Res. Chem. lntermed., Vol. 23, No. 9, pp. 801-817 (1997) �9 VSP 1997

REDOX REACTIONS OF TRANSIENT SPECIES FORMED DURING PULSE RADIOLYSIS OF 2-

PYRIDINE CARBOXALDEHYDE AND 2-PYRIDINE METHANOL IN AQUEOUS SOLUTIONS

P. DWIBEDY, G.R. DEY and K A M A L KISHORE Applied Chemistry Division, Bhabha Atomic Research Centre, Bombay -400085, India

Received 3 March 1997; accepted 27 May 1997

Abstract--Rate constants for the reactions of e~q, H and OH radicals with 2-pyridine carboxaldehyde and 2-pyridine methanol have been determined by pulse radiolysis technique. Reactions of reducing radicals such as acetone ketyl radicals and COl" with these compounds were also evaluated at various pHs. The species produced by the reaction of reducing radicals with these solutes was a strong reductant itself. While pyridinyl were produced in the case of 2-pyridine methanol, one-electron reduction of 2-pyridine carboxaldehyde led to the formation of PyCHOH radical. The one-electron reduction potential of PyCHOH radicals was estimated by establishing an equilibrium with MV + radical cations to be -0.6V vs NHE. OH radical reaction with 2-pyridine carboxaldehyde gave an OH adduct, while in the case of 2-pyridine methanol, OH radicals reacted partly by H-abstraction from the -CH2OH group. SO4" radical reaction with 2-pyridine carboxaldehyde produced a species which was reducing in nature. The rate constants for the reaction of C-,q and OH radicals are compared with similar values obtained in the case of other 2-pyridine derivatives to see if there is any electron-inductive effect.

INTRODUCTION

Pulse radiolysis studies have been carried out on a number o f pyridine derivatives

such as pyridine carboxylic acids [1], pyridinols [2] and methyl pyridines [3]. In

general, pyridinyl radicals formed by one electron reduction o f these compounds

are reducing in nature. In all these compounds, electrophilic OH radicals react by

addition to the pyridine ring preferentially at the meta position [4]. One electron

oxidation of these compounds is quite difficult. However, in the case of pyridinols,

semioxidised species have been reported to be formed [2] by the reaction o f one

electron oxidants at high pH, when the -OH group is in the deprotonated form. In

the case o f nicotinic acid [5], H atoms form only H-adducts at neutral and alkaline

pHs whereas at acidic pH, where nitrogen is protonated, pyridinyl radical is also

formed in addition to H-adducts. While the radiation chemistry ofpicol inic acid has

been studied in detail [1,6], no work is reported on 2-pyridine carboxaldehyde (2-

802 P. Dwibedy et al.

PC) and 2-pyridine methanol (2-PM) which are

structures of these compounds are given below.

its reduction products. The

ooo. o.o N N N

Picolinic acid 2-Pyridine carboxaldehyde 2-Pyridine methanol

In the present work reactions of primary radiolytic species of water viz.

e'aq , H" and "OH and some specific one electron oxidants and reductants have been

studied with 2-PC and 2-PM at different pHs and the kinetic, spectral, redox and

acid-base properties of the transient species formed by these reactions have been

determined. A comparison of the rate constants for e-aq and OH radical reactions of

various 2-pyridine derivatives is also being presented in this paper.

EXPERIMENTAL

The pulse radiolysis experiments were performed with a 7 MeV linear electron

accelerator (Ray Technologies, UK). Details of the pulse radiolysis experimental

set up have been reported earlier [7]. Single pulses of 50 nanosecond duration

(dose rate -15 Gy/pulse) were used for irradiating solutions in 1 cm 2 suprasil

quartz cuvettes. The radiation dose was measured using the KCNS dosimeter (aerated, 0.01 mol dm 3 KCNS, G.e -- 21520 dm 3 mol 1 cm 1 at 500 rim) [8]. 2-

Pyridine carboxaldehyde and 2-Pyridine methanol were from Fluka and Aldrich

Chemicals respectively and were used as received. Water from a "Barnstead

Nanopure System" having specific conductivity less than 0.1 gs cm -~ was used for

preparing all solutions. All the gases used to saturate the solutions viz. N 2 0 , N 2 and

02 were Iolar/Instrument grade from Indian Oxygen Ltd. pH of the solutions was

adjusted by using plain HCIO4 or NaOH or buffers such as phosphate and borate.

Kinetic properties of the transients formed were analyzed by on-line acquisition of

data using Iwatsu TS 8123 storage oscilloscope coupled to an IBM compatible PC.

Some of the general equations regarding primary products of water

radiolysis and the methods to study their reactions with solutes are given below.

H20 ~ e-u, H, OH, 172, H202, H3+O (1)

For studying e'aq reactions, t-butanol is used to scavenge OH radicals and solutions

Pulse Radiolysis of 2-Pyridine Carboxaldehyde and 2-Pyridine Methanol 803

are purged with nitrogen to remove dissolved oxygen. These reactions can be

studied between at pH > 4 as below pH 4, e-aq can be converted into H-atoms.

(CHB)3COH + O'H -"* (CHB)2C'HeOH + H20 (z)

For study of OH radical reactions at pHs > 3.5, the solutions are saturated with N20

to convert e'aq into OH radicals.

N20 + e~ + H20 --* O 'H + O H (3)

H-atom yield at neutral pH is only 0.55 as compared to OH yield which in N20

saturated solutions is 5.5. Hence, the contributions of H-atoms is generally

neglected under these conditions. In acidic pH, where e,q are converted into H-

atoms by the reaction of H + ions, 02 is used to scavenge H-atoms and convert them

into HO2 radicals which are usually inert towards most solutes.

Gq + H* ~ H" (4)

H" + 02 "+ HO; (5)

RESULTS AND DISCUSSION

2-Pyridine carboxaldehyde [9] has a pK a at 3.84 while 2-pyridine methanol [10] has

a pK a at 4.86. The conjugate acid-base forms for both the compounds are given

below:

2-PC

HN•+ 3.84 ... ~ + H + CHO ~ CHO

N

(6)

804 P Dwibedy et al.

2-PM

HN•+ 4.86 .... ~ + H § (7) c.2oH - % 0 .

N

The reactions of various radicals such as e'aq , "OH, H-atom and one electron

reductants such as CO2-" and (CH3) 2 "COH and oxidants such as SO4-" with these

compounds are discussed below one by one.

Reactions of e-aq. 2-Pyridine Carboxaldehyde. The transient absorption spectrum obtained in pulse irradiated Nz purged 10 .3 mol dm 3 2-PC solutions at pH = 7 containing 1 mol dm 3

t-butanol (as OH radical scavenger) is shown in Figure 1. It shows two absorption

bands with ~'max at 320 and 380 nm. The transient species viz. the electron adduct

was found to be reducing in nature as it could reduce thionine and methyl viologen

(MV 2+) as in the case of the electron adduct of picolinic acid [6]. In the absence of

any other solute, the transient species decayed by second order kinetics. The decay

co 0 . 0 3

d 0 . 0 2

0 . 0 1

0 . 0 0 I 300

0 . 0 4

co 0 . 0 4

O

Ir 0 .02

r-,,

0 . 0 0 2

400

"~ (nm)

4 6 8 I0 12 i~H

I . 500

Figure 1. Transient spectra of the one-electron reduced species obtained from 2-pyridine carboxaldehyde at different pHs immediately after the electron pulse. O -pH = 1 {using (CH3)2COH as reductant}; �9 - pH = 7 (using e~q as reductant); A- pH > 13 {using (CH3)2CO" as reductant}. Inset: AOD32o versus pH.

Pulse Radiolysis of 2-Pyridine Carboxaldehyde and 2-Pyridine Methanol 805

rate was found to be unaffected by the ionic strength indicating the species to be

neutral in nature. The reaction of actone ketyl radicals with 2-PC at neutral pH also

gave the same spectrum indicating that identical species were formed in these two

reactions. It appears that the electron adduct is protonated very fast at pH -- 7 to

give a neutral radical. Now, the electron can add either to the ring nitrogen or to the

-CHO group. Even after addition to the ring nitrogen, the electron can be transferred

to the -CHO group due to their close proximity. The later species then gets

protonated at this pH to give Py-CHOH radical species.

P y C H O + e ~ > ( P y C H O ) - (8)

( P y C H O ) - + H+ - > P y ' C H O H (9)

This was also confirmed by studying the OH radical reaction with 2-PM

where the same species is expected to be formed by H-abstraction reaction from

-CH2OH group. These results are discussed in a later section.

The reaction of acetone ketyl radical with 2-PC at pH = 1 gave a spectrum

with absorption bands at 300 and 360 nm indicating that the species formed under

these conditions is different. Possibly, it is the protonated form of the species

formed at pH = 7. In this case the spectrum was similar to that obtained by OH

reaction with 2-PM at pH = 1. The negatively charged electron adduct was observed only at alkaline pH

values. The spectrum obtained by e-aq reaction at pH = 13 is also shown in Figure

1. This shows two bands with absorption maxima at 305 and 360 nm. At the same

pH, acetone ketyl radicals also reacted with 2-PC to give the same spectrum. The

differences in spectra of the semireduced species at various pH values indicated the

possibility of two pK's. These pK's were determined from the plot of OD versus pH

at 320 nm (see inset Figure 1) which showed two inflexions and the values were 3.5

and 10.8. While the first pK is due to the protonation of ring nitrogen, the second

pK is due to the deprotonation of-CHOH group. The latter pK value is quite close

to the value reported for the C'H2OH-C'H20 equilibrium [11]. The conjugate acid-

base equilibria are as follows:

3.5 10.8 [ P y H C H O H ] + ~, P y C H O H + H § ,=, P y C H O - + 2H + (10)

806 P. Dwibedy et al.

The kinetic and spectral data are given in Table 1.

It has been mentioned earlier that the semireduced species produced from

2-PC can reduce methyl viologen. However, in this case, the observed rate constant

was found to be dependent on the concentration of 2-PC indicating an equilibrium

reaction. The equilibrium reaction is:

P y C H O H + M V 2+ *~ M V +" + P y C H O + H + (11)

In such systems [12], kob~ = kf [MV 2§ + k b [PyCHO], and hence a plot ofkobs/[2-PC ]

vs [MVZ+]/[2-PC] (see Figure 2) is a straight line whose slope is equal to kf and

intercept is equal to kb. The value o f the equilibrium constant K (= kdkb) is thus

Table 1

Spectral and kinetic characteristics of the transient species produced from various pHs.

2-PC at

Reacting pH ~m~x ~ kf 2k/el species (nm) (dm 3 mol%m -t ) (dm3mol% "1) (s "t)

e-~ 7 320 21000 1.0xl0 t~ 1.0xl05 380 2500 9.0x10 ~

13 305 22000 1.0xl0 l~ 5.0x104 360 2200 5.0x105

(CH3)2C'OH 1 300 12500 3.9x10" 5.2x104 360 2000

(CH s )2CO'" 13 305 22000 1.0xl08 5.2x104 360 2200 5.0x10 s

OH"

0 ~

S04-"

H"

1 310 2500 5.3xl08 360 1200

7 310 3400 1.5x109 2.1x105 370 1200 5.5x105

13 310 4000 1.0xl08 2.0x10 s

370 1400 6.0x105

7 320 14500 1.0xl09 1.2x105 380 1500 1.0xl06

1 300 5400 1.5x10 g 1.0xl05 340 1600 3.3x105 520 500 -

Pulse Radiolysis of 2-Pyridine Carboxaldehyde and 2-Pyridine Methanol 807

I ' - -1

I 04

1__,1

O

2 X I0 8

I X I0 8

0 o 5 x , o - 3 x,o -2

l-Mv2+7 / E2-Pc]

Figure 2. Plot of kobs/[2-PC ] versus [MV 2+ ]/[2-PC].

equal to slope/intercept and this value was found to be 360. Thus AE, which is

equal to (RT/nF) logcK, was determined to be +0.15 V. where AE = 0.059 log K =

E01 (MVZ+/MV +) - E0 ~ (PyCHO,H+/PyCHOH) = 0.15V

This gave a value of-0.6 V vs NHE for Eo' of PyCHO,HVPyCHOH couple

taking a value of-0.45 V versus NHE for Eo'(MVZ+/MV § couple [13].

2-Pyridine methanol (2-PM). In the case of e-aq reaction with 2-PM, the electron can

add only to the ring nitrogen to give a pyridinyl type of radical. The transient

spectrum obtained in N2 purged 10 .3 mol dm 3 2-PM solutions containing 1 mol dm 3

t-butanol at pH = 7 is shown in Figure 3. It shows absorption bands with maxima

at 300 and 360 nm and the spectrum is similar to that of pyridinyl radicals [1]. The

extinction coefficient values are however smaller than those obtained in the case of

picolinic acid but comparable to those obtained in the case of pyridine or methyl

pyridines [3]. The transient species was found to be neutral as indicated by kinetic

salt effect studies. It appears that in this case also the initial electron adduct gets

protonated at neutral pH to give pyridinyl type of radical. The spectrum obtained

808 P. Dwibedy et al.

0 . 0 0 5

. o.oo41 o . o o 5

d

O. 0 0 2

0 001 t

0 0 0 0 I 3 0 0 4 0 0 5 0 0

'~ ( nm)

Figure 3. Transient spectrum obtained by the reaction of e~q with 2-pyridine methanol at pll = 7 immediately after the electron pulse.

by CO2" radical reaction with 2-PM at pH = 2 and pH = 7 was also found to be

similar to that obtained by e-aq reaction at pH = 7. At pH = 13, e-aq reaction with 2-

PM gave an identical spectrum. Acetone ketyi radicals could react with 2-PM at

acidic pH where 2-PM is in the protonated form or at pH > 13 where these radicals

are present as (CH3)2CO. In these cases, the spectrum of the transient species was

identical to that obtained by the e'aq reaction at pH = 7. At neutral pH acetone ketyl

radicals were unable to reduce 2-PM. Thus the one electron reduction potential of

neutral form of 2-PM lies between -1.3 and -1.9V versus NHE, i.e it is more

negative than that of acetone ketyl radicals but less negative than that of CO2-"

radicals. The semireduced 2-PM was found to be a good reductant as indicated by

its fast reaction with M V 2+ ions to give MV + radical cation (k = 4 x 10 9 dm 3 mol 1

sl). No equilibrium was established in this case indicating that the reduction

potential of the semireduced species of 2-PM is much higher than that of methyl

viologen and also semireduced species produced from 2-PC. The reaction scheme

at pH = 7 can be written as:

At pH = 7:

PyCH20H + e , , v + H + - . . . . . > ( P y H C H 2 O H ) (12)

Pulse Radiolysis of 2-Pyridine Carboxaldehyde and 2-Pyridine Methanol 809

P y C H 2 0 H + CO 2" + H + . . . . . . > P y H C H 2 0 H + CO 2 (13)

At pH = 2:

(PyHCH2OH) + + CO 2" > P y H C H 2 0 H + CO 2 (14)

At pH = 13:

P y C H 2 0 H + e,q/(CH3)2C0 + H20 . . . . . . >

+ (CH3)2C0 + O H -

P y H C H 2 0 H (15)

All kinetic and spectral data regarding transient species formed from 2-PM are given

in Table 2.

Reactions o f O H Radicals

2-Pyridine carboxaldehyde. The transient spectrum obtained in pulse irradiated

NzO-saturated 10 .3 tool dm -3 2-PC solutions at pH = 7 is shown in Figure 4. It shows

bands with absorption maxima at 320 and 370 nm. The species was neither

oxidising nor reducing in nature as indicated by its inertness towards ascorbic acid

and methyl viologen or thionine. It decayed by second order kinetics and the decay

was unaffected by ionic strength indicating the species to be neutral in nature. It

appears that OH radicals react with 2-PC by addition to the pyridine ring and the

spectrum is similar to that obtained in the case of OH adduct of picolinic acid [6].

The transient spectrum obtained at pH = 1 in 02 saturated solutions also shown in

Figure 4 is different from that obtained at pH = 7. The species was found to react

with O 2 with a rate constant of 2 x 107 dm 3 mol 1 s q. However, it was established

that HO'2 radicals formed by the reaction of H-atoms with 02 were inert towards 2-

PC and 2-PM. This was done by pulse irradiating 02 saturated solutions of these

solutes containing t-butanol as OH radical scavenger where no absorbing species

were produced.

The difference in spectra at the two pH values indicated the presence of a

pKa in the pH region 1 - 7. A pl0t o fOD vs pH at 320 nm is shown in Figure 4 inset.

All the measurements of OD for this purpose were carried out in O2 saturated

solutions and OD measured was the initial OD. From this plot, the pka for the acid-

base conjugates of2-PC-OH adduct was determined to be 3.9. The OH adduct was

810 P. Dwibedy et al.

Table 2

Spectral and kinetic characteristics of the transient species produced from 2-PM

at various pHs

Reacting pH ~max C kf 2k/el species (nm) (din 3 mol%m "l) (dnl~mollst) (s "l )

e 7 300 1950 7.0x10 '~ 6.0x105

350 500

13 300 2000 6.5x10 '~ 5.6x105

350 500

(CH3)zC'OH 1 300 2000 6 . 0 x l 0 7

(CH3)2CO'" 13 300 2000 2.0x10 ~

350 500

CO2" 2 310 2000 1.0xl08

360 1400

OH"

6.4xi0 s

1 300 1.5x10" first order

360 decay'

7 320 2.0x10 '~ 1.0xl05

380

O" 13 305 8.0x10 x 4 .5X10 4

360

H' 1 300 2700 4.0x10 ~ 5.0x105

340 1600 560 600

SO4" 7 310 4000 4.5x 1 ()x 4.0x 10 s

"Due to reaction with oxygen.

f o u n d to r e a c t w i t h O2 e v e n at p H = 7. T h e r e a c t i o n s c h e m e s at p H = 7 and 1 are :

At pH = 7:

P y C H O + O H - - - > P y ( O t l ) C t t O (16 )

[PyHCHO] + + O H - > [PyH(OH)CHO] +" (17)

Pulse Radiolysis of 2-Pyridine Carboxaldehyde and 2-Pyridine Methanol 81 1

(.9 C} 0

0.070 t 0 . 0:5

0.050

0.030

0 . 0 0 0

l oo 0.02 -

0 . 0 1 -

0 .00 I I

500 4O0 5OO 600 "~ ( n m )

Figure 4. Transient spectra obtained by OH radical reaction with 2-pyridine carboxaldehyde at different pHs. �9 - pH = 1 (air saturated solution), 7 gs after the electron pulse; O - pH = 7 (N20 saturated solution), 2 las after the electron pulse. Inset: Plot of AOD32(I versus pH.

O-' radical anion also reacts with 2-PC at pH = 13 to give a spectrum similar to that obtained by OH radical reaction at pH = 7. Thus, in this case also an OH-adduct is

formed. The reaction is:

P y C H O + 0 -" + H20 > Py(OH)CHO + OH- (18)

2-Pyridine methanol(2-PM). OH radical reaction with 2-PM was studied at pH =

7 in N20 saturated solutions. The transient spectrum (Figure 5) showed two bands

with absorption maxima at 320 and 380 nm similar to those obtained by e-aq reaction with 2-PC at the same pH. This indicated the possibility of OH radicals reacting

with 2-PM by abstracting an H-atom from the CH2OH group besides the usual mode of addition to the pyridine ring. It was also found that the transient species produced

812 P Dwibedy ct al.

0.15

O.IO

d C5

0 . 0 5

0 . 0 0 5 0 0 4OO 5O0

(nm)

Figure 5. Transient spectra obtained by the reaction of 2-pyridine methanol with OH radicals immediately after the electron pulse. At pH = 1 (O) and at pH = 7 (O), 2 its after the electron pulse; and by the reaction of O" radical anion with at pH = 13 (A), 5 ~ts after the electron pulse.

by OH radical reaction could reduce methyl viologen. However , the yield o f MV §

radical cation as determined from its absorbance at 605 nm was only one third o f

GoH. This shows that only about 30% of OH radicals react with 2-PM by H-

abstract ion and the rest react by other pathways such as addition. The reaction

scheme is:

P y C H 2 0 H + O H - - > P y C ' H O H § 1120 (19)

P y C H 2 0 H + O H . . . . . . > P y ( O H ) C H 2 0 H (20)

Pulse Radiolysis of 2-Pyridine Carboxaldehyde and 2-Pyridine Methanol 813

At pH = 1, in air saturated solutions of 2-PM, the spectrum obtained by the reaction

of OH radicals with 2-PM was similar to that of the semireduced species produced

from 2-PC at the same pH (see Figure 5) in addition, the yield of the reducing

species formed via OH radical reaction with 2-PM was about one third of GoH.

Reaction of O-" radical anion was also studied with 2-PM at pH = 13. In this case

the transient spectrum obtained was similar to that of the semireduced species

formed from 2-PC at the same pH. However, the yield as determined from the

concentration of MV § radical cations was about 70% of G(O-'). O-" radical anions

also seem to react with 2-PM mainly by abstracting H-atom from the -CH2OH group

giving PyCHOH radicals which deprotonate at this pH to produce the PyCHO-

radical anion.

At pH = 1

[ P y H C H 2 0 H ] + + O H . . . . . . > [ P y H C H O H ] ~" + H 2 0 (21)

At pH = 1 3

P y C H g H + 0 " - . . . . . > P y ' C H O H + O H (22)

P y ' C H O H > P y ' C H O - + H § (23)

The spectral and kinetic data on the transient species are given in Table 2.

R e a c t i o n s o f O n e E l e c t r o n O x i d a n t s

Reaction of SOl" radicals with 2-PC was studied at pH = 7 and the spectrum

obtained is shown in Figure 6. It shows two absorption bands with maxima at 320

and 380 nm. The species was found to be reducing in nature and could reduce methyl viologen with a rate constant of 4 x 109 dm 3 mol 1 s ~. The yield of MV §

radical cation was equal to G(e-~q). It appears that the species produced by one

electron oxidation of 2-PC is the deprotonated form of the species produced by one-

electron reduction of picolinic acid at acidic pH values [6]. Such a species has been

proposed in the case of nicotinic acid [5]. The reaction scheme is:

P y c H o , so4" + O H > PyC(OH) 2 + SO[ (24)

0.08

0 . 0 6

d d O . O 4

0 .02

0 . 0 0 :500 4 0 0

~(nm)

814 P Owibedy et al.

500

Figure 6. Transient spectrum obtained by the reaction of SO4"" radicals with 5 x 10 -4 tool dm -3 2- pyridine carboxaldehyde solution at pH 7, containing 0.01 tool dm 3 K2S20 K and 0.1 mol dm -3 t-butanol 5 gs after the electron pulse.

Other one-electron oxidants such as N3" radical were not reactive with 2-PC. In the

case of 2-PM only SO4-" radicals were able to react at pH = 7 giving a transient

species with/%max a t 3 I0 nm. The spectrum was not similar to that formed by e'aq

reaction with 2-PC as in the case of OH radical reaction. This species could be the

cation radical of 2-PM.

H-atom Reactions

H-atom reactions were studied with both 2-PC and 2-PM at pH = 1 in N2 purged

solutions containing t-butanol as Oil radical scavenger. The spectra obtained in

both the cases are given in Figure 7. Both of them appear to be quite similar in

nature with three absorption bands having maxima at 300, 340 and around 520-560

nm. Thus it appears that for both compounds the mechanism of reaction of H-atoms

is similar and proceeds by addition of H to the pyridine ring. These spectra are

similar to that obtained by H-atom reaction with pyridine [3]. It appears that in this

case also pyridinyl radicals are not formed during this reaction and only H-adducts

are formed.

Substituent Effect On the Reactivity

Although, substituents in the ortho position do not show a regular electron inductive

Pulse Radiolysis of 2-Pyridine Carboxaldehyde and 2-Pyridine Methanol 815

0 . 0 2

0 . 0 I 0

0 . 0 0 5 0 0 4 0 0 5 0 0 6 0 0

"~ (rim)

Figure 7. Transient spectra obtained by the reaction of H-atoms with N2 purgcd 10 .3 mol dm ~ solutions of 2-PC (O) at pll = 1, 25 Its after the electron pulse and with 2-PM (0) at pH = 1, 10 gs after the electron pulse. I tool dm -3 t-butanol was used as OH radical scavenger in these solutions.

e f fec t due to s ter ic factors , tile rate cons tants for the react ion o f e , q and O H rad i ca l s

wi th a n u m b e r o f p y r i d i n e de r iva t ives wi th subs t i tuen t s in the 2 -pos i t i on ind ica t e

such a t rend as shown in Table 3. In general , e lec t ron dona t ing groups should m a k e

the e lec t ron add i t i on reac t ion s l ower w h e r e a s e lec t ron w i t h d r a w i n g g roups shou ld

Table 3 A comparison of rate constants for e~q and OH radical reactions with 2-substituted pyridines.

Substitucnts pl I e~q reaction pH OH radical reaction Ref. (dm 3 mol -I s -1) (din 3 mol -~ s -s)

-H 95 7.7x109 9.8 3,0x109 [3]

-NH2 9.0 2.0x10 s 9.0 6.0xl09 [14]

-CH 2 Oil 7.0 7.0x109 7.0 2.0x109 -

--CHO 7.0 1.0xl0 m 7.0 1.5x109

-COOI I 3.9 1.7x10 l~ 3.8 2.0x108 [6]

-COO- 9.0 5.0x109 9.0 2.0x109 [6]

-OH 5.0 1.4x10 I~ 6.8 l. Ixl0 t~ [2]

816 P. Dwibedy et al.

facililitate this reaction. This appears to be the trend as seen from Table 3. While

the rate constant value for e'aq reaction is lower in the case of 2-amino pyridine and

2-pyridine methanol, it is higher in case of picolinic acid, pyridinol and 2-pyridine

carboxaldehyde.

In the case of OH radical reaction, the trend is expected to be opposite to

that in the case of e'aq reaction as OH radical is electrophilic in nature. Table 3

shows that the rate constants generally increase with increasing electron donating

power of the substituent except in the case of 2-pyridinol, where the rate constant

value is quite high. This may be due to the fact that in this case, due to keto-enol

tautomerism, almost 100% of the molecule exists in the keto form and the ring

nitrogen is present in >NH form. This might have been responsible for the

anomalous result seen in the case of 2-pyridinol.

C O N C L U S I O N

The one electron reduction of 2-pyridine carboxaldehyde (2-PC) and 2-pyridine

methanol (2-PM) leads to the formation of reducing species. While in the case of 2-

PC, the electron addition takes place in the -CHO group giving Py-CHOH radical,

pyridinyl radicals are formed in the case of2-PM. Pyridinyl radicals formed in the

case of 2-PM were very strong reductants as compared to Py-CHOH radicals. OH

radical reaction with 2-PC leads to the formation of an adduct where OH is added

to the pyridine ring. In the case of 2-PM, OH radicals react both by addition as well

as abstraction mechanisms. The H-abstraction from -CH2OH group gives the same

species as produced by the e-aq reaction with 2-PC. One-electron oxidation of 2-PC

with SO4" radicals gave a transient species which was identical to that obtained by

e-aq reaction with picolinie acid which is a strong reductant. In the case of 2-PM,

SO4" radical reaction possibly gives a radical cation. A comparison of the e'aq and

OH radical reaction rate constants for a number of 2-pyridine derivatives has been

performed which shows that the differences in these values can be explained on the

basis of electron inductive effects.

R E F E R E N C E S

1,

2.

3.

4. 5.

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