Concentration carbonyl compounds, effect

The reaction can, however, be made preparative for (91) by a continuous distillation/siphoning process in a Soxhlet apparatus equilibrium is effected in hot propanone over solid Ba(OH)2 (as base catalyst), the equilibrium mixture [containing 2% (91)] is then siphoned off. This mixture is then distilled back on to the Ba(OH)2, but only propanone (b.p. 56°) will distil out, the 2% of 2-methyl-2-hydroxypentan-4-one ( diacetone alcohol , 91, b.p. 164°) being left behind. A second siphoning will add a further 2% equilibrium s worth of (91) to the first 2%, and more or less total conversion of (90) — (91) can thus ultimately be effected. These poor aldol reactions can, however, be accomplished very much more readily under acid catalysis. The acid promotes the formation of an ambient concentration of the enol form (93) of, for example, propanone (90), and this undergoes attack by the protonated form of a second molecule of carbonyl compound, a carbocation (94) ... 

Vardanyan [65,66] discovered the phenomenon of CL in the reaction of peroxyl radicals with the aminyl radical. In the process of liquid-phase oxidation, CL results from the disproportionation reactions of primary and secondary peroxyl radicals, giving rise to trip-let-excited carbonyl compounds (see Chapter 2). The addition of an inhibitor reduces the concentration of peroxyl radicals and, hence, the rate of R02 disproportionation and the intensity of CL. As the inhibitor is consumed in the oxidized hydrocarbon the initial level of CL is recovered. On the other hand, the addition of primary and secondary aromatic amines to chlorobenzene containing some amounts of alcohols, esters, ethers, or water enhances the CL by 1.5 to 7 times [66]. This effect is probably due to the reaction of peroxyl radicals with the aminyl radical, since the addition of phenol to the reaction mixture under these conditions must extinguish CL. Indeed, the fast exchange reaction... 

In the absence of such activation, weak nucleophiles, e.g. H2O , may react only very slowly, but strong ones, e.g. CN, do not require such aid. Additions may also be base-catalysed, the base acting by converting the weak nucleophile HY into the stronger one, Y , e.g. HCN - -base— CN. Further, while acids may activate the carbonyl carbon atom to nucleophilic attack, they may simultaneously reduce the effective concentration of the nucleophile, e.g. CN -1- HA — HCN -f-A , RNHj + HA— RNHj -t- A . Many simple addition reactions of carbonyl compounds are thus found to have an optimum pH this can be of great importance for preparative purposes. 

One possible explanation for the lack of correspondence between emitting and reacting states is reaction of the singlet. In the case of benzophenone, there is little question that the reaction involves only the triplet state, since triplet quenchers can effectively inhibit the reaction. This need not be the case with all carbonyl compounds. For example, it has been shown that both the n,ir singlet and triplet states can be involved in the Norrish type II cleavage of alkyl ketones (25-27). At high concentrations, piperylene quenches only that part of the 2-hexanone cleavage which arises from the triplet. The rate constants for... 

Loss of the carbonyl component was measured as an indication of its reaction with BPA. Results are illustrated as functions of the various parameters in Tables I and II. At each concentration level of carbonyl compound, increasing temperatures were accompanied by increases in loss of the carbonyl component. Increasing the lysine MGA to 100 1, in effect creating conditions for a first order reaction, decreased the influence of temperature, yet even at 20°C more than 75% of the MGA reacted. 

Flavor threshold values for carbonyl compounds are influenced by factors such as the number of carbon atoms, degree and location of unsaturated double bonds, isomerism, additive and/or antagonistic effects of mixtures of carbonyls and the medium in which the carbonyls exist (Day et al., 1963 Meijboom, 1964). Many carbonyls have up to 100 times greater flavor potency in an aqueous medium (e.g., liquid milk) than in a fat or oil (e.g., butteroil). Hence, off-flavor tends to be noticeable at lower concentrations of carbonyl compounds in liquid milk than in butter. 

Day, E.A., Lillard, D.A., Montgomery, M.W. 1963. Auto-oxidation of milk lipids. III. Effect of flavor of the additive interactions of carbonyl compounds at sub-threshold concentrations.. / Dairy Sci. 46, 291-294. 

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