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 ,  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. Rotation about single bonds is fast at room temperature, but there is nonetheless a barrier to rotation in ethane (A), for example, of about 12 kJ mol⁻¹.

The energy barrier for rotation about the single bond in butadiene (B) is slightly larger because of the weak conjugation between the double bonds, but the barrier to rotation about the genuine double bond in but-2-ene (D) is enormous and no rotation is seen. The energy barrier to the rotation about the C–N bond in an amide such as DMF (C) is usually about 80 kJ mol⁻¹, translating into a rate of about 0.1 s⁻¹ at 20 °C. The conjugation in amides is well developed, and the C–N bond has significant double-bond character.

مواضيع ذات صلة

E1 reactions can be stereoselective

The role of the leaving group

Substrate structure may allow E1

E1 and E2 mechanisms

Size

Elimination happens when the nucleophile attacks hydrogen instead of carbon

Axially and equatorially substituted rings react differently

Steroid

Decalins

What happens with more than one substituent on the ring

Substituted cyclohexanes

The ring inversion (flipping) of cyclohexane