Physical and chemical properties of amides

Except for formamide, which is liquid, other amides are mostly colorless crystals, and monoalkyl-substituted amides are often liquid. Due to the strong intermolecular hydrogen bonding ability of amide molecules and the large polarity of amide molecules, its melting and boiling point is even higher than that of carboxylic acids with similar molecular weights. When the hydrogen atom on the amino group is replaced by a hydrocarbyl group, its melting and boiling point is also reduced due to the reduction of the intermolecular hydrogen bond association.
Liquid amide can dissolve not only organic compounds, but also many inorganic compounds, and is a good solvent. For example, N,N-dimethylformamide and N,N-dimethylacetamide can be mixed with water, most organic solvents and many inorganic liquids in any proportion, and are good aprotic polar solvents.
Amides can undergo alcoholysis reaction with strong acids, and the adducts formed by the reaction, such as CH3CONH2·HCl, are very unstable and completely hydrolyzed when exposed to water. Amides can also form metal salts. Most metal salts are completely hydrolyzed when they meet water, but (CH3CONH)2Hg is quite stable. Amides can be hydrolyzed into carboxylic acids and ammonia (or amines) when heated for a long time in the presence of strong acids and bases. The amide is converted to the nitrile by careful heating in the presence of the dehydrating agent phosphorus pentoxide. Amides can be reduced to amines by catalytic hydrogenation or by reaction with lithium aluminum hydride. Amides can also react with hypohalites to form primary amines with one less carbon atom.
Amides can be prepared by partial dehydration of carboxylic acid ammonium salts, or by aminolysis of acid halides, acid anhydrides, and esters; nitriles can also be partially hydrolyzed, stopping at the amide stage.
Low molecular liquid amides such as N,N-dimethylformamide and N,N-dimethylacetamide are excellent aprotic polar solvents and can also be used as plasticizers, lubricant additives and organic synthesis reagents. Long-chain fatty acid amides, such as stearic acid amide, can be used as a waterproofing agent for fiber fabrics, and erucic acid amide is a lubricant in the extrusion of polyethylene and polypropylene. N,N-Dihydroxyethyl long-chain fatty acid amides are nonionic surfactants and plasticizers for vinyl chloride-vinyl acetate copolymers. N-sulfoalkyl-substituted long-chain fatty acid amides are softeners for synthetic fibers. Polyamides formed by polycondensation of dicarboxylic acids and diamines are synthetic fibers with excellent properties.
Acidity and basicity: Amides are generally near-neutral compounds, but under certain conditions they can show weak acid or weak base. Amides are acyl derivatives of ammonia or amines with amino or hydrocarbon amino groups in the molecule, but their basicity is much weaker than that of ammonia or amines. The basicity of amides is very weak, because the unshared electron pair on the amino nitrogen in the molecule forms a conjugated system with the π electrons of the carbonyl group, which reduces the electron cloud density on the nitrogen and thus weakens the ability to accept protons. At this time, the C-N bond has a certain degree of double bond. However, the decrease in electron cloud density on nitrogen increases the polarity of the N-H bond, thus showing weak acidity. If two hydrogen atoms in the ammonia molecule are replaced by an acyl group of a dibasic acid, a cyclic imino compound (imide) is formed. Due to the electron-withdrawing effect of the two carbonyl groups, the polarity of the N-H bond of the imino group is significantly increased, and the hydrogen atom on the nitrogen is more easily turned into a proton, which is weakly acidic. E.g:
Hydrolysis: Amides are generally more difficult to hydrolyze. When heated in the presence of an acid or base, the reaction is accelerated, but is much slower than the hydrolysis of carboxylic acid esters. N-substituted amides can also undergo hydrolysis to yield carboxylic acids and amines.
Reaction with nitrous acid: The amide reacts with nitrous acid to generate the corresponding carboxylic acid and releases nitrogen.