Furfural

FURFURAL (furfural, furylmethanal, furol, alpha-furyl aldehyde), a derivative of furan. It is obtained by boiling pentoses, pentosans, and glucuronic acid with dilute mineral acids. In small quantities, furfural is also formed during the acid hydrolysis of proteins, nucleosides, and during the dry distillation of inositol. The chemistry of the formation of furfural from pentoses can be represented as follows: ... Furfural melts at -36.5°, boils at 162°, and at ordinary temperature is a colorless liquid that easily distills with steam; it easily condenses with various substances, for example, phenols; its compound with phloroglucinol, which is sparingly soluble and red in color, serves for the quantitative determination of pentoses. Upon the action of KCN, furfural condenses into furoin C₄H₃O-CH(OH)-CO-C₄H₃O; with ammonia it gives hydroamide (C₅H₄O)₃N₂; upon the action of an alcoholic solution of alkali, it gives (the Cannizzaro reaction) the corresponding acid (pyromucic) and alcohol (furyl). Furfural has a rather pleasant odor; its presence is partly responsible for the smell of fresh rye bread. Furfural is a very inexpensive product. At present, it is prepared on an industrial scale from such low-value products as sunflower husks and oat grain hulls, heating them in closed boilers at a pressure of 4 atmospheres with 5% H₂SO₄. It is also obtained from paper-manufacturing waste. Due to its low cost, furfural is used in various industries: for the preparation of artificial resins (bakelites—condensation products of furfural with phenols), for the protection of wood against decay, for imparting aroma to tobacco, as a nitrocellulose solvent, in perfumery, etc.; hydrofuramide is used to protect cereal and beet seeds from harmful fungi.—Methylfurfural (a liquid with a boiling point of 187°) is similar to furfural and gives the same condensation products.—Oxymethylfurfural CH₂OH-C=CH-CH=C(CHO)-O is formed by heating hexoses with acids. It is contained in artificial honey. When resorcinol and strong hydrochloric acid are added to it, a red coloration is obtained. This reaction is used to distinguish artificial honey from natural honey. When oxymethylfurfural is heated with HCl or dilute H₂SO₄, levulinic and formic acids are formed almost quantitatively: C₆H₆O₃ + 2H₂O = H-COOH + CH₃-CO-CH₂-CH₂-COOH. The formation of furfural or oxymethylfurfural forms the basis of a number of reactions for pentoses and glucuronic acid (Plaisance's reaction, the orcinol test, the reaction with phloroglucinol and HCl).