Ru(III) catalysis in N-bromoacetamide oxidation of ethylene glycol and glycerol: A kinetic and...

Ru(II1) Catalysis in N-Bromoacetamide Oxidation of Ethylene Glycol and Glycerol:

A Kinetic and Mechanistic Study

BHARAT SINGH," DEEPIKA SINGH, and A. K. SINGH Chemistry Department, University of Allahabad, Allahabad-211002, India

Abstract

Kinetics of oxidation of ethylene glycol and glycerol by acidic solution of N- bromoacetamide (NBAI in the presence of ruthenium (111) chloride as a homogeneous catalyst and mercuric acetate as scavenger in the temperature range of 30-50°C have been reported. The reactions follow identical kinetics, being zero-order in substrate and first-order in Ru(II1). First order dependence of the reaction on NBA at its low concentrations tends to zero order in the higher concentration range. Positive effect of [H.1 and [CI I has been observed. A negative effect of acetamide and ionic strength of the medium is observed while D,O and mercuric acetate show zero effect on the reaction velocity. Various activation parameters have been computed. The main product of the oxidation is corresponding acid. (H20Br)' has been postulated as the oxidizing species. A suitable mechanism in conformity with the kinetic data has been proposed.

Introduction

NBA is a less familiar potent oxidant which is of special interest for studying reaction mechanisms as it behaves as both a halogenating and oxidizing agent. Although the oxidative capacity of this analytical reagent has been widely utilized for the quantitative direct and indi- rect estimations of a large number of compounds [l l , only a few kinetic studies of the uncatalyzed reactions have been made. Mechanism of oxidation of primary alcohols, dimethyl sulfoxide, and a few ketones by acidic solution of N-bromoacetamide has been studied by Mukherjee and Banerji [2], Radhakrishnamurthi and Sahu, [31 and Singh and coworkers [4-71, respectively.

There seems to be no Eeport about its mode of oxidation in catalyzed reactions. This prompted us to report here the kinetics of Ru(II1) catalyzed oxidation of ethylene glycol and glycerol by NBA in perchloric acid media in the presence of mercuric acetate as scavenger as a part of our broad program on mechanistic studies of catalyzed reactions 18-101.

*To whom correspondence should be addressed.

International Journal of Chemical Kinetics, Vol. 20. 501-511 (1988) G 1988 John Wiley & Sons, Inc. CCC 0538-80661881070501-11$04.00

502 SINGH, SINGH, AND SINGH

Experimental

NBA aqueous solution was prepared afresh each day from a G. R., S. Merck sample of the reagent, and its strength was checked by iodometric titration of active bromine. Ethylene glycol and glycerol of Analar grade and E. Merck (Germany) sample of mercuric acetate were used. Solutions of these polyhydroxy alcohols were prepared by weigh- ing their samples. Ruthenium(II1) chloride (Johnson Matthey) solution was prepared by dissolving the sample in hydrochloric acid of known strength. Deuterium oxide (purity 99.4%) was supplied by BARC, Bombay (India). Perchloric Acid (E. Merck) was used as a source of hydrogen ions. While the effect of the varying concentrations of perchloric acid on the reaction rate was studied, ionic strength was kept constant by using sodium perchlorate (E. Merck). All other re- agents, namely potassium chloride and acetamide, were of Analar grade. Triple-distilled water was used throughout the course of the reaction and the reaction bottles were opaqued to light to avoid any photochemical effect.

Appropriate amounts of the substrate, perchloric acid, ruthenium(II1) chloride, potassium chloride, mercuric acetate, and sodium perchlorate solutions and water were taken in a reaction bottle and thermostated at 35°C for thermal equilibrium. A measured amount of NBA solution also thermostated at the same temperature was rapidly added to the reaction mixture to initiate the reaction. The progress of the reaction was monitored by iodometric determination of unreacted amount of NBA at regular time intervals. The course of the reaction was studied for two half lives.

Stoichiometry and Product

Varying ratios of NBA to the substrate were equilibrated at 35°C for 24 h under the kinetic conditions. Estimation of residual oxidant showed that one mol of alcohol consumes two mols of NBA according to the stoichiometric eq. (1)

(1) 2CH3CONHBr + RCHzOH + H20 __*

2CH3CONH2 + RCOOH + 2HBr

where R stands for - CHzOH and - CHOHCHzOH in ethylene glycol and glycerol, respectively, whose oxidation products, glycollic acid and DL-glyceric acid were detected [113 by conventional method.

Results and Discussion

When the concentration of the substrate is in excess over [NBAI, the plot of unconsumed [NBA] against time shows that the reaction is composed of two successive reactions. The initial slow reaction is fol-

A KINETIC AND MECHANISTIC STUDY 503

lowed by a faster reaction. Such an observation has also been recorded in N-bromosuccinimide oxidation 1121 and the faster reaction has been attributed to bromine oxidation. We also observed a yellow color (per- haps of liberated bromine) in the reaction mixture after the reaction has proceeded to about 20%. The appearance of yellow color and the second faster reaction were suppressed by the addition of 0.005 M mercury(I1) acetate. All reactions were, therefore, carried out in the presence of mercury(I1) acetate. The rate of the reaction in the presence of mercury(I1) acetate was nearly the same as the rate of the slow initial reaction without Hg(I1).

The kinetics of oxidation of the title alcohols by NBA in the presence of perchloric acid and Ru(II1) was investigated at several initial concentrations of the reactants (Table I). First order dependence in NBA at its

TABLE I. M , [Ru(III)I =

11.52 x 10 M , [KCl] = 1.00 x M , [Hg(OAc)J = 4.00 x 10 M (unless otherwise stated), and ionic strength (KL) = 2.30 x 10 ' M (unless otherwise stated) at 35°C.

Effect of [Reactants] on reaction rate: [HClO,] = 1.00 x

L NBA 3 x 103u [Suhsttate] x 102M kQbr x lo2 5"

fithylene g l y c o l Glycerol

0.34

0.40

0.50

0.67

0.80

1 .oo 1.25

2.00

2.00

2.00

2.00

2.00

2.00

2.00

0.95 -- 1.16 0.91

1.45 1.17

1.86 1.23

2.07 1.45

2.24 1.81

2.17 1.81

1.67 2.00 2.40 2.17

2.00 2.00 2.53 2.40

2.50 2.00 2.52 2.40

3.34 2.00 2.47 2.43

1 .oo 13.33* 2.74 1.68

1 .oo 10.00* 2.60 1.61

1 .oo 5.00, 2.89 1.44

1 .oo 3.33* 2.74 1.44

1 .oo 2.00* 2.22 1.57

1 .oo 1 .oo* 2.82 1.59

1 .oo 0.50* 2.46 -- " l H g ( 0 A ~ ) ~ l = 1.25 x M and p 1.48 x M


low concentrations tended to zero order a t its higher concentrations. The shifting of order in NBA from first to zero is also obvious from the plot (Fig. 1) of (-dc/dt) values against [NBA"] (*indicating the concentration of NBA a t which -dc/dt value was determined). The reaction was found to be independent of the concentration of substrate, indicating zero order dependence on substrate. At constant ionic strength plots of log(-dc/dt) against log[Ru~III~l gave slopes 1.06 for ethylene glycol and 1.02 for glycerol (Fig. 2C and 2D) which indicate first order kinetics in Ru(II1). The rate increased with increase in [acid] (Table 11) and a plot of log(-dc/dt) vs. log[H ' I was linear (Fig. 2) with slopes 0.72 and 0.80 for ethylene glycol and glycerol, respectively in HC10, medium, indicating positive effect of [H'] on reaction rate. In HC1 and H2S04 media the combined effect of tH ' I and their anions was observed. Hence no attempt was made for log(-dc/dt) vs. log[H'] plots in such media. Table I11 contains the effect of addition of acetamide (reduction product), mercuric acetate, variation of ionic strength of the medium, and chloride ions concentration. A plot of log(-dc/dt) vs. log [NBHI gave slopes -0.77 (ethylene glycol) and -0.47 (glycerol), indicating thus negative fractional order in NBH for both substrates. Posi- tive effect of [Cl-1 was observed while negative effect of ionic strength was exhibited. The kinetic data indicated that addition of mercuric acetate does not interfere with the reaction. The reaction was studied in D,O (50%) medium. The values were k o b s (D,O) = 2.21 x 20-2 S-' and hobs (H,O) = 2.17 X S-' for ethylene glycol and hobs (D20) =

2.86 x lo-' S-' and bobs (H,O) = 2.80 x S-' for glycerol oxi- dations, thus ruling out all the possibilities of protonation of the substrate prior to slow and rate determining step. The reactions were studied at different temperatures (30-50°C) and values of energy of

Figure 1. tions of Table I in oxidation of (A) ethylene glycol and (B) glycerol.

Plot of zero order constant i.e., ( -dc/dt) vs. [NBA*l under the condi-


Figure 2. M, (KCII =

1.00 x lo-:' M, [Substrate] = 2.00 x M and temperature 35°C. A and B: Plot of log( -dc ld t ) vs. log[H-I in oxidation of (A) ethylene glycol and (B) glycerol; IRu(1II)J = 11.52 (A) x M. C and D: Plot of log( -dc /d t ) vs. log[Ru(III)1 in oxidation of (C) ethylene glycol and (DJ glycerol; IHClO,] = 1.00 x 10 M and

[NBA] = 1.00 x 10 ,'M, [Hg(OAcJ21 = 1.25 x

M and 7.68 (B) x lo-" M and p = 10.50 x 10

= 1.48 x lO-'M.

TABLE 11. M, ISub- strate] = 2.00 x lO-'M, IKCll = 1.00 x M , IHg(OAc)21 = 1.25 x M , and IRu(III)] = 7.68 x 1 0 ~ ~ 6 M (unless otherwise stated).

Effect of [Acid] on reaction rate at 35°C: INBAI = 1.00 x

[Acid] kobs x 1 OL 5-' - X I 02M nao; HC1** Hp34***

-------- SlYcole, _--I--_--- slYcol------__-_-_____ sl. Y 501 ___________ e t h y l e n e g l y c e r o l e i n y l e n e g l y c e r o l b e t h y l e n e g l y c e r o l

b b

0.34 - - - - 6.51 9.53 - - - - 0.50 1.30 - - 7.81 11.93 - _ - _ 0.67 - - - - 13.1 2 15.63 - - -_ 1.00 2.02 0.87 22.99 30.36 3.26 2.60 1.25 2.31 1.21 - - - - - - - - 1.33 - - - - - - - _ 3.91 3.07 1.67 2.09 1.41 40.68 43.38 4.32 4.33

2.50 3.76 1 .a0 - - - - -- - - 5.00 6.22 2.88 - - - - 6.80 6.07 6.67 7.23 - - - - - _ - - - -

10.00 - - 4.34 - _ - - 7.1 9 9.22

++ p = 10.50 x M. **+ p = 2.48 x lO-'M. ***- p = 14.48 x 10-2M. "IRu(I1I)l = 11.52 x 10-'M. "3.84 x 10 ti M.


TABLE 111. Effect of variation of IC1-l and ionic strength ( p ) of the medium and addition of acetamide and mercuric acetate on reaction rate a t 35°C: [NBAI = 1.00 x M , [substrate] = 2.00 x M , IHCIO,] = 1.00 x 10 M , [Hg(OAc),l = 1.25 x M , (unless otherwise mentioned), and [Ru(III)I = 7.68 x lo-' M (unless otherwise stated).

[Acetamide) x 1 ~ 3 ~ [ K C ~ J X 1 o 3 ~ ~ ~ I O ' M kobr x 102S" ethylene g&ol glycerol

1 .oo 1 .oo 1 .oo 1 .oo 3.33 2.50 2.00 1.33 1 .oo 0.80

1 .oo 1.00 1 .oo 1 .oo 1.00 1 .oo

1.48

1.48

1.48

1.48 1.71 1.71 1.71 1.71 1.71 1.71 2.50 4.50 8.50 3. 25g 3.2!jh 3.25'

f 0.65

0.36*

- - 0.30f 6.07' 5.1 3a 4.77. 3.49'

1 .9ed 1.83d

2.45' - -

1 .61d 2.18 2.17 2.16

-- O . s g e

0.54. 0.33.

12.74b -- - _ 3.72b

2.60c

1 .91b 1 .74b

2.06' 1.74'

1 .m 1.77 1.78

"IRu(III)] = 3.84 x 10 b5.76 x M . '9.60 x M . d11.52 x M . '15.36 x M . '19.20 x M . p[Hg(OAc)21 = 1.50 x 1 O P M . h2.00 x M . 12.50 x 10-3 M.

M .

activation (AE), Arrhenius factor (A), entropy of activation (AS ) and free energy of activation (AF ) are computed (Table IV).

It appears from the identical kinetic results in oxidation of ethylene glycol and glycerol that both reactions have common mechanism. Zero effect of mercuric acetate on reaction rate excludes its involvement in NBA oxidation and thus it azts only as a scavenger 113,141 for any Br- formed in the reaction. It eliminates completely the oxidation by Br2 which would have been formed by the interaction of HBr and NBA as follows:

(2) HBr + CH3CONHBr - Br2 + CH3CONH2

the oxidation purely through NBA. Presence of mercuric acetate in the reaction mixture thus ensures


TABLE IV. Effect of temperature variation on reaction rate and values of activation parameters in Ru(II1) Catalyzed NBA Oxidation of ethylene glycol and glycerol: [NBAI = 1.00 x M , [Substrate] = 2.00 x lo-' M , [HClO,] = 1.00 x M , [Hg(OAc)J = 1.25 x 10 M , and f i = 1.48 x M .

M, [Ru(III)] = 7.68 x M , [KCll = 1.00 x

Rate :ons tan t / ithylene 91ycol Glycerol

A c t i v a t i o n p a r a m e t e r s ---------_-__-_-__--__-_-_--__-_--_--____-___________---_- 1 .51 I .20 -2 s-l kobs ( ~ O O C J x 10

)bbs CSSoC) x 102 5-l 2.1 7 1-80

kobS 14OoC) X 102 5-l 3.52

4.35 bobs (45OC) x lo2 5-l

2.41

3.68

bobs (5OoC) x l o2 5" 7.06 4.92

AE (Kcal/mol) 14.10 14.93

A (l /mol s e c ) 2.38 x l od 7.64 X 10 8

As- (a) -21 .oea -1 ti. 77a

A F # ( K cal/rnol) 20.5ya 20.71 a

a+ 35°C.

NBA has been reported [2-71 to exist in acidic media in the following two alternative sets of equilibria. It is not possible to distinguish kineti- cally between the two alternatives.

(3) CH3CONHBr + HzO e CH3CONH2 + HOBr

14) HOBr + H30+ (H20Br)+ + H,O

or

(5) CH3CONHBr + H30t e (CH3CONHzBr)+ + H20

(6) (CH3CONH2Br)+ + H,O e CH3CONH, + (H20Br)+

Either of two sets of equilibria, (3,4) or (5,6) may be operative in the present investigation. It appears that either NBA itself, protonated NBA, HOBr, or cationic bromine i.e., (H20Br)' may be the active species.

If NBA or protonated NBA is taken as active species the rate law obtained fails to explain the negative effect of acetamide and ionic strength of the medium. Hence none of these can be taken to be reactive species of NBA. If HOBr is supposed to be involved in the reaction as active species, then reaction should proceed even in the absence of mineral acid contrary to our observation. Hence its involvement is ruled out as such although negative effect of acetamide is explained on this basis. The only choice now left is cationic bromine i.e., (H20Br) +

species which when taken as active species, leads to the rate law ca-


pable of explaining all kinetic orders and other effects. Hence on the basis of above arguments, we propose (H20Br) ' as oxidizing species under the experimental conditions.

Bearing the above facts in mind and considering the kinetic results, the following reaction scheme, involving interaction of (H20Br) with [RuCl6I3- in the rate determining step, has been proposed (where NBH is acetamide).

(I) NBA + H,O e NBH + HOBr

(11) HOBr + H 3 0 4 (H20Br) ' + H,O

(111) [RuC1,H2OI2 + C1V & [RuCl6]'- + H,O

K1

K 2

1 Br 1 Forward reaction16 is slowest and the rate determining step.

(A)

A- [RuC1,HI4- + Product

[RuCl6Hl4- + H20Br ' ~a;t' [RuCI6l3- + H 2 0 + HBr

Application of the steady state treatment with the reasonable approxi- mations yields the rate law (7).

(7) - hlk4 K2 K,[NBA] [H '1 [Ru(III)],[CIV]

k-l[NBH] + K,[Cl-] [H '1 (kz + k l Kz KJNBA]) -

- d[NBA] dt

where K 2 = kp/k-2 and K, = k,/k-, The rate law (7) conforms well to the observed kinetic orders and

effects. The rate eq. (7) can be rearranged in the following manner:


1 - 1 - - “BHI (8) __ - rate -d[NBA] K , K2K3h4[NBA] [H’] [Ru(III)]T[C~-]

dt

(k2 + kiK2KdNBAl) + ~,L~K~[NBA][Ru(III)] ,

where K, = kl/k- l A plot of l / ra te against l/[Cl-l at constant LNBAI, [RU(III)IT and

IHtl should yield a straight line with a positive intercept on l /rate axis and this was found to be so (Fig. 3). Positive effect of chloride ions on rate for the reaction is thus proved. This observation favors the equilibrium(II1) to the right side [15] and supports the assumption that dissociation possibility of chloride ions does not exist and thus [RuCl,J- is the real active species of ruthenium(II1) chloride.

The proposed mechanism is well supported by the values of energy of activation and thermodynamic parameters (Table IV). High positive values of the free energy of activation (AF 1 indicate that the transition state is highly solvated while the fairly high negative values of entropy of activation ( A S ) suggest the formation of an activated complex with a reduction in the degree of freedom of molecules.

A detailed mode of oxidation of the alcohols by NBA in the presence of acidic solution of ruthenium(II1) chloride is shown here.

[RuClS * H20]*- + C1- [RuClJ- + HZO

1 (O>O) 0-4 0.8 f . 2 1.6

(i/[ti -3) x 10” M-L Figure 3. of (A) ethylene glycol and (B) glycerol.

Plot of l /Rate vs. l /[Cl-] under the conditions of Table 111 in oxidation


Forward reaction is slowest and rate determining

+ R- C -OH + [RuC1,HI4- '~ast

I Br

Carbonium ion

H20 I Fast 1 eH OH

[RuC1,HI4- + HzOBr+ Fast\ [RuCl6I3- + H20 + HBr

Acknowledgment

The authors wish to thank CSIR, New Delhi,'Government of India for providing financial assistance to Miss Deepika Singh.

Bibliography

[ l l R. Filler, Chern. Reu., 63, 21 (1963). 121 J. Mukherjee and K. K. Banerji, J . Org. Chern., 46, 2323 (1981). [31 P. S. Radhakrishnamurthi and N. C. Sahu, Indian J . Chern., 20A, 269 (1981). 141 Bharat Singh, A.K. Samant, and B.B.L. Saxena, Tetrahedron, 40, 3321 (1984). [51 Bharat Singh and Rohit Srivastava, Current Science, 54, 415 (1985). 161 Bharat Singh and Rohit Srivastava, Oxidation Cornrnunication, 9, 1 (1986). 171 Bharat Singh and Rohit Srivastava, Tetrahedron, 42, 2749 (1986). [81 Bharat Singh, A. K. Singh, N. B. Singh, and B. B. L. Saxena, Tetrahedron, 40,5203

[91 Bharat Singh, A. K. Singh, and N. B. Singh, Kinetika Z Kataliz, 27, 93 (1986). (1984).

[lo1 Bharat Singh, A. K. Singh, V. K. Singh, A. K. Sisodia, and M. B. Singh, J . Molecular

[ l l ] F. Feigl, Spot Tests i n Organic Analysis, Elsevier, New York, 1966. [12] N. Venkatasubramanian and V. Thiagarajan, Can. J . Chern, 47, 694 (19691; Tet-

Catalysis, 40, 49 (1987).

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[131 J. C. Bailer, The Chemistry of Coordination Compounds, p. 4 Reinhold, New York,

1141 G. Gopal Krishnan, B.R. Rai, and N. Venkatasubramanian, Zndian J . Chem.,

1151 D. A. Fine, Ph.D. Thesis, University of California, Berkeley, 1960 as quoted by R. R.

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Received May 21, 1987 Accepted January 4, 1988

Ru(III) catalysis in N-bromoacetamide oxidation of ethylene glycol and glycerol: A kinetic and...

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