Solvent Effects on Homolytic Bond Dissociation Energies of Hydroxylic Acids

Abstract

The homolytic bond dissociation energies (BDEs) of the O−H bonds in DMSO solution for (a) phenol and a number of its derivatives, (b) three oximes, (c) three alcohols, (d) three hydroxylamines, and (e) two hydroxamic acids have been estimated by eq 1: BDEHA = 1.37pK HA + 23.1E ox(A-) + 73.3 kcal/mol. For most of these hydroxylic acids, the BDEs of the O−H bonds estimated by eq 1 are within ±2 kcal/mol of the literature values in nonpolar solvents or in the gas phase. There is no reason to believe, therefore, that these BDEs are “seriously in error because of failure to correct for solvent effects” as has been claimed on the basis that BDEs in highly polar solvents estimated for the O−H bond in phenol by photoacoustic calorimetry must be so corrected.

Formation of Ammonia from an Iron Nitrido Complex

“…Complex 1 was found to react with PhCH 2 SH (pK a = 15.4 in DMSO) [18] but not with phenol (pK a = 18.0 in DMSO). [18] Therefore proton transfer from TEMPO-H (pK a = 31.0 in DMSO) [19] to 1 is also thermodynamically uphill, with DG PT > 18 kcal mol À1 . The free energy for hydrogen-atom transfer to 1 was estimated in a similar manner.…”

Determination of Bond Dissociation Enthalpies in Solution by Photoacoustic Calorimetry

“…Braslavski and Heibel 19 suggested a time resolution (τ eff ) of 1.47τ a or 2τ a : all heat released within this time is integrated and included in the prompt heat release f obs . For a laser beam width of 1 mm and υ a varying from 900 to 1400 m s -1 in organic solvents, τ eff is around 10 -6 s. Hence, the lifetime for the tert-butoxyl radical in the reaction sequence employed, 1/(k 8 [RH]), should be at most 0.2τ eff to ensure that more than 99% of the reaction heat is detected. By varying τ eff (i.e., changing the laser beam radius), one can cause different parts of the exponential decay to fall within the prompt heat domain which can be used to determine the reaction time scales.…”

“…Despite their importance, BDEs in more complex species are more difficult to measure and thus less reliable. 7 Experimental approaches for the direct and indirect determination of BDEs in the gas phase 4,7 or in solution (electrochemical cycles 8 and PAC) as well as new computational methods such as density functional theory 9 have led to a dramatic increase in the number of reliable BDEs for organic and organometallic compounds. For consistency, all of the data are normally extrapolated to standard state conditions (i.e., 298 K in the gas phase).…”

Pharmacokinetics and Metabolism of the Reactive Oxygen Scavenger α-Phenyl-N-tert-butylnitrone in the Male Sprague-Dawley Rat

“…Phenyl-N-tert-butylhydroxamic acid was prepared via a slight modification of a previously published method (Bordwell and Liu, 1996). Benzoyl chloride (2.4 ml, 20 mmol) was added dropwise to N-tert-butylhydroxyl amine ⅐ HCl salt (1.1 g, 10 mmol) and triethyl amine (5.6 ml, 40 mmol) in anhydrous acetonitrile (20 ml) at 0°C under nitrogen.…”

“…The crude hydroxamic acid was recrystallized from ethyl acetate/hexanes (5:95) to afford the desired product as a light brown solid (1.4 g, 70%). Melting point 112 to 113°C (Lit melting point ϭ 110 to 113°C; Bordwell and Liu, 1996).…”