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An amide is a class of organic compounds containing a nitrogen atom attached to an acyl group. Amides are considered a derivative of carboxylic acids and can also be prepared from amines, anhydrides, esters, and nitriles. All peptide linkages in proteins are amides.
A carboxylic acid and an amine react to form amide. Organic acids are characterized by an end carbon bonded to a carbonyl group (-C=O) and a hydroxyl group (-OH), collectively written as -COOH. The amines have organic groups bonded to a nitrogen atom. The nitrogen atom may have one, two, or three side groups called primary, secondary, and tertiary amines. Only primary and secondary amines can react with the carboxylic acid to form an amide.
The reaction to form an amide from the acid and the amine is a two-step, high-temperature process. Other synthesis routes will form the amide more readily. In order from the most reactive to the least reactive feedstock, the reactants are the acyl halide, the acid anhydride, the ester, and the carboxylic acid. Compared to the carboxylic acid formula (-COOH), an acyl halide replaces the hydroxyl group with a halide, such as chlorine (-C=OCl), while in an ester the replacement is another hydrocarbon (-COOC-). An acid anhydride is the result of two carboxylic acids reacting, eliminating a molecule of water, and bonding to form a central structure(-COOOC-) where the two center carbons each have a double bond to an oxygen atom and a single bond to a shared central oxygen atom.
An amide is a stable compound due to its configuration. The C-O and C-N bonds are all in the same plane. The C-N bond acts as a partial double bond due to resonance, the sharing of electrons across multiple bonds. This resonance shortens the length of the C-N bond, making a higher energy barrier for reaction. Peptide bonds in proteins are very stable.
Urea is another biological amide. It is the end product of the metabolic breakdown of proteins by mammals. During this process, the amine group is removed from the hydrocarbon and converted to toxic ammonia. The liver converts the ammonia to urea (NH2-CO-NH2), and the kidney filters the urea out of the blood and eliminates it with urine. Commercial fertilizers use urea to promote plant growth due to its high nitrogen content.
Acetaminophen is an example of a synthetic amide. Amides are widely used in industry as surfactants, stabilizers, and release agents for plastics and soldering fluxes. Many amides are carcinogenic, and their use is restricted to non-human contact.