Indian Journal of Chemistry Vol. 40A, May 2001, pp. 474-478 Kinetics and mechanism of the oxidation of aliphatic alcohols by benzy ltrimethy lammoni urn dichloroiodate Poonam Gupta & Seema Kothari* Department of Chemistry, J.N.V. University, Jodhpur, 342 005, India Received 14 June 2000; revised 2 January 200/ The oxidation of a series of aliphatic alcohols by benzyltrimethylammonium dichloroiodate (BTMACI), in glacial acetic acid in the presence of zinc chloride, leads to the formation of the corresponding carbonyl compounds. The reaction is first order each wi th respect to the alcohol, zi nc chloride and BTMACI. Addition of the benzyltrimethylammonium chloride enhances the rate slightly. The oxidation of deuteriated ethanol indicates the presence of a substantial kinetic isotope effect. [PhCH 2 Me 3 Nt[IZn 2 CI 6 r is postulated to be the reactive oxidizing species. The reaction is susceptible to both polar and steric effects of the substituents. A mechanism involving transfer of a hydride ion from the alcohol to the oxidant has been proposed. Benzyltrimethylammonium polyhalides are widely used as halogenating reagents in synthetic organic chemistryl.2. Recently, polymeric benzyltriethylammonium dichloroiodate and dibromoiodate have been used for the addition of halogens to olefins 3 However, they have been scantly used as oxidizing agents in synthetic organic chemistr/ 6. These compounds are more suitable than molecular halogens because of their solid nature, ease of handling, stability, selectivity and excellent product yields. We are interested in the kinetic and mechanistic studies of the newer oxidizing agents and have reported the oxidation of thioacids 7, oxyacids of phosphorus 8, substituted benzyl alcohols 9 and aliphatic aldehydes 10 by benzyltrimethylammonium <.lichloroiodate (BTMACI). In the present note, we report the kinetics of the oxidation of a series of aliphatic alcohols by BTMACI, in glacial acetic acid in the presence of zinc chloride. Attempts have been made to correlate rate and structure in this reaction. Mechanistic aspects are discussed. Materials and Methods BTMACI was prepared by the reported method 1 and its purity was checked by an iodometric method. The solutions of BTMACI were freshly prepared in the presence of zinc chloride. The alcohols were commercial products and were purified by the usual methods. [I, 1-2 H 2 ]ethanol was prepared by the reported method". Its isotopic purity, as ascertained by its 1 H NMR spectrum was 90±5%. Acetic acid was refluxed with chromic oxide and acetic anhydride for 6 h and then fractionated. BTMACI is only slightly soluble in acetic acid at room temperature. However, an addition of zinc chloride renders it readily soluble in acetic acid. We found that in the absence of zinc chloride, the strength of a saturated solution of BTMACI in acetic acid is 0.0005 mol dm 3. Addition of zinc chloride (0.002 mol dm. 3 ) increased the solubility of BTMACI and a saturated solution of BTMACI, under these conditions, has strength of 0.0016 mol dm 3. Product analysis Product analysis was carried out under kinetic conditions. In a typical experiment, ethanol (0.3 mol) and BTMACI (0.02 mol) were made up to 100 ml in glacial acetic acid in the presence of 0.06 mol of zinc chloride. The reaction mixture was allowed to stand in the dark for ca. 6 h to ensure completion of the reaction. The solution was then treated with an excess (200 ml) of a saturated solution of 2,4-dinitrophenylhydrazine in 2 mol dm- 3 HCI and kept overnight in a refrigerator. The precipitated 2,4-dinitrophenylhydrazone (DNP) was filtered off, dried, weighed, recrystallized from ethanol, and weighed again. The yields of DNP before and after recrystallization was 1.39 g (90%) and 1.30 g (82%) respectively. The DNP was found identical (mp and mi xed mp) with the DNP of acetaldehyde. l:1 similar experiments, wi th
GUPTA et al. : KINETICS OF OXIDATION OF ALIPHATIC ALCOHOLS 475 other alcohols, the yield of the corresponding carbonyl compounds wa:; in the range of 80-91%, after recrystallization. Stoichiometry To determine the stoichiometry, BTMACI (0.005 mol) and ethanol (0.00 I mol) were made up to 100 ml in glacial acetic acid in the presence of zinc chloride (0.015 mol dm \ The reaction was allowed to stand for ca. 10 h to ens-ure the completion of the reaction. The residual BTMACI was determined spectrophotometrically at 364 nm. Several determinations, with alcohols showed that the stoichiometry is 1: I. Spectral studies UV-vis spectra of 0.0005 mol dm- 3 of BTMACI alone and in the presence of 0.002, 0.003 and 0.006 mol dm- 3 of zinc chloride were obtained using a HP-diode array spectrophotometer (Model 8452A), at 300±3 K. The solvent and blank were glacial acetic acid. The scanning speed was 600 nm s' 1 Kinetic measurements The reactions were carried out under pseudo-first order conditions by maintaining a large excess of the alcohol (x15 times or more) over BTMACI, at constant temperature (±0.1 K). The solvent was glacial acetic acid. The reactions were carried out in the presence of zinc chloride (0.003 mol dm- 3 unless mentioned otherwise) and were followed by monitoring the decrease in [BTMACI] spectrophotometrically at 364 nm for at least three half-lives. The pseudo-first order rate constant, kobs was evaluated from the linear (r 2 > 0.995) plots of log [BTMACI] against time. Duplicate kinetic runs showed that the rate constants were reproducible to within ±3%. The experimental third order rate constant, k 3, was determined from the relationship : k3= kobsf[alcohol] [ZnC1 2 ]. Results and Discussion The rate and other experimental data were obtained for all the alcohols. Since the results are similar, only representative data are reproduced here. The oxidation of aliphatic alcohols by BTMACI results in the formation of the corresponding carbonyl compounds as the main product.. Analyses of products and stoichiometric determinations indicate the following overall reaction. RCH 2 0H+[PhCH 2 Me 3 N]IC1 2 ~ RCHO +PhCHzMe 3 NCl + HCl +HI... (1) The reaction is of first order with respect to BTMACI. Further, the pseudo-first order rate constants do not depend on the initial concentration of BTMACI. The reaction rate increases linearly with an increase in the concentration of the alcohol (Table I). The oxidation of alcohols, under a nitrogen atmosphere, failed to induce polymerization of acrylonitrile. Further, an addition of acrylonitrile had no effect on the rate (Table 1 ). The rate of oxidation of alcohols were determined at different temperatures and the activation parameters were calculated (Table 2). To ascertain the importance of the cleavage of an a-c-h bond in the rate-determining step, the oxidation of deuteriated ethanol (MeCD 2 0H) was studied. The results (Table 2) showed that the reaction exhibited a substantial kinetic isotope effect (khiko= 5.99 at 298 K). With an increase in the concentration of zinc chloride the reaction rate increases, for example, under the conditions [BTMACI]=0.001 mol dm- 3, [EtOH]= 1.00 mol dm- 3 at 318 K, when 1 0 3 [ZnC1 2 ] was increased from 2.0 to 10.0 mol dm- 3, the corresponding 10 4 kobs increased from 9.95 to 51.0 s 1 A plot of kobs versus [ZnC1 2 ] is linear (r 2 =0.9960) and passes through the origin. An addition of benzyltrimethylammonium chloride (BTMACI) enhances the reaction rate slightly. Under similar conditions as above, a variation in 10 3 [BTMACl] from 0.0 to 4.0 mol dm- 3, resulted in an increase in the 10 4 kobs from 15.6 to 33.0 s- 1 Table I-Rate constants for the oxidation of ethanol by BTMACI at 318 K 10 3 [BTMACI]/mol dm- 3 [EtOH]/ mol dm- 3 10 4 kob/s' 1 1.0 0.05 0.81 1.0 0.10 1.60 1.0 0.20 3.1 1 1.0 0.30 4.87 1.0 0.50 7.85 1.0 1.00 15.6 1.0 2.00 31.7 1.0 0.50 7.8o 2.0 1.00 15.9 4.0 1.00 14.9 6.0 1.00 15.5 8.0 1.00 15.2 *Contained 0.005 mol dm' 3 acrylonitrile
476 INDIAN J CHEM. SEC. A, MAY 2001 Subst. (R) H Me Et Pr Bu Pri CICH 2 MeOCH 2 Bu' MeCD 2 0H kh 1 ko Table 2-Rate constants and activation parameters of the oxidation of alcohols by BTMACI I 0 2 k 3 (dm 6 mo1" 2 s ') 6. H 6. s 6. c 298 K 308 K 318 K 328 K (kj mo1" 1 ) "" (J mor' K" 1 ) (kj mo1" 1 ) 0.16 12.7 24.9 37.2 47.4 65.2 0.085 1.22 620 2.12 5.99 0.47 1.15 26.1 52.0 49.1 95.1 70.0 132 87.1 158 115 220 0.20 0.52 2.71 5.78 890 1420 4.44 9.06 5.88 5.74 2.39 93.6 152 230 266 395 0.98 10.4 2190 16.6 5.64 70.8±1.9 51.8±0.5 47.0±1.3 47.1±0.3 - \ 44.4±0.8 46.6±1.0 64.9±1.9 55.9±1.1 32.0±1.2 53.5±0.5-61±6-89±2-99±4-96±1-103±1-9:,_ 3-86±6-94±4-123±4-98±2 88.8±1.5 78.1±0.4 76.4±1.0 75.5±0.2 74.9±0.2 74.2±0.8 90.5±1.5 83.9±0.9 68.6±0.9 82.6±0.4 A comparison of the UV-vis spectra (Fig. 1) of BTMACI alone and in the presence of different concentrations of ZnC12 showed that the nature of the spectra is not much different in the presence and absence of zinc chloride. However, there is an initial sharp decrease in the absorbance followed by a regular but gradual decrease in the absorbance of BTMACI on further addition of increasing amounts of ZnC12. This clearly showed that a strong complex is formed initially which undergoes further complexation whose concentration increases with an increase in the concentration of ZnC12. From our data on the solubility of BTMACI in the absence and presence of zinc chloride, the value of the equilibrium constant 9, Kh comes to be ca. 2400 mor' dm 3. This indicates that even at the lowest concentration of zinc chloride used, almost whole of BTMACI will be in the form of complex (A) (Eq. 2). The linear increase in the rate with an increase in the concentration of zinc chloride points to a further complexation (Eq. 3). The role of ZnCh is to coordinate with ICh-. Interhalogen compounds are known to form complexes with Lewis acids like zinc chloride 12. K, [PhCH2Me3NtiCh-+ZnCh "'=7 [PhCH2Me3Nt [IZnCl4r (A)... (2) K2 (A)+ZnCh """' [PhCH2Me3Nt [IZn2Cl6r... (3) (B) In the complexes (A) and (B), the formal oxidation state of iodine is + 1. Despite the lack of evidence for the existence of discrete I+ ions, its stable complexes 0 169~3 i 0 14037 0 110 91 w u z ;3 0 08146 a: 0 Ill ~ 0 06200 0 022 55-0 00891 300 WAVELENGTH Fig. 1-UV-vis spectra of [A] (0.0005 mol dm- 3 BTMACI), [B] ([A]+0.002 mol dm- 3 ZnCI 2 ), [C] ([A]+0.003 mol dm- 3 ZnCI 2 ) and [D] ([A]+0.006 mol dm- 3 ZnCI 2 ); Solvent: glacial acetic acid; Temperature : 300±3 K with donors have been known for a long time 12 13. The formation of positive iodine species in the sulphuric acid medium has been reported recently 14. Acetic acid is a relatively poor ionizing solvent and formation of ion-pairs in it is a distinct possibility. Therefore, it is probable that complexes (A) and (B) exist as ion-pair in the solvent. The observed dependence on the concentration of zinc chloride indicates that the equilibrium between (A) and (B) is rapid, that the equilibrium constant, K 2, is small and the reaction is not complete even at high concentration of ZnC1 2, and that only the complex (B) is reactive. The small rate-enhancing effect of BTMACl suggests that iodine monochloride (Eq. 4) is not involved in the oxidation process. 400
GUPTA eta/.: KINETICS OF OXIDATION OF ALIPHATIC ALCOHOLS 477 KJ [PhCH2Me 3 Nt ICl2 "=r [PhCH2Me3N]Cl + ICl... (4) Therefore, (B) is 1 ' the only reactive oxidizing species in the oxidation of alcohols. The formation of the complex is supported by the spectral studies also. The existence of the anion [Zn2CI 6 r 2, in tertahydrofuran, has been confirmed by X-ray crystallography 15 Various metallic salts of [Zn2Cl 6 r 2 are known 16. The linear correlation between log k 3 at 298 K and 328 K (r 2 =0.9990; slope=0.862±0.010) for the nine alcohols shows that an isokinetic relationship exists in the oxidation of alcohols by BTMACI 17 The value of the isokinetic temperature is 927±22K. An isokinetic relationship is a necessary condition for the validity of linear free energy relationships. It also implies that all the alcohols so correlated are oxidized by the similar mechanism 17 The rates of oxidation of the alcohols failed to yield a significant correlation separately with Taft's 18 a * and E. (Eqs. 5 and 6). log k 3 =- 2.70 ± 0.35 a - 0.68 r 2 =0.8949; sd=0.44; n=9 ; temperature=298 K... (5) log k 3 =- 1.30 ± 0.47 E.- 1.34 /=0.5248; sd=0.94; n=9; temperature=298 K... (6) The rates were, therefore, correlated in terms of the Pavelich-Taft 19 dual substituent-parameter Eq. (7). log k 3 = p*a * + 8 Es+ log k 0. (7) The values of the substituent constants were obtained from the compilation by Wiberg 18 The correlations are excellent and reaction constants being negative (Table 3). There is no significant collinearity (r 2 = 0.2136) between a andes of the nine substituents. The negative polar reaction constant indicates an electron-deficient carbon centre in the transition state of the rate-determining step. The negative steric reaction constant shows a steric acceleration of the reaction. This may be explained by relief of steric Table 3-Temperature dependence of the reaction constants in the oxidation of alcohols by BTMACI T!K p' 8 R 2 sd 298-2.22±0.02-0.66±0.01 0.9998 0.021 308-2.14±0.02-0.57±0.01 0.9997 0.022 31 8-2.03±0.01-0.53±0.01 0.9998 0.01 4 328-2.00±0.01-0.50±0.01 0.9998 0.013 crowding as the reaction proceeds from an sp 3 hybridised carbon atom in the alcohol towards an sp 2 hybridised carbon atom in the product. Mechanism A hydrogen abstraction mechanism leading to the formation of free radicals may be discounted in view of the failure to induce polymerization of acrylonitrile 20 The cleavage of the a-c-h bond in the rate-determining step is confirmed by the presence of a substantial kinetic isotope effect: The correlation analysis of the substituent effect indicated the presence of a highly electron-deficient reaction centre in the rate-determining step. Therefore, the transfer of a hydride-ion from the alcohol to the oxidant is indicated (Scheme 1). A linear transition state, implied in a hydride-ion transfer through a bimolecular reaction, is supported by the relatively higher magnitude of kinetic isotope effect. RCH20H+[PhCH2Me3Nt[IZn2CI6r ~ + RCHOH + RCHOH + H+ + r + PhCH 2 Me 3 N + 2CI - fa st RCHO+H+ Scheme I We were unable to study the effect of polarity, which might have given supportive evidence to our mechanism, because of the decomposition of the oxidant in water and its insolubility in any other suitable solvent. The proposed mechanism is, however, supported by the observed negative entropy of activation. As the charge separation takes place in the transition state, the two ends become highly solvated. This results in an immobilization of a large number of solvent molecules, reflected in the loss of entropy. Further the bimolecularity of the transition state is also in accordance with the observed negative entropy of activation. Acknowledgement Thanks are due to Prof. K.K. Banerji for his useful suggestions and to the UGC (India) for financial support. References I Fujisaki S, Kajigaeshi S, Kakinami T, Yamasaki H & Okamoto T, Bull chem Soc Japan, 61 ( 1988) 600. 2 Kajigaeshi S, Kakinami T, Moriwaki M, Tanaka T, Fujisaki S & Okamoto T, Bull chem Soc Japan, 62 (1989) 439. 3 Mitra S & Sreekumar K, Indian J Chem, 36B ( 1997) 133.
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