Kinetic Analysis of the Methylene Blue Oxidations of Thiols - ACS

15 Kinetic Analysis of the Methylene Blue Oxidations of Thiols E A R L S. HUYSER and HSIAO-NEIN TANG

Downloaded by CORNELL UNIV on August 6, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0069.ch015

Department of Chemistry, University of Kansas, Lawrence, KS 66044

K i n e t i c s t r a d i t i o n a l l y have been among the more powerful t o o l s a v a i l a b l e f o r the study o f organic r e a t i o n mechanisms. Much o f our present understanding o f the behavior o f f r e e r a d i c a l s as r e a c t i o n intermediates can be a t t r i b u t e d to data accumulated by various competitive k i n e t i c i n v e s t i g a t i o n s (e.g. the determination o f t r a n s f e r and copolymerization constants i n v i n y l polymerization r e a c t i o n s ) . Somewhat l e s s e x p l o i t e d as a means o f gaining i n s i g h t i n t o the mechanistic d e t a i l s of f r e e radical chain r e a c t i o n s has been the examination of the r a t e laws o f such r e a c t i o n s . Neglect o f t h i s area of i n v e s t i g a t i o n is s u r p r i s i n g because those r e a c t i o n s that have been subjected t o such s t u d i e s (e.g., v i n y l polymerizations, brominations, a u t o x i d a t i o n s ) have y i e l d e d valuable information concerning the o v e r a l l mechanisms o f f r e e r a d i c a l chain r e a c t i o n s . The l a c k o f extensive e f f o r t i n t h i s area may be due, at l e a s t i n p a r t , t o the f a c t that the experimentally determined r a t e constants are combinations o f the r a t e constants o f the s e v e r a l i n d i v i d u a l steps i n the o v e r a l l r e a c t i o n . G e n e r a l l y , these observed r a t e constants are not r e a d i l y r e s o l v a b l e i n t o the r a t e constants o f the i n d i v i d u a l steps o f the r e a c t i o n and thereby a v a i l a b l e f o r the classical examination the a c t i v a t i o n parameters o f these r e a c t i o n s . Furthermore, the observed r a t e laws o f t e n i n v o l v e f r a c t i o n a l k i n e t i c orders o f the r e a c t a n t s (and sometimes the p r o d u c t s ) . Further, these k i n e t i c orders o f t e n depend on experimental c o n d i t i o n s , i n p a r t i c u l a r the r e l a t i v e concentrations of t h e r e a c t a n t s (and sometimes the products). While such f a c t o r s as i n c o n s i s t e n t k i n e t i c orders may appear a t the outset to be detrimental t o the study of a r e a c t i o n mechanism by k i n e t i c a n a l y s i s , these same f a c t o r s may, i n some instances, prove t o be v a l u a b l e probes f o r i n v e s t i g a t i o n o f the mechanisms of f r e e radical chain r e a c t i o n s . The work presented here d e s c r i b e s such an i n v e s t i g a t i o n .

©0-8412-0421-7/78/47-069-258$05.00/0

Pryor; Organic Free Radicals ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

15.

HUYSER A N D TANG

Methylene

Blue

Oxidations

of

Thiols

259

Methylene Blue O x i d a t i o n s o f T h i o l s The o x i d a t i o n s o f t h i o l s t o d i s u l f i d e s , r e a c t i o n s encountered i n a v a r i e t y o f b i o l o g i c a l processes, have been accomplished i n the l a b o r a t o r y by a v a r i e t y o f o x i d i z i n g agents 2 RSH + Ox

->

RSSR + OxH

r

(e.g. m o l e c u l a r oxygen, f e r r i c y a n i d e , quinones, f l a v i n s , and azocompounds) (1). I n t h i s study, methylene blue (MB ) was used as t h e o x i d i z i n g agent ( 2 ) . The t h i o l s subjected t o o x i d a t i o n by


+

RSSR + (CH ) N 3

2

+ H (CH ) N3

(CH )

2

3

2

MB were mercaptoethanol (RSH) and d i t h i o e r y t h r i t o l ( R i S H ^ ) which were o x i d i z e d t o t h e corresponding d i s u l f i d e s (RSSR and RS25 r e s p e c t i v e l y ) with concurrent r e d u c t i o n o f MB t o leucomethylene blue (LMB). +

2H0CH CH SH

MB+

2

HOCH CH S

LMB

2

2

2

+ H HOCH CH S 2

RSH

2

RSSR CH SH

CH-

2

MB

— OH

LMB

OH

+

- OH

+ H

OH CH:

CH SH 2

RS„

R(SH),

The r e a c t i o n r a t e s o f the o x i d a t i o n s o f mercaptoethanol and d i t h i o e r y t h r i t o l were determined a t v a r i o u s c o n c e n t r a t i o n r a t i o s of t h e t h i o l s w i t h r e s p e c t t o MB over a range of pH s and i o n i c s t r e n g t h s i n both water and D 0 by f o l l o w i n g t h e decrease i n t h e +

f

o


ORGANIC F R E E RADICALS

260 +

absorption maximum o f MB a t 650 nm. The high molar e x t i n c t i o n c o e f f i c i e n t o f MB (ε = 51,200) allows f o r r a t e determinations at very low concentrations o f t h i s reagent. That the r e a c t i o n s i n v e s t i g a t e d a c t u a l l y do occur was e s t a b l i s h e d by i s o l a t i o n o f both leucomethylene blue and the corresponding d i s u l f i d e s as r e a c t i o n products. +

MB* Oxidations o f Mercaptoethanol. A mechanism f o r the o x i d a t i o n o f mercaptoethanol with MB that i s c o n s i s t e n t with our data f o r t h e r e a c t i o n i s shown i n the sequence 1-10. This mechanism evolved f o r the most part from our k i n e t i c s t u d i e s o f the r e a c t i o n . Steps 2-4 o f the r e a c t i o n scheme comprise a f r e e Downloaded by CORNELL UNIV on August 6, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0069.ch015

+

+

RS- + MB

—

Κ

1 —>

RS- + MB-

(1)

2

RSSR

(2)

MB- + RSSR

(3)

LMB + RS-

(4)

k RS- + RS"

—

->

k

-2 k — 3 —>

+

RSSR + MB

k

MB- + RSH

— k

u

—-> k 6 RS- + MB* — k RS· + RSSR — 7 k MB· + RSSR — 8 k 2 MB- + H - 9 k ι ιη 2 RSSR — — > 2 RS-

+

—>

R

(5)

RSSR

—>

RS" + MB

(6)

—>

RS" + RSSR

(7)

—>

MB

—>

MB

+

+

+ 2 RS"

(8)

+

+ LMB

(9)

2 RS" + RSSR

(10)

r a d i c a l chain sequence that accounts o f the stoichiometry o f the o x i d a t i o n o f mercaptoethanol by MB . The s u l f u r - s u l f u r l i n k a g e of the d i s u l f i d e i s e f f e c t e d i n the formation o f the r a d i c a l anion RSSR, a r a d i c a l which i s p a r t i t i o n e d between fragmenting t o the species from which i t was formed and t r a n s f e r i n g o f an e l e c t r o n t o MB t o y i e l d the d i s u l f i d e as a r e a c t i o n product and the methylene blue derived r a d i c a l MB-. The l a t t e r propagates the chain by a b s t r a c t i n g the sulfur-bonded hydrogen from the t h i o l y i e l d i n g leucometnylene blue (LMB) and a c h a i n - c a r r y i n g t h i y l r a d i c a l ( R S O . The o v e r a l l r e a c t i o n , which occurs a t room temperature, i s i n i t i a t e d (step 1) by an i n t e r a c t i o n ( p o s s i b l y an e l e c t r o n t r a n s f e r ) between a s u l f i d e i o n and MB . There are s i x termination r e a c t i o n s p o s s i b l e (steps 5-10) f o r a chain sequence i n v o l v i n g three d i f f e r e n t c h a i n - c a r r y i n g r a d i c a l s . Our r a t e data i n d i c a t e that only those shown i n 5-8 are l i k e l y termination processes f o r the r e a c t i o n . +

+

+


15.

HUYSER AND T A N G

Methylene

Blue

Oxidations

of

Thiols

261

Examination of t h i s mechanism r e v e a l s that both the t h i o l and the s u l f i d e i o n are r e q u i r e d as r e a c t a n t s . The pH of the medium, which determines the r e l a t i v e amounts of these components, t h e r e f o r e , has a s i g n i f i c a n t i n f l u e n c e on the r a t e of the o x i d a t i o n r e a c t i o n . Table I shows the o x i d a t i o n r a t e s of mercaptoethanol by MB are pH dependent i n that the r e a c t i o n r a t e +

Table I E f f e c t o f pH on MB^-Oxidation


%ates of Mercaptoethanol pH

3

Rate χ 10

6

M sec

8.20

1.01

(0.05)

8.60

2.14

(0.08)

9.00

4.89

(0.08)

9.30

6.51

(0.26)

9.60

7.40

(0.08)

9.90

7.66

(0.13)

10.30

8.36

(0.09)

10.70

7.66

(0.32)

a.

—1

A l l solution Buffered with borate. adjusted to i o n i c strength = 0.4 with

KC1.

i s g r e a t e s t a t a pH roughly one pH u n i t greater than that r e q u i r e d f o r about equal d i s t r i b u t i o n of the t h i o l and s u l f i d e ion as d i c t a t e d by the i o n i z a t i o n constant o f mercaptoethanol (pK = 9.34) (3_). The lower o x i d a t i o n r a t e s above pH 10.3 i n d i c a t e involvement of the t h i o l i n a r a t e - l i m i t i n g step when the t h i o l concentration i s low. The observed r a t e laws f o r the o x i d a t i o n o f mercaptoethanol by methylene blue under d i f f e r e n t r e a c t i o n c o n d i t i o n s are c o n s i s t e n t with the steady-state r a t e laws derived from the proposed mechanism. I f step 2 o f the r e a c t i o n sequence, namely formation of the d i s u l f i d e r a d i c a l anion from a t h i y l r a d i c a l and a s u l f i d e i o n , i s the r a t e l i m i t i n g step of the chain sequence, and t h e r e f o r e only termination by 6 (coupling o f t h i y l r a d i c a l s ) occurs, the derived r a t e law f o r the r e a c t i o n i s Eq. 11. This r a t e law, which t a k e s - i n t o account the d i s t r i b u t i o n of the t h i o l and s u l f i d e as determined by the K o f the t h i o l and the a c i d i t y of the medium, p r e d i c t s that the observed r a t e law would be h a l f order i n MB and three halves order i n mercaptoethanol. This i s the r a t e law expected, however, only i f the concentration o f MB i s s u f f i c i e n t l y high so that the second term i n the denominator i s n e g l i b l e . At pH s below the pH maximum, and at a s u f f i c i e n t l y A

A

+

+

T



262

1 / 2

+

1 / 2

+

(k / 2 k . ) k [ M B ] ( K T R S H ] / [ H ] ) —± 5 ± à 1 + k_ /k [MB ]

3 / 2

Q

Rate =

(11)

+

2

3

+

high c o n c e n t r a t i o n of MB , the formation of the d i s u l f i d e r a d i c a l anion may w e l l be expected to be the only l i m i t i n g r e a c t i o n i n the chain sequence s i n c e the r e a c t a n t s f o r the other two steps (MB and RSH) are present i n s u f f i c i e n t concentrations t o obviate t h e i r p a r t i c i p a t i o n i n r a t e l i m i t i n g steps. Table I I shows the k i n e t i c orders of MB and RSH f o r the o x i d a t i o n r e a c t i o n at pH 9.9.* The r a t e law f o r the r e a c t i o n a t pH 9.9 very c l o s e l y f o l l o w s the derived r a t e law shown i n Eq. 11 i n that at the higher concentrations of MB , the r e a c t i o n i s t h r e e halves order i n RSH and h a l f order i n MB*. However, the d e r i v e d r a t e law i n d i c a t e s t h a t at lower concentrations o f MB the second term i n the denominator could be s i g n i f i c a n t and the observed k i n e t i c order of MB+ would be greater than one h a l f . The increase i n k i n e t i c order o f MB observed at lower concentrations of MB i s t h e r e f o r e l i k e l y a r e f l e c t i o n o f the l e s s e f f e c t i v e p a r t i o n i n g of the RSSR t o the MB . At pH > 10.3, the c o n c e n t r a t i o n of RSH r e l a t i v e t o RS~ i s small and r e a c t i o n 4, the hydrogen atom a b s t r a c t i o n from RSH by the methylene blue derived r a d i c a l ΜΒ·, i s a r a t e l i m i t i n g f a c t o r . I f r e a c t i o n 4, along with the d i s u l f i d e r a d i c a l anion formation i n r e a c t i o n 2, are the r a t e l i m i t i n g steps i n the chain sequence, the r e a c t i o n would f o l l o w the d e r i v e d r a t e law shown i n Eq. 12. Note t h a t , i n terms of the measurable k i n e t i c +


+

+

+

+

+

+

(

k kk 1

Z

4

+

+

[MB ]([RSH]-K.[RSH]/[H ])\ I k ( l + k_ /k [MB ]) /

1 / 2

Κ [RSH]/[H ]

+

6

2

(12)

A

3

+

orders of MB and RSH, the expected observed r a t e law at pH 10.7 i s not d i s t i n g u i s h a b l e from the observed r a t e law f o r the r e a c t i o n at pH 9.9, namely t h a t the r e a c t i o n i s h a l f order i n MB and three-halves order i n mercaptoethanol. Table I I I shows that at the higher concentrations of MB , the r e a c t i o n a t pH 10.7 +

+

{

*'A word concerning these k i n e t i c orders and those i n subsequent Tables i s warranted. A l l k i n e t i c orders were determined by the method of i n i t i a l r a t e s . The k i n e t i c order l i s t e d at each c o n c e n t r a t i o n of MB i s the average c a l c u l a t e d from at l e a s t three ( g e n e r a l l y four or f i v e ) determinations o f the r e a c t i o n r a t e a t t h a t c o n c e n t r a t i o n of MB . The value i n parenthesis f o l l o w i n g the k i n e t i c order i n d i c a t e s the o u t s i d e l i m i t s of c e r t a i n t y based on the c a l c u l a t i o n of the k i n e t i c order from the f a s t e s t and slowest r a t e s at t h a t p a r t i c u l a r concentration. +

+


15.

HUYSER A N D T A N G

Methylene

Blue

Oxidations

of

263

Thiols

Table II K i n e t i c Data f o r MB* of Mercaptoethanol Rate = k

+

Q b s

+


at pH a

6

e

a

i

.47

5.92

.50 (.12)

4.36

.48 (.10)

3.05

.50

1.66

.74 (.05)

.78

.89 (.18)

.59

1.07

a

Orders

7.84

a. b. c.

9.9

[MB ] [RSH]

Kinetic 1] χ i o

Oxidations

b

1.48

(.n)

1.49

(.13)

1.48

(.11)

1.50

(.12)

1.48

(.13)

1.50

(.16)

1.52

(.10)

1.49

(.11)

c

(.12)°

(.10)

(.18)

Borate b u f f e r e d , μ = 0.40. [RSH] = 8.15 χ 10-3 d 5.78 χ 10~ . Extreme l i m i t s of r e l i a b i l i t y . 3

a n

i s indeed three halves order i n RSH At lower concentrations o f MB , order of t h i s reagent r e f l e c t s ^ l e s s the d i s u l f i d e r a d i c a l anion RSSR to decrease i n the k i n e t i c order o f RSH +

+

and h a l f order i n MB . the increase i n k i n e t i c favorable p a r t i t i o n i n g MB as noted at pH 9.9. The at lower MB concentrations +

+

Λ. i n d i c a t e s that RSSR i s p o s s i b l y i n v o l v e d i n a termination reaction. The derived r a t e law f o r the r e a c t i o n i f steps 3 and 4 are r a t e l i m i t i n g i s shown i n Eq. 13 and i n d i c a t e s that i f r e a c t i o n / k k k (K [RSH]/[H ])\ 1/2 Rate = I J [MB ][RSH;r^ (13) , k ( l + k_ /k [MB ]) +

1

d

H

A

+

+

Q

2

3

3 i s a r a t e l i m i t i n g step, the k i n e t i c order o f RSH

i s unity.

MB* Oxidations o f D i t h i o e r y t h r i t o l . Although the k i n e t i c r a t e laws observed f o r the mercaptoethanol o x i d a t i o n s are


ORGANIC F R E E

264

RADICALS

Table I I I Rate Data f o r Mercaptoethanol Oxidations at pH Rate = k


[MB*]

a. b.

χ 10

+

v

obs

10.7 a

a

[MB ] [RSH]

6

3

3

a

7.84

0.50

(.16)

5.92

0.48

(.13)

4.36

0.55

(.11)

3.05

0.51

(.09)

1.66

0.72

(.05)

.78

0.81

(.16)

.59

0.93

(.04)

.44

1.02

(.12)

.30

1.17

(.16)

1.50

(.10)

1.49

(.09)

1.50

(.08)

1.47

(.08)

1.45

(.08)

1.38

(.10)

1.35

(.06)

1.28

(.04)

1.25

(.13)

1.18

(.13)

Borate b u f f e r e d , μ = 0.40. [RSH] = 6.38 χ 10" and 3.89 3

B

χ 10

,

c o n s i s t e n t with those expected f o r the proposed mechanism, these s t u d i e s were somewhat l e s s than s a t i s f y i n g because the d e r i v e d r a t e laws p r e d i c t the same observed r a t e laws a t pH s above and below the pH o f the r a t e maximum. The o x i d a t i o n s o f d i t h i o e r y t h r i t o l are more informative because, owing to the u n i m o l e c u l a r i t y o f the d i s u l f i d e r a d i c a l anion formation ( i n c o n t r a s t t o the b i m o l e c u l a r i t y o f the d i s u l f i d e r a d i c a l anion formation i n the case o f mercaptoethanol), the observed and derived r a t e laws are pH-dependent. The mechanism proposed f o r the d i t h i o e r y t h r i t o l o x i d a t i o n s by MB i s shown i n the sequence 14-23. As i n the case of the mercaptoethanol o x i d a t i o n s , the o x i d a t i o n r a t e s o f d i t h i o e r y t h r i t o l are pH-dependent showing a maximum at pH ^ 9.6 (see Table IV). The lower pH observed f o r the maximum r a t e r e l a t i v e t o that observed f o r mercaptoethanol (see Table I) p a r a l l e l s the lower ρ Κ f o r the f i r s t i o n i z a t i o n T

+

Δ


15.

HUYSER AND TANG

Methylene

.SH

MB

(14)

+

Γ

+

^^s

MB

+

265

Thiols

.SH

15 -15 ,S-

of

14 -> MB- + R: k

I

Oxidations

k

+ R' SH

Blue

.^S R ^ I

+

MB-

(16)

'S .SH

-SH

MB2

+

R'

k

17 -> LMB

SH

R^

il_>

R:

(17)

^R

(18)

*SH HS*

*SH K

+ MB-

'SH

3

+ Rt

19

-

S

(19)

^*SH

.ST +

R

—-—> R C J CT *SH .ST

I

^ S H

MB- + H

+

(20)

-SH +

—

+ MB

>

(21)

'S

+ 2 R ^

I

+

2 MB-

+

H

H

+

.SH

k 9

9

——>

——>

R ^

I

+

Κ

+

LMB + MB

(22)

'S~

(23)

constant o f d i t h i o e r y t h r i t o l (9.0) (40 r e l a t i v e t o t h a t o f mercaptoethanol. Not shown i s a second r a t e maximum observed a t pH ^ 11 r e f l e c t i n g the d i b a s i c character o f d i t h i o e r y t h r i t o l (second p K = 9.9) (40. For t h e present study, o n l y the r a t e maximum a s s o c i a t e d w i t h the f i r s t i o n i z a t i o n i s p e r t i n e n t . I f the d i t h i o e r y t h r i t o l o x i d a t i o n s do indeed f o l l o w t h e same mechanistic path as the mercaptoethanol o x i d a t i o n s with respect to r e l a t i v e reagent c o n c e n t r a t i o n s , a t pH s below t h a t observed f o r t h e maximum r a t e and a t t h e high concentrations o f MB, o n l y the unimolecular d i s u l f i d e r a d i c a l anion formation ( r e a c t i o n 15) would be r a t e l i m i t i n g . The d e r i v e d r a t e law based on only A

T

+


ORGANIC FREE RADICALS

266

Table IV +

E f f e c t o f pH on MB -Oxidation Rates o f D i t h i o e r y t h r i t o l

3

—

6 1 Rate χ 10 M sec


pH 8.21

0.11 (0.01)

8.44

0.28 (0.03)

8.86

0.62 (0.02)

9.28

1.08 (0.09)

9.58

1.37 (0.13)

9.88

1.12 (0.09)

10.24

1.03 (0.02)

10.56

0.80 (0.02)

a.

I o n i c strength = 0.40.

t e r m i n a t i o n by r e a c t i o n 18, the c h a i n - c a r r y i n g r a d i c a l i n the r a t e l i m i t i n g s t e p , i s Eq. 24. This derived r a t e law p r e d i c t s that a t a pH below 9.6 and r e l a t i v e l y high concentrations MB t h e +

R a

te=

( k

14

/ 2 k

18

) 1 / 2 k

1

15

( [ M B + ] K

A

Î R ( S H )

2

] / [ H + ] ) 1 / 2 (

Ά

)

+

+

k_ /k [MB ] 1 5

1 6

+

observed r a t e law would be h a l f order i n both MB and d i t h i o e r y t h r i t o l , a p r e d i c t i o n that i s c o n s i s t e n t w i t h r a t e law obtained a t pH 9.0 (Table V) when the MB+ i s s u f f i c i e n t l y high. However, the derived r a t e law i n d i c a t e s that the k i n e t i c order o f MB should i n c r e a s e w i t h decreasing c o n c e n t r a t i o n o f MB . The l a t t e r e f f e c t , as i n the mercaptoethanol r e a c t i o n s , r e f l e c t s t h e l e s s e f f e c t i v e p a r t i t i o n i n g o f the d i s u l f i d e r a d i c a l anion toward the MB a t t h e lower concentrations o f MB . In the pH range above t h a t observed f o r the r a t e maximum, hydrogen atom a b s t r a c t i o n by MB- from the t h i o l ( r e a c t i o n 17) i s a r a t e l i m i t i n g step i n the chain sequence. I f t e r m i n a t i o n occurs by r e a c t i o n 19 and the derived r a t e law f o r the r e a c t i o n (Eq. 25) p r e d i c t s that the r e a c t i o n would be h a l f order i n MB +

+

+

+

+

/k k .k Rater

lu it ir7 1 4 1 5 1 7

+

+

+

[ M B ] ( C R S H ] - K r R ( S H ) ] / [ H ] ) ( K . [R( SH) . ]/[H ] ) \ A 2 A 2

1 / 2

0

k

19

U

+

k

-15

/ k

16

[ M B + ] )

(25)


15.

HUYSER AND TANG

Methylene

Blue

Oxidations

of

Thiols

267

Table V K i n e t i c Data f o r MB* Oxidations of D i t h i o e r y t h r i t o l a t pH 9.0 +

a

Rate = k [ M B ] [ R ( S H ) ] obs 2. v

a

3

0

K i n e t i c Orders +


[MB ] x 1Q

6

a

7.84

.52 (.14)

5.92

.74 (.13)

4.36

.86 (.12)

3.05

1.08 (.10)

1.66

1.25 (.07)

1.13

1.38 (.08)

b

3 .52 ( .04) .51 ( .03) .52 ( .04) .51 ( .04) .51 ( .05) .49 ( .05) .45 ( .12)

Borate b u f f e r , μ = 0.40. 3. [RSH] = 2.59 χ 10~ and 1.,01 χ 10" 3

and f i r s t order i n d i t h i o e r y t h r i t o l . The k i n e t i c orders observed f o r the d i t h i o e r y t h r i t o l o x i d a t i o n by MB+ a t pH 10.3 (Table V I ) show the r e a c t i o n i s indeed f i r s t order i n d i t h i o e r y t h r i t o l and h a l f order i n MB a t the higher concentrations o f the l a t t e r reagent. The k i n e t i c order o f MB+ a t lower concentrations o f MB i s g r e a t e r than h a l f order and i n d i c a t e s the involvement o f the f r a c t i o n a l term i n the denominator o f derived r a t e law r e s u l t i n g from l e s s e f f e c t i v e p a r t i t i o n i n g o f the d i s u l f i d e r a d i c a l anion toward the MB a t low concentrations o f t h i s reagent. The observed r a t e laws f o r the methylene blue o x i d a t i o n s o f both mercaptoethanol and d i t h i o e r y t h r i t o l a t the d i f f e r e n t pH s are c o n s i s t e n t with the mechanisms proposed f o r these o x i d a t i o n s . The most s i g n i f i c a n t e f f e c t o f the pH on the r e a c t i o n i s t h a t o f e s t a b l i s h i n g the r e l a t i v e amounts o f t h i o l and s u l f i d e i o n . I n i t i a t i o n o f the chain sequence by the r e a c t i o n o f a s u l f i d e i o n with MB would be expected t o be more r a p i d , and t h e r e f o r e the o v e r a l l o x i d a t i o n f a s t e r , i f the pH o f the medium i s increased. However, a t the higher pH's where the r e l a t i v e amount o f u n d i s s o c i a t e d t h i o l i s s m a l l , the r e a c t i o n r a t e diminishes i n d i c a t i n g not only that the t h i o l i s l i k e l y a r e a c t a n t i n the o v e r a l l r e a c t i o n but that i t i s involved i n a r a t e l i m i t i n g step of the r e a c t i o n a t higher pH s. While not as supportive as the +

+

+

T

+

f



268 Table VI +

K i n e t i c Data f o r MB

Oxidations

of D i t h i o e r y t h r i t o l a t pH 10.3 Rate = k

+

o b s

a

[MB ] [R(SH) ]

C

3

2

K i n e t i c Orders


"] χ 1 0

6

a

7.84

.49 (.12)

5.92

.50 (.14)

4.36

.53 (.14)

3.05

.67 (.13)

1.66

.79 (.09)

1.13

.89 (.13)

.99

(.04)

.98

(.03)

.98

(.04)

.99 (.05)

a. b.

1.01

(.06)

.98

(.03)

.99

(.07)

Borate b u f f e r , y = 0.40. [RSH] = 1.56 χ 10" and 5.58 χ 10 3

.

r a t e laws f o r the d i t h i o e r y t h r i t o l o x i d a t i o n s , the observed r a t e laws f o r the mercaptoethanol o x i d a t i o n are c o n s i s t e n t with the hypothesis that hydrogen atom a b s t r a c t i o n from the t h i o l i s a r a t e l i m i t i n g step a t higher pH s where the concentration o f t h i o l i s small. Finding hydrogen atom a b s t r a c t i o n from the t h i o l i s the chain propagating step that produces the c h a i n - c a r r y i n g t h i y l r a d i c a l r a t h e r than e l e c t r o n t r a n s f e r from a s u l f i d e i o n i s not unexpected i n terms o f the expected chemical behavior o f the methylene blue derived r a d i c a l Μ Β · . The most probable s t r u c t u r e T

RSH (CH ) 3

2

(CH ) 3

->

LMB

+

RS-

2

RS(CH ) N 3

*(CH )

2

3

MB


2

15.

HUYSER A N D T A N G

Methylene

Blue

Oxidations

of

269

Thiols


of MB- i s the hybrid r a d i c a l having as a major c o n t r i b u t o r to the hybrid the s t r u c t u r e with the unpaired e l e c t r o n l o c a l i z e d on heterocyclic nitrogen. I f e l e c t r o n t r a n s f e r from RS" to t h i s species d i d occur, i t would y i e l d MB", the conjugate base of the weakly a c i d i c leucomethylene blue. On the other hand, hydrogen atom a b s t r a c t i o n from the t h i o l not only y i e l d s a more s t a b l e ΜΒ· derived r e a c t i o n product, namely LMB, than e l e c t r o n t r a n s f e r but the r e a c t i o n i t s e l f involves a favorable p o l a r e f f e c t i n that the t h i o l i s a good e l e c t r o n acceptor substrate and the MB- a good e l e c t r o n donor r a d i c a l .

Solvent Isotope E f f e c t s . The involvement o f the a b s t r a c t i o n of the hydrogen atom from the t h i o l by MB- as a r a t e l i m i t i n g f a c t o r at the higher pH s i s supported by comparison of the o x i d a t i o n r e a c t i o n r a t e s i n H 0 and D2O at d i f f e r e n t pH s. Table VII l i s t s the observed r a t e constants f o r the o x i d a t i o n s o f both mercaptoethanol and d i t h i o e r y t h r i t o l a t d i f f e r e n t acidities. Examination of the solvent isotope e f f e c t s as measured by k i / k ^ (where, i n the case o f mercaptoethanol at each pH, k i s the observed r e a c t i o n r a t e constant c a l c u l a t e d from the r a t e law ν = k [RSH]3/2 +]l/2 and i n the case of d i t h i o e r y t h r i t o l , k i s c a l c u l a t e d from the r a t e law ν = k [ R ( S H ) o ] [ M B ] at the lower pH's and from ν = k b s C R ( S H ) ] [ M B ] ^ t the highest pH) shows that an isotope e f f e c t i s observed only under those c o n d i t i o n s where breaking and S-H bond i s a r a t e l i m i t i n g f a c t o r , namely at the higher pH s where the f r e e t h i o l concentrations are low. The inverse isotope e f f e c t s at the lower pH's i n both cases can be a s c r i b e d to the d i f f i c u l t i e s encountered i n comparing the s u l f i d e to t h i o l r a t i o s i n H2O and D 0 at a designated pH ( 5>). I t i s p o s s i b l e that at the lower pH s the higher observed r a t e constant i n D 0 r e f l e c t s an increased r a t e of i n i t i a t i o n owing to a higher concentration of s u l f i d e ion i n D 0 r e l a t i v e to that i n the H 0 s o l u t i o n with which i t i s compared. At the higher pH, t h i s f a c t o r i s outweighed by the p o s i t i v e isotope e f f e c t r e s u l t i n g from the involvement o f the f

!

2

f

o b s

[ M B

1

1 / 2

+

1 / 2

o b s

+

0

2

1

a

T

2

f

2

2

2


ORGANIC FREE RADICALS

270

Table V I I Solvent Isotope E f f e c t s £H

k

H_

/k

H D


Mercaptoethanol 9.90

1.69

10.6

1.88

0.92

10.30*

2.08

11.0*

2.44

0.85

10.80

2.05

11.2

1.83

1.12

Dithioerythritol 9.30

1.06

9.90

1.17

0.91

9.60*

1.32

10.20*

1.43

0.92

9.90

1.20

10.60

0.91

1.31

n

* pH o r pD" a t which maximum r a t e i s observed. breaking o f the S-H bond i n a r a t e l i m i t i n g step. Thus, the observed isotope e f f e c t at the higher pH's as measured by the k /k£> values i n Table V I I l i k e l y are considerably smaller than the a c t u a l isotope e f f e c t operative i n the hydrogen atom a b s t r a c t i o n from the t h i o l s by the methylene blue derived r a d i c a L H

I o n i c Strength Considerations. Reactions i n v o l v i n g charged r e a c t a n t s may be expected t o be i n f l u e n c e d by the i o n i c s t r e n g t h of the medium i n which the r e a c t i o n occurs. The most profound e f f e c t s are observed i n r e a c t i o n s between two charged s p e c i e s , e i t h e r both w i t h the same charge or each o p p o s i t e l y charged. The e f f e c t o f a change i n the i o n i c strength o f the medium on t h e r a t e constant o f a r e a c t i o n between i o n i c species can be expressed i n the Eq. 26 which i s a combination o f Br#nsted equation and the Debye-Huckel r a t e l i m i t i n g law (6_). This equation, where k and k are the r a t e constants a t i n f i n i t e Q

In k = I n k + 2 Ζ Ζ α

Kinetic Analysis of the Methylene Blue Oxidations of Thiols - ACS

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