Effect of substitutions on thermal behavior of symmetric double Schiff bases of 1, 1’- bis (4-amino phenyl) cyclohexane

. Thermo gravimetric analyses of some symmetric double Schiff bases of 1, 1’- bis (4-amino phenyl) cyclohexane (SDSB-1 to SDSB-6) was carried out at 10 0 C/min in nitrogen atmosphere. Schiff bases are thermally stable up to 200-285 0 C and followed two step degradation kinetic. Various kinetic parameters such as order of degradation (n), energy of activation (E a ), frequency factor (A) and entropy change (ΔS) were derived accordingly Anderson-Freeman method interpreted in light of nature of the substituents. The nature, size and position of substituents affected thermal properties of the Schiff bases.


INTRODUCTION
Thermal methods of analyses are most widely used to study industrially important products such as polymers, pharmaceuticals, metals, minerals, alloys, clays and various metal complexes etc. Jin et. al [1] have reported the thermal stabilities of the polyimide derived from 1,1' bis (4-amino phenyl) cyclohexane and its derivatives with PMDA, BTDA and HFDA. Polyamides showed glass transition ranging from 290-372 0 C, stable up to 500 0 C and 30% residue at 800 0 C. The glass transition temperature was found to increase with increasing methyl groups in 1, 1' bis (4amino phenyl) cyclohexane because of steric restriction on the main chain rotation motion.
Schiff bases are useful as analytical reagents [2] fine chemicals, medical substrates and ligands for metal complexes having industrial importance as antifungal and biological applications [3]. They are also used as drug intermediates for the synthesis of drugs like antibiotics, antiallergic, antiphlogistic and antitumor To our best knowledge no work has been reported on thermal behaviour of substituted Schiff bases, which encouraged us to study effect of hydroxy, chloro, phenoxy, nitro, N,N'-dimethyl and styrene substituents on thermal degradation kinetics.

Materials
Solvents and chemicals used in the present investigation were of LR grade and used either as received or purified by fractional distillation. The Schiff bases were repeatedly recrystallized from appropriate solvent systems. 1, 1' Bis (4-aminophenyl) cyclohexane [Scheme-1] was synthesized and recrystallized according to reported work

Instrumentation and method
The Thermo gravimetric measurements were made on Perkin Elmer TGA Model No Pyris-I, at the heating rate of 10°C/min in nitrogen atmosphere.

RESULTS AND DISCUSSION
TG thermograms of SDSB-1 to SDSB-6 at the heating rate of 10°C/min in nitrogen atmosphere are presented in Figs.1 from which it is observed that SDSB-1 to SDSB-6 followed two steps decomposition reactions. Several temperature characteristic thermal properties such as initial decomposition temperature (IDT), temperature of 10 % weight loss (T 10 ), temperature of 50 % weight loss (T 50 ), final decomposition temperature (FDT), temperature of maximum weight loss (T max ) and the decomposition range were derived from these thermograms and are reported in Table  1. From which it is clear that Schiff bases are thermally stable up to about 200-285 0 C and different T max values of 333-380 0 C suggested different degradation mechanism. Observed thermal stability order is SDSB-5 > SDSB-3 > SDSB-4 = SDSB-6> SDSB-2 > SDSB-1.

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ILCPA Volume 63    From this trend it is concluded that the nature, the position (Ortho, Meta and Para) and size of substituents affected thermal properties considerably Associated kinetic parameters such as energy of activation (Ea), frequency factor (A), and order of reaction (n) were determined according to Anderson-Freeman method [13]. The least derived kinetic parameters are reported in Table 2. The entropy change (ΔS*) was determined at corresponding T max and also reported Table 2. Schiff bases followed either fraction or integral order of degradation kinetics. The first step decomposition of SDSB-2, SDSB-3, SDSB-4 and SDSB-6 followed fraction order (0.3, 0.2, 0.7 and 0.4) degradation kinetics while SDSB-1 and SDSB-5 followed third order degradation kinetics. The second step decomposition of SDSB-3 followed first order kinetics while both SDSB-4 and SDSB-5 followed second order degradation kinetics. The second step decomposition of SDSB-6 followed 0.3 order degradation kinetics. The magnitude of E a and A found different due to different nature and steric effect of the substituents. High magnitudes of these parameters suggest rigid nature of the Schiff bases.
A large and positive magnitudes of ΔS* confirmed less ordered transition states while negative magnitudes of ΔS* confirmed ordered transition state [13][14][15][16]. The side substituents are weak points in the Schiff bases which on heating forms free radicals which undergo further recombination, branching, rearrangement, cross linking reaction and further degrade at elevated temperatures.

CONCLUSION
Schiff bases possess fairly good thermal stability. The Schiff bases followed two step degradation kinetics either fraction order or integral order kinetics. The nature, size and position of the substituents affected thermal properties to considerably extent.