Comparative thermal stability studies of some isomeric phenolic β-diketones and their phenolic pyrazoles by thermogravimetric analysis

Mehwash Zia, Muhammad Zia-ul-Haq


The thermal degradation and thermal stability of some isomeric phenolic β-diketones I(a-b) their phenolic pyrazoles II(a-b) and III(a-b) were studied with thermogravimetric analysis TGA. The isomeric phenolic β-diketones I(a-b) were synthesized via modified Baker-Venkataraman rearrangement. Isomeric β-diketones were converted to their phenolic pyrazoles II(a-b) and III(a-b) on reaction with phenyl hydrazine. Factors effecting thermal stability based on the initial decomposition temperature (IDT), temperature of maximum weight loss (Tmax), decomposition temperature ranges and activation energy (Ea) of the decomposition reaction were studied. Activation energy, enthalpy and entropy of all the synthesized products were calculated by Horowitz and Metzger’s method, an integral method for the determination of reaction order from the thermogravimetric data. On the basis of IDT, ortho isomer of phenolic β-diketones (Ib) was found to be more stable than that of its para counterpart (Ia). Among the phenolic pyrazoles, compound IIIa exhibited the highest initial decomposition temperature (IDT) followed by IIb, IIa and IIIb (in decreasing order). 


Phenolic β-diketones; phenolic pyrazole; thermal stability; thermogravimetric analysis; activation energy.

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