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.<sec.< tert. and the order is reversed in the case of aro- matic hydroperoxides. Thus the following con- clusion: the larger the value (absolute) of Taft's 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<EHP<CHP and 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.