The carbonyl group (C=O) is polar because oxygen is more electronegative than carbon, so the carbon carries a partial positive charge and the oxygen a partial negative charge. This makes the carbonyl carbon electrophilic, so it is readily attacked by nucleophiles, leading to nucleophilic addition. For example, with hydrogen cyanide (HCN), the addition gives a cyanohydrin: R-CHO + HCN -> R-CH(OH)-CN. Aldehydes are more reactive than ketones towards nucleophilic addition due to less steric hindrance and a greater positive charge on the carbonyl carbon.

R-CHO + HCN -> R-CH(OH)-CN

Carboxylic acids are more acidic than phenols because the carboxylate ion formed after the loss of the proton is stabilised by resonance in which the negative charge is spread equally over two electronegative oxygen atoms, making it very stable. In the phenoxide ion the negative charge is mainly on one oxygen and partly on less electronegative ring carbons, so it is less stabilised. Electron-withdrawing groups (like -Cl or -NO2) increase acidity because they further disperse and stabilise the negative charge of the carboxylate ion (for example, chloroacetic acid is stronger than acetic acid), while electron-releasing groups decrease acidity.

(1) Reduction of nitro compounds: a nitro compound is reduced (with H2/Ni, or Sn/HCl, or Fe/HCl) to a primary amine; for example, nitrobenzene is reduced to aniline, C6H5NO2 + 6[H] -> C6H5NH2 + 2H2O. (2) Hofmann's bromamide degradation: an amide is treated with bromine and a strong alkali (Br2 + NaOH/KOH) to give a primary amine with one carbon atom less than the amide, R-CONH2 + Br2 + 4NaOH -> R-NH2 + Na2CO3 + 2NaBr + 2H2O.

R-CONH2 + Br2 + 4NaOH -> R-NH2 + Na2CO3 + 2NaBr + 2H2O

Amines are basic because the nitrogen atom has a lone pair of electrons which it can donate to a proton (or to a Lewis acid), forming a bond; thus amines can accept a proton. In the gaseous phase (or a non-aqueous medium), only the inductive effect of the alkyl groups operates: more alkyl groups increase the electron density on nitrogen, so the basic strength follows the order tertiary > secondary > primary > ammonia. In aqueous solution the order changes because of the combined effect of inductive effect, solvation (hydrogen bonding of the cation) and steric hindrance.

Benzenediazonium chloride is prepared by diazotisation: aniline is treated with nitrous acid (formed from sodium nitrite and hydrochloric acid) at a low temperature of 0 to 5 degrees C, C6H5NH2 + NaNO2 + 2HCl -> C6H5N2Cl + NaCl + 2H2O. In the Sandmeyer reaction, the diazonium group is replaced by a halogen or cyanide using the corresponding copper(I) salt; for example, C6H5N2Cl + CuCl -> C6H5Cl + N2. Diazonium salts are very useful for introducing many groups into the benzene ring.

C6H5NH2 + NaNO2 + 2HCl -> C6H5N2Cl + NaCl + 2H2O

Proteins are naturally occurring biomolecules that are polymers of alpha-amino acids joined by peptide bonds (-CO-NH-); they are essential for growth and maintenance of the body and act as enzymes, hormones and structural materials. Denaturation of a protein is the process in which a protein loses its natural three-dimensional shape (its secondary and tertiary structure) due to heat, change in pH, or addition of chemicals, while the primary structure (sequence of amino acids) remains intact. On denaturation the protein loses its biological activity; for example, the coagulation of egg white on boiling.