Kinetics and mechanism of Ru(III) catalysed oxidation of some polyhydric alcohols by N -bromosuccinimide in acidic media

-

Tetrahedron Vol.42,No. IO,pp.2739390 2741.19% RintiinGrcatBritnin.

Pergpmon

53.00 + 40 Journals Ltd.

KINETICS AND MECHANISM OF 'Ru(II1)CATALYSED OXIDATION OF SOME POLYHYDRIC ALCOHOLS'BY Y-BROMOSUCCINIMIDE IN ACIDIC MEDIA

J.P. SHARMA*, R.N.P. SINGH, A.K. SINGH and BHARAT SINGH

Department of Chemistry, University of Allahabad, Allahabad 211 002, U.P., India.

(Received in UK 8 January 1986)

Abstract - Kinetics of oxidation of ethylene glycol, glycerol, erythritol and dulcitol by acidic solution of N-bromosuccinimide (NBS) in presence of ruthenium(II1) chloride as a homogeneous catalyst and mercuric acetate as scavenger in the temperature range of 30-50°C have been reDOrt.ed. The reactions follow identical kinetics. being Zero effect first order in each NBS, substate and Ru(II1). of [H'], [mercuric acetate] and ionic strength has been observed. A negative effect of succinimide and acetic acid is observed while [Cl-l shows the positive effect on reaction velocity. Various activation parameters have been computed. The products of the reaction were identified as the coresponding acids. A suitable mechanism consistent with the experimental results has been proposed.

INTRODUCTION NBS is a potent oxidant and has been used in the quantitative estlmation1-3 of a large number of organic as well as inorganic compounds. Relatively little work has been done on the oxidation kinetics involving NBS, and the mechanistic interpretations of the results are obscure. Some investigations on oxidation kinetics involving NBS and alcohols,' esters,5 and a few ketones,6-9 in acidic media have been reported. The mode of NBS oxidation in catalysed reactions 1s unknown and so far none has attempted to probe the role of ruthenium(II1) chloride.-ascatalyst in NBS oxidations. This prompted us to undertake the present lnvestlgation which constitutes the studies of kinetics and mechanism of Rut1111 catalysed NBS oxidation of aforesaid polyhydric alcohols in perchlorlc acid media in presence of mercuric acetate. EXPERIMENTAL An aqueous solution of NBS was prepared afresh each day from a G.R., S. Merck sample of the reagent, and its strength was checked by the lodometrlc method." Ethylene glycol, glycerol, erythritol and dulcitol of AnalaR grade and E. Merck (Germany) sample of mercuric acetate were used. The solutions of the polyhydric alcohols 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 perchlorio acid. glacial acetic acid and sodium perchlorate, were of AnalaR grade. Triple 2739

J.P.SHARMA et

2748

al.

distilled water was used throughout the investigations. The reaction stills (Jena glass) were blackened Prom outside. All the kinetic measurements were carried out at constant temperature The reaction was initiated by rapid addition of NBS to the reaction (i O.lOC). mixture containing appropriate quantities of alcohol, glacial acetic acid, perchloric acid, ruthenlum(II1) chloride, mercuric acetate and water and mixing them by vigorous shaking. The progress of the reaction was monitored by estimating the amount of unconsumed NBS at regular time intervals iodometrically. RESULTS Stoichiometry OP the reaction was ascertained by equilibrating the reaction mixture containing an excess of NBS over alcohol (in varying ratios) at 50°C for 48 hrs, and estimations of residual NBS in different sets showed that one mole of alcohol consumes two moles of NBS according to the stoichiometric equation (1). CH2-CO

CH2-CO \

2 CH2-

NBr

+

RCH20H +

H20

-

2

/ CO

CH2+

RCOOH

+

'NH / CO

2HBr

(1)

where R stands for CH20H, CH20HCHOH, CH20H(CHOH)2, and CH20H(CHOH)4 in ethylene glycol, glycerol, erythritol and dulcitol whose oxidation products, glycollic acid, dl-glycerlc acid, erythronic acid and galactonic acid were detected by conventional methods."

TABLE 1

Effect of Concentrations of Reactants on Reaction Rate

[Hg(OAc)21

-

2.00 x 1O-3 H, pH - 2.58,

CRu(III)] 0.88 x 10W6 H and [Cl']

[NBSII03 M 0.6 0.8 1.0 1.2 1.4 ::; 1.0 1.0 1.0 1.0 1.0

r [Subs&rate] 10 M 1.0 1.0 1.0 1.0 1.0 1.0 ii; 1.0 1.5 ;:"5

-

3.60

x 10e3

M

1

lo4 kl set-'

Ethylene glycol 30° 2.60 2.64 2.70

2.62 2.74 2.68 2.74 1.39 2.70 3.67 5.10 6.88

35O I 4.30 4.31 4.40 4.38 4.44 4.36 4.38 2.17 4.38 6.28 8.44 10.10

Glycerol

Erythritol

3o"

30" 5.17

3.92

3.68 3.69 4.38 3.80 3.82 :% 3:69

350 6.07 6.12 -6.26

5.11 5.16

35O 7.80 :z.

'6-E 6:14 6.20 '6% 9:16 12.28 15.46

2.68 3.91 5.17 7.95 8.09 11.60 lo.48 15.84 13.04 19.76

Dulcitol 300 6.78 7.12 6.86 6.93 7.04 7.04 6.88 3.36 6.86 lo.19 13.40 16.91

350 9.70 9.77 9.90 9.78 9.82 ;*:: 4:96 9.90 14.86 ;;.;; .

The kinetics OP the oxidation of polyhydric alcohols was investigated at several initial concentrations of the reactants (Table 1). First order dependence in NBS was Pollowed at all initial concentrations oP NBS. Pseudo first order constants (k,) were calculated Prom the slopes of log-time Further, a linear increase in the Plrst order rate constants in plots (Fig. 1). NBS with an increase in initial concentrations of polyhydric alcohols was

zl41

Ru(II1) catalyscdoxidationof some pblyhydricalcohols observed.

The order

[substrate]

plots

glycerol

and 0.95

follow of

first

that

with

linearly

(Table

proves that

kinetics

first

pH of

(ionic

Successive

it

reaction

rate

of

constant

parameters

addition

value, 2 also

rate

hydrogen

OP mercuric

which reveal

constants

in

ion

acetate

and sodium

was observed. acid

decreased

ion

the

in the

in NBS decreased

were computed

was observed

constant order

of

reactions

in UBS increased

OP Table

the first

log

The rate It

constants

effect

kl s and

alcohols.

had a fairly

zero

log

that

by [RU(III)~. rate

interaction

chloride

of

glycol

established

The results

of

acetic

slopes

polyhydric

mixtures

effect

45O and 50°C and are

400,

all

order

thereby

an anion-dipole

on increasing

activation

first

variation)

addition

order

increased

350,

the

strength

in NBS, indicating

to

influenced

on [RU(III)].

Insignificant

perchlorate

This

3) and k,/[Ru(III)]

indicating

unchanged,

concentration.

The first

the

dependence

on increasing

NBS remain

respect

be highly

[Ru(III)]

2 and Fig.

order

Prom the

be 0.96 Por ethylene

and dulcltol.

with

was Pound to

increasing

was computed

2) and was Pound to

Por erythritol

order

the reaction

in substrates

(Fig.

order

on addition

concentration

Prom the

reported

first of

(Table

study

in Table

of

rate

constants

rate-determining

rate

step.

succlnimide while The values of

3).

measurements

at 30”.

4.

DISCUSSION The results

of

dulci to1 , recorded and thus acetate

appear

the

here,

of

OP ethylene

revealed

that

rules

for

out

any Br-

oxidation

by Br2,

its

formed which

glycol,

the

glycerol,

reactions

involvement in the would

in

identical

effect It

been formed

and kinetics

0P mercuric

oxidation

NBS

reaction. have

erythritol

have

InsignLPLcant

common mechanism.

rate

as a scavenger12s13 the

have

to have

on reaction

completely

oxidation

and acts

only

suppresses by the

interaction

HBr and NBS as follows: CH2-CO

CH2-CO

\ /

Mercuric

acetate

Positive

effect

+

It

NBr

\ /

right

known to H*

+

H+

i

+

Thus NBS 1tselP oxidlsing

2-

the oxidation ions

t’

that

rate

ruthenium(II1)

IRuClg13

be assumed

Br2

(2)

purely

on reaction

in acidic

side”(

NH +

/ CO

+

through suggests chloride

NBS. that

equilibrium

solution.

Hz0

CRUC~,~]~- is

(3) the active

species

of

chloride.

l

NBr

ensures

chloride

may, therefore,

NBS is \

to

thus of

[RuC15.H201

ruthenlum(II1)

\

HBr ---f CH2-

(3) is Pavoured

cl-

+

I

I

CH2-CO

/

NBr

species

exist \

/

media

in the

following

equilibria:

iHBr

/

\

in acidic

NH +

(4)

Br*

or protonated

in acidic

media.

(5)

NBS, i.e.

~~BSHor Br* may be the

possible

2742

Zero molecule

effect

(retarding These

of

in the

involve

effect

facts

of

in the

and Ru(II1)

rate

first

to

involved

Ru(II1)

the

species

of

NBS

complexation require

first

observed

itself

NBS

of

molecule

because

involvement

of

in its

alcohol

During

concluded species

that

and negative

choice

to

followed

to form

the

by its

will

on nitrogen gets

to

dissociation

towards

its

proceeds

with

with

to

be more

and then

leading of It

Cl-

ion

tries the

reaction

paths

the are

above

statements

proposed,

CRuC15.H2012

+

and kinetic

where M is

Cl-

_!L

does

between

Ru(II1)

results,

the

following

succinimide.

[RuC1613-

\k_

H20

l

1

[RuC~~]~-

A

+

NBS

l

RCH20H

L&

7

LL

\

A (Complex)

k-2

x

+

(ii)

M

(iii)

k-3

X

CRUC~~H]~-

slow and rate determining

[RuC~~HI~Here X stands

for

Application approximations

+

NBS

l

H+

%’

+

Products

[RuC1613-

(iv)

+

HBr

+

M

the activatedcomplex. of

yields

the steady the

rate

state law (6).

treatment

with

the

of

reasonable

to

chemical

therefore,

is,

formation

to

and NBS. Considering

of

the

power on account

NBS

possibility complex

with

bond formation.

reagent,

association.

This

remote. Formation

seems

in coordinate

to

addition.

is

species.

greater

in

would

contrary

suhcinimide

complex

the

as

involved

species

interaction

exert

complexed

nearer

species

species

reactive

activated

so formed

process,

reaction

be the

or Br’ of

involvement

charged

of

form

Hence Br* as reactive

succinimide

effect

any

alcohol

molecule.

direct

reactive

as reactive

species

state

is

of

and of

Complexatlon

negatively

effect

polyhydric

protonated

effect.

NBS as the

desired

and zero

pair

(3)

the

and its

molecule

this

as equilibrium

to be unreasonable

the

and lack

as neutral

or its

constant

reciprocal substrate

alcohols,

alcohol

itself

ion.

as neutral

involving

between

must

charged

Michaelis-Menten

species.

complex

electron

wlth

NBS acting

8 coordination

polyhydric

reaction.

the

of

step

alcohol

formation

Ru(II1)

H* ions

as the only

determining

formation

NBS

Protonated

protonated

alcohol

polyhydric

either

to give

Ru(II1)

complex

dielectric

out.

by NBS with

reasonable,

exist

Br+ seems positive

in Hi ions

order of

bring

involvement

Ru(II1)

effect

Ru(II1)

the

ruled

is

zero

complex

suggest

formation

Thus possibility leaves

the

complex

of

a neutral effect

and a negatively

in polyhydric

for

the

with

requirement

that

rate

Absence

the complex

evidence

with

negate

for

the

at least dielectric

NBS or polyhydric

step.

proof

of

positive

molecule

either

conclusion

in the

species

such will

shows that of

order

involvement

Observed

a neutral

controlling

species

leads

acid)

between

(a kinetic

spectrophotometric

the

acetic

suggests

step.

involvement

strict

catalyst),

strength

controlling

require

relatlonship15

This

ionic

rate

an interaction

molecule

is

J. P. SHARMA et al.

-

(v)

not

2l43

Ru(III) catalyzedoxidationof some polyhydricalcohols K k k,, [NBS1

2~

d[NBS]

’

dt

where

Kl

The rate The equation

(6)

K3

is

K2

/

+

k,,K,

[Ru(III)lT

[Cl-l

(6)

[Cl-l

tir k-2

-

in excellent

can be rearranged

agreement

in the

with

following

the experimental

results.

form:

dlNBS1 dt

-

1

_

2K,K2K3k4 [NBS]

--

c

1

CkH,OHl

CRU(III)IT

1

2K2k3 [NBS]

+

where

and

k-l

law

1 rate

k_3 [Ml

kl

-

(6)

’

CRCH30Hl

tRCH20Hl

ccl-l

CRU(III>~~

(7)

k?

-

k-3

A plot should

was found is

thus

to

be so

line

(Fig. This

proved. the

ruthenium(II1) may be assigned

l/Ccl-l with

at

observation

assumption

favours

that to

structures

(I)

or

of

[RCH20Hl and CRU(III)I~

on l/rate

chloride

is

of its

the active

the

complex

hydride

ion

axis

ions

the equilibrium

[RUC~~]~-

explain

[NBS],

intercept

effect

The structure

in order

constant

a positive

Positive

4).

chloride.

known, but

not

against

a straight

and supports is

l/rate

of

yield

(3)

to the

species

of

(A)

and this

on reaction

formed

rate

right

in step

abstracting

side (ii)

capacity,

it

(II).

3-

0

II

CH2-C

CH2-C\ JBP

\

or

I

/N”‘R~C1

N-B

/

CH2- C II 0--+RuClg

t (Complex)

A

II The lone-pair Ru(III)

of

of

electrons

to form

(RuC16)3-

n-electrons

of

giving

rise

to a n-n complex

become

electron

towards

one of

and,

considerably

after

Jones

et -existence

all8 of

In the therefore,

which

transfer

and Freeman -et al.lg Structure (I) or (II) light

of

the above

be pictured

as given

Consequently,

leading

to

its

theory It with

is

is

the

the

capacity

also

present the

N-Br bond shift

NBS is

with to

Ru(II1).

electrophilic of

supported

possible

the

the N atom will of

interaction

not

discussion,

bond with case,

electron-pair

abstracting

with

Alternatively,

In either

make the

ion

NBS can coordinate

I).

form a coordinate

positive.

complexation

atom of

(Structure

11).

will

the hydride ion

nitrogen

groups

(Structure

Br slightly

hence,

The hydride

alcohol.

carbonyl

deficient.

N, making

character

the

on the

a n-n complex

enhanced

polyhydric

by Littler,16 establish

Lee,17

the

data.

sequence

of

the

reaction

may,

in the Scheme. ACKNOWLEDGEMENT

The authors wish to thank Government of India for financial

the Unive5ity assistance.

Grants

Commission,

New Delhi,

2744

J.P.SHARMA~~U~. [RuC15.H203 2-

+

=

cl-

[R~C163~-

+

H20

Or

Structure

(II)

RCH20H slow and rate determining step

4R---OH

+

[RuC16H]

I Br

N<:J::

Activated complex

Carbonlum ion Fast

H20

OH

OH H+

luCl6LR-$lj-+

(

+

'KY R-C-O-H

j

R-C-O

l

HBr

CLr CH2-CO \ [RuC~~H]~-

+

H+

+

NBS

Fast\

CRUCIFIX-

NH

+

I

l

HBr

/

CH2-CO SCHEME

TIME, MINUTE

FIGURE 1

First order ra e plots at 30°C - 1.20 x 10-5 M, [Ru(III)] - 0.88 x 1O-6 M, ccl-l - 3.60 x 10-3 H, L$(OAc) ] - 2.00 x 10-3 M [Substrate] - 1.00 x 10 M in 1 (Ethylene glycol), B (Glycerol), C (Erythritol) and D (Dulcitol) CNBSI

Ru(III) catalysed oxidation of some polyhydric alcohols

2.4-

2745

36.Or

1.8-

B 1

0.6o.o1.201)

#!? Q6 3+1og

, 1.2

1.8

24

l&bstr8tr/MJ

Plot of_;"" k, ~9 log FIGURE 2 [substrate] at 35O M, pH - 2.58, [NBS3 - 1.00 x 10 ERu(III)l - 0.88 x 10m6 M, [Cl-i 3.60 x 10-3 M, and CHg(OAc)21 - 2.00 x 10-3 M, A (Ethylene l3lYCOl), B (Glycerol), C (Erythritol), D (Dulcltol).

FIGURE 3 Plot of kl vs [RU(III)I at Conditions as-m Table 2, A 3ov. (Ethylene glycol), B (Glycerol), C (Erythritol), D (Dulcitol).

Plot of l/rate vs l/Ccl-1 at 30°C. Conditions as in Table 3, FIGURE 4 A (Ethylene glycol),T (Glycerol), C (ErYthritol), D (Dulcitol).

2746

J.P. SHARMA etal.

TABLE 2

Effect of [Ru(III)l, pH, CH~(OAC)~I and ionic strength (u) variation on

reaction rate at 3OOC.

[NBS] - 1.00 x 10-3 M. [Substrate] - 1.00 x lo2 M, pH 2.58 (unless otherwise state and CH~(OAC)~I - 2.00 x 10e3 M (unless otherwise mentioned,

[Cl-] - 3.60 x 10V3 H k, x 104, set -1 Ethylene glycol

0.88 1.76 2.64 2% 0:88a 0.88b 0.88C 0.884 0 :88e 0.88f 0.888 0.88h 0.881 ;.$ o:ss1 0.88m

Glycerol

2.67 5.52 8.40 10.88 13.66 2.72 2.io 2.68 2.60 2.67 2.64 2.70 2.66 2.64 2.66 2.65 2.67 2.66

3.72 3.70 ;*:2 .

; PH - 1.42;

b pg 1;11.;6;o ' pH - 2.08; [Hg(OAc)21 x 10 " " 6:ooj : 8.00. "2 'j (IJ) 10 M - ;.$ n k n 2:ooj 1 " 2.48; m " 3.00.

TABLE 3

Erythritol

Dulcitol

5.16 10.38 15.88 20.81

6.86 13.88 20.22 28.12

':% 5100 5.10 5.20 5.16 5.12 5.22 5.18 5.16 5.19 5.16 5.17 5.19

'Xi 6:94

d pH - 2.20;

i% 6:86 6.60 6.70 6.80 z!: 6:78 6.76 6.78 e pH - 2.78.

Effect of addition of succinimide, acetic acid and chloride ions on

reaction rate at 30°C, [NBS]

-

1.00 x 10m3 M,

[Substrate] - 1.00 x 10V2 M,

[Ru(III)] - 0.88 x 10e6 M, pH - 2.58, [Acetic acid] (v/v) - 2% (unless otherwise stated) and CHg(OAc)2] - 2.00 x low3 M k, x 104, set -1 ccl-lMx 103

3.60 4.40 5.20 6.00 6.80

Ethylene glyc01

Glycerol

2.67 6.61 4.11 1.96 1.46 1.05 0.72 2.03 1% 1:02

1.91 1.36 3.05 2.50 2.06 1.62

[Acetic acid] (v/v) - 16a, 30b, 44' and 60Xd; [Succlnimide]

x lo3 H - 0.8~. l.Of, l.2g and l.4h

Erythritol

Dulcitol

5.16 5.66 6.22 6.76 7.55 3.68 2.60 1.85 1.32 4.06

6.86 7.05 7.98 9.12 10.22 5.04 3.60 2.57 1.84 5.20 4.10 3.28 2.48

z2 2:06

Ru(II1) catalysed oxidation of some polyhydric alcohols

TABLE 4

Activation

parameters for NBS oxidation of polyhydric alcohols AS*

Substrate

(l/mtl set)

Ethylene glycol Glycerol Erythritol Dulcltol

2747

4.57 x 10"

19.48

(e.u.1

*

*

(Al!cal/mol)

( Kcznol)

-6.12

17.85

19.76

-8.03 -13.53 -16.64

17.12 15.20 14.13

19.62 19.43 14.33

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