Journal of Molecular Catalysis, 48 (1988) 207 - 215 207

MECHANISM OF Ru( III) CATALYSIS. IN N-BROMOACETAMIDE OXIDATION OF SOME GLYCOLS IN PERCHLORIC ACID MEDIA

BHARAT SINGH*, A. K. SINGH and DEEPIKA SINGH

Department of Chemistry, University of Allahabad, Allahabad-211002 (India)

(Received September 9,1987; accepted June 3,1988)

summary

The kinetics of ruthenium(II1) chloride catalysis in the oxidation of diethylene glycol (DG) and methyl diethylene glycol (MDG) by N-bromo- acetamide (NBA) in perchloric acid media are reported. The reactions follow identical kinetics, showing zero-order dependence on NBA, but first-order on each of H+, Ru(II1) and Cl- ions. The first-order kinetics with respect to glycol at low concentrations shifts to zero-order at higher concentrations. A negative effect of ionic strength is observed, while succes- sive addition of acetamide, DzO and mercuric acetate shows zero effect on the reaction rate. Various activation parameters have been computed. The products of the reactions were identified as the corresponding acids. A suitable mechanism consistent with the kinetic observations has been pro- posed.

Introduction

NBA is used in preparative organic chemistry as an oxidising and halogenating agent [l], although there seems to be little information in the literature about its mode of oxidation [2 - 81. Thus far, no attempt has been made to probe the role of ruthenium(II1) chloride as catalyst in NBA oxidations, and thus the mode of NBA oxidation in the catalysed reaction is unknown. This prompted us to undertake the present investiga- tion involving kinetic study of the Ru(III)-catalysed NBA oxidation of the aforesaid glycols in perchloric acid media in the presence of mercuric acetate. Mechanistic conclusions are discussed.

Experimental

An aqueous solution of NBA was prepared fresh each day from a G.R. sample of the reagent from Merck and its strength was checked by

*Author to whom correspondence should be addressed.

0304-5102/88/$3.50 @ Elsevier Sequoia/Printed in The Netherlands

208

the iodometric method [9]. Diethylene glycol and methyl diethylene glycol of BDH (A.R. grade) and E. Merck (Germany) sample of mercuric acetate were used. The solutions of the glycols were prepared by weighing their samples. Ruthenium(II1) chloride (Johnson Matthey) solution was prepared by dissolving the sample in hydrochloric acid of known strength. All other reagents, namely sodium perchlorate and perchloric acid, were of AnalaR grade. Pure hypobromous acid [lo] was prepared by shaking a mixture of bromine (3 ml) AnalaR, water (1 1) and silver sulphate (15 g) until the heavy precipitate settled, leaving a clear, pale straw-coloured supernatant solution. After filtration, the aqueous solution of hypobromous acid was distilled under reduced pressure (cu. 15 mm) on a water bath at 40 - 50 “C. Total hypobromous acid was estimated by direct titration with standard&d so- dium thiosulphate in the usual way. Triple distilled water was used through- out the investigations. The reaction stills (Jena glass) were blackened from outside. All the kinetic measurements were carried out at constant tem- perature (+O.l “C). The reaction was initiated by rapid addition of NBA to the mixture containing appropriate quantities of glycol, mercuric acetate, perchloric acid, ruthenium(II1) chloride, potassium chloride and water, and mixing them by vigorous shaking. The progress of the reaction was monitored by estimating iodometrically the amount of remaining NBA at regular time intervals.

Results

The stoichiometry of the reaction was ascertained by equilibrating the reaction mixture containing an excess of NBA over glycols at 35 “C for 43 h. Estimation of unconsumed NBA showed that 4 moles and 2 moles of NBA were respectively consumed by 1 mole of diethylene glycol and methyl diethylene glycol, according to the stoichiometric eqns. (1) and

(2). /CH&H20H

4CHsCONHBr + 0, + 2Hz0 CHzCHzOH

/CH&OOH - 4CHsCONH, + 0, + 4HBr

CH&OOH (1)

/ CH2CH20CH3 2CHsCONHBr + 0, + Hz0

CH&H,OH

- 2CH&ONH2 + 0 /CH2CH20CHs

‘CH,COOH + 2HBr (2)

The products dicarboxylic acid of DG and methyl digolic acid of MDG were detected by conventional methods [ 111.

The kinetics of the oxidation of glycols were investigated at different initial concentrations of the reactants (Table 1, Figs. 1 and 2). Zero-order dependence in NBA was followed at all initial concentrations of NBA. Zero-order rate constants, (-dc/dt) or ko, were calculated from the slopes

TABLE 1

Effect of [ NBA] and [ KCl] on reaction rate

[KC11 [NBAI (~10~ M) (~10~ M)

Ionic strength (cc) k. x 10’ (~10~ M) (mol 1-i s-l)

DG MDG DG MDG

1.00 0.80

1.00 1.00 1.00 1.25 1.00 1.66 1.00 2.00 1.00 2.50 1.00 3.34 0.66 1.00 0.80 1.00 1.00 1.00 1.25 1.00 1.32 1.00 1.66 1.00 2.00 1.00 2.50 1.00 3.34 1.00 5.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

3.30 3.30 3.30 3.30 3.30 3.30 3.30 2.62

-

2.62 2.62

-

2.62 -

2.62 - -

2.50 3.50 5.00 7.50

10.00 10.48e 10.48’ 10.48s 10.488 10.48’

3.30 3.30 3.30 3.30 3.30 3.30 3.30

-

1.88d 1.88d

-

1.88d -

1.88d -

1.8Bd 1.88d 2.00d 3.50d 5.50d 8.20d

11.50d 6.38d 6.3ad 6.38d 6.3ad 6.3gd

4.03 4.45 4.45 4.43

-

4.75 4.00 4.33

-

6.66 8.33

-

11.66 -

16.66 - -

5.50b 3.66b 2.70b 2.08b 1.53b 1.39c 1.83C 3.33c 5.33a 7.94

3.15 3.09 3.33 3.53 3.33 3.61 - -

1.33a 1.80a -

2.33a -

3.668 -

6.10a 9.10s 5.33b 4.58b 3.00b 1.3Bb 0.29b -

1.66’ 2.21s 3.75h 7.66’

Conditions: [HC104] = 2.00 x 10m2 M unless stated otherwise, [substrate] = 2.00 (DG) and 5.00 (MDG) x lop2 M, [Ru(III)] = 9.60 x 10h6 M unless otherwise stated, [Hg- (OAC)~] = 4.00 x 10m3 M unless otherwise mentioned and temp. 35 “C! unless otherwise stated. a[Ru(III)] = 5.76 x 1O-6 M. bkRu(III)] = 15.36 x lo+ M. c[Ru(III)] = 11.52 x 1O-6 M. d[HC104] = 1.00 x 1O-2 M. “[Hg(OAc)?] = 1.25 x 1O-3 M, T = 25 “C. f[Hg(OAc)2] = 1.25 x 1O-3 M, T = 30 “C. s[Hg(OAc)2] = 1.25 x 1O-3 M. h[Hg(OAc)2] = 1.25 x 1O-3 M, T = 40 “C. i[Hg(OAc)2] = 1.25 x 1O-3 M, T = 45 “C.

210

6+ log&u (a)] ----t

l.SO 1.25 1.00 0.75 CJ 2.5 I I I

I -2.4 4 8 f2 16 1.1 t.3 1.5 1.7

[substrate7 xlo2M+ 3 t lOgLH+1-+

Fig. 1. Plot between zero-order constant ie. (-dc/dt) and concentration of substrate at 35 “C; [NBA] = 1.00 x 1O-3 M, [HC104] = 2.00 x 1O-2 M, [Ru(III)] = 9.60 x 1O-6 M, [KC11 = 1.00 x 10e3 M, [Hg(OAc)z] = 1.25 x 10m3 M and p (ionic strength) = 2.48 x

10M2 M; A = diethylene glycol, B = methyl diethylene glycol.

Fig. 2. Plot of log(-dc/dt) against log[H+] or log[Ru(III)] in oxidation of diethylene glycol (curves A and C) and methyl diethylene glycol (curves B and D) at 35 “C; [NBA] = 1.00 x 10m3 M, [KC11 = 1.00 X 10e3 M, [substrate] = 2.00 (A, C) and 5.00 (B, D) X lop2 M, [Hg(OAc)z] = 1.25 x low3 M, [Ru(III)] = 11.52 (A) and 5.76 (B) X 10m6 M, [HC104] = 2.00 (C) and 1.00 (D) X 10h2 M and p = 10.48 (A), 6.38 (B), 2.48 (C) and 1.48 (D) x 1O-2 M.

of plots of unconsumed [NBA] and time for varying concentrations of NBA. Figure 1 shows first-order kinetics with respect to each glycol at lower concentrations, but the reaction rate tends to zero-order at higher concentrations of the substrate. The rate of the reaction was found to be highly influenced by [Ru(III)]. It was observed that with increasing [Ru- (III)] the zero-order rate constants in NBA increased linearly, which proves first-order dependence on [Ru(III)]. This is obvious from the slopes (1.04 for DG and 1.02 for MDG) of the plots (Fig. 2, C, D) of log(-dc/dt) us. log[Ru(III)]. First-order kinetics with respect to hydrogen ions is con- firmed by the slope value (0.98 for DG and 1.05 for MDG oxidations) obtained from the plots (Fig. 2 A, B) of log(-dc/dt) us. log[H+].

The zero-order rate constants in NBA increased linearly with the in- crease in chloride ion concentration (Table l), indicating first-order kinetics in Cl- ions. The addition of mercuric acetate, acetamide and D,O (solvent isotope variation) had a negligible effect. Successive additions of sodium perchlorate (ionic strength variation) decreased the zero-order rate constant in NBA (Table 1). The values of activation parameters were computed from the study of rate measurements at 30, 35, 40 and 45 “C and are reported in Table 2.

211

TABLE 2

Values of activation parameters in Ru(III)-catalysed NBA oxidation of glycols

Substrate Eal mol-‘)

A x lo’* As* AF* (1 mole1 s-l) (e.u.) (kcal mol-‘)

diethylene glycol 17.00 1.66 -4.67 18.44

methyl diethylene glycol 17.90 11.50 -0.82 18.15

Discussion

The results of the oxidation of diethylene glycol and methyl diethylene glycol, recorded here, have revealed that the reactions have identical kinetics and thus their oxidations appear to have a common mechanism. The insig- nificant effect of mercuric acetate on the reaction rate rules out its involve- ment in NBA oxidation, and it acts only as a scavenger [12,13] for any Br- formed in the reaction. It eliminates completely the oxidation by BrZ, which would have been formed by the interaction of HBr and NBA as fol- lows:

CHsCONHBr + HBr - CH&ONH2 + Brz (3)

The presence of mercuric acetate thus ensures that the oxidation proceeds solely through NBA.

First-order dependence of the reaction on chloride ions suggests that equilibrium (4) is shifted to the right side [14] in acidic ruthenium(II1) chloride solution.

Cl- + [RuC15*HzO12 - = [RuCl,]‘- + Hz0 (4)

It may, therefore, be assumed that [RuCl,13- is the active species of ruthenium(II1) chloride.

NBA is known to exist [2 - 81 in acidic media in the following two sets of equilibria (5), (6) and (7), (ES), neither of which can be distinguished on the basis of the present kinetic data.

CHsCONHBr + Hz0 e CHsCONH2 + HOBr

(NBA) WW

HOBr + Hs+O == H20Br+ + H20

(5)

(6)

CH,CONHBr + H,+O e (CH-$ONH2Br)+ + H20 (7)

(CH$ONH2Br)+ + H,O e CH,CONH2 + H,OBr+ (3)

In either of the two alternative equilibria, it seems that NBA itself, protonated NBA, HOBr or a cationic bromine species, i.e. (H,OBr)+, may

212

be the possible oxidising agent of NBA in acidic media. Zero-order depen- dence of the reaction on NBA suggests that an NBA species is involved in the fast step after the ratedetermining step, and hence any of the afore- said possible species of NBA may be suggested to participate in the oxida- tion process. In the absence of mineral acid, oxidation of glycols does not occur, neither by NBA nor by HOBr obtained from another source [lo]. This proves that neither NBA as such, nor HOBr (produced through equi- librium 5), is involved in the oxidation of glycols. Of the remaining two species, H20Br+, and not protonated NBA (Le. NBAH+), should be the oxidising one. In order to confirm this, experiments were carried out using HOBr as the oxidant [lo] in acidic medium. In acidic medium, HOBr gives H,OBr+ according to equilibrium (6). The kinetic data show that the reac- tion is fractional order in glycols and first order in hydrogen ions in NBA and HOBr oxidations. The similarities suggest that it is only H,OBr+ and not NBAH+ which is the active species. If NBAH+ were the active species, the kinetic data in NBA oxidation would not be similar to that obtained in HOBr oxidation (Table 3).

The oxidation of glycols by NBA in an atmosphere of nitrogen, how- ever, failed to induce polymerisation of acrylonitrile, which indicates that a one-electron oxidation is highly unlikely.

Considering the above statements and kinetic results, the following reaction paths are proposed (where R = -CH2CH20H and -CH2CH20CHs in DG and MDG, respectively).

TABLE 3

Effect of [HC104] and substrate on rate of HOBr oxidation at 35 “C!

WC1041 (~10~ M)

[Substrate] (~10~ M) k. x 10’ (mall-’ s-l)

DG MDG DG MDG

0.66 2.00 5.00 0.36 1.76

1.00 2.00 5.00 0.55 2.72

1.25 2.00 5.00 0.61 3.46

1.66 2.00 5.00 0.94 4.38

3.34 2.00 5.00 1.92 9.06

5.00 2.00 5.00 2.74 13.84

2.00a 1.00 1.00 1.68 1.44

2.008 1.34 1.34 2.24 1.88

2.00a 2.00 2.00 3.39 2.80

2.00a - 3.34 - 3.06 2.008 4.00 - 3.80 -

2.00a - 5.00 - 3.41 2.00a 6.66 - 5.12 -

2.00a 10.00 10.00 5.78 3.72

Conditions: [HOBr] = 1.00 x 10e3 M, [Ru(III)] = 9.60 x 10v6 M, [KC11 = 1.00 x low3 M, [Hg(OAc)s] = 1.25 x lop3 M and /J (ionic strength) = 10.48 (DG) and 6.38 (MDG) x 10m2 M unless otherwise mentioned. a~ = 2.48 x 1O-2 M.

213

[RuCl,.H,O]*- + Cl-- +- [RuC1J3- + H20 (I)

(Cl) 1

(C2)

[ RuCl,] 3 - + ROCH,CH,OH + [RuCl, t- 0CH2CH20R] 3 - (II)

(S)

2

l!I (C3)

H

C3+H+3 [RuC16 - d+CH2CH20R]*-

4 (A)

(III)

A -& [RuC~&I]~- + ROCH,CHO + 2H+

rate-determining step

(IV)

OH

C ROCH2CH0 + H,OBr+ s ROCH, , -OH + H+

Br

(V)

OH

ROCH2 -aH c:

IL?

s ROCH2COOH + HBr (VI)

ROCH&OOH Ru(II1) species + H+ ,CH,COOH

H20Br+ 20

‘CH,COOH (VII)

[RuC~&I]~- + H,OBr+ fastr [RuC1J3- + H20 + HBr (VIII)

Formation of [RuC~$I]~- has been recently reported [15]. Step (VII) is valid for diethylene glycol oxidation only.

In the above reaction scheme, the dissociation possibility of Cl- ion does not exist, as equilibrium (I) is shifted towards its association.

The rate of the reaction in terms of consumption of NBA may be written as:

d[NBA] - dt

= k[A] (9)

From step (III) we have K3 = [Al

[C31 W+l

or [Al = KS31 WI

Considering eqns. (9) and (10) we have

(10)

(11)

The total concentration of Ru(III), i.e. [Ru(III)] r, may be written as eqn. (12):

FWWIT = [CA + [C21 + [&I (12)

Consideration of steps (I - III) and eqns. (11) and (12) and application of the steady state treatment with the reasonable approximation yields the rate law (13).

d[NBA] - = kk,~,~,[Ru(III)l,[Cl-l [H+l [sl

dt bW201 + k2b31 (13)

where K2 = k2/k_2. The rate law (13) accords well with the experimental results. The

rate law (13) can be rearranged into the following form:

1 1 - =I/- d[NBA]

rate dt = k’K1K2K3[Ru(III)]T[Cl-] [S] [H+]

kz + kk,K2K3[Ru(III)]T[Cl-] [H+]

(14)

where.Ki = k,/k_l and k’ = k/[H20]. A graph of l/rate us. 1/ [substrate] should yield a straight line (Fig.

3A and B) with an intercept on the l/rate axis; this was found to be so in the oxidation of both glycols, which confirms the formation of a complex between glycol and ruthenium(II1) chloride species prior to the ratedeter- mining step.

3 0; , 0.4 0.8 f.2 1.6 (~/~.SUBSTRATE])X fo-2r4-‘L

Fig. 3. l/rate us. l/[substrate] plot in oxidation of diethylene glycol (A) and methyl diethylene glycol (B) under the conditions of Fig. 1.

215

Acknowledgement

The authors wish to thank CSIR, New Delhi, Govt. of India, for pro- viding financial assistance to (Miss) Deepika Singh.

References

1 2 3 4 5 6 7 8 9

10 11 12

13

14

15

R. Filler, Chem. Rev., 63 (1963) 21. J. Mukherjee and K. K. Banerji, J. Org. Chem., 46 (1981) 2323. P. S. Radhakrishnamurthi and N. C. Sahu, Indian J. Chem., 20A (1981) 269. B. Shah, A. L. Jain and K. K. Banerjee, Znd. J. Chem., 22B (1983) 720. B. Singh and R. Srivastava, Curt-. Ski., 54 (1985) 415. B. Singh and R. Srivastava, Oxidation Commun., 9 (1986) 1. B. Singh and R. Srivastava, Tetrahedron, 42 (1986) 2749. B. Singh and R. Srivastava, J. Zndian Chem. Sot., 63 (1986) 735. M. 2. Barkat and M. F. Wahab Abdel, Anal. Chem., 26 (1954) 1973. M. K. Reddy, C. S. Reddy and E. V. Sundaram, Tetrahedron, 41 (1985) 3071. F. Feigl, Spot Tests in Organic Analysis, Elsevier, New York, 1966. J. C. Bailer, The Chemistry of Coordination Compounds, Reinhold, New York, 1956, p. 4. G. Gopalkrishnan, B. R. Rai and N. Venketasubramanian, Indian J. Chem., 19-B (1980) 293. D. A. Fine, Ph.D. Thesis, University of California, Berkeley, 1960, as quoted by R. R. Buckley and E. E. Mercer, J. Phys. Chem., 70 (1966) 3103. J. P. Sharma, R. N. P. Singh, A. K. Singh and B. Singh, Tetrahedron, 42 (1986) 2739.