Cannizzaro reaction controlled

Several cases of spontaneous ignition after exposure to air of fine coke particles removed from filter strainers on a petroleum refinery furfural extraction unit have been noted. This has been associated with the use of sodium hydrogen carbonate (bicarbonate) injected into the plant for pH control, which produced a pH of 10.5 locally. This would tend to resinify the aldehyde, but there is also the possibility of a Cannizzaro reaction causing conversion of the aldehyde to furfuryl alcohol and furoic acid. The latter, together with other acidic products of autoxidation of the aldehyde, would tend to resinily the furfuryl alcohol. Pyrolysis GLC showed the presence of a significant proportion of furfuryl alcohol-derived resins in the coke. The latter is now discarded into drums of water, immediately after discharge from the strainers, to prevent further incidents. 

One obvious candidate for an electrophilic but non-enolisable compound is formaldehyde CH2=0 but it is simply too electrophilic to be well controlled. A trivial example is its reaction with acetaldehyde and hydroxide ion. The first aldol gives the expected product 43 but a second gives 44 and a third follows. Now hydroxide adds to another molecule of formaldehyde and delivers a hydride ion 45 in the Cannizzaro reaction (the other product is formate ion HCO2-) to give pentaerythritol 46, a useful compound in polymer chemistry for cross-linking but not much use to us. We need to moderate the unruly behaviour of this useful one-carbon electrophile. 

Potassium hydroxide Sterically controlled intramolecular Cannizzaro reaction ... 

This reaction continues independently. Fortunately, methanol (CH3OH) one of the byproducts of the Cannizzaro reaction, tends to shift the equilibrium of the reaction in Eq. (31.2) to the left and thus prevents the decrease of formaldehyde concentration by an unproductive reaction. By controlling the plating conditions properly, the wasteful consumption of formaldehyde by the Cannizzaro reaction can be retained within 10 percent of the consumption in the reaction in Eq. (31.1). 

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]. 

Branched-chain versus normal-chain selectivity is evidently a function of the aldose-ketose reaction equilibrium, and the driving force and degree of approach to that equilibrium this, in turn, is a function of the pH of the reaction and species concentrations, including that of the catalyst. Alditol-glycose selectivity is apparently controlled by the cross-Cannizzaro catalyst, as sodium hydroxide gives alditols, whereas calcium hydroxide affords glycoses. This, again, is probably more an effect of pH than of catalyst. 

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