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AMIDE

In chemistry, amide usually refers to organic compounds that contain the functional group consisting of an acyl group (C=O) linked to a nitrogen atom (N). The term refers both to a class of compounds and a functional group within those compounds. The term amide also refers to deprotonated form of ammonia (NH3) or an amine, often represented as anions R2N-. The remainder of this article is about the carbonyl-nitrogen sense of amide. For discussion of these "anionic amides," see the articles sodium amide and lithium diisopropylamide.

Structure and bonding

The simplest amides are derivatives of ammonia wherein one hydrogen atom has been replaced by an acyl group. The ensemble is generally represented as RC(O)NH2. Closely related and even more numerous are amides derived from primary amines (R'NH2) with the formula RC(O)NHR'. Amides are also commonly derived from secondary amines (R'RNH2) with the formula RC(O)NR'R. Amide are usually regarded as derivatives of carboxylic acids in which the hydroxyl group has been replaced by an amine or ammonia.

Amide resonance:

The lone pair of electrons on the nitrogen is delocalized onto the carbonyl, thus forming a partial double bond between N the carbonyl carbon. Consequently the nitrogen in amides is not pyramidal. It is estimated that acetamide is described by resonance structure A for 62% and by B for 28% [1]

 

Nomenclature

In the usual nomenclature, one adds the term "amide" to the stem of the parent acid's name. Thus, the simplest amide derived from acetic acid is named acetamide (CH3CONH2). IUPAC recommends ethanamide, but this and related formal names are rarely encountered. When the amide is derived from a primary or secondary amine, the substitutents on nitrogen are indicated first in the name. Thus the amide formed from dimethylamine and acetic acid is N,N-dimethylacetamide (CH3CONMe2, where Me = CH3). Usually even this name is simplified to dimethylacetamide. Cyclic amides are called lactams; they are necessarily secondary or tertiary amides. Functional groups consisting of -P(O)NR2 and -SO2NR2 are phosphonamides and sulfonamides, respectively.

 

Pronunciation

Some chemists make a pronunciation distinction between the two, saying /əˈmiːd/ for the carbonyl-nitrogen compound and /ˈæmaɪd/  ( listen) for the anion. Others substitute one of these with /ˈæmɨd/, while still others pronounce both /ˈæmɨd/, making them homonyms.

 

Properties

 

Basicity

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 of electrons on the nitrogen is delocalized by resonance. On the other hand, amides are much stronger bases than carboxylic acids, esters, aldehydes, and ketones (conjugated acid pKa between -6 and -10). It is estimated in silico that acetamide is represented by resonance structure A for 62% and by B for 28% [2]. Resonance is largely prevented in the very strained quinuclidone.

Because of the greater electronegativity of oxygen, 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 and nitrogen atoms 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.

Owing to their resonance stabilization, amides are relatively unreactive under physiological conditions, even less than similar compounds such as esters. Nevertheless, amides can undergo chemical reactions, usually through an attack of an electronegative atom on the carbonyl carbon, breaking the carbonyl double bond and forming a tetrahedral intermediate. When the functional group attacking the amide is a thiol, hydroxyl or amine, the resulting molecule may be called a cyclol or, more specifically, a thiacyclol, an oxacyclol or an azacyclol, respectively.

The proton of an amide does not dissociate readily under normal conditions; its pKa is usually well above 15. However, under extremely acidic conditions, the carbonyl oxygen can become protonated with a pKa of roughly -1.

 

Solubility

Thus amides have water solubilities roughly comparable to esters. Typically amides are less soluble than comparable amines and carboxylic acids since these compounds can both donate and accept hydrogen bonds. Tertiary amides exhibit particularly low solubility due to their inability donate hydrogen bonds to water (they can only be H-bond acceptors).

 

Characterization

Being pervasive, amides are usually the default functional group for nitrogen-containing organic compounds that are nonbasic. They can be distinguished from nitro and cyano groups by their IR spectra. Amides exhibit a moderately intense nCO band near 1650 cm-1. By 1H NMR spectroscopy, CONHR signals occur at low fields. In X-ray crystallography, the three atoms attached to the C(O)N center characteristically define a plane.

 

Applications and occurrence

Amides are pervasive in nature and technology as structural materials. The amide linkage is easily formed, confers structural rigidity, and resists hydrolysis. Nylons are polyamides such as Aramid, Twaron, and Kevlar). 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. Low molecular weight amides, such as dimethylformamide (HC(O)N(CH3)2) are common solvents. Many drugs are amides, for example penicillin and LSD.

 

Amide synthesis

Amides are commonly formed via reactions of a carboxylic acid with an amine. Many methods are known for driving the unfavorable equilibrium to the right:

RCO2H + R'R"NH \overrightarrow{\leftarrow} RC(O)NR'R" + H2O

For the most part, these reactions involve "activating" the carboxylic acid and the best known method, the Schotten-Baumann reaction, which involves conversion of the acid to the acid chlorides:

Amide bond formation

Other methods include:

  • Cyclic amides are synthesized in the Beckmann rearrangement from oximes.
  • Amides also form from ketones and hydrazoic acid in the Schmidt reaction
  • Amides can be prepared from aryl alkyl ketones, sulfur and morpholine in the Willgerodt-Kindler reaction
  • Other amide-forming multicomponent reactions are the Passerini reaction and the Ugi reaction
  • In the Bodroux reaction an amide RNHCOR' is synthesized from a carboxylic acid R-COOH and the adduct of a Grignard reagent with an aniline derivative ArNHR' [3][4]
  • In the Chapman rearrangement (first reported in 1925[5]) an aryl imino ether is converted to a N,N-diaryl amide:
Chapman rearrangement
The reaction mechanism is based on a nucleophilic aromatic substitution.[6]
  • The seemingly simple direct reaction between an alcohol and an amine to an amide was not tried until 2007 when a special ruthenium-based catalyst was reported to be effective in a so-called dehydrogenative acylation:[7]
Synthesis of Amides from Alcohols and Amines with Liberation of H2
The generation of hydrogen gas compensates for unfavorable thermodynamics. The reaction is believed to proceed by one dehydrogenation of the alcohol to the aldehyde followed by formation of a hemiaminal and the after a second dehydrogenation to the amide. Elimination of water in the hemiaminal to the imine is not observed.

 

Amide reactions

Amides are noteworthy for their resistance to reactions with mild acids and bases and their stability toward hydrolysis. They can be hydrolysed in hot alkali, as well as in strong acidic conditions. Acidic conditions yield the carboxylic acid and the ammonium ion while basic hydrolysis yield the carboxylate ion and ammonia.

Amides are also versatile precursors to many other functional groups.

  • In the Vilsmeier-Haack reaction an amide is converted into an imine.
  • Hofmann rearrangement of primary amides to primary amines.

Amides will react with nitrous acid (HONO) forming the carboxylic acid and yielding nitrogen. Nitrous acid is formed by addition of a strong acid to a nitrite salt in solution at temperatures of between 0 and 10 degrees.[citation needed]

Amides undergo the Hofmann rearrangement in which an amine with one less carbon atom is produced upon reaction with bromine and sodium hydroxide. On the other hand, reacting the amide with the strong reducing agent lithium aluminium hydride yields an amine with the same number of carbon atoms.

Amides are dehydrated with phosphorus (V) oxide forming the nitrile. Care should be taken when performing such a reaction since phosphorus (V) oxide smoulders when in contact with organic matter.







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