An Electron Spin Resonance Study of Intermediates Formed during

2,6-disulfonate arid -2-sulfonate were photolyzed, a singlet was observed which could be associated with the formation of the semiquinone radical. Pho...
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PHOTOSENSITIZED OXIDATION OF ALCOHOLS

3061

An Electron Spin Resonance Study of Intermediates Formed during

Photosensitized Oxidation of Alcohols'

by Peter J. Baugh,? Glyn 0. Phillips,aand Jett C. Arthur, Jr. Southern Regional Research Laboratory,4 New Orleans, Louisiana

(Received February 18, 1966)

Electron spin resonance spectroscopy was used to identify and to examine the role and kinetics of semiquinone radicals and radical ions in the photosensitized oxidation of aqueous solutions of D-sorbitol, cellobiose, D-glucose, glycerol, and methyl, ethyl, isopropyl, and n-propyl alcohols. Sodium anthraquinone-1- and -2-sulfonates and -2,6- and -l,&disulfonates were used as sensitizers. When aqueous solutions of sodium anthraquinone2,6-disulfonate arid -2-sulfonate were photolyzed, a singlet was observed which could be associated with the formation of the semiquinone radical. Photolysis of the sensitizers in the presence of carbohydrates and alcohols caused the singlet which formed on photolysis to decay with first-order dependence on concentration of alcohol and anthraquinonesulfonate salt. Second-order rate constants were calculated, and relative values compared with those of previous workers obtained by other methods. I n alkaline solution of anthraquinone salts containing carbohydrates, anthrasemiquinone radical ions were formed and were the same whether produced by photolysis and/or thermolysis. From the observed spectra the hyperfine splitting constants were calculated and possible assignments proposed in relation to the structure of the anthrasemiquinone radical ions. Attempts have been made to identify the precursor of the semiquinone radicals formed during photolysis of aqueous solutions of sodium anthraquinone-2-sulfonate and -2,6-disulfonate, containing carbohydrates or alcohols. It is possible that the precursor of the radicals may be the triplet state.

Introduction The commonly accepted mechanism of the photosensitized oxidation of ethanol is that proposed initially by Bolland and Cooper,s and extended by Wells.6 The initial reactions proposed in neutral oxygenated solutions were

A + hv--tA*

A*

+ CH~CH~OH

AH.

(1)

+ CH~CHOH (2)

A represents the sensitizer; A*, the excited state of the sensitizer; and AH-, the semiquinone radical. A series of additional reactions were proposed to account for the participation of oxygen, perhydroxyl radicals, and sensitizer. I n a series of kinetic investigations based on oxygen absorption, Wells' proposed that the deactivation of

the sensitizer proceeds in two ways only, as shown in reactions 2 and

A*&A Oxygen absorption obeyed the expression

-

(3)

dt - 1 + -1 - -1 ko d[Ozl I I [RH] ki

(1) Presented in part a t the 151st National Meeting of the American Chemical Society, Pittsburgh, Pa., March 22-31, 1966. (2) Resident Postdoctoral Research Associate. (3) University College, Cardifi, Wales. (4) One of the laboratories of the Southern Utilization Research and Development Division, Agricultural Research Service, U. S. Department of Agriculture. (5) J. L.Bolland and H. R. Cooper, Proc. Roy. SOC.(London), A225, 405 (1954). (6) C. F.Wells, Trans. Faraday SOC.,57, 1703 (1961). (7) C. F. Wells, {bid., 57, 1719 (1961); Discussions Faraday Soc., 29, 219 (1960); J. Chem. SOC.,3100 (1962).

Volume 70,Number 10

October 1966

P. J. BAUGH,G. 0. PHILLIPS, AND J. C. ARTHUR,JR.

3062

where [RH] is the concentration of alcohol and I the rate of activation proportional to the intensity of light. Using this relationship kl/lco can be calculated. The values of kl/ko for a series of alcohols using the same quinone give the related reactivities of these alcohols to hydrogen transfer promoted by the excited sensitizer. Based on actual product estimations, Phillips, Barber, and Rickardss found that the effective kinetics for Dsorbitol (RCHzOH) degradation sensitized by sodium anthraquinone-2-sulfonate or -2,6-disulfonate could be represented by

-

dt d[RCH20H]

= 1- + 1I

1

RCH~OH AH. * -2RCHOH

k0

+ AH^

(4) is of particular significance in the photodegradation process where AH.* may be the semiquinone in its ground or excited state.

Experimental Section Materials. The anthraquinones used were obtained from commercial chemical sources and were recrystallized from CcHs, CzHbOH, or distilled HzO before use. Alcohols and sugars used were reagent grade and were used without further purification. Apparatus and Procedure. The electron spin resonance spectra of the intermediates formed during the photosensitized oxidation of alcohols in solution were determined in a Varian 4502-15 epr spectrometer ~ y s t e m . ~The system was equipped with a variable temperature accessory allowing operation from about -185 to 300°, a dual sample cavity having a slotted opening in one side of one cavity, and an aqueous solution sample cell accessory. A P E K 110 Hg short-arc, point-source, high-pressure lampg was used to irradiate the solutions directly in the sample cell within the resonant cavity. The light from the lamp was focused to about 1 em2 on the slotted opening in the cavity and had a rated luminous intensity of 140,000 candles/ em2. The lamp was placed a t a distance from the sample so that the light from the lamp did not sensibly heat the sample. The spectral distribution of the The Journal of Physical Chemistry

t g :t?.oos6

I [RCHzOH] 6

We have been interested in this reaction because of its close relation to the photodegradation of cotton cellulose sensitized by anthraquinonoid vat dyes. To devise protective methods it is particularly important to elucidate the role of the semiquinone radical (AH.) relative to the excited sensitizer (A*). Electron spin resonance was used to identify and to examine the role of semiquinone radicals and radical ions in the photodegradation of D-sorbitol sensitized sodium anthraquinones. We have found that the reaction

+

I

MAGNETIC FIELD

-

Figure 1. Esr spectra of photolyzed aqueous solutions (0.037

M ) of sodium anthraquinone-2,6-disulfonate(A-1, 370 sec; A-2, 150 sec) and sodium anthraquinone-2-sulfonate (B, 150 sec).

:I

I-

#

L

A

IP

IO

0

? t

I

I

4

0

I

I

I

8

IO

I?.

I 14

I

IO

TIME,MIN.

W

r

TIME,SEC.

Figure 2. Rate of change of esr signal strength of photolyzed aqueous solutions of sodium anthraquinone-2,6-disulfonate: (A) 0.037 M , (B) 0.0037 M .

radiant energy output of the lamp contained characteristic high-pressure Hg vapor lines. Except where indicated, the solutions were irradiated a t 25". Absolute splitting constants were obtained by calibrating the system with p-benzoquinone (a = 2.37 gauss).l0

Results Sodium Anthraquinme-W,G-disulf~nate. A narrow singlet (line width about 5 gauss with g = 2.0036) was observed when an aqueous neutral solution of sodium anthraquinone-2,6-disulfonate(3.7 X M ) was (8) G. 0. Phillips, P. Barber, and T. Rickards, J. Chem. Soc., 3443 (1964). (9) Trade names are given a8 part of the exact experimental conditions and not as an endorsement of the products over those of other manufacturers. (10) B. Venkataraman, B. G. Segal, and G. K. Fraenkel, J. Chem. Phys., 30, 1006 (1959); J . Am. Chem. SOC.,7 7 , 2707 (1955).

PHOTOSENSITIZED OXIDATION OF ALCOHOLS

photolyzed with ultraviolet light (see Figure 1). The rates of formation of the radical species during photolysis of this solution and of a less concentrated solution (3.7 X M ) are shown in Figure 2. For the lower concentration of the 2,6-disulfonate on photolysis the esr signal increased to a maximum value in a few seconds. The rate of decay of the radical species when the light was extinguished was much lower in the more concentrated solution. Effects of Sugars. When an aqueous neutral solution of sodium anthraquinone-2,6-disulfonate (3.7 X M ) and wsorbitol (0.55 M ) was photolyzed (preoxygenated or in vucuo), an esr signal was observed (same as in Figure 1A). This species appeared almost immediately (within 5 sec of initiation of irradiation) and decayed rapidly as the photolysis continued (see Figure 3). The maximum free-radical concentration, obtained after about 5 see of photolysis, varied with sensitizer concentration, as shown in Figure 4, the carbohydrate concentration being kept constant. Point C in Figure 4 shows that for a given sensitizer concentration the maximum concentration of free radical is independent of the sugar used. The rate of disappearance of the radical during continuous photolysis was dependent on the concentration of D-sorbitol (see Figure 5 ) . For these data the plot of 1/ [radical concentration] us. time was linear and could therefore be used to calculate initial rates for the radical decay. The plots of log (rate),,o us. log [solute],,o are linear (see Figure 6). From the slopes of the lines it can be calculated that the rate was proportional to sorbitol]^.^^, [cellobi~se]~.~~, and It has been similarly shown that the [~-glucose]'.~~ reaction rate was proportional to the first power of the sensitizer concentration. We therefore calculated second-order rate constants using the expression: [rate of decay of radi~al],,~= k2[sensitizer][solute]. sec-l; for For sorbitol IC2 was 2 X lo-* 1. cellobiose, 4 X loF21. mole-' sec-'; and for D-glucose, 3X 1. mole-' see-'. I n alkaline solution (0.2 M NaOH) of sensitizer M ) and sugars (0.1-0.5 M ) , the esr signal (3.7 X (similar to that for neutral solution) was stronger by several orders of magnitude than that for neutral solution. The strength of the esr signal, which formed within a few seconds after the light was turned on, varied linearly with sensitizer concentration (see Figure 4A). The decay rate showed some dependence on solute concentration during continuous photolysis (see Figure 7), Sodium Anthraquinone-W-su&mate. When aqueous solutions of Elodium anthraquinone-2-sulfonate were irradiated, an esr signal was observed (see Figure 1B)

3063

A IN VACUO 0

WE-OXYOENATICD

-

I

I

100

e00

I

I

300

400

500

TIYE,SEC.

Figure 3. Rate of decay of radical species formed by continuous photolysis of aqueous solutions of D-sorbitol (0.55 M) containing sodium anthraquinone-2,6-disulfonate (0.0037M ) .

Figure 4. Effect of concentration of sodium anthraquinone-2,6-disulfonateon the initial esr signal strength in photolyzed aqueous solutions of D-sorbitol (0.4 M) and NaOH (0.2 M ) (A),of D-sorbitol (0.275M ) (B), and of D-sorbitol, cellobiose, and D-glucose (0.1375 M) (C).

I

I

100

200

I

so0

I

1

400

500

TIME, SEC.

Figure 5. Effect of varying the concentration of D-sorbitol, in the presence of sodium anthraquinone-2,6-disulfonate (0.0037 M), on rate of decay of radical species formed by continuous photolysis: (A) 0.05 M, (B) 0.1 M , (C) 0.2 M , (D) 0.4 M, (E) 0.55 M, (F) 0.75M.

Volume 70,Number 10 October 1066

P. J. BAUGH,G. 0. PHILLIPS, AND J. C. ARTHUR, JR.

3064

1.7

,

I

I

I

01

01

1.0

I

I

I

I

I

I

I

I

I

1.1

0.1 0.4

4

4

I

1.L

E+ WQ I

1.4

I

1.6

I.#

do

I

i

~ R I C ,

I

I

t

I

a

I

I

4

5

I

0

~

TIYE, YIN.

Figure 6. Order of reaction for decay of radical species formed by continuous photolysis of aqueous solutions of cellobiose (A), D-glucose (B), and D-sorbitol (C) containing sodium anthraquinone-2,6-disulfonate(0.0037 M ) .

Figure 8. Rate of decay of radical species formed during photolysis of aqueous solutions of alcohols (0.5 M ) containing sodium anthraquinone-2,6-disulfonate(0.0037 M ) (light on): (A) methanol, (B) glycerol, (C) ethanol, (D) l-propanol, (E) 2-propanol.

Aliphatic Alcohols. When sodium anthraquinone2,6-disulfonate was irradiated in aqueous alcoholic solution at pH 7, there was a pronounced difference in decay rate of the radical depending on the alcohol. Typical results for glycerol, methanol, l-propanol, 2-propanol, and ethanol are shown in Figure 8. The initial rates for the reaction were calculated at varying alcohol and sensitizer concentrations, and from the data the second-order rate constants, shown in Table I, were calculated. 01 0

I

I

900

300

I

loo

4 0

TIME, SEC.

Figure 7. Dependence of rate of decay of radical species formed by photolysis on concentration of D-sorbitol (A, 0.3 M; B, 0.1 M ) in alkaline solutions (NaOH, 0.2 M ) containing sodium anthraquinone-2,6-disulfonate (0.0037 M ) .

which decayed with a rate dependent on sensitizer and solute concentration. A kinetic expression was found for this sensitizer similar to that for the 2,6-disulfonate. The reaction was first order in D-sorbitol and sensitizer. For D-sorbitol kz, calculated as above, was 1 X lo-* 1. mole-' sec-I. Sodium Anthraquinme-l-sulfonate and -1,6disulfonak. When the l-sulfonate and 1,5disulfonate were irradiated in neutral solutions under conditions identical with those described above for the 2-sulfonate and 2,6disulfonate, no esr signal was observed. I n alkaline solution under similar conditions an esr signal was observed, probably due to the formation of the semiquinone radical ions. The Journal of Physical C h i s t r y

Table I: Rate Constants for Reaction of Semiquinone Radicals with Aliphatic Alcohols Alcohol Iso-

k2 X 10-21.mole-1 sec-l kz( ROH)/kz( EtOH)

7

n-

Methyl

Ethyl

propyl

Propyl Glycerol

1.4

3.4

6.4

5.1

2.5

0.4

1

1.9

1.5

0.7

Em Spectra of Semiquinone Ions. The esr spectra of the semiquinone ions are not obtained instantly when aqueous alkaline solutions of sodium anthraquinonesulfonates are irradiated. This behavior for the 1sulfonate (3.7 X M ) in the presence of D-sorbitol (0.4 M ) and NaOH (0.2 M ) is shown in Figure 9. The final spectra of the irradiated solutions of the sulfonate are shown in Figure 10. Less hyperfine splitting was found at the lower temperatures. The esr spectrum of the semiquinone radical ion from sodium anthraq~inone-2~6-disulfonate at - 76" is shown in Figure 11.

PHOTOSENSITIZED OXIDATIONOF ALCOHOLS

...

i

+

3065

.

Figure Rate of formation of semiquinone radical ion -y photolysis of aqueous alkaline solution (NaOH, 0.2 M ) of sodium anthraquinone-1-sulfonate (0.0037 M ) containing D-sorbitol (0.4 &f): first six sweeps, 10 gays, 30 sec; seventh sweep, 10 gauss, 1 min; eighth sweep, 10 gauss, 5 min.

MAQNETK: FIELD

-

Figure 11. Esr spectrum of radical formed by photolysis a t -76" of an aqueous alkaline solution (NaOH, 0.2 M ) of D-sorbitol (0.4 M ) containing sodium anthraquinone-2,6-disulfonate (0.0037 M ) .

the same conditions of heating gave an ill-defined spectrum. The effect on the spectrum by irradiation after heating was similar to that due to irradiation alone; that is, a 21-line spectrum was obtained. The hyperh e spacings and g values for the anthrasemiquinone radical ions are given in Table 11. Table II: Hyperfine Spacings of the Anthrasemiquinone Radicals Produced by Light and/or Heat in Alkaline Solutions of Sodium Anthraquinonesulfonates (0.0037 M )Containing D-Sorbitol (0.4 M ) l.l!wsL

Figure 10. Esr spectra of semiquinone radical ions formed by thermolysis of photolysis of aqueous solutions or sodium anthraquinonesdfonates (0.0037 M )containing D-sorbitol (0.4 M): (A) sodium anthrasemiquinone-2,6-disulfonate, heat or light; (B) sodium anthrssemiquinone-l,5-disulfonate, heat or light; (C) sodium anthrasemiquinone-1-sulfonate, after 5-min photolysis, heat or light; (D) sodium anthrasemiquinone-1-sulfonate, after 15-mh photolysis, light only or light after heat; (E) sodium anthrasemiquinone-2-sulfonate, light only (compare Table 11).

The sodium anthraquinone-2,6- and -1,bdisulfonates M ) in alkaline D-sorbitol and -1-sulfonate (3.7 X solutions (0.4 M ) on heating to about 80" for 10 min gave a wine-red coloration. The esr spectra of these solutions werje identical with those shown in Figure 10A, B, and C, respectively. On being irradiated the radical ion from the 1-sulfonate was transformed from a 12line to a 194ine spectra. This effect was similar to that produced by light alone. The 2-sulfonate under

No.

Sodium anthraquinone-

value4

2,6-Disulfonate

2.0036

l,5-Disulfonate

2.0038

1-Sulfonate

2.0039

1-Sulfonate %Sulfonate

... ...

%Sulfonate

2.0037

B

Conditions

of

lines

Heat or 11 light Heat or 9 light Heator 12 light Light 19 only Heat Not resolved Light 21

Overall hyperfine splitting, gauss

gauss

3.93

0.39

3.286 4.99

0.38 0.51 0.45

6.28

...

4.96

0.25

AH,

a Based on peroxylamine disulfonate, g = 2.00550: "EPR a t Work, No. 28," Varian Associates, Palo Alto, Calif.

The semiquinone ion derived from the 2,6-disulfonate has a spectrum consisting of 11 lines. This number of lines could result from the coupling of four a protons Volume 70, Number 10 October 1966

P. J. BAUGH,G. 0. PHILLIPS, AND J. C. ARTHUR, JR.

3066

and two p protons with some of the lines falling so close that overlapping essentially occurs. The correct number of lines would arise if the splitting constant of the CY protons was double that of the protons, which was found to be 0.39 gauss. If a, = 2ag 6 where 6 < up, the hyperfine spacings taken from the central component would be f a g 6, *2aa 6, f 3 a g 26. From the spectrum 6 26, *4ag 26, *5aa has a maximum value of 0.07 gauss, setting an upper limit of 0.85 for a,. The ion derived from sodium anthrasemiquinone1,5-disulfonate has a spectrum consisting of nine lines due to coupling with two CY protons and four p protons, but with some overlapping lines. The observed splitting constants are aa = 0.37 gauss, a, = 0.89 gauss with the hyperfine spacings taken from the central 6, f3aa 6, component equal to fag, *2ag

+

+

*4ag

+

+

+ 6.

+

+

+

+

Sodium anthrasemiquinone-1-sulfonate has a spectrum consisting of 12 equally spaced lines which could arise from the interaction of four p protons and three CY protons. If the splitting constant of the p proton was approximately double that of the a proton, 12 equally spaced lines would be observed. The splitting constant (a,) for the a proton was found to be 0.45 gauss. This sets an upper limit of 0.97 gauss for ag. The 12-line spectrum changes on continued irradiation to 19 lines. Resolution of this spectrum was not obtained. The ion derived from the sodium salt of the 2-sulfonate gave an equally spaced 21-line spectrum, AH = 0.25 gauss. Energy Transfer. I n neutral and alkaline solutions M) the addition of naphthalene or anthracene (ca. inhibited the production of anthrasemiquinone radical M) during irradiation of anthraquinone (2.75 X in degassed 2-propanol-benzene (1:4). For the light filter saturated solution of anthracene or naphthalene in CsHs used, two radical species were observed. The radical species, which could be observed a t 25 and - 50" in neutral solution, occurred a t a higher magnetic field than the other species which could be stabilized only by freezing the solution a t -50". An estimate of the protection afforded by the addition of aromatic hydrocarbons was obtained at -50". The effect of naphthalene and biphenyl, when added to a solution of anthraquinone (2.75 X M ) in 2-propanolbenzene (1:4) and then irradiated in uacuo through the appropriate filter, is shown in Figure 12.

Discussion There has been considerable discussion of the mechanism of the photooxidation of alcohols by sodium The Journal of Physical Chemistry

0

0

,

100

I

COO

I

so0 TIME, SEG.

400

BOO

Figure 12. Rate of formation of semiquinone radicals by photolysis at -50" of anthraquinone (0.0275 M ) in 2-propanol-benzene (1:4 ) as related to energy transfer. Solutions: (A) anthraquinone (0.0275 M ) ; (B) biphenyl (0.01 M ) and anthraquinone (0.0275 M ) ; (C) naphthalene (0.01 M ) and anthraquinone (0.0275 M ) . Filters: (1) saturated naphthalene in benzene, (2) saturat,ed biphenyl in benzene.

anthraquinonesulfonates.s-8i11 Although the intermediates active in this process have not previously been directly observed, radicals have been noted during the photoreduction of alkaline solutions of sodium anthraquinone-2-sulfonate12 and anthraquinonesulfonic acids.13 Elschner, et al., observed semiquinone radical ions during photoreduction in both the anthraquinone-2-sulfonic and 2,6-disulfonic acids but not in the cases of the 1-sulfonic and 1,5-disulfonic acids.13 Spectra consisting of a t least 30 lines have been observed in hydroxyanthraquinonesulfonates when photoreduced.14 It is common practicel5-l* to prepare the substituted anthrasemiquinone radical ions from alkaline organic solutions of the corresponding anthraquinones by use of heat and/or light. Attention has been directed, almost exclusively, to the conditions where ionization of the semiquinone to the radical ion occurs.

(11) F. Wilkinson, J . Phys. Chem., 66, 2569 (1962). (12) B. Mooney and H. I. Stonehill, Chem. Ind. (London), 1309 (1961). (13) B. Elschner, R. Neubert, H. Berg, and D. Tresselt, 2. Chem., 1, 361 (1961). (14) A. D. Broadbent and H. Zollinger, Help. Chim. Acta, 47, 2140 (1964). (15) R. W. Brandon and E. A. Lucken, J. Chem. Soc., 4273 (1961). (16) R. W. Elofson, K. F. Schulz, B. E. Galbraith, and R. Xewton, Can. J . Chem., 43, 1553 (1965). (17) G. Vincow and G. K. Fraenkel, J . Lhem. Phys., 34, 1333 (1961). (18) M. Adams, M. S. Blois, and R. H. Sands, ibid., 28, 774 (1958).

PHOTOSENSITIZED OXIDATIONOF ALCOHOLS

0

0

& \-.+-I-*bH

\

b0I

6 Mechanistic studies, on the other hand, have been carried out in neutral solution where the reactive intermediate (AH.) could not be detected.l9 Our initial efforts were, therefore, directed toward identifying and observing the behavior of the radical species which we readily observed in neutral and alk% line aqueous solutions of sodium anthraquinonesulfonates, on irradiation with heat or light, in the presence and absence of alcohols and carbohydrates. I n alkaline solution there can be little doubt that we are observing the esr spectra of the semiquinone radical ions. However, with the evidence available} it is not possible to identify unequivocally the individual proton splitting constants. For the semiquinones derived from the 2,6- and 1,5-disulfonates a t least two possibilities exist, and these are indicated in relation to those of substituted anthraquinones which have been studied in Table 111. For the 2,6-disulfonate the esr spectrum accords better with the splitting constants a2 = 0.39 gauss (4), al = 1.21 gauss (2). Furthermore, the splitting constants designated al and az are more consistent with those observed generally in the anthraquinone series (Table 111),and would indicate that a1 protons may be identified with p protons and az protons with a protons. Comparable values (a2 = 0.37 gauss, a1 = 1.27 gauss) are consistent with the nine-line spectrum from the 1,Bdisulfonate on the basis of two hydroxyl groups having been introduced symmetrically into the molecule. Hydroxylation during this operation is a distinct possibility and was initially proposed by Mooney and Stonehill12 to account for the colored products arising from the 2,6-disulfonate in aqueous alkali with light. This side reaction is also observed in photooxidation reactions sensitized by the 2,gdisulfonate a t low alcohol It has been suggested that OH radicals can be produced by sensitized photolysis of water and would initiate the hydroxylation process.12v20 Alternatively, OH ions may lead to hydroxylation as observed during esr examination of substituted monoand dihydric phenols.21P22

3067

Spin densities may be calculated using the expres~ i o aHi n ~ =~ QHC=p*i where aHi is the proton coupling constant for the proton a t position i, pri is the electron spin density a t carbon atom i, and Q H c is ~ a constant. A value of Q close to -22.5 satisfied Vincow and Fraenkel'sl' calculation of unpaired electron density in semiquinones by calculation and experiment and is used here. On this basis the unpaired electron density or more precisely the spin densityz4in the 2,Gdisulfonate semiquinone is p", = 0.38, pRB= 0.017 or pT1 = 0.054, pTZ = 0.017. I n each case only about 20% of the spin density is associated with the carbon atoms of the ring. A closely similar low proportion is also associated with the carbon atoms in the 1,5-disulfonate semiquinones. For alkylated phenoxy radicals the major proportion (80%) of the spin density resides in the carbon atoms of the ring not directly attached to the phenolic oxygen. On the other hand, 0- and p-benzosemiquinones have about 65 and 60%, respectively, of the spin density associated with the two oxygen atoms and more nearly resembles our observations with the anthraquinonesulfonates.21i22 Yo detailed consideration is given here to the spectra of the species produced from sodium anthraquinone-1- and -2-sulfonates. The changes in the spectrum, particularly for the 2-sulfonate to give 21 equally spaced lines, could be due to secondary photoreduction of the initial species. Broadbent and ZollingerI4 have reported 30 lines observable by esr during photoreduction of hydroxyanthraquinone-2,7disulfonic acid. The tentative assignments [a, = 0.45 gauss (3); as = 0.97 gauss (4)]for the 12-line spectrum from the 1-sulfonate is, at least, consistent with our other observations (see Table 111). In alkaline conditions the semiquinone radical ion is the species which is probably formed by irradiation of the aqueous solution containing sodium anthraquinonesulfonates and D-sorbitol. The species in which no hyperfine splitting was detectably observed in neutral solution is most probably the unionized semiquinone radical, since all other conditions are comparable. Previously, Mooney and Stonehill12 suggested that a semiquinone could be produced by photodecomposition of a molecule of hydration water (19) J. H. Sharp, T. Kuwana, A. Osborne, and J. N. Pitts, Chem. Ind. (London), 508 (1962). (20) G. K . Oster and N. Wotherspoon, J . Am. Chem. Soc., 79, 4837 (1957). (21) F. R. Hengill, T. J. Stone, and W. A. Waters, J . Chem. Soc., 408 (1964). (22) T. J. Stone and W. A. Waters, ibid., 213 (1964). (23) H. M. McConnell, J. Chem. Phys., 24, 632 (1956). (24) E. de Boer and S. I. Weissman, J. Am. Chem. Soc., 80, 4549 (1958).

Volume 70,Number 10 October 1866

P. J. BAUGH,G. 0. PHILLIPS, AND J. C. ARTHUR, JR.

3068

the relative reactivities of alcohols to photoexcited dichromate solutions, as shown in Table IV. Here Bowen26suggests that the first reaction on photochemical oxidation of the dichromate ion is H-atom abstrac-

Table III: Hyperfine Splitting Constants (gauss) of 9,lO-Anthrasemiquinone Ion and Its Derivatives Derivative

Unsubstitutedle 2,6-Dkdfonate I,&Disulfonate 1-Sulfonate %Sulfonate 2,6-Dimethyl16 1,4Dimethy116 0

az

aa

a@

0.41 (4)" 0.85 (4) 0.39 (4) 0.89 (2) 0.37 (2) 0.45(3) . . . (4) 0.47 (4) 0.33 (2)

0.83(4) 0.39 (2) 0.37 (4) 0.97(4) . . (3) 0.71 (2) 0.68(2) 0.64(2)

a1

1.21 (2) 1.29 (2)

.

Alcohol

kz(ROH)/

Number in parentheses indicates number of protons.

hydrogen-bonded to the carbonyl oxygen atom of the 2-sulfonate1a process which was strongly pH dependent leading to colored products. When an alcohol is present, on energetic grounds, reaction 2 is the most probable route to the semiquinone AH.. Our results, however, point to further reaction between AH. * or AH. and the alcohol, and it is our suggestion that reaction 2, which is fast, is followed by a slower process reaction 4 or RCH,OH

+ AH.

RCHOH

+ AH2

(6)

Reaction 4 is energetically more probable with subsequent aldehyde formation by disproportionation 2RcHOH +RCH20H

+ RCHO

(7)

Evidence from the radiation chemistry of aqueous solutions of alcohols supports reaction 7. The evidence in support of reaction 4 or 6 is, as follows. The rate of disappearance of AH. is first order in AH. and RCH2OH. Thus the process 2AH.

4AH2

+A

(8) for removal of AH- cannot be significant. The concentration of AH - is proportional to the light absorbed as required by reactions 1 and 2, and the over-all stoichiometry observed in oxygen6-*is satisfied. More sensitizer can be regenerated by the reaction AH2

+

0 2

A

+ H2Oz

(9)

and similarly the quantum yield of production of AH2 would be 1.0 in the absence of oxygen. On this basis second-order rate constants for reaction 6 have been calculated (see Table I). It is significant that the ratio of the reactivities closely paralIel those calculated by Wellss-' based on oxygen absorption measurements and attributed by him to the relative reactivities of the alcohols to A*. The ratio of ka(ROH)/ ROH -+ is in good agreement with k2(EtOH) for AH.

+

The Journal of Phfisical Chenaistry

Table IV: Reactivities of Alcohols to Transient Species Produced by Photoexcitation ISO-

n-

-

kz(Et0H)

Methyl

Ethyl

propyl

Propyl

Glycerol

This report Bowenss WelIsaJ

0.4 0.3 0.12

1 1.0 1.00

1.9 2.0 2.14

1.5 1.5 1.53

0.7 0.5 0.28

tion from the alcohol. Reaction 6 would therefore appear to be the rate-controlling reaction which determines the absorption of oxygen. The reactivity of carbohydrates, based on oxygen absorption,26 is similar also to the reactivity of D-sorbitol, D-glucose, and cellobiose with AH.. When studying the photochemical oxidation of alcohols by p-benzoquinone in CCI, a t 25", Atkinson and Di2' found it necessary to propose a reaction such as (6) to account for the dependence of quantum efficiency on alcohol concentration. To obtain some estimate of the relative values of the rate constants for reactions 6 and 2 and information also about the nature of the photoreactive A* which is the precursor of AH., we have utilized triplet energy transfer. The principles have been described by Wilkinson." The t'riplet energy levels (cm-1) of anthraquinone, anthracene, naphthalene, and biphenyl are, respectively, 22,000,28 14,700,29 21,300,29 and 22,800.29 These hydrocarbons have lower triplet levels but higher singlet levels than anthraquinone. Thus, when a suitable filter is introduced to prevent direct population of the singlet level in the hydrocarbons, only the anthraquinone is excited directly. If the triplet state of anthraquinone is produced as an intermediate in the photoinduced reaction, conditions are suitable for energy transfer to the lower lying triplet state of the hydrocarbon. These reactions may be represented

A +A* (singlet) A* +A* (triplet) (25) E. J. Bowen, Nature, 177, 889 (1956); E. J. Bowen and C. W. Bunn, J . Chem. Soc., 2353 (1927); E. J. Bowen and E. T. Yarnold, ibid., 1648 (1929). (26) G. 0. Phillips and T. W. Rickards, unpublished data. (27) B. Atkinson and M. Di, Trans. Faraday SOC.,54, 1331 (1958). (28) M. Kasha, Radiation Res. Suppl., 2 , 243 (1960). (29) G. N. Lewis and M. Kasha, J. Am. Chent. Soc., 66, 2100 (1944).

PHOTOSENSITIZED OXIDATION OF ALCOHOLS

Triplet-triplet energy transfer may therefore occur in competition with reaction 2. As the triplet energy levels of the donor and acceptor become comparable, the transfer efficiency decreases.a0 Our observations are that the rate of formation of and final equilibrium concentration of AH during irradiation of a solution of anthraquinone in 2-propanol-benzene (1:4) are reduced by the addition of M naphthalene in neutral and alkaline solution. It is possible, therefore, that the triplet state is the precursor of A H . . The M ) does not change the presence of biphenyl final concentration of AH , which would be anticipated because of its higher lying triplet state and hence making energy transfer energetically impossible. It would appear, therefore, that the ability of AH. to abstract a hydrogen atom from the alcohol or carbohydrate, in addition to its formation by reaction 2, will determine its ability to act as a sensitizer in

-

-

3069

photooxidation reactions. The 2-sulfonate is well known to be lem reactive than the 2,6-di~ulfonate,~-~ and this is confirmed by the lower k2 for the 2-sulfonate in reaction 6. The l-sulfonate and 1,bdisulfonate are poor sensitizers with quantum yields for the production of AH2 in evacuated systems