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:) [ə'mɪd] for the carbonyl-nitrogen compound and ['æmɑɪd] for the anion. Others substitute one of these pronunciations with ['æmɪd], 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: R
1(
CO)
NR
2R
3 where either or both of R
2 and R
3 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carboxylic acid in which the hydroxyl group has been replaced by an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: [R
1NR
2]
-, 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.
*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 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.
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).
Sulfonamides are analogs of amides in which the atom double bonded to oxygen is
sulfur rather than carbon.
*Example: CH
3CONH
2 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*
IUPAC Compendium of Chemical Terminology
