Amide

In chemistry, an amide is one of two kinds of compound. It can mean the organic functional group characterized by a carbonyl group linked to a nitrogen atom or a compound that contains this functional group (pictured to the right), or it can mean a particular kind of nitrogen anion. Many chemists make a pronunciation distinction between the two, saying (IPA:) for the carbonyl-nitrogen compound and ^*," target="_blank" >while still others pronounce both as ['æmɪd, making them homonyms.

In the first sense referred to above, an amide is essentially an amine where one of the nitrogen substituents is an acyl group, represented generally by the formula: R₁(C O)NR₂R₃ where either or both of R₂ and R₃ may be hydrogen. Specifically, an amide can also be regarded as a derivative of a carboxylic acid in which the hydroxyl group has been replaced by an amine or ammonia.

Compounds in which a hydrogen atom on nitrogen from ammonia or an amine is replaced by a metal cation are also known as amides or azanides.

The second sense of the word amide is the amide anion, which is a deprotonated form of ammonia or an amine. It is represented generally by the formula: ^*^-, and is an extremely strong base, due to the extreme weakness of ammonia and its analogoues as Brønsted acids.

The remainder of this article is about the carbonyl-nitrogen sense of amide. For examples of the anionic amide, see the articles Sodium amide and Lithium diisopropylamide.

Amide synthesis

Amides are commonly formed from the reaction of a carboxylic acids with an amine.This is the reaction that forms peptide bonds between amino acids. These amides can participate in hydrogen bonding as hydrogen bond acceptors and donors, but do not ionize in aqueous solution, whereas their parent acids and amines are almost completely ionized in solution at neutral pH. Amide formation plays a role in the synthesis of some condensation polymers, such as nylon and Kevlar.
Cyclic amides are synthesized in the Beckmann rearrangement from oximes.
Other amide forming reactions are the Passerini reaction and the Ugi reaction

Amide reactions

Amide breakdown is possible via amide hydrolysis.
In the Vilsmeier-Haack reaction an amide is converted into an imine.
Hofmann rearrangement of primary amides to primary amines.

Amide properties

An amide linkage is kinetically stable to hydrolysis. However, it can be hydrolysed in boiling alkali, as well as in strong acidic conditions. Amide linkages in a biochemical context are called peptide linkages. Amide linkages constitute a defining molecular feature of proteins, the secondary structure of which is due in part to the hydrogen bonding abilities of amides.

Compared to amines, amides are very weak bases. While the conjugate acid of an amine has a pKa of about 9.5, the conjugate acid of an amide has a pKa around -0.5. Therefore amides don't have as clearly noticeable acid-base properties in water. This lack of basicity is explained by the electron-withdrawing nature of the carbonyl group, where the lone pair on the nitrogen is delocalized by resonance, forming a partial double bond with the carbonyl carbon and putting a negative charge on the oxygen, and discussed in the peptide bond article. On the other hand, amides are much stronger bases than carboxylic acids, esters, aldehydes, and ketones(conjugated acid pKa between -6 and -10).

Derivatives

Sulfonamides are analogs of amides in which the atom double bonded to oxygen is sulfur rather than carbon.

Naming conventions

Example: CH₃CONH₂ is named acetamide or ethanamide
Other examples: propan-1-amide, N,N-dimethylpropanamide, acrylamide
For more detail see IUPAC nomenclature of organic chemistry - Amines and Amides

External links

IUPAC Compendium of Chemical Terminology