Abnormal solvent effects on hydrogen atom abstractions. 1. The reactions of phenols with 2,2-diphenyl-1-picrylhydrazyl (dpph*) in alcohols

Rate constants, k(ArOH/dpph*)(S), for hydrogen atom abstraction from 13 hindered and nonhindered phenols by the diphenylpicrylhydrazyl radical, dpph*, have been determined in n-heptane and a number of alcoholic and nonalcoholic, hydrogen-bond accepting solvents. Abnormally enhanced k(ArOH/dpph*)(S) values of have been observed in alcohols. It is proposed that this is due to partial ionization of the phenols and a very fast electron transfer from phenoxide anion to dpph*. The popular assessment of the antioxidant activities of phenols with dpph* in alcohol solvents will generally lead to an overestimation of their activities.     

Absolute rate constants for some hydrogen atom abstraction reactions by a primary fluoroalkyl radical in water

A combination of laser flash photolysis and competitive kinetic methods have been used to measure the absolute bimolecular rate constants for hydrogen atom abstraction in water from a variety of organic substrates including alcohols, ethers, and carboxylic acids by the perfluoroalkyl radical, *CF(2)CF(2)OCF(2)CF(2)SO(3)(-) Na(+). Comparison, where possible, of these rate constants with those previously measured for analogous reactions in the non-polar organic solvent, 1,3-bis(trifluoromethyl)benzene (J. Am. Chem. Soc, 1999, 121, 7335) show that the alcohols react 2-5 times more rapidly in the water solvent and that the ethers react at the same rate in both solvents. A transition state for hydrogen abstraction that is more reminiscent of an "intimate ion pair" than a "solvent separated ion pair" is invoked to explain these modest solvent effects.     

The frequently overlooked importance of solvent in free radical syntheses

This tutorial review is designed to dispel the myth, still believed by many synthetic organic chemists, that radical-based syntheses are free from significant solvent effects. However, many synthetically valuable radical reactions do exhibit large kinetic solvent effects. It is therefore important to select the solvent for any proposed radical synthesis with considerable care if good product yields are to be achieved.     

Another Unprecedented Wieland Mechanism Confirmed: Hydrogen Formation from Hydrogen Peroxide,Formaldehyde, and Sodium Hydroxide

In 1923, Wieland and Wingler reported that in the molecular hydrogen producing reaction of hydrogen peroxide with formaldehyde in basic solution, free hydrogen atoms (H^.) are not involved. They postulated that bis(hydroxymethyl)peroxide, HOCH₂OOCH₂OH, is the intermediate, which decomposes to yield H₂ and formate, proposing a mechanism that would nowadays be considered as a “concerted process”. Since then, several other (conflicting) “mechanisms” have been suggested. Our NMR and Raman spectroscopic and kinetic studies, particularly the determination of the deuterium kinetic isotope effect (DKIE), now confirm that in this base‐dependent reaction, both H atoms of H₂ derive from the CH₂ hydrogen atoms of formaldehyde, and not from the OH groups of HOCH₂OOCH₂OH or from water. Quantum‐chemical CBS‐QB3 and W1BD computations show that H₂ release proceeds through a concerted process, which is strongly accelerated by double deprotonation of HOCH₂OOCH₂OH, thereby ruling out a free radical pathway.     

Thermogravimetric Investigation of Antioxidant Activity of Selected Compounds in Lipid Oxidation

Uninhibited and inhibited autooxidation of ethyl linoleate(LnEt) in bulk phase initiated by 0.04 M azoisobutyronitrile (AIBN) was investigated by thermogravimetry in isothermal mode at 35, 40 and 50°C. LnEt oxidation was inhibited by:2,6-di-t-butyl-4-methylphenol (BHT), 2-t-butyl-6-methylphenol (BMP),2-hydroxyphenylacetic acid (PAA), 2-hydroxyacetophenone (HAP), α-tocopherol,2,2'-methylene-bis-(4-methyl-6-t-butylphenol) (BIS), caffeic acid andβ-carotene. All investigated compounds showed antioxidant activity measured by induction time, values of rate of oxidation during induction period (R _inh) and values of kinetic chain length ν. At lower temperatures the monohydroxyphenols are more efficient inhibitors than dihydroxyphenols while at 50°C dihydroxyphenols have better antioxidant efficiency. This revised version was published online in July 2006 with corrections to the Cover Date.     

Autooxidation of Unsaturated Fatty Acids and Their Esters

The shape of DSC curves of non-isothermal oxidation of fats was explained. Two main exothermic effects overlapped partially are caused by hydroperoxide formation (first peak) and by further oxidation of peroxides (second peak). The oxidation of oils and lipid analogues of various peroxide concentration showed that only the start of the oxidation process is affected by initial concentration of peroxides, other temperatures determined from DSC curves are not connected with this parameter. The computer simulations gave the best agreement of theoretical and experimental data for kinetic scheme of a two-step consecutive reaction with autocatalytic start. The comparison of activation energies calculated for isothermal and non-isothermal autooxidation of unsaturated fatty acids and their esters also confirmed this interpretation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Abnormal solvent effects on hydrogen atom abstraction. 2. Resolution of the curcumin antioxidant controversy. The role of sequential proton loss electron transfer

The rates of reaction of 1,1-diphenyl-2-picrylhydrazyl (dpph*) radicals with curcumin (CU, 1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione), dehydrozingerone (DHZ, "half-curcumin"), and isoeugenol (IE) have been measured in methanol and ethanol and in two non-hydroxylic solvents, dioxane and ethyl acetate, which have about the same hydrogen-bond-accepting abilities as the alcohols. The reactions of all three substrates are orders of magnitude faster in the alcohols, but these high rates can be suppressed to values essentially equal to those in the two non-hydroxylic solvents by the addition of acetic acid. The fast reactions in alcohols are attributed to the reaction of dpph* with the CU, DHZ, and IE anions (see J. Org. Chem. 2003, 68, 3433), a process which we herein name sequential proton loss electron transfer (SPLET). The most acidic group in CU is the central keto-enol moiety. Following CU's ionization to a monoanion, ET from the [-(O)CCHC(O)-](-) moiety to dpph* yields the neutral [-(O)CCHC(O)-]* radical moiety which will be strongly electron withdrawing. Consequently, a phenolic proton is quickly lost into the alcohol solvent. The phenoxide anion so formed undergoes charge migration to produce a neutral phenoxyl radical and the keto-enol anion, i.e., the same product as would be formed by a hydrogen atom transfer (HAT) from the phenolic group of the CU monoanion. The SPLET process cannot occur in a nonionizing solvent. The controversy as to whether the central keto-enol moiety or the peripheral phenolic hydroxyl groups of CU are involved in its radical trapping (antioxidant) activity is therefore resolved. In ionizing solvents, electron-deficient radicals will react with CU by a rapid SPLET process but in nonionizing solvents, or in the presence of acid, they will react by a slower HAT process involving one of the phenolic hydroxyl groups.     

DSC study of linolenic acid autoxidation inhibited by BHT,dehydrozingerone and olivetol

Non-isothermal oxidation of linolenic acid (LNA) in bulk phase was monitored by differential scanning calorimetry. The kinetic parameters E _a, Z and k (activation energies, pre-exponential factors, and rate constants, respectively) were calculated by Ozawa-Flynn-Wall method for the first detectable exothermic effect of uninhibited LNA oxidation. The kinetic parameters were also calculated for LNA oxidation inhibited by 2,6-di-tert-butyl-4-methylphenol (BHT), and two natural compounds, 1,3-dihydroxy-5-pentylbenzene (olivetol), and 4-(4’-hydroxy-3’-methoxyphenyl)-3-buten-2-one (DHZ, dehydrozingerone) at various concentrations. For oxidation processes at 25, 90 and 180°C the plots of logk values vs. concentration of phenolic compounds indicated that optimal concentration of inhibitor determined for one particular temperature cannot be extrapolated to other temperatures.