Aldehyde Cannizzaro reaction

Keywords aldehyde, Cannizzaro reaction, barium hydroxide, microwave irradiation, alcohol, carboxylic acid... 

Cannizzaro reaction Two molecules of many aldehydes, under the influence of dilute alkalis, will interact, so that one is reduced to the corresponding alcohol, while the other is oxidized to the acid. Benzaldehyde gives benzyl alcohol and benzoic acid. Compare the aldol condensation. 

Action of sodium hydroxide. Does not undergo the Cannizzaro reaction. It dissolves in dil. NaOH solution, giving a yellow solution from which the aldehyde is precipitated unchanged on acidification. If heated with cone. NaOH solution, salicylaldehyde slowly undergoes atmospheric oxidation to salicylic acid. 

Aromatic aldehydes react with the dimedone reagent (Section 111,70,2). All aromatic aldehydes (i) reduce ammoniacal silver nitrate solution and (ii) restore the colour of SchifiF s reagent many react with sodium bisulphite solution. They do not, in general, reduce Fehling s solution or Benedict s solution. Unlike aliphatic aldehydes, they usually undergo the Cannizzaro reaction (see Section IV,123) under the influence of sodium hydroxide solution. For full experimental details of the above tests, see under Ali-phalic Aldehydes, Section 111,70. They are easily oxidised by dilute alkaline permanganate solution at the ordinary temperature after removal of the manganese dioxide by sulphur dioxide or by sodium bisulphite, the acid can be obtained by acidification of the solution. 

By the Cannizzaro reaction. This consists in the action of a concentrated aqueous solution of sodium or potassium hydroxide upon an aldehyde (see detailed discussion before Section IV, 123), for example ... 

Only half of the aldehyde is reduced to the alcohol, the other half being oxidised to the acid. By using a slight excess (say, 1 -3 mols) of aqueous formaldehyde, practically the whole of the aromatic aldehyde is converted into the alcohol the formaldehyde Is simultaneously oxidised to formic acid. This is sometimes termed a crossed Cannizzaro reaction. The example given is ... 

Aromatic aldehydes, and also aliphatic aldehydes containing no a-hydrogen atom, undergo the Cannizzaro reaction (see Section IV,123) when treated with aqueous alkali, for example ... 

Ahphatic aldehydes can also be prepared in moderate yields by distilling the aldehyde from the mixture rapidly to avoid aldol addition or a Cannizzaro reaction (53). 

The adduct can be prepared and decomposed as described for aldehydes. Alternatively, because no Cannizzaro reaction is possible, it can also be decomposed with 0.5N NaOH. 

In general, the carbonyl derivatives of isothiazole behave normally and condense readily with carbonyl reagents. The aldehydes reduce ammoniacal silver nitrate and undergo the Cannizzaro reaction. ... 

Aldehydes can react through a hydride transfer as in the Cannizzaro reaction. 

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 key step of the Cannizzaro reaction is a hydride transfer. The reaction is initiated by the nucleophilic addition of a hydroxide anion to the carbonyl group of an aldehyde molecule 1 to give the anion 4. In a strongly basic medium, the anion 4 can be deprotonated to give the dianionic species 5 ... 

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. 

The applicability of the Cannizzaro reaction may be limited, if the substrate aldehyde can undergo other reactions in the strongly basic medium. For instance an a ,a ,a -trihalo acetaldehyde reacts according to the haloform reaction. 

The Cannizzaro reaction takes place by nucleophilic addition of OH- to an aldehyde to give a tetrahedral intermediate, which expels hydride ion as a leaving group and is thereby oxidized. A second aldehyde molecule accepts the hydride ion in another nucleophilic addition step and is thereby reduced. Benzaldehyde, for instance, yields benzyl alcohol plus benzoic acid when heated with aqueous NaOH. 

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

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Cannizzaro Reaction. In the Cannizzaro reaction (19-60) aldehydes without an a hydrogen are reduced to alcohols. 

The reaction can be stopped at this point, but more often a second mole of formaldehyde is permitted to reduce the newly formed aldol to a 1,3-diol, in a crossed Cannizzaro reaction (19-60). If the aldehyde or ketone has several a-hydrogens, they can all be replaced. An important use of the reaction is to prepare pentaerythritol from acetaldehyde ... 

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