Indian Journal of Chemistry Vol. 47A, August 2008, pp. 1218-1221
Oxidation of diols by cetyltrimethylammonium dichromate Sabita Patela, Dae Dong Sungb,* & B K Mishraa,* a
Centre of Studies in Surface Science and Technology, Department of Chemistry, Sambalpur University, Jyoti Vihar 768 019, India Email:
[email protected] b Department of Chemistry, Faculty of Chemistry and Biology, College of Natural Science, Dong-A University, Saha-Gu, Busan, 604-714, Korea Email:
[email protected] Received 29 November 2007, revised 3 July 2008. The oxidation kinetics of some diols have been investigated using cetyltrimethylammonium dichromate as the oxidant in dichloromethane in the presence of acetic acid and a cationic surfactant. A tentative mechanism has been proposed on the basis of (i) observed rate constant dependencies on the reactants, (ii) high negative entropy change and (iii) the kobs dependencies on surfactant concentration. IPC Code: Int. Cl.8 C07B33/00
DNA damage due to Cr(VI) has generated a wide interest among researchers to study the mechanistic details of the related chemical processes1. Though Cr(VI), the abundantly occurring species of chromium, is the parent reagent in the oxidation process, the Cr(V) formed in situ plays a vital role in the DNA damage. To investigate the mechanism, Goodgame et al.2 studied the oxidation of various nucleotides, i.e., the monomeric units of nucleic acids, by Cr(VI). The oxidation of ribonucleotides reduced Cr(VI) to Cr(V) which is not observed during the oxidation of deoxyribonucleotides. This observation proposes the involvement of cis diol present in ribonucleotides in entrapping the nascent Cr(V). In an in vivo experiment of Cr(VI) reduction by NADPH and microsomes, Jennette detected Cr(V)-diol species3, which was further confirmed by the EPR spectroscopy analysis results of the investigation of Cr(VI) metabolism in rats4. Codd and Lay5, during their search for involvement of diol in speciation of Cr(V), reported that a cis-diol can trap the less stable Cr(V) while a trans-diol can not. Chromates or dichromates with onium ions as the counter ion exhibit relatively low oxidation potential and behave as mild oxidants for organic substrates.
They are, thus, capable of reacting in non-aqueous solvents6. Kuotsu et al.7 have investigated the oxidation of trans-1,2-cyclohexanediol and 1,5-pentanediol by quinolinium dichromate and have reported the corresponding hydroxyl ketone as the oxidised and Cr(III) as the reduced product. In continuation of our work on phase transferring oxidants, we have synthesized cetyltrimethylammonium dichromate (CTADC) and used it for oxidizing various organic substrates in dichloromethane(DCM)6,8-13. In the absence of acid, the oxidant produced some unusual products like diazo compounds from amines, disulphides from thiols, nitriles from oximes and 7-dehydrocholesterol from cholesterol. However, in all the reactions the Cr(VI) is finally converted to Cr(III). The present study aims at investigating the role of extra hydroxy group in the oxidation of diols by CTADC. Experimental Cetyltrimethylammonium dichromate (CTADC) was prepared by the method reported earlier8 and its purity checked by estimating Cr(VI) iodometrically14. Diols were obtained in analytical grade and distilled before use. Glacial acetic acid (Merck) was used without further purification. The surfactant, cetyltrimethylammonium bromide (CTAB) (Siscochem, Mumbai, India) was purified by recrystallization from methanol solution and its purity was checked by measuring the conductance in aqueous medium. Kinetic studies
The oxidation of diols by CTADC in presence of acetic acid was studied by variation of [CTADC], [diol] and [CTAB]. The temperature in the reaction cell was controlled by circulating water by using an INSREF thermostat within a temperature fluctuation of ±0.1 K. The reactions were performed under pseudo-first order conditions by taking a large excess (100 times or more) of the diol with respect to CTADC. The rate of disappearance of the Cr(VI) species was followed spectrophotometrically by monitoring the absorption band at 350 nm from which the first order rate constant, kobs, was obtained from the linear plot of log[CTADC] against time for up to
NOTES
1219
Table 1―Observed rate constants and activation parameters of the oxidation of various diols by CTADC in DCM. {[Substrate] = 0.5 M; [CTADC]=1.89 × 10-4 M; [Acetic acid]= 3.24 M}
298 K
103kobs (s-1) at 303K
2.6 4.6 8.2 0.2
3.1 5.8 9.2 0.3
Substrate
Ethane-1,2-diol Propane-1,2-diol 2-Methyl-2,4-pentane diol Diethylene glycol
75% completion of the reaction (r = 0.99). The values reported are the average of at least duplicate runs and are reproducible within ±4% error. Some of the experiments were run under nitrogen atmosphere, and the data were found to be almost the same (within an error of ±2%) as those under normal atmospheric conditions. Hence, most of the reactions were carried out without a nitrogen environment. The reaction mixture of CTADC, diols and acetic acid in appropriate ratio in DCM was initially kept for 12 h and then the volume of the reaction mixture was reduced to a pasty mass under low pressure. The mass thus obtained was treated with petroleum ether and the extract was evaporated. To the contents, 2,4-dinitrophenyl hydrazine was added and the reaction mixture was kept on a water bath at 50-60oC to obtain orange precipitates of corresponding phenylhydrazone. From the melting points of the hydrazones obtained, the oxidised products of diols were found to be the corresponding hydroxy carbonyl compounds. The stoichiometry of the reaction was determined by performing the experiments at 303K, under the condition of [CTADC]≈[ethane-1,2-diol] at varying ethane-1,2-diol concentrations. The disappearance of Cr(VI) was followed, until the absorbance values become constant. The [CTADC] was estimated after 48 h. A stoichiometry ratio, ∆[CTADC]/∆[ethane-1,2diol] ≈ 0.36 was observed, which confirmed a 1:3, CTADC:ethane-1,2-diol relationship. It also indicates that only one hydroxy group is oxidized in the present reaction condition. Results and discussion The diols in the present study feature primary, secondary and tertiary hydroxyl groups. Under the experimental conditions, one of the hydroxyl groups is oxidized to yield the corresponding carbonyl group. Ethane-1,2-diol (I), with two primary hydroxyl groups produced hydroxyl ethanal; propane-1,2-diol (II) containing one primary and one secondary
308K
∆H≠ (kJ mol-1)
-∆S≠ (J mol-1K-1)
∆G≠ (kJ mol-1)
3.8 7.8 10.5 0.5
25.4 37.8 16.8 67.4
209 163 230 90
87.7 86.3 84.9 94.1
Table 2―Effect of [diol], [CTADC] and [acetic acid] on the oxidation of ethane-1,2-diol by CTADC at 303K [CTADC] × 104 [Ethane-1,2-diol] [Acetic acid] (M) (M) (M) 0.47 0.94 1.89 2.83 4.70 1.89 1.89 1.89 1.89 1.89 1.89
0.5 0.5 0.5 0.5 0.5 0.01 0.1 1.0 0.05 0.05 0.05
3.24 3.24 3.24 3.24 3.24 3.24 3.24 3.24 1.62 6.48 8.10
kobs × 103 (s-1) 6.0 4.3 2.8 2.5 1.8 0.3 0.8 5.9 2.2 4.8 5.7
hydroxyl groups yielded hydroxy acetone; 2-methyl 2,4-propane diol (III) having a secondary and one tertiary alcohol produced 2-hydroxy-2-methyl pentan4-one; and diethylene glycol (IV) with two primary alcohols with five atoms in between yielded the corresponding hydroxyl aldehyde groups . Thus, the reaction preferentially occurs at secondary alcohol. The rate constants are found to be in the order: III > II > I > IV (Table 1). The stoichiometry of the oxidation is found to be 3:2 with respect to diol and Cr(VI). Accordingly, the following reaction equation can be proposed: 3 Diol + 2 Cr(VI)
3 Carbonyl hydroxy derivative + 2 Cr(III) + 6H+
The existence of Cr (III) in the product mixture was established from the absorption maximum at 580 nm. However, the change in the intensity at 580 nm was not reliable to study the rate of formation of Cr(III). The perfect linearity (r2= 0.99) in the plot for determination of rate constant also suggest the rate determining step is a simple reaction with no complexity. The reaction was found to be catalysed by acid. The linear relationship of kobs with [Acetic acid] (Table 2) indicates a first order dependence of rate on
INDIAN J CHEM, SEC A, AUGUST 2008
1220
acid with an un-catalysed rate constant of 1.3×10-3 s-1. The catalytic effect may be attributed to the formation of protonated dichromate (Scheme 1), which triggers the complexation between the substrate and oxidant. -
O
OCrO2O CTA Cr
O
+
OCrO2O CTA
+
O
CTA
+
OH
+
−
Scheme 1
k+1 k
[Complex]
… (1)
1
k2 [Complex] → Products
… (2)
Thus by applying the steady-state approximation OH R CH OH
OCrO2O CTA
O H
+
R
Cr O
R'
-
O +
-
O CTA
k k d [Complex ] 1 × = k obs = +1 2 [diol] dt [CTADC] k −1 + k 2 … (4)
The effect of [substrate] on the rate constant is found to be linear and the rate is found to be first order with respect to substrate, which is contrary to the earlier observation on the oxidation reactions of cholesterol, benzyl alcohol and other alcohols11-13. The reaction kinetics in the aforesaid cases maintains a Michaelis-Menten type relationship, where at higher concentrations the rate constants show a plateau. The plateau is ascribed to the saturation of the substrates at the reversed micellar surface. In the present case the continuous increase in rate constant may be ascribed to the partitioning of the oxidant into the bulk, whereby the oxidant is distributed in both the micro-domain of reversed micelle as well as in the bulk for the availability of the substrate. In earlier reported reactions, Cr(V) may be a transient species in the reaction process but due its instability it may not be contributing to the rate equation. However, in the present case the entrapment of Cr(V) by the diol stabilizes the oxidant Cr(V), which may contribute to the rate equation equally as Cr(VI). This may be responsible for the linear relationship of the rate constant with the [substrate], r2 being 0.998. The rate constant is found to decrease with increase in [CTADC]. This is consistent with the earlier observations on the oxidation reactions of cholesterol11 and alcohols12. The decrease in rate
The rate constants of the diols are found to be higher than the corresponding mono-hydroxy derivatives. Since a single hydroxy compound is being oxidized by the oxidant, the ester complex is formed due to the condensation of one hydroxy group with the oxochrome group of the oxidant. The enhancement of the reactivity may be attributed to the assistance of the second hydroxy group in the rate determining cleavage of the ester intermediate (Scheme 2). It forms a bicyclic intermediate i.e., a five membered ring fused with another five membered ring for 1,2 diols and a five-membered ring fused with a six membered ring in 1,3-diol. The latter being thermodynamically more stable, the rate enhancement is also more prominent for 1,3-diol derivative. As this is not possible for diethylene glycol, the reactivity of this species is found to be low. The rate expression for the mechanism given in Scheme 2 is given in Eqs 1-4. diol + CTADC
… (3)
+
Cr
H
+ -
O CTA
-
O +
d [Complex] = k 2 [Complex] dt k k = +1 2 [diol ][CTADC] k −1 + k 2
Rate = −
HO
-
OCrO2O CTA
+
C O Cr R' H O +O CTA
+
Compex(C) Complex (C) O H
O H R
+
R
O-
HO OCO2OCTA C O Cr R' O H
-
HO OCO2OCTA C O Cr R' H O O + CTA
CTA
+
Scheme 2
+
OH R R'
C
HO + O HO
-
Cr
OCrO2O CTA -
O CTA
+
+
NOTES
1221
Acknowledgement The authors thank University Grants Commission and Department of Science and Technology, New Delhi for financial support through DRS and FIST programmes respectively. SP thanks Council of Scientific and Industrial Research, New Delhi for an Extended Senior Research Fellowship. References 1
2 3 4
Fig. 1―Plot of 104 kobs versus [CTAB] in the oxidation reaction of diols with CTADC at 303K. [1, ethane-1,2-diol; 2, propane-1,2diol; 3, 2-methyl-2,4-pentanediol; 4, diethylene glycol].
5 6 7 8
constant with increase in [CTADC] may be ascribed to the formation of reversed micelle wherein the dichromate ion is enveloped by CTA+ and diol is more partitioned to bulk DCM. Thus, the effective concentration of diol in the proximity of dichromate decreases. This inference gets support from the change in rate constants due to the addition of CTAB. On addition of CTAB the rate constants suffers a sharp fall within a short range of CTAB concentration and then remains almost constant (Fig. 1). CTAB forms reversed micelles in dichloromethane and can help in partitioning the oxidant and the substrates into two different domains and thus affecting the rate.
9 10 11 12 13 14 15
16
Nieboer E & Shaw S L, in Chromium in the Natural and Human Environments, edited J O Nriagu & E Nieboer, (Wiley Interscience, New York) 1988, p. 399. Goodgame D M L, Hayman P B & Hathway D E, Polyhedron, 1 (1982) 497. Jennette K W, J Am Chem Soc, 104 (1982) 874. Liu K J, Shi X, Jiang J, Goda F, Dalal N & Swartz H M, Ann Clin Lab Sci, 26 (1996) 176. Codd R & Lay P A, J Am Chem Soc, 121 (1999) 7864. Patel S & Mishra B K, Tetrahedron, 63 (2007) 4367. Kuotsu B, Tiewsoh E, Debroy A & Mahanti M K, J Org Chem, 61 (1996) 8875. Patel S, Kuanar M, Nayak B B, Banichul H & Mishra B K, Synth Commun, 35 (2005) 1033. Patel S & Mishra B K, Tetrahedron Lett, 45 ( 2004) 1371. Sahu S, Patel S & Mishra B K, Synth Commun, 35 (2005) 3123. Patel S & Mishra B K, J Org Chem, 71 (2006) 3522. Patel S & Mishra B K, J Org Chem, 71 (2006) 6759. Patel S & Mishra B K, Int J Chem Kinet, 38 (2006) 651. Vogel A I, in A Textbook of Quantitative Inorganic Analysis, 3rd Edn; (ELBS and Longmans, UK), 1961. Dictionary of Organic Compounds, edited by J R A Pollock & R Stevens, (Eyre & Spottiswoode Publishers Ltd, London) 1965, Vol. 3, p. 1537. Vogel’s Textbook of Practical Organic Chemistry, revised by B S Furniss, A J Hannaford, P W G Smith & A R Tatchell, (Pearson Education, New Delhi) 2005, pp. 1336.
