THERMAL DECOMPOSITION OF BENZALDEHYDE

When a few torr of NO is present, an inhibiting effect can be observed, while at NO pressures above 15 torr catalysis occurs Inhibition of the thermal decomposition of p-methyl- and p-chlorobenzaldehyde by NO has also been reported One of the features distinguishing the decomposition of benzaldehyde from that of other substances dealt with so far is that the inhibition curve is independent of the initial aldehyde concentration. Thus, it follows that one of the chain carriers decomposes within the chain cycle. 

Smith and Hinshelwood - interpreted the maximally inhibited reaction as the intramolecular decomposition of the aldehyde. As already mentioned, such a conclusion must be treated with reserve. The participation of free radicals has been confirmed experimentally by Ingold and Lossing , who identified the phenyl and benzoyl radicals by mass-spectrometry. 

On the basis of their experimental results. Smith and Hinshelwood posed the following scheme 

From the first mechanism, a first-order rate equation can be deduced, which was found to be valid by Smith and Hinshelwood in the pressure range 100-300 torr. On energetic grounds, however, initiation reaction (5) is more probable. Detection of the phenyl and benzoyl radicals also favours the latter mechanism. One has to admit that experiental data available at present do not render possible the setting up of a detailed reaction scheme. 

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Thermal decomposition of this salt also provides heptafulvalene [247]. It is interesting that (XXXI) also arises from thermal decomposition of benzaldehyde tosylhydrazone presumably the phenylcarbene which is generated rearranges to cycloheptatrienylidene [247,248]. Other ways of generating heptafulvalene include the reaction between trimethylsilyltropylium tetrafluoroborate and tetra-n-butylammonium fluoride [249], and the thermal decomposition of 7-acetoxycyclohepta-triene or 7-acetoxynorbornadiene [250]. 

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