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The thermal decomposition of fluorinated esters

✍ Scribed by P. G. Blake; B. F. Shraydeh

Publisher
John Wiley and Sons
Year
1981
Tongue
English
Weight
446 KB
Volume
13
Category
Article
ISSN
0538-8066

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✦ Synopsis

Abstract

The kinetics of thermal decomposition of ethyl, isopropyl, and t‐butyl trifluoroacetates have been studied in the gas phase. In each case initial decomposition follows the normal ester route to give an olefin and trifluoroacetic acid, and elimination of hydrogen fluoride does not occur. However, trifluoroacetic acid is thermally unstable at ethyl and isopropyl ester decomposition temperatures, and further products result, including those from the difluorocarbene produced by decomposing trifluoroacetic acid. Placing a CF~3~ group at an ester's γ carbon increases the polarity of its transition state and decreases its thermal stability. The activation energies of the ethyl and isopropyl esters are lowered by 3.8 and 4.7 kcal/mol compared to the corresponding acetates, and the primary decomposition kinetics, which are homogeneous and of the first order, are expressed by
equation image
α‐Methylation enhances the reactivity of the trifluoroacetates, and the t‐butyl ester, the transition state for which is sufficiently polar for heterogeneous decomposition to occur, shows signs of thermal instability at room temperature. The equilibrium
equation image
was also investigated and gave Δ__H__° = +13,580 cal/mol and Δ__S__° = +31.07 gibbs/mol in the forward direction. The results obtained extend and support the known structure–rate correlations in the gas‐phase elimination of esters.

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