Benzaldehyde Cannizzaro reaction

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. 

Sodium hydroxide solution cannot be used at this stage since it may produce benzoic acid by the Cannizzaro reaction (Section IV,123) from any unchanged benzaldehyde. If, however, the reaction mixture is diluted with 3-4 volumes of water, steam distilled to remove the unreacted benzaldehyde, the residue may then be rendered alkaline with sodium hydroxide solution. A few grams of decolourising carbon are added, the mixture boiled for several minutes, and filtered through a fluted filter paper. Upon acidifying carefully with concentrated hydrochloric acid, cinnamic acid is precipitated. This is collected, washed and purified as above. 

Furfural undergoes the Cannizzaro reaction (compare Benzaldehyde Section IV,123) when treated with sodium hydroxide solution ... 

Prior to the commercial development of this process benzyl alcohol was obtained from benzaldehyde [100-52-7] which undergoes the Cannizzaro reaction (2) upon treatment with potassium hydroxide. High yields of benzyl alcohol and potassium benzoate are obtained by this route which cannot compete with the present day process because of the high cost of benzaldehyde (qv). 

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. 

Heavy water, DzO, has often been used in a rather similar way. Thus in the Cannizzaro reaction of benzaldehyde (p. 216),... 

If HB is a weak acid, this reaction is rather slow, but it nevertheless takes place. Substances that are unable to react with O2 can also be involved when the solvent-reactant mixtures contain water or other proton donor compounds, even in traces. Thus, benzaldehyde is absolutely resistant to the action of the superoxide ion. However, benzaldehyde transforms into benzylic alcohol and benzoic acid on the action of O2 in the presence of moisture. Sawyer and Gibian (1979) described the following superoxide variant of the Cannizzaro reaction 202 + H2O —>62 + HOO + OH and 2PhCHO + OH + H2O PhCOOH + PhCH20H. 

Formylfuran behaves in a very similar manner to benzaldehyde and undergoes the usual reactions of an aromatic aldehyde, e.g. (i) the Cannizzaro reaction with cone, sodium hydroxide to give furan-2-ylmethanol and the sodium salt of furoic acid, (ii) the Perkin reaction with acetic anhydride and sodium acetate to yield an aldol product that dehydrates to 3-(furan-2-yl)propenoic acid, and (iii) a condensation with potassium cyanide in alcoholic solution to form furoin (under these conditions, benzaldehyde undergoes the benzoin condensation) (Scheme 6.32). 

Problem 15.21 For the Cannizzaro reaction, indicate (a) why the reaction cannot be used with aldehydes having an a H, —CHCHO (b) the role of OH and OD (Problem 15.20) (c) the reaction product with ethanedial, 0==CH—CH==0 (d) the reaction products of a crossed-Cannizzaro reaction between (i) formaldehyde and benzaldehyde, (ii) benzaldehyde and p-chlorobenzaldehyde. ... 

Benzaldehyde. The addition of less than stoichiometric quantities of benz-aldehyde to CoH (H2 atmosphere) did not result in hydrogen absorption. However, when this procedure was carried out with CoH containing added alkali (KOH, 2x cobalt concentration), hydrogen was taken up, 1.0 atom of hydrogen being absorbed per mole of substrate. Since benzyl alcohol was isolated in 66% yield, it is assumed that a portion of the product may have been formed via a competitive Cannizzaro reaction. Reinforcing this assumption is the observation of an apparent depletion of alkali during the run. 

The lower reactivity of benzaldehyde with respect to acetaldehyde was found also in the vapour phase aldolisation over lithium phosphate [390]. Over the same catalyst, the reactivity order in the self-condensations of aldehydes could be estimated as CH3CHO > CH3CH2CHO (CH3)2-CHCHO. The reactivity of isobutyraldehyde in the self-condensation was almost undetectable, probably due to steric hindrance on the a-carbon, but this substance was able to react as a hydrogen acceptor with cyclohexanone. With propionaldehyde over a calcium hydroxide catalyst, a Cannizzaro-type reaction occurred to some extent simultaneously with the aldolisation [390]. This unexpected result was also recorded by other authors [391], who established that the tendency to aldolisation decreased, and the tendency to the Cannizzaro reaction increased, with... 

Condensation of dimethyl sulfone with benzaldehyde under PT conditions yeilds 3,5-diphenylthiadioxane S,S-dioxide (38). The product may be obtained in good yield, using TEBA or 18-crown-6.64 Gokel65 has shown that when the catalyst is a crown ether the reaction is complicated by the production of benzoic acid, presumably via a Cannizzaro reaction. 

Cyclocitral (540) behaves as a nucleophile towards benzaldehyde and in the presence of sodium hydroxide the fused pyran-2-oI (541) is formed through an aldol (Scheme 202) (81JHC549). However, when sodium ethoxide is used as the catalyst, the pyran-2-one is produced. It is proposed that oxidation arises through a crossed Cannizzaro reaction with benzaldehyde (and much benzyl alcohol is observed in support of this idea), followed by a carbanion attack on more benzaldehyde and subsequent cyclization. 

Microwave irradiation of only 15 s was reported to give almost quantitative yields of both the alcohol and the carboxylic acid in the sodium hydroxide catalysed Cannizzaro reaction of substituted benzaldehydes and other aromatic heterocyclic aldehydes (Scheme 4.18). Basic alumina was used as the solid support. The alcohol was selectively... 

In an attempt to couple halobenzaldehydes with amines, A1203 was pre-absorbed with the substituted benzaldehydes and imidazole or piperidine as a base and irradiated with microwaves. However, the corresponding benzylic alcohols and benzoic acids were unexpectedly obtained by the Cannizzaro route (Scheme 4.20). The products of Cannizzaro reactions were also obtained as the main products, when microwave-assisted condensation reactions of benzaldehydes with vinyl acetate using barium hydroxide as the catalyst were attempted40. 

Dismutations of the same type, but involving two unlike aldehyde molecules, will be classed as crossed Cannizzaro reactions. The reduction of benzaldehyde to benzyl alcohol by means of formaldehyde and alkali is an example. 

It has been shown by Kharasch and Foy22 and confirmed by Urushi-bara and Takebayashi28 that the presence of peroxides markedly accelerates the heterogeneous Cannizzaro reaction. Highly purified benzaldehyde undergoes dismutation to the extent of only 2-4% under conditions which result in 25-80% reaotion with ordinary benzaldehyde. 5-Bromofurfural dismutates only slowly with 30% sodium hydroxide in ether, but the reaotion is accelerated markedly by the addition of a trace of hydrogen peroxide.24 Variousjexplanations have been advanced to account for the effect of peroxides,26 260 but the problem is still obscure-. 

Hydrohydrastinine does not arise by ring olosure of the corresponding alcohol during the reaction, since the alcohol is stable to alkali under the conditions of the reaction. Further examples of the Cannizzaro reaction for polysubstituted benzaldehydes are given in Table V. 

Radical intermediates were proposed at an early stage in the history of the Cannizzaro reaction, and this possibility has been resurrected to account for the detection of some 20% of a-D-benzyl alcohol in the products of the Cannizzaro reaction of a-D-benzaldehyde in aqueous alkaline dioxan. This has been rationalized in terms of formation of the benzaldehyde radical anion, which abstracts a H-atom from the solvent (Chung, 1982). Epr spectroscopy of reacting solutions of p-Cl-,/ -NO 2 and / -CF3-benzaldehyde as well as benzaldehyde itself in THF HMPA (9 1) yielded spectra of the aldehyde radical anions identical to those produced by the action of metallic... 

Which step would be rate-determining for this mechanism It could not be step 1 since, if this were the case, then the rate law would be first-order with respect to the aldehyde rather than the observed second-order relationship. Also, if the reaction is carried out in water labelled with oxygen-18, the oxygen in the benzaldehyde exchanges with the 180 from the solvent much faster than the Cannizzaro reaction takes place. This can only be because of a rapid equilibrium in step 1 and so step 1 cannot be rate-determining. 

We said earlier that we can never prove a mechanism—only disprove it. Unfortunately, just as the correct mechanism seems to be found, there are some observations that make us doubt this mechanism. In Chapter 39 you saw how a technique called electron spin resonance (ESR) detects radicals and gives some information about their structure. When the Cannizzaro reaction was carried out with benzaldehyde and a number of substituted benzaldehydes in an ESR spectrometer, a radical was detected. For each aldehyde used, the ESR spectrum proved to be identical to that formed when the aldehyde was reduced using sodi-... 

Before leaving the Cannizzaro reaction, look at these rates of reactions for aromatic aldehydes with different substituents in the para position. These aldehydes maybe divided into two classes those that react faster than unsubstituted benzaldehyde and... 

Alkaline hydrolysis of lignin increases the number of reactive benzylic hydroxyl groups and may also be important in further depolymerizing the lignin once the oxidative-cleavage reaction has occurred. The formation of a p-electron-withdrawing -CHO substituent on aryl lignin units should increase the rate of hydrolysis of the ether bonds (26). Hydrolysis also forms p-phenylate ions, which then protect the benzaldehyde from further reaction via the Cannizzaro reaction, as mentioned earlier. 

The effects of micellization on reactivity have been investigated for a wide variety of ionic organic reactions other than those discussed previously in Sections IV and V, e.g. the Cannizzaro reaction, racemiza-tion, acid catalyzed enolization, base catalyzed hydrolysis of a,]8-un-saturated ketones, and coupling of quinonediimines with phenols. In the case of the Cannizzaro reaction of benzaldehyde (equation 43), the cationic surfactants eicosanyltrimethylammonium bromide and octa-decyltrimethylammonium bromide increased and the anionic surfactant... 

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