Structure and Property of Hydroperoxide in Solution*

by Shin'ichi Kato**, Takamasa Ishihara** and Fujio Mashio**

Summary: Structureand propertyof hydroperoxidein solutionwere studied in connectionwith the epoxidationof olefins. ThepKa valuesof n-, sec-and tert-butylhydroperoxide and that of benzyl-, ethylbenzene-and cumene-hydroperoxidewere measured by UV spectrophotometricmethod and thepKa valueswere plotted against Taft polar substituentconstants. A linear relationwas obtained and p* value was +4.1 for the aliphatic series, but the linefor aromaticseries was differentfrom that for aliphatic series even thoughhaving almost the same slope. In the case of aromatichydroperoxides,

intramolecularπ-H bonding between peroxy H and π-electron of aromatic ring was observed by IR spectrumand thefact well explainswhy a differentline from that of aliphaticseries is obtained. The abilities of associationand solvationof six hydroperoxidesexamined in this study were evaluatedin termsof the shift of VOHand the relationwith pKa is discussed. Equilibrium constants of the di-

meric association and the intra molecular. π-H bonding of hydroperoxide is also estimated. The rates of epoxidationof 1-octeneby the six hydroperoxideswere measuredin thepresence of a solvent-soluble molybdenumcatalyst and a possible configurationof the transition state complexis suggested.

sociation, especially H-bonding characteristic of 1 Introduction the hydroperoxides, were also measured. The In the case of epoxidation of olefin by hydro- interrelation among these values and the re- peroxide as a source of electrophilic oxygen1), activities of hydroperoxides in epoxidation are the rate is slow and the yield is low when catalyst discussed. is not used2). The authors3) reported previously

that the reaction proceeds in the presence of MoO3 2 Experiments at an adequate rate and an epoxide is selectively 2.1 Materials obtained. The following scheme was proposed Normal-butyl, sec-butyl and benzyl hydro- in the previous paper. peroxide were prepared by autoxidation of the ROOH+Catalyst→←[Complex] (1) corresponding alkyl or benzyl cadmium chloride at -70～-80℃6). Ethylbenzene hydroperoxide was prepared by autoxidation of ethylbenzene. Tertiary-butyl and cumene hydroperoxide were obtained from commercial scurces. Each hydro- Sheng et al.4)studied the effectof catalysts,sol- vents,reaction temperaturesand olefinstructure peroxide was purified by the following procedure: the sodium salt was washed with alcohol tho- on the epoxidation,and assumed almost the same roughly, being dissolved in water and the aqueous scheme, but the structureof complex is somewhat solution was acidified with acetic acid and ex- different(seebelow). tracted with ether. The ethereal layer was Hydroperoxides have a nature of association dried over anhydrous sodium sulfate and ether with each other and with solvent (solvation). was evaporated in vacuum. The purity was The rate of epoxidation is slown down by H- above 97% by iodmetric titration. bonding solvation as in the case of peracids. Preparation of Soluble Molybdenum Catalyst: Acidity constants,pKa, of three aliphatichydro- Permolybdic acid (PMA)- Five percent (w/v) peroxides (n-,sec- and tert-butylhydroperoxide) of molybdenum trioxide were added to aqueous and three aromatic hydroperoxides (benzyl-, 0.2M H2O2 solution and the mixture was stirred ethylbenzene- and cumene-hydroperoxide) were for 10hrs, at 50℃. About 0.1M solution of measured and the degrees of solvation, and as- permolybdic acid solution (yellow) was obtained by removing the residual insoluble MoO3. The * Received December 8, 1969. ** Kyoto Technical University (Matsugasaki , Sakyo-ku, 10-3～10-4M solution diluted by tert-butanol Kyoto) was an effective homogeneous catalyst for epoxi-

Volume 12-May 1970 118 Kato, Ishihara and Mashio: Structure and

dation. Diethoxymolybdenyl Chloride (DEM): Molybdenum oxychloride Mo2O3Cl68) was obtained by heating the mixture of hydrated sodium molybdate (Na2MoO4 2H2O) and an excess thionylchloride. Four equivalent amounts of sodium ethylate in alcohol was added to the oxychloride and diethoxymolybdenum oxychloride assumed to have the following composition (Mo2

O3Cl2(OEt)4, Mo: 39.4％, Cl: 15.1%) was Fig. 1 Plot. of lonization Constants of HP and Taft σ* obtained. The 10-3～10-4M solution of the

Mo-salt in tert-butanol is also an effective catalyst. were used. Linear relation of Taft equation (3) 2.2 Experimental Methods holds good for the aliphatic series, and p*= The pKa value of hydroperoxides was measured +4.1 (r=0.913) was obtained. by the UV spectrophotometric method9). The logK/K0=ρ*σ* (3) conditions of measurement are as shown below: This shows that the dissociation constant Ka [HP] is 10-3M, the ionic strength 0.45 and the reaction temperature is 20±0.5℃ in the system (Eq. 4) is governed by I-Effect of substituent R. Ka HP-KOH-KCl. The key band is 260mμ in ROOH→←ROO-+H+(4) aliphatic hydroperoxide and 280mμ in aromatic The relation for the aromatic series is different, hydroperoxide. Hitachi double beam UV but the slope is almost the same, and a particular spectrophotometer type 124was used. Stretching association of aromatic hydroperoxide is suggested.

vibration band of free OH (VOH,free) and as- In the following section, an intramolecular π-H sociated OH (VOH, assoc.) were measured in 3μ bonding between undissociated peroxyhydrogen region of the IR spectrum. The product of and aromatic ring, is shown by IR spectrum. hydroperoxide concentration and the length of The ρ* value of +4.1 is larger than ρ* (+1.721

light path was set up to be constant by using at 25℃)11) for the aliphatic carboxylic acid and a convertible NaCl cell. IR spectra measure- this fact shows that -O- is much larder than -C- ments were performed on a Nippon Bunko IR O spectrophotometer model DS-402G. in the transmission of I-Effect to the terminal hydrogen. 3 Results and Discussion 3.2 Intra- and Intermolecular Hydrogen 3.1 Dissociation Constant of HP Bond of Hydroperoxide

Some pKa values of hydroperoxides are reported IR spectra in 3μ region are shown in Fig. 2 by Everett10), but pKa values of hydroperoxides and Fig. 3. The maximum frequency of free used in this study are not known. The data deter- OH (VOH,free) is at 3,553cm-1 in the case, of mined are shown in Table 1. Fig. 2-A. The relative intensity of VOH, free and

Table 1 The pKa values of HP

The pKa value of hydroperoxide increases in the order of primary, secondary and tertiary in both aliphatic and aromatic series. In order to examine the substituent effect on the acidity of HP, pKa values were plotted against Taft polar substituent constants in Fig. 1. The pKa values of methyl and ethyl hydroperoxide of Everett10) Fig. 2-A 2-B

Bulletin of The Japan Petroleum Institute Property of Hydroperoxide in Solution 119

hydroperoxide are shown in Table 2. Suffix

to Δν shows the type of association and the

solvent solvated to the hydroperoxide. The

νOH, free of each hydroperoxide appears within

the range of 3,5433～553cm-1. The magnitude of the shift resulted from the intermolecular

associaton of hydroperoxides, Δν assoc., is the following order in aliphatic HP: prim.

σ* notwithstanding the sing, the stronger the

association. The intramolecular π H-bond ob- served in the aromatic hydroperoxides is readily

Fig. 3-A 3-B explained by the geometrical model shown in Fig. 4. When the plane containing three atoms νOH, assoc., under the condition whereby the -O-OH is oriented perpendicular to the aromatic

product of hydroperoxide concentration and the ring, the distance between the center of the length of light path are kept constant, clearly aromatic ring and the terminal H atom is esti-

changes in accordence with the hydroperoxide mated to be 2.1～2.2Å and this well agrees

concentration. That is to say, the intensity of with the H-bonding distance. The shift, Δν intra π, νOH, assoc. is attributed to an intermolecular as- increases in the order of BHP

sociation of hydroperoxide. An absorption band the acidity of hydroperoxide is in the opposite

of intramolecular H-bond (νOH, intra π) is observed

between νOH, free and νOH, assoc. in the case of

CHP (Fig. 2-B). The ratio of intensities of νOH, free.

and νOH, intra π does not vary with hydroperoxide concentration and has a definite value according to the structure of aromatic HP, that is 44/56 (BHP), 49/51 (EHP) and 55/45 (CHP). The

absorption band of νOH , intra π is characteristic for

aromatic hydroperoxide and is assigned to in-

tramolecular π H-bond between the aromatic π electron and perhydroxy hydrogen. H-bonding absorption bands between hydroperoxide and solvent (benzene or dioxane) are shown in Fig.

3-Aand 3-B, and νOH, solv. is its wave number

for maximum absorption. The strength of the intra- and intermolecular H-bond is evaluated relatively by the shift of

frequency (Δν) from νOH, free. The data for each Fig. 4

Table 2 Shift of OH Vibration Frequency (cm-1) Temp. 22±1℃

Volume 12-May 1970 120 Kato, Ishihara and Mashio: Structure and order. This fact can be attributed to a change concentration and length of light path is con- of bond angle at α-carbon by a steric hindrance stant). Kassoc.of aliphatic hydroperoxide is cal- of its substituents. culated by using Eq. 7 and shown in Table 3. The shift due to solvation , Δν solv.. increases In the case of aromatic HP, Kassoc. was with the basicity of solvent and the acidity of calculated by a similar treatment, taking the hydroperoxide. presence of intramolccularπ-H bonding into 3.3 Association of Hydroperoxide consideration. The results are shown in Table Assocation of hydroperoxide in a dilute solution 4. Calculated values of Kassoc. in Table 3 and initially forms dimeric associate and a higher Table 4 reach asymptotically to a constant value order aggregate is formed in a more concentrated as Po decreases. The existence of higher order solution as shown below . aggregate may be neglected approximately below Kassoc. 10-2 molar hydroperoxide concentration. Inter- 2ROOH→←(ROOH)2 (5) K'assoc. poated values of Kassoc to [Po]iim→o(real kassoc) (ROOH)n-1+ROOH→←(ROOH)n (6) are also shown in Table 5. Walling et al.13) assumed the following cyclic 3.4 Effect of Hydroperoxide Structure on associate as H-bonding dimer constitution . In Olefin Epoxidation The epoxidation rates of 1-octene were measured by using the six hydroperoxides in the presence of Mo catalyst, PMA or DEM. The results are shown in Fig. 5 and the reaction the sufficiently dilute solution of HP, Kassoc, is conditions are shown in Fig. 5. Yields of epoxide given by the Eq. 7, in which Po is the total are quantitative for the consumed hydroperoxide. concentration of hydroperoxide and Pf is the Reactionrates are of the firstorder to[HP]and concentration of unassociated hydroperoxide in [Olefin]and the reactionrate constants,k, are equilibrium. calculated from the data. The reactivity of Kassoc.=(P0-Pf)/2pf2 (7) hydroperoxide for epoxidation is not much affected

Pf can be calculated in terms of Efree (absorp- by its structure and does not coincide with its tion intensity of νOH, free at [HP] lim→o in the pKa value. The order of reactivity is tert.< condition in which the product of hydroperoxide prim.

Table 3 Calculation of Equilibrium Constant of Association of Aliphatic HP Solvent: CCl4 C.l=0.012 Temp.22±1℃

Bulletin of The Japan Petroleum Institute Property of Hydroperoxide in Solution 121

Table 4 Calculation of Equilibrium Constant of Association of Aromatic HP Solvent= CCl4, C.l=0.012 Temp. 22±1℃

Table 5

ning of this paper should be rewritten as follows.

K solv. ROOH+solvent→←(ROOH)solv . (9)

(ROOH)solv.+catalyst→←[Ｃomplex]+solvent (10) Sheng et al.4) proposed I or II below for the structure of HP-catalyst-Olefin complex in the transition state of epoxidation. Giving attention to the fact that many of the active molybdenum catalysts have molybdenyl oxygen and con- sidering the polar nature and bond distance of Mo-O, III seems the most reasonable structure, and it is well elucidated that peroxide (ROOR) does not act as an epoxidizing reagent. Fig. 5-A 5-B

is more reactive than PMA, as the latter was made in aqueous solution and so that the hydration of HP or catalyst would take place to retard the epoxidation reaction. As for solvent, dioxane is more basic than tert-butanol and dioxane deacti- Various kinds of H-bonding characteristics and vates HP by solvation. Generally speaking, pKa of hydroperoxide should have important solvation of hydroperoxide retards the formation effects on the bonding between catalyst and of active complex which is formed from hydro- hydroperoxide, but these characteristics also peroxide and catalyst, thus strong donor-type govern the intermolecular association and sol- solvents reduce considerably the rate of epoxi- vation which retard complex formation. Conse- dation. Consequently, Eq. 1 shown at the begin- quently the characters of hydroperoxide do not

Volume 12-May 1970 122 Kato, Ishihara and Mashio: Structure and Property of Hydroperoxide in Solution

correlate directly with the rate of epoxidation. Ser., 76) 1968 Am. Chem. Soc., Washington D. C. 5) Renolen, P., Ugelstad, J., J. Chem. Phys., 57, 634

Acknowledgements (1960). 6) Huch, H., Ernst, F., Chem. Ber., 92, 2710 (1959); The authors wish to express their thanks to Mr. Walling, C., Bucker, S. A., J. Am. Chem. Soc., 77, 6032 (1955). Y. Tashiro and Mr. Y. Fujimoto for their assistance 7) Crouthamel, C. E., Johnson, C. E., Anal. Chem., 26, in the experimental work. This work was sup- 1284 (1954). ported in part by a Grant in Aid of Scientific 8) Gmelins Handbuch der Anorganischen Chemie. Research from the Ministry of Education. syst. No. 53, p. 173 (1938). 9) Albert, A., Serjeant, E. P., "Ionization Constants of Acid and Bases, A Laboratory Manual", Japa- Literature nese Translation by Matsuura, pp. 63, (1963) Maru- 1) Lee, J. B., Uff, B. O., QuarL. Rev., 21, 429 (1967). zen. 2) Brill, W. F., Indictor, N., J. Org. Chem., 29, 710 10) Everett, A. J., Minkoff, G. J., Trans. Faraday Soc., (1964). 49, 410 (1953). 3) Mashio, F., Kato, S., J. Syn. Org. Chem., Japan, 11) Taft, Jr., R. W., J. Am. Chem. Soc., 74, 3120 (1952); 26, 367 (1968); Memoirs of Faculty of Kyoto 75, 4231 (1953). Technical Univ., Science and Technology, 16, 79 12) Fox, J. J., Martin, A. E., Trans. Faraday Soc., 36, (1967). 897 (1940). 4) Sheng, M. N., Zajacek, J. G., "Oxidation of Organic 13) Walling, C., Heaton, C. D., J. Am. Chem. Soc., 87, Compounds", Vol. 2, pp. 418, (Adv. in Chem., 48 (1965).

Bulletin of The Japan Petroleum Institute