Preservation (butylated hydroxytoluene).112 The coverage in this section is not intended to be complete, but is rather focused on representative cases where there are extensive pKa, E, and bond strength data. A reader interested in a particular substituted derivative that does not appear in Table 4 is encouraged to check the references cited there, and reference 56, as many of the primary papers cover a range of substituents.NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author ManuscriptChem Rev. Author manuscript; available in PMC 2011 December 8.Warren et al.Page5.2.1 Phenol (PhOH)–Phenol has been widely studied as the simplest of the aromatic hydroxylic compounds. The gas-phase O BDE in phenol has been a subject of much discussion.62,113,114 Heats of formation from the NIST Chemistry WebBook, Hf as(PhO? = 13 ?1 kcal mol- and Hf as(PhOH =-23.03 ?0.14 kcal mol-1, give BDEg(PhOH) = 88.0 ?1 kcal mol-1.49,70 This value is in between alternative values of 86.7 kcal mol-1 114 and 88.7 kcal mol-1.62 A clearer value for this important benchmark compound would be valuable. A wealth of thermochemical data is available for phenols, in particular their acidity [pKa(ArOH)] and the phenoxyl radical/phenoxide RR6MedChemExpress RR6 reduction [E?ArO?-)]. Protonated phenoxyl radicals are typically high energy species with aqueous pKa values > 0.115 The most extensive studies of E?ArO?-) are by Bordwell et al. for DMSO solutions116 and by Lind et al. and Steenken and Neta in aqueous media.117,118 The aqueous measurements take advantage of the phenol potential becoming independent of pH above its pKa (see Section 3.2 above). Phenols readily react by hydrogen atom transfer (HAT) and this pathway is implicated in the antioxidant properties of phenols both in vivo and in vitro (see below).119 For the more acidic phenols, or under basic conditions, a mechanism of sequential proton loss then HS-173 biological activity electron transfer (SPLET) can occur.11?213 It is less common for phenols to react by initial outer-sphere electron transfer because of the high E?PhOH?/0) potentials. The ArO? ArOH potentials (or, better, BDFEs) are often above the thermodynamic requirement for water oxidation, as is necessary for the function of Tyrosine Z in photosystem II, mediating hole transfer from the chlorophyll radical cation to the oxygen evolving complex. 5.2.2 2,4,6-Tri-tert-butylphenol (tBu3PhOH)–4-Substituted-2,6-di-tert-butyl-phenols are widely used in the research lab and as food preservatives, especially `butylated hydroxytoluene’ (BHT, 4-Me) and `butylated hydroxyanisole’ (BHA, 4-MeO). 2,4,6-tBu3PhOH is an especially interesting and useful reagent for studies of PCET reactions because of the exceptional stability of the phenoxyl radical (tBu3PhO?.120 The radical is easily prepared from the corresponding phenol using NaOH and K3Fe(CN)6, and can be isolated as dark blue crystals.120 As discussed for TEMPOH above, we have recently reevaluated the solution BDE of tBu3PhO?in C6H6 to account for recent revision of the thermochemistry of the originally used diphenylhydrazine/azobenzene couple.40 Our preferred value is 81.6 ?0.4 kcal mol-1. The tBu3PhO(?H) PCET couple is a very useful benchmark for the determination of bonds strengths in other phenols. The clearest example is Pedulli and co-workers’ EPR method to measure equilibrium constants for ArOH + tBu3PhO?121 Please note that here and in Table 4, we have slightly adjusted Pedulli’s reported BDEs to reflect our recent critical evaluation of the BDE.Preservation (butylated hydroxytoluene).112 The coverage in this section is not intended to be complete, but is rather focused on representative cases where there are extensive pKa, E, and bond strength data. A reader interested in a particular substituted derivative that does not appear in Table 4 is encouraged to check the references cited there, and reference 56, as many of the primary papers cover a range of substituents.NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author ManuscriptChem Rev. Author manuscript; available in PMC 2011 December 8.Warren et al.Page5.2.1 Phenol (PhOH)–Phenol has been widely studied as the simplest of the aromatic hydroxylic compounds. The gas-phase O BDE in phenol has been a subject of much discussion.62,113,114 Heats of formation from the NIST Chemistry WebBook, Hf as(PhO? = 13 ?1 kcal mol- and Hf as(PhOH =-23.03 ?0.14 kcal mol-1, give BDEg(PhOH) = 88.0 ?1 kcal mol-1.49,70 This value is in between alternative values of 86.7 kcal mol-1 114 and 88.7 kcal mol-1.62 A clearer value for this important benchmark compound would be valuable. A wealth of thermochemical data is available for phenols, in particular their acidity [pKa(ArOH)] and the phenoxyl radical/phenoxide reduction [E?ArO?-)]. Protonated phenoxyl radicals are typically high energy species with aqueous pKa values > 0.115 The most extensive studies of E?ArO?-) are by Bordwell et al. for DMSO solutions116 and by Lind et al. and Steenken and Neta in aqueous media.117,118 The aqueous measurements take advantage of the phenol potential becoming independent of pH above its pKa (see Section 3.2 above). Phenols readily react by hydrogen atom transfer (HAT) and this pathway is implicated in the antioxidant properties of phenols both in vivo and in vitro (see below).119 For the more acidic phenols, or under basic conditions, a mechanism of sequential proton loss then electron transfer (SPLET) can occur.11?213 It is less common for phenols to react by initial outer-sphere electron transfer because of the high E?PhOH?/0) potentials. The ArO? ArOH potentials (or, better, BDFEs) are often above the thermodynamic requirement for water oxidation, as is necessary for the function of Tyrosine Z in photosystem II, mediating hole transfer from the chlorophyll radical cation to the oxygen evolving complex. 5.2.2 2,4,6-Tri-tert-butylphenol (tBu3PhOH)–4-Substituted-2,6-di-tert-butyl-phenols are widely used in the research lab and as food preservatives, especially `butylated hydroxytoluene’ (BHT, 4-Me) and `butylated hydroxyanisole’ (BHA, 4-MeO). 2,4,6-tBu3PhOH is an especially interesting and useful reagent for studies of PCET reactions because of the exceptional stability of the phenoxyl radical (tBu3PhO?.120 The radical is easily prepared from the corresponding phenol using NaOH and K3Fe(CN)6, and can be isolated as dark blue crystals.120 As discussed for TEMPOH above, we have recently reevaluated the solution BDE of tBu3PhO?in C6H6 to account for recent revision of the thermochemistry of the originally used diphenylhydrazine/azobenzene couple.40 Our preferred value is 81.6 ?0.4 kcal mol-1. The tBu3PhO(?H) PCET couple is a very useful benchmark for the determination of bonds strengths in other phenols. The clearest example is Pedulli and co-workers’ EPR method to measure equilibrium constants for ArOH + tBu3PhO?121 Please note that here and in Table 4, we have slightly adjusted Pedulli’s reported BDEs to reflect our recent critical evaluation of the BDE.
