Studies about thermal decarboxylation of hydroxyl benzoic acids used in thermal imaging

Six isomers of dihydroxyl benzoic acid and three isomers of trihydroxyl benzoic acid were analysed systematically with thermogravimetric analysis. They, the starting temperatures (Ts), the peak temperatures (Tp) and the end temperatures (Te) in the decarboxylation reactions,were worked out based on the integral curves (TG) and the differential curves (DTG) in thermogravimetric analysis of these compounds. The carbon atoms of benzene rings connected with carboxyl groups of these compounds are numbered as C1, and the carbon atoms of carboxyl groups are numbered as C7. The chemical shift values of C1 (δ1) and C7 (δ7) are from the NMR spectra of these compounds, and it is found that there is a linear relation between Tp and δ1 or (δ7 - δ1). The net charge values of C1 (q1) and C7 (q7) are from the calculations of quantum chemistry. After compared δ1 and(δ7-δ1) with q1 and(q7-q1) respectively, it is also found that there are linear relations between them. These prove that there is a close relation among δf, qi and the reactivity of i-position on the benzene ring of a phenolic compound. ©1999TAPJ.