Cannizzaro reaction side reactions

A crossed Cannizzaro reaction is similar to a normal Cannizzaro reaction however, two different aldehydes are reacting. Normally one of the aldehydes is formaldehyde because there are fewer chances for side reactions. (It also has the advantage of being cheap.) The reactions in Figures 11-34 and 11-35 are crossed Cannizzaro reactions using an excess of formaldehyde. The excess of formaldehyde increases the probability of the initial attack by the hydroxide being on the formaldehyde instead of the other aldehyde. Figure 11-35 shows the synthesis of pentaerythritol. 

Aldol condensations were originally carried out in the liquid phase and catalysed homogeneously by acids or bases this way of operation is still predominant. Solid-catalysed aldol reactions can also be performed in the liquid phase (in trickle or submerged beds of catalyst), but in many cases vapour phase systems are preferred the factors determining the choice are the boiling points and the stability of the reactants at elevated temperatures. At higher temperatures, the formation of a, j3-unsaturated aldehydes or ketones [reactions (B) and (C)] is preferred to aldol (ketol) formation [reaction (A)]. A side reaction, which may become important in some cases, is the self-condensation of the more reactive carbonyl compound if a mixed condensation of two different aldehydes or ketones is occurring. The Cannizzaro reaction of some aldehydes or polymerisation to polyols or other resin-like products can also accompany the main reaction. 

The Cannizzaro Reaction should be kept in mind as a source of potential side products when aldehydes are treated under basic conditions. 

Typically, reactions are carried out open to air, at room temperature (rt), using a mixture of tBuOH/water (9 1) or MeCN/water (9 1) as solvent and sodium or potassium hydroxide as base. The presence of water suppresses undesired side reactions (e.g., Cannizzaro reaction, Williamson alkylation, solvent reactivity) [37]. Benzyl bromides are most commonly reported as the halide component. 

MPa H2. To suppress the isomerization of D-glucose to D-mannose and D-fructose (Lobry de Bruyn-van Ekenstein transformation) (Scheme 5.2) and the Cannizzaro reaction, which were both promoted in an alkaline medium, the pH value was maintained between 5.5 and 6.5. Under the conditions that were optimized to minimize the side reactions, the formation of gluconic acid and mannitol was reduced to less than 1% each at 99.5-99.6% conversion, while with a normal nonpromoted Raney Ni 1.5-2.1% of gluconic acid and 1.3-1.9% of mannitol were formed at 99.5-99.7% conversion. 

Milder reaction conditions have been developed for the HWE reaction of phosphonate-stabilized carbanions to increase yields, accommodate sensitive substrates and to minimize undesired side reactions such as double bond migrations, the Cannizzaro reaction, Knoevenagel condensation and Michael addition. For example, a number of different bases have been employed to generate the carbanion. These include sodium hydroxide under phase-transfer conditions, potassium carbonate, barium hydroxide, diisopropylethylamine and l,8-diazabicyclo[5.4.0]undec-7-ene (see Protocol 10).22... 

Investigation of the mechanism of these reactions has suggested ways in which the yields can be improved. Acidic conditions (pH 2) will prevent Cannizzaro rearrangement of any glyoxal-type species and also serve to hydrolyse any Schiff bases which result from side reactions of aldehyde and amine. Conditions should be adjusted so that the rate of hydrolysis of linear products is equal to the rate of cyclocondensation, allowing accumulation of the imidazole products. From glyoxal, formaldehyde and ammonium chloride the yield of imidazole can be inereased to 85% by careful control of the conditions. With an appropriate alkylammonium chloride, 1-substituted imidazoles are also accessible (e.g. 1-methyl (56%), 1-isopropyl (46%), 1-cyclohexyl (49%), 1-n-butyl (55%), 1-t-butyl (25%)). The process may have some applications, but yields drop off with branched alkyl compounds [22 j. Imidazolium salts are also available under similar conditions when two molar equivalents of a primary alkylamine are used [23]. 

K2CO3. No side reactions like the Cannizzaro or Knoevenagel reactions are observed on treatment with C-200. In the reaction with benzaldehyde and furfural the products 71 (R = Ph and 2-furyl) are formed nearly quantitatively. Bulky aromatic aldehydes such as pyrene-l-carboxaldehyde and aliphatic aldehydes are also suitable substrates for this process. ( )-acyclic a,/ -unsaturated ketones 74 can be prepared by use of a similar HWE reaction of 2-oxoalkanephosphonates 72 with aliphatic aldehydes 73 under the influence of barium hydroxide C-200 (Scheme 5.14) [36]. The HWE procedure employing activated Ba(OH)2 is applicable to the structurally complex, base-sensitive aldehydes which are susceptible to elimination and/or epimerization under the traditional basic conditions of the HWE reaction using NaH. For instance, when phosphonate 75 and aldehyde 76 are exposed to 0.8 equiv. activated Ba(OH)2 in aqueous THF at 20°C, (//-enone 77 is obtained in 70% yield (Scheme 5.14) [37]. 

New C-0 bonds are formed in the CO/H2 synthesis when CO is converted to CO2 by the WGS reaction (3) and in the synthesis of esters. Only the latter will be discussed here. Primarily methyl esters are formed, and they are significant side products over the (Cs)/Cu/ZnO catalysts but not over the alkali/(Co)/M0S2 catalysts. The mechanism for methyl ester formation has been suggested (ref. 39) to occur via a coupling of a Cn aldehyde with a Ci aldehyde by the Cannizzaro reaction or by a nucleophilic attack of a Cn aldehyde by methoxide (Tischenko reaction). The exception is the formation of methyl formate that occurs via a nucleophilic attack of CO by adsorbed methoxide e... 

A troublesome side reaction encountered in the manufacture and use of amino resins is the conversion of formaldehyde to formic acid. Often the reaction mixture of amino compound and formaldehyde must be heated under alkaline conditions. This favors a Cannizzaro reaction in which two molecules of formaldehyde interact to yield one molecule of methanol and one of formic acid. 

The aldol condensation is a very attractive route to a,p-unsaturated carbonyl compounds. The application of this reaction is nevertheless rather limited, since numerous side reactions usually occur amongst these are self-condensation of the ketone, Michael-type addition to the newly formed product, or Cannizzaro reactions. As a consequence, poor yields are obtained in most cases [90]. In the enol ether condensation, described earlier, these side reactions are less troublesome. A disadvantage of the enol ether condensation compared to the aldol condensation is that strongly acidic conditions have to be used to cleave the intermediate in the enol ether synthesis. 

It was quite surprising that the secondary amine-functionalized polymers could be prepared quantitatively by normal addition of 1.1 equivalents of neat N-benzylidenemethylamine to PSLi solutions as shown in eqn [21]. The similar functionalization reaction of PSLi with N-benzylide-netrimethylsilylamine (see eqn [19]) formed significant amounts of amine-functionalized dimer and acetophenone-type functionalized polymer due to Cannizzaro-type side reactionsas shown in Scheme 6. Both of these benzalde-hyde imines have an a-hydrogen that could participate in Cannizzaro-type hydride-transfer reactions. The absence of this side reaction in the functionalization with N-benzylidenemethylamine could be rationalized by the fact that Si is more electropositive and polarizable than C and this facilitates hydride transfer. 

Cannizzaro Reaction and Formaldehyde Concentration. While this main reaction, represented by Eq. (31.1), takes place, other undesirable side reactions proceed, also in... 

Fehlings-Type Reactions. In addition to the Cannizzaro reaction, the following side reaction also competes for formaldehyde ... 

Most reports remark that the best reaction environment is a (sometimes triphasic) mixture of polar solvents, like t-BuOH/H O or CH CN/H O in proportions of 9 1, in presence of the bases like NaOH or KOH at room temperature [146]. These conditions reduce the possibilities of side reactions (e.g. Cannizzaro reaction, Williamson alkylation, reaction with solvent) [181]. Kinetic studies demonstrated that this reaction has two slow steps the alkylation (formation of sulfonium salt from the catalytic sulfide) and the addition of the ylide to the carbonyl compound [184]. 

Undergoes Cannizzaro s reaction, giving benzyl alcohol and benzoic acid (p. 229). Warm gently for several minutes, cool, add a few ml. of water and then cone. HCl, and cool again. Crystals of benzoic acid separate out on scratching the sides of the tube with a glass rod. 

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