Photochemical initiated carboxylic acid decomposition

Decomposition of chloroformic acid formed photochemically (see below) was studied in the temperature range 288-343 °K. The mechanism shown below was suggested. 

It was suggested that the observed activation energy, = 10 exp (-14,000/1 7 ) see , indicated that the rate limiting step was cw- trans isomerization (i.e., the reaction coordinate is an internal rotation), viz. 

The observed rotational barrier in formic acid is 17 kcal.mole , and this corresponds reasonably well with the observed activation energy (14 kcal.mole ). From an analysis of the reaction energetics it was concluded that BrCOOH, formed photochemically, should also decompose rapidly by a similar four-center mechanism, whereas HCOOH and FCOOH should be relatively stable. 

The first-order decarboxylation of acids will be considered in this section. As it was indicated in the introduction, the effect of solvent imposes a difficulty in the 

Of the acids for which kinetics of decarboxylation have been reported, those that have an electronegative site two (Y) or three atoms (Z) removed from the carboxyl group (ZYX-COOH) undergo facile decarboxylation. In addition, the capability of developing a double bond between atoms X and Y in the transition state also facilitates decarboxylation. 

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