Amide

Applications

Amides are pervasive in nature and technology. Proteins and important plastics like nylons, aramids, Twaron, and Kevlar are polymers whose units are connected by amide groups (polyamides); these linkages are easily formed, confer structural rigidity, and resist hydrolysis. Amides include many other important biological compounds, as well as many drugs like paracetamol, penicillin and LSD. Low-molecular-weight amides, such as dimethylformamide, are common solvents.

Structure and bonding

The lone pair of electrons 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 orbitals occupied by delocalized electrons, forming a conjugated system. Consequently, the three bonds of the nitrogen in amides is not pyramidal (as in the amines) 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 results in a more flexible bulk material.

The C-C(O)NR₂ 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 between two alternative structures: neutral (A) and zwitterionic (B).

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). Resonance is largely prevented in the very strained quinuclidone.

In their IR spectra, amides exhibit a moderately intense ν_CO band near 1650 cm⁻¹. The energy of this band is about 60 cm₋₁ lower than for the ν_CO of esters and ketones. This difference reflects the contribution of the zwitterionic resonance structure.

Basicity

Compared to amines, amides are very weak bases. While the conjugate acid of an amine has a pK_a of about 9.5, the conjugate acid of an amide has a pK_a 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 bases than carboxylic acids, esters, aldehydes, and ketones (their conjugate acids' pK_as are between −6 and −10).

The proton of a primary or secondary amide does not dissociate readily; its pK_a is usually well above 15. Conversely, under extremely acidic conditions, the carbonyl oxygen can become protonated with a pK_a 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 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 N,N-dimethylformamide, exhibit low solubility in water.

Reactions

Amides do not readily participate in nucleophilic substitution reactions. Amides are stable to water, and are roughly 100 times more stable towards hydrolysis than esters. Amides can, however, be hydrolyzed to carboxylic acids in the presence of acid or base. The stability of amide bonds has biological implications, since the amino acids that make up proteins are linked with amide bonds. Amide bonds are resistant enough to hydrolysis to maintain protein structure in aqueous environments but are susceptible to catalyzed hydrolysis.

Primary and secondary amides do not react usefully with carbon nucleophiles. Instead, Grignard reagents and organolithiums deprotonate an amide N-H bond. Tertiary amides do not experience this problem, and react with carbon nucleophiles to give ketones; the amide anion (NR₂⁻) is a very strong base and thus a very poor leaving group, so nucleophilic attack only occurs once. When reacted with carbon nucleophiles, N,N-dimethylformamide (DMF) can be used to introduce a formyl group.

Here, phenyllithium 1 attacks the carbonyl group of DMF 2, giving tetrahedral intermediate 3. Because the dimethylamide anion is a poor leaving group, the intermediate does not collapse and another nucleophilic addition does not occur. Upon acidic workup, the alkoxide is protonated to give 4, then the amine is protonated to give 5. Elimination of a neutral molecule of dimethylamine and loss of a proton give benzaldehyde, 6.

A new class of amide reactions was discovered in 2015 by the research teams of Neil Garg and Ken Houk, showing that amides can be converted to esters using nickel catalysis. The nickel catalyst breaks the ordinarily strong amide C-N bond through oxidative addition. Many other amide cross-couplings were subsequently developed using nickel or palladium catalysis, including Suzuki-Miyaura couplings, allowing for amides to be readily converted to numerous other functional groups.

Hydrolysis

Amides hydrolyse 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. The protonation of the initially generated amine under acidic conditions and the deprotonation of the initially generated carboxylic acid under basic conditions render these processes non-catalytic and irreversible. Electrophiles other than protons react with the carbonyl oxygen. This step often precedes hydrolysis, which is catalyzed by both Brønsted acids and Lewis acids. Peptidase enzymes and some synthetic catalysts often operate by attachment of electrophiles to the carbonyl oxygen.

Synthesis

From carboxylic acids and related compounds

Amides are usually prepared by coupling a carboxylic acid with an amine. The direct reaction generally requires high temperatures to drive off the water:

RCO₂H + R'₂NH → RCO−2 + R'₂NH+2

RCO−2 + R'₂NH+2 → RC(O)NR'₂ + H₂O

Esters are far superior substrates relative to carboxylic acids.

Further "activating" both acid chlorides (Schotten-Baumann reaction) and anhydrides (Lumière–Barbier method) react with amines to give amides:

RCO₂R" + R'₂NH → RC(O)NR'₂ + R"OH

RCOCl + 2R'₂NH → RC(O)NR'₂ + R'₂NH+2Cl⁻

(RCO)₂O + R'₂NH → RC(O)NR'₂ + RCO₂H

Peptide synthesis use coupling agents such as HATU, HOBt, or PyBOP.

From nitriles

The hydrolysis of nitriles is conducted on an industrial scale to produce fatty amides. Laboratory procedures are also available.

Specialty routes

Many specialized methods also yield amides. A variety of reagents, e.g. tris(2,2,2-trifluoroethyl) borate have been developed for specialized applications.