Carbonyl compounds, condensation reactions Cannizzaro reaction

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 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. 

Originally, formaldehyde was exclusively employed as the carbonyl component, but more recently, higher aldehydes and ketones have also been used. However, the usefulness of the latter compounds is limited by aldol-type condensations, Cannizzaro reactions, enamine formation, and steric hindrance. 

All derivatives of furan-2-carbaldehyde are reactive substances. They are easily oxidised by oxygen to the corresponding acids, which, together with the appropriate alcohols, are also formed from furan-2-carbaldehyde derivatives by Cannizzaro reaction. Derivatives of furan-2-carbaldehyde condense together or with other carbonyl compounds and enter into non-enzymatic browning reactions. Oxidation of 5-hydroxymethylfuran-2-carbaldehyde, for example, yields 5-hydroxymethylfuran-2-carboxylic acid (4-198). Condensation... 

Aldehydes and Ketones. Pyrrole aldehydes and ketones are somewhat less reactive than the corresponding benzenoid derivatives. The aldehydes do not undergo Cannizzaro or Perkin reactions but condense with a variety of compounds that contain active methylene groups. They also react with pyrroles under acidic conditions to form dipyrryhnethenes (26). The aldehydes can be reduced to the methyl or carbinol stmctures. The ketones undergo normal carbonyl reactions. 

Whilst formylpyrroles behave in many ways as would be expected, some anomalous properties have attracted attention. Thus, 2-formylpyrrole gives an oxime, arylhydrazones and other typical carbonyl derivatives , and condenses with methyl ketones, hippuric acid and other reactive methylene compounds . 168, 169 It does not give the usual aldehyde reactions with Schiff s, Fehling s or Tollen s reagent ", nor does it form a cyanhydrin or take part in the Cannizzaro or Perkin reactions . ... 

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