HALOGEN ANHYDRIDES

HALOGEN ANHYDRIDES, derivatives of organic acids in which the hydroxyl group of the acid is replaced by a halogen. Of the various halo-anhydrides, the greatest importance belongs to chloroanhydrides—compounds containing the COCl or SO2Cl group. Chloroanhydrides are usually obtained by the action of phosphorus chlorides (PCl3 or PCl5) on the corresponding acids or their alkali salts: CH3.COOH+PCl5 = CH3.CO.Cl+POCl3 + HCl. Lower halo-anhydrides are colorless liquids with a pungent odor, fuming in air; higher ones are solid bodies, stable in the absence of moisture. Halo-anhydrides boil at a lower t° than the corresponding acids. The halogen in halo-anhydrides is very loosely bound and therefore capable of entering into various kinds of interactions. For example, with water, chloroanhydrides react vigorously, decomposing into the corresponding acid and HCl: R.CO.Cl + H2O = R.COOH + HCl; with ammonia or amines they give amides or alkylated amides of acids: R-CO-Cl + NH3 = R-CO-NH2 + HCl; with salts of organic acids—acid anhydrides: R-CO-Cl+NaOOCR' = R-CO-0-CO-R' + NaCl. Thus, chloroanhydrides are substances by means of which acid residues (acyls) are introduced into organic compounds. This operation is of very great importance in solving various kinds of synthetic and research problems. Acylation is particularly often used for the determination and isolation of substances with hydroxyl and amino functions. The most important halo-anhydrides are: acetyl chloride (CH3.COCl), benzoyl chloride (C6H5.COCl), benzene sulfochloride (C6H5.SO2Cl) and paratoluene sulfochloride (CH3.C6H4.SO2Cl).