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Electron Paramagnetic Resonance Study of Radicals from Aliphatic Formate Esters' Peter Smith, Richard A. Kaba,* Luis M. Dominguez, and Stephen M. Denning Paul M. Gross Chemical Laboratoty, Department of Chemistty, Duke University, Durham, North Carolina 27706 (Received August 19, 1976)
The TiC13-H202radical-generating method has been used within an aqueous continuous-flow system to study by EPR at ca. 25 "C the following substrates: allyl, ethyl, isopropyl, isobutyl, and tert-butyl formates. Attention was directed to the characterization of radicals having the following structures: -C-UOC-H and .C-C44C-H. Few radicals of these two structural types have been characterized before in aqueous solution. In general, each type of radical showed a formyl-proton coupling, but the size of this coupling in radicals of the same type depended on radical structure, e.g., for -CH2CH200CHand CH2CH(CH3)00CH,respectively, a?$cH was 2.2 and 0.8 G. A possible explanation of the experimental data for these y-and 6-CH formyl-proton couplings has been obtained with the use of INDO molecular-orbital calculations.
Introduction There seem to have been few electron paramagnetic resonance, EPR, studies of aliphatic formates as substrates by means of the TiCl3-HZO2continuous-flow, radicalgenerating m e t h ~ d . ~In - ~an earlier investigation4 of a series of aliphatic carboxylic acids and their esters as substrates with use of the TiC13-H202system, methyl and ethyl formate were found to give only the ?F radicals CHzOOCH, 1, and CH3CHOOCH, 2, respectively; the spectrum for each radical showed a small (2.5 G) y-CH formyl-proton coupling. These two substrates have since been reinvestigated by others5 using this same radicalgenerating system. These workers5confirmed the earlier results4 but, in addition, observed for each ester a broad singlet which was assigned to the u radical formed by formyl-hydrogen-atom abstraction. Vinyl formate and allyl formate have also been examined in dilute solution as substrates within the TiC13-H202system at ca. 25 "C. The former gave6 only HOCH,cHOOCH, 3, its spectrum showing a small y C H formyl-proton coupling similar in size to those in both 1 and 2.495 The latter yielded7 only HOCH2CHCH200CH, 4, and *CH2CH(OH)CHZOOCH,5. The spectrum of 4 had a small (2.8 G) 6-CH formyl-proton coupling which, to our knowledge, is of a long-range type not previously reported. In this present work, the earlier studies of aliphatic formates4y5J as substrates within the TiCl3-H2O2radical-generating system have been extended with special emphasis on long-range formyl-proton couplings. The substrates examined were allyl, ethyl, isopropyl, isobutyl, and tert-butyl formates. For the case of allyl formate, the results obtained were in general agreement with those from the previous study7and particular attention was given to investigating the temperature dependence of the coupling constants of 4. In studying the other substrates, it proved possible to characterize all radicals bearing either an aor P-(-OOCH) group which could conceivably be formed by hydrogen-atom abstraction from the alkoxy group. A possible explanation of the y- and 6-CH formyl-proton couplings observed in this present study and the earlier investigation^^,^,^ has been obtained by means of INDO molecular-orbital calculations. Experimental Section Except when the tert-butyl compounds were examined, the experimental arrangement and procedures were as given elsewhere7except that a Varian F-BOA X-Y recorder was used for the temperature-dependence study of 4. For the tert-butyl compounds, the only difference was that the The Journal of Physical Chemistty, Vol. 8 1, No. 2, 1977
EPR system was the Varian E-9 spectrometer and accessories described previously.8 Spectra were recorded as the first derivative and g values taken as before against aqueous potassium peroxyiaminedisulfonate, which also served as the field ~ t a n d a r d . ~The field-modulation amplitude was normally ca. 0.1-0.4 G, but was raised to as high as ca. 2 G when an effort to detect a u radical was made. One of the two streams was an aqueous solution of TiC13 containing added HzSO4 and the other, aqueous HzOz. Normally, to minimize hydrolysis of the ester^,^ the substrate was included only in the H202stream and this was kept free of added HzS04. However, occasionally HzSO4 was added to this stream to favor hydrolysis and thus facilitate study of the possible contribution of alcohol-derived radicals to the spectrum observed in the absence of added HzS04. This procedural variation was useful since, in general, a given alcohol tended to be more reactive than its formate, as e ~ p e c t e d .Unless ~ specified otherwise, the results reported refer to the use of the normal procedure. The following esters were examined without further purification: allyl formate (Eastman, reagent grade), ethyl formate (Eastman, practical grade), and isopropyl formate (City Chemical and K & K). Isobutyl formate (Eastman, reagent grade) was found by GLC analysis to contain ca. 1 mol % isobutyl alcohol. Triple distillation of this substrate from P205reduced the alcohol impurity below detection l e ~ e l . ~However, J~ control EPR spectra showed the unpurified and purified substrate to give about the same proportion of radicals derived from isobutyl alcohol. tert-Butyl formate was prepared by the method of Stevens and van Esl' and used without further purification, since no sign of impurity was detected by analysis of its 60-MHz NMR spectrum (that this substrate was adequately free of tert-butyl alcohol was shown by EPR measurements, see later). The reaction conditions were as follows: total flow rate, 2-4 ml s-l equally divided between the two streams, the results obtained being not noticeably dependent on flow rate over this range; the reaction temperature, 25 f 2 "C, unless stated otherwise; the reducing stream, 0.004 and 0.2 M in TiC13 and H2SO4, respectively; and the oxidizing stream, 0.1 M in HzOz and, normally, with no added H2S04. As mentioned before, substrates were included only in the oxidizing stream, with concentrations as follows: allyl formate, 0.03 M; isopropyl formate and isobutyl formate, 0.1 M; tert-butyl formate and ethyl formate, 0.2 M.
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EPR of Radicals from Aliphatic Formate Esters
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I C
I I
I /
I I
I 1
I 1
a":
II
1 1 I 1
A
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I
-
-
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bein equal, within ex erimental-error, but because afscH aN/2. Ethyl Formate, CH3CH200CH. Under normal fieldmodulation-amplitude conditions, the spectrum obtained was completely interpretable in terms of 2 and .CH2CH200CH,7, from the ester and CH3CHOH,8, from ethyl alcohol1*present as impurity.15 As expected>52 gave a quartet (1:3:31)of doublets (1:l) of small (2.5 G) doublets (l:l), the last coupling from the formyl p r ~ t o n . Radical ~,~ 7, not observed in the earlier studies4y5of ethyl formate, gave a triplet (1:2:1) of triplets (1:2:1) of small (2.2 G) doublets (1:l). The two triplets were assigned by comparison with those in related radicals studied in this present work and elsewhere.16 The small doublet was attributed to the formyl proton. Figure 1 illustrates the spectrum from ethyl formate: the spectra from 2 and 7 are both intense and overlapped. In their previous study of ethyl formate, Metcalfe and Waters' found not only 2 but also a broad singlet of g value 2.0005 f 0.0003 assignable to the u radical CH3CH200C., 9. Such a signal was not found under the normal field( a g ~ o o -c agB"z> ~
modulation-amplitude conditions used here or in the earlier study reported from this l a b ~ r a t o r y .However, ~ upon raising the field-modulation amplitude, one of the lines increased in intensity at the expense of all others, see Figure 1. This line was assigned to 9, with a g value of 2.0008. Isopropyl Formate, (CHd,CHOOCH. This substrate gave an organic-radical absorption of rather low intensity and the signal from the titanium-radical complex17 was never completely suppressed. This situation could not be avoided since the oxidizing stream was saturated with respect to the ester. However, by time-averaging techniques, this absorption was completely analpable in terms of .CH2CH(CH3)OOCH, 10, (CH3)2COOCH, 11, and (CH3)2CHOOC-,12, with no evidence for radicals from isopropyl al~0h0l.l~ Radical 10 gave a quartet (1:33:1) of small (1.8 G ) quartets (1:3:3:1) of small (0.8 G ) doublets (1:l). The larger quartet was attributed to the chance equivalence of the a-CH2and 0-CH proton couplings and the small quartet, to the yCH3 protons. Although end lines were not observed, the spectrum of 11 appeared to be a septet (1:615:2015:61) of small (1.3 G) doublets (1:l). The small doublets in both 10 and 11 were assigned to the formyl proton. A broad (ca. 1 G peak-to-peak) absorption observed at a g value of 2.0008 was assigned to u radical 12. Isobutyl Formate, (CH,),CHCH,OOCH. The spectrum observed was fully accountable in terms of .CH2CH(CH3)CH200CH, 13, (CH3)2&!H200CH, 14, and (CH3)2CHcHOOCH, 15, from the formate and CH2CH(CH3)CH20H,16, and (CH3)2cCH20H,17, from isobutyl alcohol14present as impurity. All attempts to observe the u radical from this substrate by procedures similar to those employed for 9 failed. Also, no evidence for the presence of (CH3),CHeHOH, 18, was found. However, when HzSO4 was included in the oxidizing stream so that the substrate reacting was largely isobutyl alcohol, the predominant radical observed was 18, as e x p e ~ t e d . ~Since J ~ this seemingly different selectivity of hydrogen-atom abstraction from the alcohol in the presence of the formate was irrelevant to the present study, it was not investigated further. It should be noted that 13 and 1614would be expected to yield similar, overlapping patterns of lines, each a triplet (1:21) of doublets (1:l). Only one such triplets-of-doublets pattern was observed, this finding being in line with the presence of either one or both of 13 and 16. Based on the amount of alcohol in the reaction mixture, as crudely inferred from the signal intensity of 17, which was small in comparison with that from 14 or 15, it appears unlikely that this pattern could have arisen solely from 16. Thus, both 13 and 16 were suspected to be present. However, since this point was unimportant to the chief thrust of the present work, and because of the previously noted question of the seeming change in the selectivity of hydrogen-atom abstraction from the alcohol in the presence of the formate, no attempt was made to pursue it further. The spectrum of 14 was a septet (1:6:15:20:15:6:1)of triplets (1:2:1) of small (3.2 G)doublets (1:l); that of 15 was a doublet (1:l) of doublets (1:l) of small (2.4 G) doublets (1:l). For 14 and 15, the small doublets were assigned to the formyl proton. The two larger doublet splittings for 15 were assigned by comparison with the couplings in the analogous radical 2 since these are known unambiguously. tert-Butyl Formate, (CHJ,COOCH. Even though the oxidizing stream was saturated with respect to the substrate, the organic-radical absorption produced was of somewhat low intensity and there was a residual signal The Journal of Physical Chemktv, Vol. SI, No. 2, 1977
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TABLE I : Spectroscopic Data for Formate-Derived Radicalsa Coupling constants, G Radical a-CH 8-CH Other g value 2, CH,CHOOCH~ 19.42 24.28 2.4gC 2.00275 4, HOCH,CHCH,OOCH~ 21.82 17.87,e 23.42s 2.80g* 21.75 17.96; 23.40f 2.80g 2 -0024i 21.74 18.13,e23.40s 2.78gj 5, CH,CH(OH)CH,OOCH~ 22.25 24.52 2.0024 6, H,NCH,CHCH,OOCHk 22.39 19.64,' 22.395 2.78: 5.56m 2.0024 7 , .CH,CH,OOCH 22.14 24.59 2.18g 2.0024 9, CH,CH,OOC. 2.0008n90 0.8 3 ,g 1.82p 2.0024 1 0 , .CH,CH(CH,)OOCH 22.18 22.18 11, (CH,),COOCH 22.29 1.27c 2.00265 1 2 , (CH,),CHOOC. 2.0008" 13, *CH,CH(CH,)CH,OOCHQ 21.87 27.50 2.0025 14,(CH3),CC13.,00CH 23.22,' 12.985 3.22g 2.00255 15, (CH,),CHCHOOCH 18.70 23.24 2.4OC 2.0025 1 9 , *CH,C(CH,),OOCHS 22.13 OB$t, 1.3BP 2.0026 a All data are mean values based o n several full field scans. Unless specified otherwise, at 25 i: 2 "C and with the maxiThese a values are in reasonable agreemum uncertainties for a and g values being 0.10 G and ca. 0,0001, respectively. ment with the literature considering that the previous workers did not allow for the presence of overlapping lines from 7, see Fi ure 1. a y . ~ O o d H . The assignment of the 8-CH, proton couplings in 4 is justified elsewhere.' e ~ 8 . ~ ~ ~ f a -H 'H200CH. g All data taken a t 9 i 1 "C, the standard deviation of the mean being 0.04, 0.03, 0.05, and 0.61 G, respectively, left t o right. All data taken a t 23 ;1O C , the standard deviation of the mean for the a values being 0.03, 0.02, 0.02, and 0.01 G, respectively, left to right. I All data taken a t 44 * 1 'C, the standard deviation of the mean being 0.04, 0.03, 0.05, and 0.01 G, respectively, left to right. All data taken from ref 7. (Ip.HCH2"2. U N . " In agreement with the l i t e r a t ~ r e . ~O. ~Maximum ~ uncertainty 0.0002. (ly.HCH3. Q The data attributed here t o 1 3 refer to a mixture of 13 and 16, see the text. ap.HCH3. In order to make the most reliable comparison, the results for 19 and 20 (see ref 20) were taken in close succession under the same recording conditions. Not resolved;
