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Amide synthesis
- Amides are commonly formed from the reaction of a carboxylic acid 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 Aramid (Twaron / Kevlar). In biochemistry peptides are synthesized in solid phase peptide synthesis. The Schotten-Baumann reaction describes the formation of amides from amines and acid chlorides.
- 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' [1] [2]
- In the Chapman rearrangement (first reported in 1925) an aryl imino ether is converted to a N,N-diaryl amide:
- The reaction mechanism is based on a nucleophilic aromatic substitution. [3]
- 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 be effective in a so-called dehydrogenative acylation [4]:
- 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
- Amide breakdown is possible via amide hydrolysis. Such hydrolysis can occur under basic or acidic conditions. Acidic conditions yield the carboxylic acid and the ammonium ion while basic hydrolysis yield the carboxylate ion and ammonia.
- In the Vilsmeier-Haack reaction an amide is converted into an imine.
- Hofmann rearrangement of primary amides to primary amines.
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.
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 nitrate (III) salt in solution at temperatures of between 0 and 10 degrees.
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.
Amide linkage (peptide bond)
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.
Amide properties
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, thus forming a partial double bond with the carbonyl carbon and putting a negative charge on the oxygen. 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% [5]. Resonance is largely prevented in the very strained quinuclidone.
Solubility
Amides contain carbonyl (C=O) and ether (N-C) dipoles arising from covalent bonding between electronegative oxygen and nitrogen atoms and electro-neutral carbon atoms. Primary and secondary amides also contain two- and one N-H dipoles, respectively. Because of the pi-bonding arrangement of the carbonyl and 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
While hydrogen bonding may enhance the water solubility of amides relative to hydrocarbons (alkanes, alkenes, alkynes and aromatic compounds), amides typically are regarded as compounds with low water solubility. They are significantly less water soluble than comparable acids or alcohols due to: 1). their non-ionic character 2). the presence of nonpolar hydrocarbon functionality, and 3). the inability of tertiary amides to donate hydrogen bonds to water (they can only be H-bond acceptors). 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, and can ionize at appropriate pHs to further enhance solubility
Derivatives
Sulfonamides are analogues of amides in which the atom double-bonded to oxygen is sulfur rather than carbon.
Cyclic amides are called lactams.
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
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References
- ^ Bodroux F., Bull. Soc. Chim. France, 1905, 33, 831;
- ^ Bodroux reaction at the Institute of Chemistry, Skopje, Macedonia Link
- ^ Advanced organic Chemistry, Reactions, mechanisms and structure 3ed. Jerry March ISBN 0-471-85472-7
- ^ Direct Synthesis of Amides from Alcohols and Amines with Liberation of H2 Chidambaram Gunanathan, Yehoshoa Ben-David, David Milstein Science 10 August 2007: Vol. 317. no. 5839, pp. 790 - 792 doi:10.1126/science.1145295
- ^ "Amide Resonance" Correlates with a Breadth of C-N Rotation Barriers Carl R. Kemnitz and Mark J. Loewen J. Am. Chem. Soc.; 2007; 129(9) pp 2521 - 2528; (Article) doi:10.1021/ja0663024
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