Cannizzaro aldehyde disproportionation

This involves hydride transfer from an aldehyde molecule lacking an a-H atom, e.g. HCHO, R3CCHO, ArCHO, to a second molecule of either the same aldehyde (disproportionation) or sometimes to a molecule of a different aldehyde ( crossed Cannizzaro). The reaction requires the presence of strong bases, and with, for example, PhCHO the rate law is found to be,... 

Aldehydes 1 that have no a-hydrogen give the Cannizzaro reaction upon treatment with a strong base, e.g. an alkali hydroxide.In this disproportionation reaction one molecule is reduced to the corresponding alcohol 2, while a second one is oxidized to the carboxylic acid 3. With aldehydes that do have a-hydrogens, the aldol reaction takes place preferentially. 

The synthetic importance of the reaction is limited, because as a consequence of the disproportionation, the yield of the alcohol as well as the carboxylic acid is restricted to 50%. However good yields of alcohols can often be obtained when the reaction is carried out in the presence of equimolar amounts of formaldehyde. The formaldehyde is oxidized to formic acid and concomitantly reduces the other aldehyde to the desired alcohol. This variant is called the crossed Cannizzaro reaction. 

Cannizzaro reaction (Section 19.12) The disproportionation reaction of an aldehyde to yield an alcohol and a carboxylic acid on treatment with base. 

In the Cannizzaro reaction [110, 111] two aldehyde functionalities disproportionate into the corresponding hydroxyl and carboxyl functions, either as separate compounds or as an ester (Scheme 20.25). The reaction conditions needed are rather harsh, except when R1 or R2 is a phenyl group. Typically, an excess of so-... 

Cannizzaro Reaction The formation of an acid and an alcohol through the base-catalyzed disproportionation of an aliphatic or aromatic aldehyde with no a-hydrogen atoms. 

The Cannizzaro reaction, that is, the base-catalysed disproportionation of a carbonyl compound to an alcohol and a carboxylic acid, has gained some importance as an economically viable alternative to the reduction with borohydrides. However, the reaction is restricted to carbonyl compounds without any a-hydrogen, which do not undergo competing aldol reactions. Thus, mainly aromatic aldehydes are used for this kind of transformation. The protocols developed for microwave applications typically involve solvent-free conditions using alumina as the solid support. Under these conditions, a significant acceleration of the reaction was achieved. 

This dismutation or disproportionation reaction is known as the Cannizzaro reaction. The mechanism of the reaction involves the production of the anion (1) which may transfer a hydride ion to a carbonyl carbon atom in another aldehyde molecule. The reaction sequence is completed by a proton transfer to yield the carboxylate anion and the alcohol. 

The dianion becomes a much more stable carboxylate monoanion, and a second molecule of aldehyde has been reduced to an alcohol. This is the Cannizzaro reaction in this case it takes the form of a disproportionation of two molecules of aldehyde to one HO as nucleophile of carboxylate and one of alcohol. 

Disproportionation (or dismutation) — Reaction in which species with the same oxidation state react with each other to yield one species of higher oxidation state and one of lower oxidation state, e.g., 3Au+ -> Au3+ + 2Au, or 4KCIO3 -> KC1 + 3KCIO4, or the d. reaction of an aldehyde in alkaline medium into the respective carboxylic acid and alcohol compounds, known as Cannizzaro reaction 2R - CHO + NaOH - R - COONa + R-CH2OH. In practice, d. happens rarely spontaneously, but is usually achieved by a catalyst application. 

A mechanistically similar example is the Cannizzaro reaction, which is in essence a disproportionation between two aldehydes lacking a-hydrogen with a parallel addition of water to yield an alcohol and a carboxylic acid [144]. Aldol condensation cannot take place since the a-hydrogen is absent. Most likely the formation of aldehydrates takes place in the zeolite pores. NaX and NaY have been reported to be successful catalysts [150]. 

The conversion of aldehydes into carboxylic acids as a result of disproportionation occurs in the Cannizzaro and Guerbet reactions. The Cannizzaro reaction takes place when aromatic aldehydes are treated with alkali hydroxides and results in the formation of carboxylic acids and the corresponding alcohols. Yields of the acids can therefore be 50% at a maximum. The reaction is catalyzed by silver [363] and silver oxide [362], which is sometimes generated in situ from silver nitrate [364] see equation 357). 

The Cannizzaro reaction takes place by nucleophilic addition of OH" to an aldehyde to give a tetrahedral intermediate, which expels hydride icn as a leaving group. A second aldehyde molecule accepts the hydride ion in another nucleophilic addition step, resulting in a simultaneous oxidation and reduction, or disproportionation. One molecule of aldehyde undergoes a substitution of H by OH" and is thereby oxidized to an acid, while a second molecule of aldehyde undergoes an addition of H" and is thereby reduced to an alcohol. Benzaldehyde, for instance, yields a 1 1 mixture of benzoic acid and benzyl alcohol when heated with aqueous NaOH. 

Cannizzaro reaction A disproportionation reaction between two molecules of aldehydes that usually have no a-hydrogens in the presence of base to give an alcohol and an acid. Called a crossed Cannizzaro reaction if the reaction occurs between different aldehydes. 

A wide variety of reaction conditions have been proposed for the reaction of starch with aldehydes. Early studies entailed use of alkaline solutions, perhaps in order to gelatinize starch.1276 Because of the known behavior of aldehydes in alkaline media (Cannizzaro disproportionation and aldol condensations of aldehydes with active active a-methylene groups), this reaction has no simple outcome. However, a process of acetalation in alkaline solutions was subsequendy patented.1293 An exothermic reaction was observed within 3 4 min using an excess of formaldehyde. It was also reported that a,(3-unsaturated aldehydes reacted readily in slightly basic media.1294... 

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