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Structure of Amides

Amides are neutral compounds -- in contrast to their seemingly close relatives, the amines, which are basic. The amide linkage is planar -- even though we normally show the C-N connected by a single bond, which should provide free rotation. Figure 1 below shows this common drawing of an amide.

Figure 1. An amide; usual representation. The amide shown here, and in Figure 2, is the primary amide from ethanoic acid (acetic acid); the amide is called ethanamide (acetamide).

To help understand these properties, we need to look at a more complex -- but better -- representation of the amide structure. This is shown in Figure 2:

Figure 2. Resonance structures for an amide. Remember that the molecule does not actually switch between these structures. Instead, the actual structure is somewhere in between the structures shown. It can be thought of as some average of these structures.

Why is this resonance system better? A qualitative argument is that the O, which is very electronegative, draws electrons toward it. In this case, it draws electrons from the lone pair of the N. Note that in the right hand form, the electrons of the N lone pair have moved in to the double bond (giving the N a + charge), and electrons of the C=O double bond have moved out to the O (giving it a - charge).

The resonance system shown in Figure 2 is based on measurements of the properties of amides. That is, detailed study of amides shows that the properties are better explained by Figure 2 than by Figure 1. As examples:

  • The bond length measured for amides is about half way between that typical for C-N single bonds and C=N double bonds. This is easily explained by the resonance system shown in Figure 2, which suggests that the actual bond between C and N is about a 1 1/2 bond.
  • A double bonded structure, or a structure with a substantial contribution of double bonding, would be expected to be planar, without free rotation about the C-N bond. This fits with observation.
  • The left hand structure in Figure 2 might look like it would accept an H+ on the N, thus acting as a base. However, the right hand structure has no lone pair, and even has a positive charge on the N. These features argue against the N being basic. A resonance system with a substantial contribution of the right hand structure would not be expected to be basic.

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