Carbon-nitrogen bonds single, rotation around

Amides have a planar geometry. Even though the carbon-nitrogen bond is normally written as a single bond, rotation around that bond is restricted because of resonance. 

The second very important conclusion is related to the nature of the covalent bond between the nitrogen and the carbon atom. This bond has a single bond character in one of the resonance structures and a double bond character in the other. Hence, the amide bond has a bond order of 1.5, i.e. it has an intermediate character between a double and a single bond. In the chapter on alkanes we have mentioned that while the rotation around a single bond is free, the rotation around a double bond is forbidden. Because the amides bond has a partial double bond character the rotation around it is hindered. This evidence is important in the study of the stereochemistry of complex molecules with amide groups, such as polyamides and polypeptides. 

The molecule has two chiral elements. There is a chiral axis along the bond from the carbon at the 3-position of the thiophene to the nitrogen. This is because there is not free rotation around this single bond. There is also an asymmetric carbon at the methine carbon attached to nitrogen. Because of two chiral elements, there can be 1 or four possible stereoisomers. Dimethenamid with S-configuration at the chiral carbon can be prepared from S-methoxyisopropylamine. This single enantiomer amine is isolated from a racemic mixture of amines by enantioselective enzymatic acylation [58]. The enzyme selectively acylates only one enantiomer and the resultant amide can be readily separated from the unreacted free amine. The undesired enantiomer can then be racemized to provide a source of (after further separation) more of the desired enantiomer. 

In each of the resonance structures shown below, one of these bonds can be drawn as a double bond, indicating their partial double-bond character. Each bond is thus a hybrid between a single and double bond the partial double bond character results in partially restricted rotation around these bonds. As a result of the sj/ hybridized carbon atom (just to the left of the nitrogen) atom, the two bonds on the left (shown above) have only single-bond character and can freely rotate. 

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