Thermochemistry of 2,2,5,7,8-pentamethylchroman-6-ol (PMC) and 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (trolox)

Abstract

2,2,5,7,8-Pentamethylchroman-6-ol (PMC, CAS number 950-99-2) and 6-hydroxy-2,5,7,8-tetramethyl chroman-2-carboxylic acid (trolox, CAS number 53188-07-1) are perhaps the two most widely used model compounds for α-tocoferol, the main active antioxidant form of vitamin E.  Because the antioxidant activity of α-tocoferol, and of phenol derivatives (ArOH) in general (such as PMC and trolox), is intimately related to the breaking of the ArO−H bond, considerable efforts have been made to investigate how the energetics of that bond is linked to the structure and energetics of both the ArO· radical and the ArOH parent compound, which are reflected by the corresponding enthalpies of formation.  In this work the standard molar enthalpies of formation of crystalline PMC (triclinic, space group P1 phase) and trolox (monoclinic, space group P21/c phase), at T = 298.15 K, were obtained as ∆_fH^o_m(PMC, cr) = -(513.7±3.4) kJ/mol and ∆_fH^o_m(trolox,cr I) = -(892.5±4.1) kJ/mol by combustion calorimetry.  The results of drop-sublimation Calvet microcalorimetry experiments and vapour pressure versus temperature measurements by the Knudsen effusion method, combined with C^o_p,m (cr) and C^o_p,m (g) data obtained by differential scanning calorimetry and calculated at the B3LYP/6-31+G(d,p) level of theory, respectively, led to ∆_subH^o_m (PMC) = 107.4±0.8 kJ/mol and ∆_subH^o_m (trolox) = 136.9±2.5 kJ/mol at T = 298.15 K.  From the obtained ∆_subH^o_m and ∆_fH^o_m values it was possible to derive ∆_fH^o_m (PMC, g) = -(406.3±3.5) kJ/mol and ∆_fH^o_m (trolox, g) = -(755.6±4.8) kJ/mol at T = 298.15 K.  These results led to the conclusion that, in terms of the homodesmotic stabilization enthalpy criteria the replacement of a CH₃ group in PMC by a COOH group in trolox approximately conforms to group additivity with no significant contribution from the possible presence of a weak intramolecular hydrogen bond in trolox.

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