Rosin and Cannizzaro Reaction

ROSIN (Colophonium, s. Resina Colophonium, State Pharmacopoeia), a resin obtained from turpentine after the distillation of oil of turpentine from it. Producing plants: pine (Pinus silvestris L.) provides Russian, Swedish, and German rosin; Pinus pinaster Soland provides French rosin; Pinus taeda L. and Pinus australis provide American rosin. Regardless of its origin, rosin has almost identical appearance and properties. It consists of glassy, transparent, shiny pieces covered with powder, with a conchoidal fracture, and is without taste or odor. Specific gravity is 1.068–1.100. The melting point is variable depending on the grade (from 100° to 130°). Upon melting, rosin yields a transparent liquid; upon stronger heating, it ignites and burns with a smoky flame. It is insoluble in water and easily soluble in alcohol. The main component of American rosin is abietic acid anhydride (formerly called sylvic acid) (C18H27CO)2O or (C19H29CO)2O. French rosin consists of pimaric acid C20H30O2. Application: formerly used internally for catarrhal conditions of the respiratory tract. Externally: in the form of a dusting powder for hemorrhages. It is more often included in the composition of plasters and ointments: Emplastrum adhaesivum (State Pharmacopoeia), Empl. oxycroceum (Austria, Switzerland, France), Empl. Cantharidis (Pharmacopoeia VII), Empl. Plumbi compositum (Pharmacopoeia VII), Empl. Meliloti (State Pharmacopoeia), Empl. saponatum (State Pharmacopoeia), and Unguentum basilicum. Rosin is used in industry for the preparation of varnishes, sealing wax, soap, flypaper, glue for cementing dishes, etc.

CANNIZZARO REACTION (Cannizzaro), or dismutation, is a coupled reaction consisting of the simultaneous oxidation and reduction of 2 molecules of aldehyde with the addition of a water particle. In this process, 1 molecule of aldehyde serves as a hydrogen acceptor and is converted into the corresponding alcohol, while the second plays the role of an oxygen acceptor and is oxidized by the oxygen of water into an acid. CH3CHO + H2O = CH3CH2OH + CH3COOH acetaldehyde + water = ethyl alcohol + acetic acid

Along with such a simple Cannizzaro reaction, dismutations are possible in which 2 molecules of different aldehydes (and ketones) participate—a mixed Cannizzaro reaction. The Cannizzaro reaction proceeds spontaneously, although slowly, in alkaline solutions of aldehydes. More important is the Cannizzaro reaction occurring under the influence of oxidation-reduction enzymes, so-called aldehydrases or aldehydemutases, to which, for example, the Schardinger enzyme of milk belongs. The Cannizzaro reaction is only one of the particular cases of dismutation produced by oxidation-reduction enzymes (oxidoreductases) and plays a most essential role in various types of fermentation and in intermediate metabolism.

Examples: the dismutation of acetaldehyde with the formation of acetic acid is the final stage of acetic acid fermentation; the formation of pyruvic acid and ethyl alcohol as a result of a mixed Cannizzaro reaction between methylglyoxal and acetaldehyde during the alcoholic fermentation of sugar: methylglyoxal CH3.CO.CHO + H2O + acetaldehyde CH3.CHO = CH3.CO.COOH pyruvic acid + CH3.CH2OH ethyl alcohol

Dismutation can also occur between two carbonyl groups located in the same molecule, with one of them being converted into an alcohol group and the other into a carboxyl group (internal Cannizzaro reaction). A biologically important case of this kind is the conversion of methylglyoxal into lactic acid—the final phase of lactic acid fermentation and glycolysis (see):

CH3

CH3

H2

CO + O

CHOH

CHO

COOH

methylglyoxal

lactic acid

The enzyme producing this conversion has received the name ketoaldehyde mutase or glyoxalase. A conversion close in type to the Cannizzaro reaction is the dismutation of purines, for example, xanthine into hypoxanthine and uric acid. This process, leading to the formation of uric acid in the mammalian organism, is attributed to the action of the enzyme xanthine dehydrogenase; the latter is likely identical to aldehydrases.

In enzymatic dismutations, the second molecule of aldehyde or purine can be replaced by other hydrogen acceptors (e.g., methylene blue), and since there are always reducing substances in tissues that play the role of H-acceptors, reactions of this type show that aldehydemutases do not represent a fundamental difference from other oxidoreductases.