Editing Amide (section)

==Structure and bonding==
[[File:CSD CIF ACEMID06.jpg|thumb|288 px|Structure of acetamide [[hydrogen-bond]]ed dimer from [[X-ray crystallography]].  Selected distances: C-O: 1.243, C-N, 1.325, N---O, 2.925 Å. Color code: red = O, blue = N, gray = C, white = H.<ref>{{cite journal |doi=10.1107/S1600536803019494 |title=A new refinement of the orthorhombic polymorph of acetamide |date=2003 |last1=Bats |first1=Jan W. |last2=Haberecht |first2=Monika C. |last3=Wagner |first3=Matthias |journal=Acta Crystallographica Section E |volume=59 |issue=10 |pages=o1483–o1485 }}</ref>]]
The lone pair of [[electron]]s on the nitrogen atom is delocalized into the [[Carbonyl group]], thus forming a partial [[double bond]] between nitrogen and carbon. In fact the O, C and N atoms have [[molecular orbital]]s occupied by [[delocalized electron]]s, forming a [[conjugated system]]. Consequently, the three bonds of the nitrogen in amides is not pyramidal (as in the [[amine]]s) but planar. This planar restriction prevents rotations about the N linkage and thus has important consequences for the mechanical properties of bulk material of such molecules, and also for the configurational properties of macromolecules built by such bonds. The inability to rotate distinguishes amide groups from [[ester]] groups which allow rotation and thus create more flexible bulk material.

The C-C(O)NR<sub>2</sub> core of amides is planar.  The C=O distance is shorter than the C-N distance by almost 10%.  The structure of an amide can be described also as a [[resonance structure|resonance]] between two alternative structures: neutral (A) and [[zwitterionic]] (B).
:[[File:Amide resonance v2.svg|300px|thumb|none]]

It is estimated that for [[acetamide]], structure A makes a 62% contribution to the structure, while structure B makes a 28% contribution (these figures do not sum to 100% because there are additional less-important resonance forms that are not depicted above). There is also a hydrogen bond present between the hydrogen and nitrogen atoms in the active groups.<ref name = Kemnitz>{{Cite journal|doi=10.1021/ja0663024|title="Amide Resonance" Correlates with a Breadth of C−N Rotation Barriers|year=2007|last1=Kemnitz|first1=Carl R.|last2=Loewen|first2=Mark J.|journal=Journal of the American Chemical Society|volume=129|issue=9|pages=2521–8|pmid=17295481}}</ref> Resonance is largely prevented in the very strained [[quinuclidone]].
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barrier from DMF, and a more general ref than this JACS rpt
comment on syn and anti secondary amides-->

In their IR spectra, amides exhibit a moderately intense ''ν''<sub>CO</sub> band near 1650&nbsp;cm<sup>−1</sup>.  The energy of this band is about 60&nbsp;cm<sub>−1</sub> lower than for the ''ν''<sub>CO</sub> of esters and ketones.  This difference reflects the contribution of the zwitterionic resonance structure.

===Basicity===
Compared to [[amine]]s, amides are very weak [[base (chemistry)|base]]s. While the [[conjugate acid]] of an [[amine]] has a [[pKa|p''K''<sub>a</sub>]] of about 9.5, the [[conjugate acid]] of an amide has a p''K''<sub>a</sub> around −0.5. Therefore, compared to amines, amides do not have [[acid–base]] properties that are as noticeable in [[water]]. This relative lack of basicity is explained by the withdrawing of electrons from the amine by the carbonyl. On the other hand, amides are much stronger [[Base (chemistry)|base]]s than [[carboxylic acid]]s, [[ester]]s, [[aldehyde]]s, and [[ketone]]s (their conjugate acids' p''K''<sub>a</sub>s are between −6 and −10).

The proton of a primary or secondary amide does not dissociate readily; its p''K''<sub>a</sub> is usually well above 15. Conversely, under extremely acidic conditions, the carbonyl [[oxygen]] can become protonated with a p''K''<sub>a</sub> of roughly −1. It is not only because of the positive charge on the nitrogen but also because of the negative charge on the oxygen gained through resonance.

===Hydrogen bonding and solubility===
Because of the greater electronegativity of oxygen than nitrogen, the carbonyl (C=O) is a stronger dipole than the N–C dipole. The presence of a C=O dipole and, to a lesser extent a N–C dipole, allows amides to act as H-bond acceptors. In primary and secondary amides, the presence of N–H dipoles allows amides to function as H-bond donors as well. Thus amides can participate in [[hydrogen bonding]] with water and other protic solvents; the oxygen atom can accept hydrogen bonds from water and the N–H hydrogen atoms can donate H-bonds. As a result of interactions such as these, the water solubility of amides is greater than that of corresponding hydrocarbons. These hydrogen bonds also have an important role in the [[secondary structure]] of proteins.

The [[Solubility|solubilities]] of amides and esters are roughly comparable. Typically amides are less soluble than comparable amines and carboxylic acids since these compounds can both donate and accept hydrogen bonds. Tertiary amides, with the important exception of [[Dimethylformamide|''N'',''N''-dimethylformamide]], exhibit low solubility in water.