In aqueous Solutions in the presence of carbon monoxide, the Fenton-like reactions between Fe(H2O)32+ and alkyl hydroperoxides (RC(CH3)2OOH, R = CH3, C2H5) readily produce acyl radicals, R&bull;CO. For R = CH3, the resulting acetyl radicals reduce Fe(H2O)63+, k = 6 × 105 mol-1 dm3 s-1. The reaction of Fe(H2O)63+ with &bull;CH3 is much slower, k &asymp; 4 × 103 mol-1 dm3 s-1. Halide ions have an accelerating effect and lead to the formation of halomethanes. The rate constant for the reaction Fe(H2O)5Br2+ + #CH3 -> CH3Br + Fe(H2O)62+ is greater than 108 mol-1 dm3 s-1

Andreja Bakač

English

Hrčak - Portal of scientific journals of Croatia

ISSN-0011-1643CCA-2756 Original Scientific PaperEffects of Halide Ions and Carbon Monoxideon the Reactions of Iron(III) with Alkyl Radicals*Andreja Baka~Ames Laboratory, Iowa State University, Ames, Iowa 50011, USA(E-mail: bakac@ameslab.gov)Received September 22, 2000; revised March 26, 2001; accepted April 2, 2001In aqueous solutions in the presence of carbon monoxide, the Fen-ton-like reactions between Fe(H2O)62+ and alkyl hydroperoxides(RC(CH3)2OOH, R = CH3, C2H5) readily produce acyl radicals, RCO.For R = CH3, the resulting acetyl radicals reduce Fe(H2O)63+, k =6 105 mol–1 dm3 s–1. The reaction of Fe(H2O)63+ with •CH3 is muchslower, k  4 103 mol–1 dm3 s–1. Halide ions have an acceleratingeffect and lead to the formation of halomethanes. The rate constantfor the reaction Fe(H2O)5Br2+ + •CH3  CH3Br + Fe(H2O)62+ isgreater than 108 mol–1 dm3 s–1.Key words: carbon monoxide, Fenton reaction, alkyl hydroperoxide,iron, alkyl radicals.INTRODUCTIONTransition metal complexes react with alkyl radicals by a variety of me-chanisms, including electron transfer, atom transfer, and metal-carbon bondformation. Because of the transient nature of most radicals, which typicallydisappear in self-reactions at close to diffusion-controlled rates, only fast re-actions with chosen substrates can compete.Despite the reasonably high reduction potential for the couple Fe(H2O)63+ /Fe(H2O)62+, the ion Fe(H2O)63+ does not react readily with alkyl radicals, pre-sumably because of the lack of a suitable mechanism. The Fenton-like reac-tions between Fe(H2O)62+ and alkyl hydroperoxides of the type RC(CH3)2OOHCROATICA CHEMICA ACTA CCACAA 74 (3) 633¿640 (2001)* Dedicated to Professor Smiljko A{perger on the occasion of his 80th birthday.thus proceed with an overall 1:1 stoichiometry as shown for tert-butylhydro-peroxide ((CH3)3COOH or t-BuOOH) in Eqs. (1) to (3).Fe2+ + (CH3)3COOH  FeOH2+ + (CH3)3CO• (kFe = 18.7 mol–1 dm3 s–1)1 (1)(CH3)3CO••CH3 + (CH3)2CO (k = 1.4  106 s–1)2 (2)2 •CH3  C2H6 (k  109 mol–1 dm3 s–1)3 (3)In this paper we explore the effect of two types of reagents – halide ionsand carbon monoxide – on the mechanism and stoichiometry of the reactionsof Fe(H2O)62+ with (CH3)3COOH and C2H5C(CH3)2OOH. The role of addedreagents is to modify either the metal (by complex formation with halideions) or the alkyl radicals (by addition of CO) to facilitate the FeIII-radicalreaction and make it competitive with the self-reaction of Eq. (3).EXPERIMENTALtert-Butylhydroperoxide (Aldrich) and carbon monoxide (Air Products) were used asreceived. C2H5C(CH3)2OOH was synthesized according to a literature procedure.4 Solu-tions of Fe(H2O)62+ were prepared by the reduction of Fe(H2O)63+ with zinc amalgam.Organic products (hydrocarbons, biacetyl, and alkyl halides) were analyzed byuse of a Hewlett-Packard 5790 A gas chromatograph equipped with a FID detectorand packed VZ-10, OV-101, and Porapak-Q columns (Alltech). In most of the experi-ments the samples were withdrawn from the head space in the spectrophotometriccell after the completion of the reaction. Biacetyl was determined quantitatively byinjecting 1–2 microliters of liquid samples onto the column of Porapak-Q. Despite aserious effort, we were not successful in detecting acetic acid, the presumed productof the reaction between Fe(H2O)63+ and acetyl radicals. Standards, including somewith much higher concentrations of CH3COOH than those expected in the samples,also produced no observable peaks in gas chromatograms.Kinetic measurements were conducted at 240 nm, where Fe(H2O)63+ has an absorp-tion maximum,  = 4160 mol–1 dm3 cm–1. The amount of Fe(H2O)63+ produced in eachkinetic run was calculated from the absorbance change and related to the initial con-centration of the hydroperoxide. These data, combined with a reasonable estimatefor the rate constant for radical self-reactions, were used to calculate the rate con-stant for the reaction of Fe(H2O)63+ with CH3CO/CH3C(OH)2.RESULTS AND DISCUSSIONUnder typical conditions (Fe2+ = (0.5–5)  10–3 mol dm–3, t-BuOOH =(0.5–1)  10–4 mol dm–3, 0.1 mol dm–3 HClO4, argon atmosphere, 25 °C),Fe(H2O)62+ reacts with t-BuOOH as in Eqs. (1) to (3). Less than 3% of •CH3634 A. BAKA^produced in reaction (2) engages in hydrogen atom abstraction from thehydroperoxide, Eq. (4).1•CH3 + (CH3)3COOH  CH4 + (CH3)3COO• (4)The 1:1 stoichiometry, required by Eqs. (1–3), is obeyed closely, as shownby the measured yields of Fe(H2O)63+ in Table I. The yields get smallerwhen large concentrations of Fe(H2O)63+ are present initially. This situationwill be discussed later.Effect of COWhen the argon atmosphere was replaced by CO, the yields of hydrocar-bons (CH4 + C2H6) and Fe(H2O)63+ decreased, and the apparent rate constantincreased, Table I. A new peak, corresponding to biacetyl, appeared in gaschromatograms. These observations are consistent with the formation of acetylradicals and their subsequent self-reactions and oxidation by Fe(H2O)63+,Eqs. (5) to (8). Using kCO = 2.0  106 mol–1 dm3 s–1,5 we calculate that >95%of •CH3, produced in reactions (1–2), reacts with CO in CO-saturated solu-tions (0.96 mmol dm–3 at 1 atm and 25 °C).6 The reaction with Fe(H2O)63+may involve either CH3CO or the hydrated form, CH3C(OH)2, produced rap-idly in aqueous solutions (k6  104 s–1).5,7•CH3 + CO  CH3CO kCO (5)CH3CO + H2OM CH3C(OH)2 K  17 (6)CH3CO/CH3C(OH)2 + Fe3+ CH3COOH + Fe2+ (7)CH3CO/CH3C(OH)2  CH3COCOCH3 + CH3CHO + CH3COOH	 (8)The oxidation by Fe(H2O)63+ and self-reactions of acetyl radicals are com-petitive processes in the absence of significant amounts of externally addedFe(H2O)63+. The kinetic effect of CO on Fe(H2O)62+/t-BuOOH reaction, moni-tored at an absorption maximum for Fe(H2O)63+ at 240 nm, Table I, is thuscaused by gradual decrease in CH3COss (and increased importance of reac-tion (7) relative to reaction (8)) as the reaction proceeds. In the later stagesthe greater fraction of the radicals reacts with Fe(H2O)63+, leading to a de-crease in the rate of growth of Fe(H2O)63+ and producing unrealisticallyhigh values of k1. Because of the subtle interplay between reactions (1), (7)and (8), the kinetic curves do not deviate significantly from those expectedIRON(III) AND ALKYL RADICALS 635636 A. BAKA^TABLEIEffectofCOonthekineticsandproductsofthereactionbetweenFe(H2O) 62+andt-BuOOHaconc./mmoldm–3AtmosphereFe3+b/mmoldm–3k/s–1Fe2+0t-BuOOH0Fe3+0cobs.calc.dobs.calc.d1.00.100.01Ar0.1070.100.01820.0178CO0.0530.0520.02910.02961.00.100.089CO0.0400.0300.0310.0331.00.101.0Ar0.0890.01880.0178COv.small0.500.100.006Ar0.1030.100.008550.00842CO0.0450.0490.01610.01620.500.100.053Ar0.0990.100.008560.00842CO0.0330.0300.01580.01620.500.04670.014CO0.0210.0200.01710.0158aConditions:0.10moldm–3HClO4,25°C,solutionssaturatedwitheitherArorCO(0.96mmoldm–3).bConcentrationofFe(H2O) 63+producedduringthereaction,calculatedfromtheabsorbancechangeat240nm.cIntroducedwithFe(H2O) 62+solutionoraddeddeliberately.dObtainedfromthetracessimulatedwithk1=18.7mol–1dm3s–1 ,k7=6105mol–1dm3s–1 ,andk8=1109mol–1dm3s–1 .for a first order product build-up, but the calculated rate constant is nolonger the pure k1, as shown in the kinetic simulations described below.Consistent with the proposed mechanism, the build-up of Fe(H2O)63+ isdiminished when small amounts of Fe(H2O)63+ are initially present in thereaction mixtures. In an experiment having Fe(H2O)63+0 = Fe(H2O)62+0 =1 mmol dm–3 and t-BuOOH0 = 0.10 mmol dm–3, there was no absorbancechange at 270 nm (Fe3+ = 1960 mol–1 dm3 cm–1), the shortest wavelength thathad an acceptably low background absorbance caused by added Fe(H2O)63+.This result shows that the reaction has become purely catalytic such thatthe formation of Fe(H2O)63+, Eq. (1), and the consumption, Eq. (7), canceleach other out.Experiments with large amounts of added Fe(H2O)63+ provide convincingsupport for the proposed mechanism, but are not useful for the determinationof k7. This determination relies on the competition between reactions (7) and(8) and had to be carried out at low Fe(H2O)63+. As a result, Fe(H2O)63+varied during each experiment that was designed for kinetic purposes. Foreach run the calculations, Eq. (9), used the observed increase in Fe(H2O)63+,denoted as Fe3+, as a measure of CH3CO that disappeared in self-reac-tions, and an average concentration of Fe(H2O)63+ (Fe3+av) to convert theapproximate pseudo-first order rate constants into second order rate cons-tants. The concentration of acetyl radicals was calculated by use of a steadystate approximation and taking k8 = 1  109 mol–1 dm3 s–1, a value typicalfor aliphatic radicals. These calculations yielded an estimate for k7 of  106mol–1 dm3 s–1.k7 =2 8 3333kt +++[ ][ ] D[ ]D[ ][ ]CH COBuOOH FeFeFeSS0av (9)Kinetic simulations were carried out using the program Chemical Kinet-ics Simulator (IBM). The initial concentrations of all the reactants and therate constants k1 (18.7 mol–1 dm3 s–1) and k8 (1  109 mol–1 dm3 s–1) werefixed, and k7 was floated until the calculated rate constants and absorbancechanges matched the observed values. The best agreement was obtainedwith k7 = 6  105 mol–1 dm3 s–1, reasonably close to the value of  106, esti-mated by use of Eq. (9). Table I lists the agreement between the measuredand calculated values for kobs and Fe3+.From the reduced yield of Fe(H2O)63+ in a single experiment under argonin the presence of added Fe(H2O)63+ (1 mmol dm–3), we calculate k  4  103mol–1 dm3 s–1 for the reaction between •CH3 and Fe(H2O)63+. The keto form ofacetyl radicals, CH3CO, is probably not much more reactive than •CH3 towardFe(H2O)63+, which leads us to suggest that the reduction of Fe(H2O)63+ inIRON(III) AND ALKYL RADICALS 637the Fe(H2O)62+/t-BuOOH/CO reaction is carried out by CH3C(OH)2. The greatincrease in the reducing power of acetyl radicals upon hydration has beendemonstrated previously.5,7In the reaction between C2H5C(CH3)2OOH and Fe(H2O)62+, which gener-ates ethyl radicals in a scheme analogous to Eqs. (1–2), the combined yieldsof hydrocarbons produced in the self-reactions (ethane, ethene and butane)are significantly lower under CO than under argon. An experiment having1.5 mmol dm–3 Fe(H2O)62+ and 0.6 mmol dm–3 C2H5C(CH3)2OOH in 0.2 moldm–3 HClO4 was run at 0 °C to increase the solubility of CO to 1.6 mmoldm–3.6 The total yield of hydrocarbons was only 10% of the yield obtained inan otherwise identical experiment that was run under argon. These resultsare comparable to those observed with t-BuOOH, suggesting that the rateconstants for the capture of CO by •CH3 and•C2H5 are comparable. Similarobservations were made earlier in the gas phase, where the two rate con-stants differ by only a factor of two, even though the absolute values areseveral orders of magnitude smaller than in solution.8Effect of Halide IonsWhen the reaction between t-BuOOH and Fe(H2O)62+ was conducted inthe presence of 0.10 mol dm–3 NaBr, the combined yield of CH4 and C2H6 de-creased dramatically compared to the experiments under argon. Only onenew peak, corresponding to CH3Br, appeared in gas chromatograms, sug-gesting that bromide ions facilitate only one major new pathway. Because ofthe different solubilities and volatilities of alkanes and alkyl halides in wa-ter, the peak areas for the two types of compounds are not a direct measureof solution concentrations. Instead, a decrease in CH4 + C2H6 was taken torepresent the amount of CH3Br produced. Similar experiments were con-ducted in the presence of chloride ions.The large effect of halides on the yield and chemical identity of products,Table II, strongly suggests that Fe(H2O)5Br2+ and Fe(H2O)5Cl2+ react rapidlywith methyl radicals as in Eq. (10).FeX2+ + •CH3  Fe2+ + CH3X X = Br, Cl (10)Solutions of FeIII containing halide ions absorb strongly in the UV, whichprevented us from using an excess of FeIII. This and the lack of precise spec-tral data for various FeIII species involved made it impossible to determinethe relevant concentrations needed to calculate the rate constants for reac-tion (10). A limit of >108 mol–1 dm3 s–1 can be, however, safely placed on thereaction of Fe(H2O)5Br2+, given that 0.1 mol dm–3 Br– reduces the yield ofCH4 + C2H6 to only  5% of that observed in the absence of Br–. Less than638 A. BAKA^10% of FeIII is present as Fe(H2O)5Br2+ under our conditions.9 Similar argu-ments applied to the data in chloride solutions lead to k10 > (5  106) mol–1dm3 s–1 for Fe(H2O)5Cl2+.We rule out another mechanistic option whereby the accelerating effectof halide ions would be caused by the formation of (CH3X)•– and its rapid re-action with Fe(H2O)63+. Alkyl halide radical anions are extremely short-livedto the point of being practically non-existent, as demonstrated by the factthat the reduction of aliphatic halides takes place by dissociative electrontransfer to produce R• and X– directly without the involvement of observ-able (RX)• – intermediates.10CONCLUSIONSBoth carbon monoxide and halide ions change the outcome of Fe(H2O)62+-hydroperoxide reactions. In the presence of CO, alkyl radicals are convertedto acyl radicals which either undergo self-reactions or reduce Fe(H2O)63+. Thelatter reaction prevails in the presence of externally added Fe(H2O)63+ so thatthe Fe(H2O)62+/hydroperoxide/CO reaction becomes catalytic, much like theclassical Fenton reaction involving Fe(H2O)62+, H2O2 and oxidizable substrates.Halide ions greatly accelerate the reaction between iron(III) and alkylradicals. The reaction yields alkyl halides, most likely by halogen atomtransfer between Fe(H2O)5X2+ and the radicals.Acknowledgment. – This work was supported by the U.S. Department of Energy,Office of Basic Energy Sciences, Division of Chemical Sciences under contract W-7405-Eng-82.IRON(III) AND ALKYL RADICALS 639TABLE IIEffect of Br– and Cl– on the yields of hydrocarbons inthe reaction of FeIII with •CH3aHalide (conc. / mol dm–3)CH4 + C2H6b– 1.3 105NaBr (0.10) 6 103HCl (0.20) 4 104HCl (0.20) 9 103 ca FeIII and •CH3 were generated in the reaction between 10 mmol dm–3Fe2+ and 1 mmol dm–3 t-BuOOH in 0.20 mol dm–3 HClO4 at 25 °C.b Integrated GC areas, arbitrary units.cFe(H2O)63+0 = 4.5 mmol dm–3.REFERENCES1. A. Baka~ and W.-D. Wang, Inorg. React. Mech. 1 (1998) 65–73.2. M. Erben-Russ, C. Michael, W. Bors, and M. Saran, J. Phys. Chem. 91 (1987)2362–2365.3. P. Neta, J. Grodkowski, and A. B. Ross, J. Phys. Chem. Ref. Data 25 (1996) 709–1050.4. N. A. Milas and D. M. Surgenor, J. Am. Chem. Soc. 68 (1946) 643–644.5. A. Baka~, J. H. Espenson, and V. G. Young Jr., Inorg. Chem. 31 (1992) 4959–4964.6. R. W. Cargill, Solubility Data Series. Carbon Monoxide, Pergamon Press, Oxford,New York, 1990, p. 2.7. M. N. Schuchmann and C. von Sonntag, J. Am. Chem. Soc. 110 (1988) 5698–5701.8. K. W. Watkins and W. W. Word, Int. J. Chem. Kinet. 6 (1974) 855–873.9. L. G. Sillen, Stability Constants of Metal-Ion Complexes. Section I: Inorganic Lig-ands, The Chemical Society, London, 1964, p. 321.10. J. M. Saveant, Adv. Phys. Org. Chem. 35 (2000) 117–192.SA@ETAKEfekt halogenidnih iona i ugljikova monoksida nareakciju `eljeza(III) s alkilnim radikalimaAndreja Baka~U modificiranoj Fentonovoj reakciji izme|u Fe(H2O)62+ i alkilhidroperoksida(RC(CH3)2OOH, R = CH3, C2H5), u vodenoj otopini u prisutnosti ugljikova monoksidaprete`ito nastaju acilni radikali, RCO. Za R = CH3, nastali acetilni radikali reducirajuFe(H2O)63+, k = 6 105 dm3 mol–1 s–1. Reakcija Fe(H2O)63+ s •CH3 mnogo je polaga-nija: k  4 103 dm3 mol–1 s–1. Halogenidni ioni djeluju ubrzavaju}e, {to dovodi do na-stajanja halogenmetana. Konstanta brzine reakcije Fe(H2O)5Br2+ + •CH3  CH3Br +Fe(H2O)62+ ve}a je od 108 dm3 mol–1 s–1.640 A. BAKA^

Effects of Halide Ions and Carbon Monoxide on the Reactions of Iron(III) with Alkyl Radicals

Abstract

Similar works

Full text

Available Versions

Hrčak - Portal of scientific journals of Croatia