7675
Effects of Methyl Substitution on the Photoreactivity of Phenyl Ketones. The Inapplicability of Hammett (T Values in Correlations of Excitation Energies Peter J. Wagner,* M. J. Thomas, and E. Harris Contributionfrom The Department of Chemistry, Michigan S t a t e Unicersity, E a s t Lansing, Michigan 48824. Received M a y 4, 1976
Abstract: The photoreactivity of various ring-methylated butyrophenones and valerophenones is correlated with the effects of methyl substitution on the triplet energy of benzoic acid. meta-Methyl substitution stabilizes the r,r*triplets of the benzoyl chromophore somewhat more than does para substitution. para-Methyl substitution destabilizes n , r triplets whereas metamethyls have a t most a slight effect. As the energy difference AET between n,r* and K,K* triplets of the methylated ketones increases, the observed rate constant for triplet-state hydrogen atom abstraction decreases and is proportional to exp(-AET/ R T ) . No evidence for vibronic mixing induced reactivity in H,K* ketone triplets is apparent: all reactivity seems to arise from equilibrium levels of the upper n,K* states. The results illustrate the inapplicability of Hammett u values in correlations of excited-state reactivity.
The quantum yields of 4-methyl- and 3,5-dimethylbutyroIt is now well established that ketones with a,a* lowest phenone and of 3,4-dimethylvalerophenonedecreased slightly triplets are appreciably less reactive than those with n,a* with increasing ketone concentration over the range 0.05-0.2 triplets in hydrogen-abstraction reactions, both intermolecuM. The quantum yields for the other ketones were invariant lar'-3 and i n t r a m ~ l e c u l a r .In ~ .cases ~ where the two triplets are over that concentration range. close enough in energy to equilibrate thermally before Spectroscopic Studies. Uv spectra of each ketone in heptane decaying, hydrogen abstraction can occur from low concenenergies are listed in Table 11. All ketones trations of n,a* triplets even when the a,a*triplets are l ~ w e r . ~ . ~were measured; ,A phosphoresce strongly a t 77 K, but our apparatus could not It has also been suggested3 that vibronic mixing7q8of the two resolve the band structure in the more heavily substituted ketriplets might induce n,a*-like reactivity in the lowest triplet. tones. Phosphorescence spectra for the various methylated Despite the lack of any theoretical or experimental evidence benzoic acids were recorded in ethanol. These all showed vion the quantitative extent of state mixing induced by vibronic brational spacings of 1020 and 1540 cm-'. The 0-0 band coupling, it is certain that coupling is maximal when the energy energies are recorded in Table 11. The phosphorescence of ethyl gap separating the two triplets is minimal. Methyl groups are benzoate at 77 K was identical in a rigid MCIP glass and in an the most weakly electron-donating substituents in terms of their effects on the K,K* excitation energies of b e n ~ e n e . ~ , ~isopentane slush. Consequently we have systematically varied the number and Discussion positions of methyl groups on butyrophenone and valeropheTriplet-state rate constants were determined as usualiofrom none. Normal analysis of triplet lifetimes and type I1 quantum yields5,I0 has allowed us to measure reactivity as a function of eq 1. It is assumed that pyridine prevents disproportionation 3 n , ~ *- 37r,7r* energy gaps. k H = @"ax/? (1) I/' = k H k d (2)
+
Results Quantum Yield and Quenching Measurements. Degassed benzene solutions 0.1 M in one of the ketones listed in Table I were irradiated at 313 nm to conversions of 5-10%. Quantum yields of substituted acetophenone and cyclobutanol formation were determined relative to a valerophenone actinometer. l o Quantum yields were lower when low concentrations of 2,5dimethyl-2,4-hexadiene were present. Stern-Volmer plots of $ O / $ vs. diene concentration were linear; their slopes yield k,r values. With k, equal to 5 X IO9 M-' s-I,l1 triplet decay rates (1 / r ) listed in the table were calculated. Quantum yields were also determined as a function of added pyridine concentration;I2 the maximum values (observed at 0.5-1.0 M pyridine) are listed in the table. Low concentrations ( para substitution pattern takes over. This large meta tuted ketones are even lower in polar solvent^.^^^^ It is well substituent effect on transition energies does not involve any known that n,a* phosphorescence occurs a t higher energies direct conjugation of substituents but rather the vector addition in polar solvents than in nonpolar solvents,2' so that x , , , is ~ of substituent effects on transition dipole^.^ For example, with lower in polar solvents. This solvent effect lowers type I1 two electron-accepting substituents, the para compound's quantum yields at high alcohol concentrations.5 triplet energy is considerably lower than the meta comPattern of Substituent Effects. An important sidelight of this pound'~.~~ work is the realization that the pattern of substituent effects on triplet energies differs for n,a* and a,a*triplets. A metaExperimental Section methyl produces a negligible effect on 3n,a* energies but a n Chemicals. Valerophenone was available from previous studies.I0 even larger effect than para-methyl on 3 7 r , ~ *energies. The methyl-substituted ketones were prepared by three methods. In Ground-state substituent constants are inappropriate for exall cases, the crude reaction products were first vacuum distilled on cited state properties, so Hammett plots such as Figure 1 must a IO-cm Vigreux column, passed through a short column of alumina, be interpreted very cautiously. In the present case, we included recrystallized from heptane or pentane, and then carefully distilled the plot merely to demonstrate a change in mechanism. on a Nester Faust Model N F T 61 spinning band column. In all cases, To a first approximation both n a* and a a* excitaGC-detectable impurities were less than 0.01%. tions populate much the same a* orbital. It is widely accepted Method 1. The proper methyl-substituted benzonitrile (0.1 mol) that, in phenyl ketones, the lowest a* orbital is located priwas added to 0.12 mol of propyl- or butylmagnesium bromide in 200 ml of ether. The resulting solutions were refluxed 1 h, cooled, and marily on the carbonyl and is mixed with the benzene a*,orpoured onto 300 g of ice to which 30 ml of concentrated HCI had been bitaL2' To a good approximation, an n a* excitation can be added. After all solids were dissolved, the two phases were separated, viewed as a one-electron reduction of the a system. There is and the cold aqueous phase was extracted with ether to remove any very little electron density at the meta position of the a*,ororganic residues. The aqueous solution of imine hydrochloride was bita1,28so meta substituents have negligible effects on energies. heated on a steam bath for 30-60 min. The resulting ketone was exThe reactivity of the n,a* state is dominated by the hole in the tracted into ether. The ether extract was dried over anhydrous magoxygen n orbital. Since the n orbital is orthogonal to the a nesium sulfate and then reduced on a rotary evaporator. Ketones so system, ring substituents can produce normal inductive effects prepared were: p-methylbutyrophenone, bp 72 OC (1 Torr); M S (70 on reactivity. eV) m/e 162, 134, 119, 91; 60 M H z ' H N M R (CDC13) 6 ~ 1.00~
-
+
-
l/3,/
1/12
~
~
(t, 3 H , J = 7.0 Hz, CH3-C), 1.75 (m, 2 H , C-CH2-C), 2.40 (s, 3 H, C H 3 - k ) . 2.83 (t, 2 H , J = 7.0 Hz, COCH2-), 7.22 (d, 2 H , ortho-H), 7.87 (d, 2 H , meta-H); m-methylbutyrophenone, bp 92 OC (3 Torr);
MSm/e162,134,119,91;NMR60.98(t,3H),1.73(m,2H),2.40
1/12
1/3 4 J /(1 4 I/ I /4
1/4
I13
Ts
Ta
We pointed out earlier than the lowest a,a*triplets of these phenyl ketones correspond to the La singlets, with ,,A a t 238 nm for acetophenone, and suggested that triplet reactivity may correlate with the energies of these singlet a,a* transition^.^,^^ Unfortunately, substituent effects on the 'La and 3La states differ. The La states of benzenes are combinations of A, ra* and a, a,* transitions. The presence of a conjugated elec-
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-
Journal of the American Chemical Society
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98:24
(s, 3 H ) 2.82 (t, 2 H ) , 7.25, 7.35, 7.75 (3 H , aromatic H);p-methylvalerophenone; mp 20 "C; M S m/e 176, 134, 119.91; N M R b 0.90 ( t , 3 H ) , 1.52(m,4H),2.35(~,3H),2.8O(t,2H),7.17(d,2H),7.82 (d, 2 H ) . m-Methylvalerophenone, bp 81 "C (2 Torr); M S m/e 176, 133, 119, 91; N M R 8 0.98 (t, 3 H), 1.55 (m, 4 H), 2.40 (s, 3 H), 2.87 (t, 2 H), 7.20, 7.37, 7.75 (4 H , aromatic H's). 3,3'-Dimethylbenzophenone was also prepared by this method, mp 46-47 O C . Method 2. The proper dimethyl benzoic acid (Aldrich) (0.01 mol) was refluxed with lithium hydride in dry T H F to produce the lithium benzoate. Propyl- or butyllithium was prepared by adding 0.01 mol of the alkyl bromide to a suspension of lithium ribbon in T H F at -20 OC. The alkyllithium reagent was then added slowly to the rapidly stirred suspension of lithium benzoate; the resulting mixture was refluxed under nitrogen for 2 h. The mixture was then hydrolyzed by being stirred vigorously into a cold aqueous solution containing 0.3 mol of HCI. Ketone was extracted into ether, dried, and distilled. Ketones so prepared were: 3,4-dimethylbutyrophenone, bp 105 OC (2.3Torr);MSmle 176, 148, 133, 1 0 5 ; N M R 6 1.02(t,3H), 1.85(m, 2 H ) , 2.35 (s, 6 H ) , 2.85 (t, 2 H), 7.10-7.3 (m, 3 H , aromatic H's); 3,5-dimethylbutyrophenone,bp 98 OC (2.4 Torr); M S m/e 176, 148, 133, 105; N M R b 0.98 (t, 3 H), 1.70 (m, 2 H), 2.35 (s, 6 H), 2.85 (t, 2 H), 7.15 (s, 2 H , ortho-H), 7.55 (s, 1 H , para-H); 3,4-dimethylval-
/ November 24, 1976
i
7679 erophenone, bp 110 "C ( I Torr); M S m/e 190, 148, 133, 105; N M R 60.98 (t, 3 H), 1.65 (m, 4 H), 2.35 ( s , 6 H), 2.95 (t, 2 H), 7.20-7.85 (m, 3 H); 3,5-dimethylvalerophenone:bp 106 "C (1.2 Torr); MS m/e 190, 148,133, 105; N M R 6 0.94 (t, 3 H), 1.60 (m, 4 H), 2.35 (s, 6 H), 2.93 (t, 2 H), 7.20 (s, 2 H), 7.65 (s, 1 H). Method 3.331,2,3-Trimethylbenzene (18.3 g, 0.15 mol) (Chemical Samples Company) and 10.6 g (0.08 mol) of AIC13 were stirred in 100 ml of 1,2-dichloroethane at 0 OC. Butyryl or valeryl chloride (0.30 mol) was added dropwise over 2 h. The reaction mixture was stirred for an hour until further generation of HCI was minimal; it was then poured into chopped ice containing enough added HCI to dissolve all AI(OH)3. The mixture was extracted with chloroform; the organic layer was dried and distilled. G C analysis indicated an 80:20 mixture of 3,4,5- and 2,3,4-trimethyl isomers. The latter isomers distilled over first, followed by the desired isomers ( 1 20- 130 OC, 1-2 Torr), which were then recrystallized several times: 3,4,5-trimethylbutyrophenone, mp6O0C;MSm/e 190, 162, 147, 1 1 9 ; N M R 6 l.OO(t,3H), 1.67(m, 2 H ) , 2.24 (s, 3 H , para CH,), 2.35 (s, 6 H, 3,5-CH3), 2.83 (t, 2 H), 7.52 (s, 2 H , ortho H); 3,4,5-trimethylvalerophenone, mp 37-38 "C;
G. Porter and P. Suppan, Trans. Faraday SOC.,61,1664 (1965). (a) N. C. Yang, D. S.McClure, S. L. Murov, J. J. Houser, and R. Dusenbery. J. Am. Chem. Soc., 89,5466(1967); (b) N. C. Yang and R. Dusenbery, ibid.,
90,5899 (1968). E. J. Baum, J. K. S.Wan, and J. N. Pitts, Jr., J. Am. Chem. Soc., 88,2652
(1966). P. J. Wagner, A. E. Kemppainen. and H. N. Schott, J. Am. Chem. Soc., 95,
5604 (1973). P. J. Wagner and T. Nakahira, J. Am. Chem. SOC.,95,8474 (1973). R. M. Hochstrasser and C. A. Marzzacco in "Molecular Luminescence", E. C. Lim, Ed., W. A. Benjamin, New York, N.Y., 1969,p 631. (a) T. H. Li and E. C. Lim. Chem. Phys. Left., 7, 15 (1970);(b) L. Goodman and M. Koyanagi, Mol. Photochem., 4,369 (1972). J.Petruska, J. Chem. Phys., 34, 1 1 1 1 , 1120(1961). P. J. Wagner and A. E. Kemppainen, J. Am. Chem. SOC., 94, 7495
(1972). (a) P. J. Wagner and I. Kochevar. J. Am. Chem. SOC.,90,2232 (1968); (b) W. D.K. Clark, A. D. Litt, and C. Steel, ibid., 91,5412 (1969);(c) G.Porter and M. R. Topp, Proc. Roy. Soc., Ser. A, 315, 163 (1970). P. J. Wagner, I. Kochevar, and A. E. Kemppainen. J. Am. Chem. SOC.,94,
7489 (1972). (a) 0.L. Chapman and G. Wampfler. J. Am. Chem. SOC.,91,5390 (1969); (b) D. Schuster and T. M. Weil, ibid., 95,4091 (1973);(c) M. W. Wolf, K. D. Legg, R. E. Brown, L. A. Singer, and J. H. Parks, ibid., 97, 4490
MSm/e204,162,147,119;NMR60.98(t,3H),1.65(m,4H),2.25 (1975). (a) D. I. Schuster. T. M. Weil, and A. M. Halpern. J. Am. Cham. Soc., 94, (s, 3H), 2.35 (s, 6H), 2.85 (t, 2H), 7.53 (s, 2H). 8250 (1972):(b) J. Saltiel. H. C. Curtis, and B. Jones, Mol. Photocham., 2, Solvents and quencher were purified as usual.'0 All of the benzoic 231 (1970). acids whose phosphorescence was measured were Aldrich prodL. P. Hammett, "Physical Organic Chemistry", 2nd ed, McGraw-Hill, New York. N.Y.. 1970.D 356. ucts. (16)F. B. Mallory, M. T.-Arnold, C. W. Mallory, andT. E. Colman, Abstracts, 5th Irradiation and analysis procedures were also conducted as usual.I0 Northeast Regional Meeting, American Chemical Society, Rochester, N.Y., The methyl- and dimethylacetophenones for calibrating the G C re1973,No. 26. (17)D.R. Kearns and W. A. Case, J. Am. Chem. Soc., 88,5087 (1966). sponse were available commercially. That for the trimethylaceto(18)M. J. May, unpublished work. phenone was extrapolated from those of the reactant ketones. The (19)J. Saltiel, H.C. Curtis, L. Metts, J. W. Miley, J. Winterle. and M. Wrighton. cyclobutanols were assumed to have the same response as the parent J. Am. Chem. SOC.,92,410 (1970). ketones; they were not isolated; their identity was assumed from their (20)P. J. Wagner, M. J. May, and A. Haug, Chem. Phys. Left., 13, 545 (1972). characteristic G C retention times. Pentadecane, hexadecane, and (21)D. R. Arnold, Adv. Photochem., 6, 301 (1967). heptadecane were used as internal standards for G C analysis of the (22)R. 0.Loutfy and R. 0. Loutfy, Tetrahedron, 29, 2251 (1973). acetophenone products (on 4% QF-1, 1% Carbowax 20 M col(23)J. R. Jones and J. A. Robinson, J. Electroanal. Cham., 19,297 (1968). umns). (24)(a) A. A. Lamola, J. Chem. Phys., 47,4810(1967);(b) R. M. Hochstrasser and C. Marzzacco, ibid., 49,971 (1968). Phosphorescence spectra were measured on an Aminco-Bowman (25)P. E. Stevenson in "Systematics of the Electronic Spectra of Conjugated spectrofluorimeter with a phosphoroscope attachment. Samples were Molecules. A Source Book", J. R. Platt, Ed., Wiley, 1964,Paper 32. contained in 2 mm i.d. quartz tubes. Benzoic acid derivatives were (26)J. N. Murrell. "The Theory of the Electronic Spectra of Organic Molecules", irradiated a t 270 nm; 3,3'-dimethylbenzophenone at 362 nm. Methuen, London, 1963,p 302.
Acknowledgment. This work was supported by grants from the National Science Foundation. W e thank Professor Don Arnold for some unpublished data and for several useful discussions. References and Notes (1)G. S. Hammond and P. A. Leermakers, J. Am. Chem. SOC., 84, 207 (1962).
(27)H. Suzuki, "Electronic Absorption Spectra and Geometry of Organic Molecules", Academic Press, New York, N.Y., 1967,p 438 ff. (28)P. H. Reiger and G. K. Fraenkel. J. Chem. Phys., 37,2795 (1962). (29)P. J. Wagner and G. Capen, Mol. Photochem., 1, 173 (1969). (30)N. Hirota, T. C. Wong, E. T. Harrigan, and K. Nishimoto, Mol. Phys., 29,903 (1975). (31)P. J. Wagner and M. J. May, Chem. Phys. Left., 39,350(1976). (32)D. R. Arnold, J. R. Bolton, and J. A. Pederson, J. Am. Chem. Soc.,94,2872 (1972). (33)G. Marino and H. C. Brown, J. Am. Chem. Soc., 81,5929 (1959)
Wagner, Thomas, Harris
/ Photoreactivity of Phenyl Ketones
