Investigation of the influence of hydroxy groups on the radical scavenging ability of polyphenols.
Recently, O-H bond dissociation enthalpies (BDEs) have been successfully used to express the free radical scavenging ability of polyphenolic antioxidants. In this work, the BDEs of phenol, catechol, resorcinol, hydroquinone, pyrogallol, phloroglucinol, 1,2,4-benzenetriol, and 5-hydroxypyrogallol have been calculated at B3LYP/6-311G++(3df, 3pd) and used to elucidate the effect of OH groups. Increasing the number of OH groups in the adjacent (vicinal) position decreases the BDE of phenols. Increasing the number of O-H groups in the alternative position C(1,3) as in resorcinol and C(1,3,5) as in phloroglucinol does not show any notable change in the BDEs when compared to that of OH in C(1) as in phenol. 5-Hydroxypyrogallol has the smallest BDE (250.3 kJ mol(-1)) followed by pyrogallol (289.4 kJ mol(-1)), then 1,2,4-benzenetriol (294.8 kJ mol(-1)), and then catechol (312.8 kJ mol(-1)). Overall, our results indicated that the presence of ortho and para hydroxy groups reduces the BDEs. An intramolecular hydrogen bond (IHB) develops due to the ortho arrangement of OH's and plays a dominant role in decreasing the BDEs. This key study on phenols showed that the reactive order of OH position in the benzene ring is the following: 5-hydroxypyrogallol > pyrogallol > 1,2,4-benzenetriol > catechol > hydroquinone >> phenol approximately resorcinol approximately phloroglucinol.
Thavasi
Velmurugan
V
Department of Chemistry, National University of Singapore, 3, Science drive 3, Singapore 117543.
Leong
Lai Peng
LP
Bettens
Ryan Phillip Anthony
RP
D000975:Antioxidants / Q000737:chemistry; D003198:Computer Simulation; D005419:Flavonoids / Q000737:chemistry*; D016166:Free Radical Scavengers / Q000737:chemistry*; D005609:Free Radicals / Q000737:chemistry; D006859:Hydrogen / Q000737:chemistry*; D006900:Hydroxylation; D015394:Molecular Structure; D010100:Oxygen / Q000737:chemistry; D010636:Phenols / Q000737:chemistry*; D059808:Polyphenols; D013816:Thermodynamics
D000975:Antioxidants; D005419:Flavonoids; D016166:Free Radical Scavengers; D005609:Free Radicals; D010636:Phenols; D059808:Polyphenols; D006859:Hydrogen; D010100:Oxygen