylonitrile cannot be melt processed because its decomposition temperature is close to the melting point. Fibers are therefore spun from solution by either wet or dry spinning (see Chapter 2). 

The extent of resonance can be observed directly in the structures of carboxylic acid derivatives. In the progression from acyl halides to esters and amides, the C-L bond becomes progressively shorter, owing to increased double-bond character (Table 20-1). The NMR spectra of amides reveal that rotation about this bond has become restricted. For example, W,N-dimethylformamide at room temperature exhibits two singlets for the two methyl groups, because rotation about the C-N bond is very slow on the NMR time scale. The evidence points to considerable tt overlap between the lone pair on nitrogen and the carbonyl carbon, as a result of the increased importance of the dipolar resonance form in amides. The measured barrier to this rotation is about 21 kcal moF (88 kJ moF ). 

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