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Reaction of OH radicals with nitrophenols in aqueous solution
WO-7055/78/0801-001312.00/0
Radior. Phys. Chum. 1978. Vol. 12, Pp. 13-17 @ Pergnmon Press Ltd. Printed in Great Britain
REACTION OF OH RADICALS WITH NITROPHENOLS IN AQUEOUS SOLUTION P. O’NEILL, S. STEENKEN, H. VAN DER LINDE* and D. SCHULTE-FRoHLlNDEt Institut f&r Strahlenchemie im Max-Planck-Institut f& Kohlenforschung, Stiftstr. 3&36, D-4330 Mtllheim/Ruhr, Germany (Manuscript received 20 January 1978; received for publication 8 May 1978) Abdraet-The reaction of OH radicals with 2-, 3- and 4-nitrophenol in aqueous solution was investigated using pulse radiolysis with conductivity detection, steady state radiolysis, e.s.r. and product analysis. The yield of nitrite was measured and the formation of semiquinone radicals from 2- and +nitrophenol but not from 3-nitrophenol was confirmed. The yield of nitrite from 2- and Qnitrophenol is less by a factor of two than from the corresponding phenolates and that of nitrite from 3-phenolate is eight times lower than that from Cphenolate. These and further results are interpreted in term$ of preferential addition of the electropbilic OH radical to ring positions activated by the ortko-para directing OH or O- groups. OH addition leads predominantly to the 1,2dihydroxy4-nitrocyclohexadienyl radical and to the 1,4dihydroxy4nitrocyclohexadienyi radical. The latter eliminates NH@ (k = 2.3 x Ids-‘) to yield para-semiquinone radical. The 1.2dihydroxy4nit cyclohexadienyl radical, the PK. of which is ~4, does not observably eliminate OH- to yield +nitrophenoxyl radical since 2-hydroxy4nitrophenol is formed as product. It is further shown that the nitro-group stabilizes anion free radical states of benzene derivatives more strongly than the cyano-group. This effect also leads to a much smaller elimination rate of OH- from OH adducts of nitrouhenolate as cornoared to those from OH adducts of phenolates containing other electron withirawing groups (-dN, -CHO. -COCH&
INTRODUCTION
methoxybenzenes,“’ methoxybenzoic acids”’ and methoxyphenols.” Furthermore the influence of the nitro group on the PK. values and on the rate of OH- elimination from OH adducts is discussed.
THE REACTION of the OH radical
with Cnitrophenol has been shown to l&ad mainly to 2hydroxy4nitrophenol”’ and, to a lesser extent, to denitration and formation of p-semiquinone. radical.“’ The production of 2-hydroxy4nitrophenol was explained by assuming direct addition of OH ring followed by disthe aromatic to proportionation reactions of the OH adducts.(” In contrast, Cercek and Ebert’” suggested that the the adds to primarily radical OH nitro group to form HOCaHJV03I followed by a slow rearrangement reaction to yield hydroxycyclohexadienyl radicals. The present study was undertaken in order to resolve the mechanism of reaction of OH radicals with nitrophenols and to investigate whether the mechanism involves preferential addition of the electrophG? OH radical to those ring positions which are activated by the o&o-para directing OH or Ogroup. An analogous concept was found to explain successfully the pattern of addition of OH to
EXPERIMENTAL N20 saturated solutions containing 2x lo-* and lo-’ mol dm-’ substrate were irradiated &ith “‘CO y-rays using doses from 1.0~ lO”eVn- to 1.16~ 10”eV P-* at a d& rate of 2.9 x 10” eVH* min-‘. The irra&ted solutions were analysed for Na- using the method of Barnes and Folkard.m The 3 MeV van de Graaff accelerator and the conductivity detection system have previously been described.“) Solutions saturated with N20 were irradiated at 20 + X with electron pulses of 1 c(s duration. Dosimetry was performed using C(NO&m The pH of the solutions was adjusted using HClO, or NaOH. With the conductivity system, measurements were limited to 4 s pH s 11. The in situ E.S.R. experiments were performed using the method described by Eiben and Fessenden.“lo) The substrates (“puriss” quality) were obtained from Fluka with the exception of Qcyanophenol (>97%), which was obtained from Aldrich. They were used as received.
*Permanentaddress: Atomic Energy Board, Pelindaba, Private Bag 256. Pretoria, South-Africa. tcorrespondence should be sent to Prof. Dr. D. Schulte-Frohlmde. Institut fOr Strahlenchemie im MaxPlanck-Institut fllr Kohlenforschung, Stiftstr. 34-36. D 4330 MtUheii/Ruhr. Germany.
RESULTS
AND DISCUSSION
(a) Yields of nittite in alkaline solutions
In agreement with previous findings,O’ reaction of OH radicals with nitrophenols leads to formation 13
14
P. O’NEILLd al.
Reaction (1) is an oxidative replacement reaction analogous to those observedo*“’ in the reaction of OH with other substituted aromatics. The rate and yield of NOZ- elimination was pH IO’ pH lob Substance PH’ measured by performing pulse radiolysis experi2-nitrophenol 0.4 (7%)s 0.8 (15%) 0.6 (11%) ments using conductometric detection. N20 3-nitrophenol 0.2 (4%) 0.2 (4%) $25 (5%) saturated 2 x lo4 mol dm-” solutions of the nitro4-nitrophenol 0.8 (15%) 1.6 (29%) 1.8 (33%) phenols were irradiated at pH 9.5-10.4 with 1 ps ‘G(N9-) determined chemically (see experimental pulses of -2oOrad. As shown in Fig. la, a decrease of conductivity was observed. This section). bG(NQ-) determined from conductivity changes using decrease is explained in terms of replacement of pulse radiolysis. and OH- ion by the less mobile NO, ion (the ‘Percentage values in parentheses arc defined as proton produced in reaction (1) is removed by the G(NG-) x lOO/G(OH)using G(OH) = 5.5. excess OH- present). From the rate of conductivity decrease after the pulse in the pH region of nitrite. For 2-, 3- and 4-nitrophenol the yields of 9-10.4 the rate constant for elimination of N&from OH adduct I is determined to be k, = nitrite, obtained from the linear plots of [N&-l on dose, are presented in Table 1. It is evident that 2.3(kO.S) x 10 s-‘. From the conductivity changes the yields of N&- were calculated assuming that the yields of nitrite from reaction of OH with the and 2- and 4-nitrophenolates at pH 10 (PK., (nitro- the ion mobilities of nitrophenolate phenols) -7-g) are larger by a factor of -2 than semiquinone anion are equal. The values thus those from reaction of OH with the nitrophenols at determined (Table 1) are in agreement with chempH 4. ically determined G(N0;) obtained from The production of nitrite is suggested to proceed “Co-r experiments. The results cannot be intervia addition of OH to the ring carbon carrying the preted in terms of the assumption’” that the OH radical adds to the nitro group to yield nitro group (ipso addition) followed by elimination of HNOZ, as shown in equation (1) for the case of HOC&NOJI, the PK. of which was reported”’ Cnitrophenolate: to be 5.3. On the basis of this mechanism and this PK. value, at pH 9-10 a decrease of conductivity corresponding to G(H’) - 5.5 and occurring within NO2 1 us after the pulse should be observed whereas virtually no conductivity changes are observed within the first few microseconds. Further conkmation for reaction (1) is obtained from the observation of the ortho- and paw semiquinone radical anion on reaction of OH with 0' 2- and kitrophenol, respectively, using the ia situ I radiolysis E.S.R.” method. No semiquinone anions or aroxyl radicals are observed from 3A H+ -f NO; nitrophenol. E.S.R. signals assignable to HOC&NO,‘- were not observed although the I_ 0 postulated’” long lifetime of this species should TABLE~.Y~OFN~,EWRESSEDMTBR~~SOPGVALUESFROMace-y -IRRADIATED AQUEOUSSOLUTIONS OF lo-’ mol dms3 NITBOPHENOL SATURATED wrna NpO
a)
Fx. 1. Conductivity s&ml as a function
b)
of time on pulse irradiation of N20 satu&ed 2 x lO-‘moldm-’ solutions of Cnitrophenol at (a) pH 9.7 (sensitivity 2mVkm) and (b) pH 4.5 (sensi-
tivity 5 mV/cm). Dose/l ja pulse -0.2 krad.
Reaction of OH radicals with
permit its detection. The ES.& data are in agreement with previous findings.n’O’ (b) Nitrite yields and formation of H’ from OH adducts in acidic solutions On irradiation of a 2 x lo-’ mol dm-’ solution of 4-nitrophenol saturated with NzO at pH 5 using 1 ps pulses of - 2OOrad, an increase of conductivity was observed as shown in Fig. lb. This increase is suggested to be due to formation of a proton and a radical anion. The increase of conductivity 10 JLSafter the pulse cannot be due to formation of N02- and the corresponding H’ alone since G(N02-) is only 0.8 whereas G(H’) measured 10 CC s after the pulse is 4.9(-t 0.3). and is independent of pH (range 4-5). Measurements were not performed in the range 5 < pH < 9 since in this region conductivity studies are dilllcult to interpret due to the buffering effect of nitrophenol (PK.9 - 7). Electron spin resonance measurementso2) show that the OH radical, in addition to attachment to position 4 of Qnitrophenol, adds to position 2 of the aromatic ring to yield the 4nitro-l&Shydroxycyclohexadienyl radical (equation (2)):
finding is in agreement with the observation”’ of 2-hydroxy-4-nitrophenol as the main stable product from the reaction of OH with 4-nitrophenol. However, it is still an open question whether the decay of II or II’ leads to 2-hydroxy4 nitrophenol in a one step disproportionation or involves more complicated reactions. This
At pH 4-S the conductivity changes 10 p s after the pulse are suggested to arise from deprotonation of radical II’ to yield radical II (equation (3)). A contribution of HN02 is not expected within this time range since the rate of eliiination of
nltrophenolsin aqueous solution
15
HNt& from I’ is expected to be of the same order of magnitude as that from radical I (k, = 2.3 x lo) s-l, see section (a)). Furthermore the observed G(H*) is much larger than G(N0;) (the.difIerence between the observed G(H’) and G(OH) corresponds to the measured G(N02-), see below). The PK. for dissociation of radical II’ is estimated to be 44 since the yield of H’ does not decrease in going from pH 5 to pH 4. At pH > 9 radical II is formed directly from +nitrophenolate so that, as described in section (a), no conductivity changes occur when radical II is formed in alkaline solutions. As already mentioned, on the basis of optical measurements it was suggested”’ that the OH radical adds to the NO2 group. It was further concluded”’ that the pK,, value of the OH adduct is 5.3. This value can be excluded on the basis of the conductivity results described in the present work.
The difference between G(OH) and G(H’) at pH 4-5 lops after the pulse corresponds to 0.6. Within experimental error, this value is equal to G(N&-) as determined at the same pH values from YO-y irradiated solutions (Table 1). It is therefore suggested that at pH 4-5 radical II and radical I’ are the stable protolytic forms of the radicals. This is reason&k since the phenolic Oand the nitro group are conjugated in radical II but not in radical I (equations (3) and (4)). (c) pK. values of hydroxpcycloheradienyl radicals The pK values of OH adducts of phenols
containing electron withdrawing groups were measured using the conductivity technique. The PK. value for the OH adduct of 4-nitrophenol (radical II’, see section (b)) is low (~4) in comparison with the PK., values of the OH adducts of Qcyanophenol, Qhydroxybenzaldehyde and 4hydroxyacetophenone which were determined to be larger than 6. The PK. values of Qnitro and 4-cyanophenol difIer by only 1 unit. From the observed larger difference in the PK. values of the OH adducts it is concluded that the relative abilities of the nitro and the cyan0 group to stabilixe a negative charge depend on whether or not the anion is radical in nature. It is of interest to note
P. O’NFZLLer 01.
16
that the PK. value of the nitrobenzene radical anion (PK. = 3.2”“) is much lower than that of the radical anion of cyanobenzene (PK. = 7.2(“1 which indicates that also in this case the negative charge of the radical anion is stabilized much more e5ciently by the NO* group than by the CN group. (d) Elimination
of
OH-
from
hydroxycyclo-
hexadienyl radicals The experimental observation of 2-hydroxy4
nitrophenol as a stable product is surprising, since OH adducts of phenolic compounds are known”“s”6’ to eliminate water to yield phenoxyl radicals. If the pH of the solution is L PK. of the OH adduct, formation of phenoxyl radicals proceeds by elimination of OH- from the ionized OH adducts.‘6’ However, as judged by the absence of conductivity changes at pH r 9 except those due to production of NO*-, the 4nitro-1,2dihydroxycyclohexadienyl radical (radical II) does not observably eliminate OH-. The first order rate constant for this process is estimated to be eld s-’ which is to be compared with the value r lad s-’ for elimination of OH- from OH adducts of phenolate”” and methoxyphenolates.’ The low value for the rate constant for elimination of OHfrom radical II is suggested to be due to strong withdrawal of electron density from the conjugated system of radical II by the N02 group. In order to see whether the rate of OH- elimination from OH adducts of other phenolates is influenced by electron withdrawing substituents to a comparable degree, OH adducts of ionized 4 cyanophenol, Qhydroxybenzaldehyde and 4 hydroxyacetophenone were prepared by reaction of OH with these phenolates and investigated using optical and conductometric detection. At pH values above the p&values of the phenols (pK,,‘s - 8) the OH adducts of 4cyanopheno1, 4 4hydroxyacetohydroxybenzaldehyde and phenone eliminate OH- to yield phenoxyl radicals with rate constants of 3, 4 and 7 x 10 s-‘, respectively. Thus, the rate of elimination of OHfrom the ionized OH adduct is reduced by electron withdrawing substituents as compared to electron donating subs&tents. However, the rate of OHelimination from the OH adduct of nitrophenolate is much lower than expected on the basis of the Hammett u value for N01, as compared to those for CN, CHO and COCH,. Deviation from a Hammett relationship has also been observed”” with respect to the rate of reaction of SO4” with nitrobenzene as compared with cyanobenzene and other substituted benzenes. This shows that not
only the PK. values but also the reactivity of nitroaromatic radicals show an unusual behaviour. (e) Selectivity of OH addition
As seen in Table 1 the yield of nitrite on reaction of OH with the nitrophenols depends on the relative positions of the OH(O-) and N02 groups in the substrates. This dependence is suggested to be due to the predominant addition of the electrophilic”’ OH radical to those ring positions which are activated by the electron donating OH or O- group. Since the yield of nitrite from 4 nitrophenolate corresponds to - 33% and that”’ of 2-hydroxy4nitrophenoi to -55% of the OH radical yield, the OH radical adds predominantly to positions ortho and pam to the phenolate group as observed in the reaction of OH with methoxyphenols” and methoxybenzenes.‘n The low yield of nitrite from 3-nitrophenolate is in agreement with this concept. However, in the case of 2nitrophenolate the yield of nitrite is lower than expected. One possible explanation may be steric hindrance of OH addition to the orrho position carrying the nitro group. On protonation of the phenolate group in the case of 2- and Cnitrophenolate the yield of nitrite is reduced by a factor of -2 (Table 1). This is suggested to be due to the smaller ortho-para directive effect of the OH group in comparison with that of the O- group. Acknowledgement-We assistance.
thank G. Meyer for technical
REFERENCES L.0.
VOLKERT.G. TERMENSand D. SCHULTE-FROHLINDE, 2. &YS. Chum. N.F. 1%7. #. 261. 2. C. SUARE&i?. Lows. K. GOti& and K. RIBEN. Tetrahedron Lett. 1970,575. 3. B. CERCEKand M. RBERT.in Radiation Chanirtrp, edited by R. P. GOULD,Adu. Chem. Ser. 1%8, 81. 210. 4. M. ANBAR,D. Mm and P. NETA, 1. phgs. Chem. 1966,7& 2660. 5. P. G’NEILL, D. !kHULTJ#ROHLINDE and S. STEENKEN,J. them. Sot. Faraday Disc. 1977.0, 141. 6. P. O’Neus and S. STEENKEN, Err. Bunsmgcu. phys. Chem. 1977,81,550. 7. H. BARNESand A. R. FOLKARD,Analyst 1951. 76. 599. 8. H. G. KLEVER, Ph.D. Thesis, Ruhr-UniversitBt Bochum, 1974. 9. K.-D. ASMUS, Int. J. Radiat. Phys. Chum. 1972. 4. 417. 10. K. J&EN and R. W. -ENDEN, J. phys. Chem. 1971, 75.1186. 11. For a review, see P. NETA. Adu. pLys. org. Chum. 1976, 12,223.
Reaction of OH radicals with nitrophenols 12. K. Etem. D. SCHULTE-FROUNDE, C. SUAREZ and H. ZORN, Int. 3. Radiat. Phys. Chem. 1971, 3, 409. 13. K.-D. ASMUS, A. WIGGER and A. HENGLEIN, Ber. Bunsenges. phys. Chem. 1966.70.862. 14. B. CHIJTNYand A. J. SWUW, Trans. Faraday Sot. 1970,66,2847.
in aqueous solution
17
15. E. J. LAND and M. EBERT, Trans. Fmaduy Sm. 1%7, 63, 1181. 16. G. E. ADAMS and B. D. hfICHAEL, Trans. Faraday Sm. 1967.63, 1171. 17. P. NETA. V. M,~~AvAN, H. ZEMEL and R. W. FESSENDEN, .I. Am. them. Sot. 1977.99, 163.
Radior. Phys. Chum. 1978. Vol. 12, Pp. 13-17 @ Pergnmon Press Ltd. Printed in Great Britain
REACTION OF OH RADICALS WITH NITROPHENOLS IN AQUEOUS SOLUTION P. O’NEILL, S. STEENKEN, H. VAN DER LINDE* and D. SCHULTE-FRoHLlNDEt Institut f&r Strahlenchemie im Max-Planck-Institut f& Kohlenforschung, Stiftstr. 3&36, D-4330 Mtllheim/Ruhr, Germany (Manuscript received 20 January 1978; received for publication 8 May 1978) Abdraet-The reaction of OH radicals with 2-, 3- and 4-nitrophenol in aqueous solution was investigated using pulse radiolysis with conductivity detection, steady state radiolysis, e.s.r. and product analysis. The yield of nitrite was measured and the formation of semiquinone radicals from 2- and +nitrophenol but not from 3-nitrophenol was confirmed. The yield of nitrite from 2- and Qnitrophenol is less by a factor of two than from the corresponding phenolates and that of nitrite from 3-phenolate is eight times lower than that from Cphenolate. These and further results are interpreted in term$ of preferential addition of the electropbilic OH radical to ring positions activated by the ortko-para directing OH or O- groups. OH addition leads predominantly to the 1,2dihydroxy4-nitrocyclohexadienyl radical and to the 1,4dihydroxy4nitrocyclohexadienyi radical. The latter eliminates NH@ (k = 2.3 x Ids-‘) to yield para-semiquinone radical. The 1.2dihydroxy4nit cyclohexadienyl radical, the PK. of which is ~4, does not observably eliminate OH- to yield +nitrophenoxyl radical since 2-hydroxy4nitrophenol is formed as product. It is further shown that the nitro-group stabilizes anion free radical states of benzene derivatives more strongly than the cyano-group. This effect also leads to a much smaller elimination rate of OH- from OH adducts of nitrouhenolate as cornoared to those from OH adducts of phenolates containing other electron withirawing groups (-dN, -CHO. -COCH&
INTRODUCTION
methoxybenzenes,“’ methoxybenzoic acids”’ and methoxyphenols.” Furthermore the influence of the nitro group on the PK. values and on the rate of OH- elimination from OH adducts is discussed.
THE REACTION of the OH radical
with Cnitrophenol has been shown to l&ad mainly to 2hydroxy4nitrophenol”’ and, to a lesser extent, to denitration and formation of p-semiquinone. radical.“’ The production of 2-hydroxy4nitrophenol was explained by assuming direct addition of OH ring followed by disthe aromatic to proportionation reactions of the OH adducts.(” In contrast, Cercek and Ebert’” suggested that the the adds to primarily radical OH nitro group to form HOCaHJV03I followed by a slow rearrangement reaction to yield hydroxycyclohexadienyl radicals. The present study was undertaken in order to resolve the mechanism of reaction of OH radicals with nitrophenols and to investigate whether the mechanism involves preferential addition of the electrophG? OH radical to those ring positions which are activated by the o&o-para directing OH or Ogroup. An analogous concept was found to explain successfully the pattern of addition of OH to
EXPERIMENTAL N20 saturated solutions containing 2x lo-* and lo-’ mol dm-’ substrate were irradiated &ith “‘CO y-rays using doses from 1.0~ lO”eVn- to 1.16~ 10”eV P-* at a d& rate of 2.9 x 10” eVH* min-‘. The irra&ted solutions were analysed for Na- using the method of Barnes and Folkard.m The 3 MeV van de Graaff accelerator and the conductivity detection system have previously been described.“) Solutions saturated with N20 were irradiated at 20 + X with electron pulses of 1 c(s duration. Dosimetry was performed using C(NO&m The pH of the solutions was adjusted using HClO, or NaOH. With the conductivity system, measurements were limited to 4 s pH s 11. The in situ E.S.R. experiments were performed using the method described by Eiben and Fessenden.“lo) The substrates (“puriss” quality) were obtained from Fluka with the exception of Qcyanophenol (>97%), which was obtained from Aldrich. They were used as received.
*Permanentaddress: Atomic Energy Board, Pelindaba, Private Bag 256. Pretoria, South-Africa. tcorrespondence should be sent to Prof. Dr. D. Schulte-Frohlmde. Institut fOr Strahlenchemie im MaxPlanck-Institut fllr Kohlenforschung, Stiftstr. 34-36. D 4330 MtUheii/Ruhr. Germany.
RESULTS
AND DISCUSSION
(a) Yields of nittite in alkaline solutions
In agreement with previous findings,O’ reaction of OH radicals with nitrophenols leads to formation 13
14
P. O’NEILLd al.
Reaction (1) is an oxidative replacement reaction analogous to those observedo*“’ in the reaction of OH with other substituted aromatics. The rate and yield of NOZ- elimination was pH IO’ pH lob Substance PH’ measured by performing pulse radiolysis experi2-nitrophenol 0.4 (7%)s 0.8 (15%) 0.6 (11%) ments using conductometric detection. N20 3-nitrophenol 0.2 (4%) 0.2 (4%) $25 (5%) saturated 2 x lo4 mol dm-” solutions of the nitro4-nitrophenol 0.8 (15%) 1.6 (29%) 1.8 (33%) phenols were irradiated at pH 9.5-10.4 with 1 ps ‘G(N9-) determined chemically (see experimental pulses of -2oOrad. As shown in Fig. la, a decrease of conductivity was observed. This section). bG(NQ-) determined from conductivity changes using decrease is explained in terms of replacement of pulse radiolysis. and OH- ion by the less mobile NO, ion (the ‘Percentage values in parentheses arc defined as proton produced in reaction (1) is removed by the G(NG-) x lOO/G(OH)using G(OH) = 5.5. excess OH- present). From the rate of conductivity decrease after the pulse in the pH region of nitrite. For 2-, 3- and 4-nitrophenol the yields of 9-10.4 the rate constant for elimination of N&from OH adduct I is determined to be k, = nitrite, obtained from the linear plots of [N&-l on dose, are presented in Table 1. It is evident that 2.3(kO.S) x 10 s-‘. From the conductivity changes the yields of N&- were calculated assuming that the yields of nitrite from reaction of OH with the and 2- and 4-nitrophenolates at pH 10 (PK., (nitro- the ion mobilities of nitrophenolate phenols) -7-g) are larger by a factor of -2 than semiquinone anion are equal. The values thus those from reaction of OH with the nitrophenols at determined (Table 1) are in agreement with chempH 4. ically determined G(N0;) obtained from The production of nitrite is suggested to proceed “Co-r experiments. The results cannot be intervia addition of OH to the ring carbon carrying the preted in terms of the assumption’” that the OH radical adds to the nitro group to yield nitro group (ipso addition) followed by elimination of HNOZ, as shown in equation (1) for the case of HOC&NOJI, the PK. of which was reported”’ Cnitrophenolate: to be 5.3. On the basis of this mechanism and this PK. value, at pH 9-10 a decrease of conductivity corresponding to G(H’) - 5.5 and occurring within NO2 1 us after the pulse should be observed whereas virtually no conductivity changes are observed within the first few microseconds. Further conkmation for reaction (1) is obtained from the observation of the ortho- and paw semiquinone radical anion on reaction of OH with 0' 2- and kitrophenol, respectively, using the ia situ I radiolysis E.S.R.” method. No semiquinone anions or aroxyl radicals are observed from 3A H+ -f NO; nitrophenol. E.S.R. signals assignable to HOC&NO,‘- were not observed although the I_ 0 postulated’” long lifetime of this species should TABLE~.Y~OFN~,EWRESSEDMTBR~~SOPGVALUESFROMace-y -IRRADIATED AQUEOUSSOLUTIONS OF lo-’ mol dms3 NITBOPHENOL SATURATED wrna NpO
a)
Fx. 1. Conductivity s&ml as a function
b)
of time on pulse irradiation of N20 satu&ed 2 x lO-‘moldm-’ solutions of Cnitrophenol at (a) pH 9.7 (sensitivity 2mVkm) and (b) pH 4.5 (sensi-
tivity 5 mV/cm). Dose/l ja pulse -0.2 krad.
Reaction of OH radicals with
permit its detection. The ES.& data are in agreement with previous findings.n’O’ (b) Nitrite yields and formation of H’ from OH adducts in acidic solutions On irradiation of a 2 x lo-’ mol dm-’ solution of 4-nitrophenol saturated with NzO at pH 5 using 1 ps pulses of - 2OOrad, an increase of conductivity was observed as shown in Fig. lb. This increase is suggested to be due to formation of a proton and a radical anion. The increase of conductivity 10 JLSafter the pulse cannot be due to formation of N02- and the corresponding H’ alone since G(N02-) is only 0.8 whereas G(H’) measured 10 CC s after the pulse is 4.9(-t 0.3). and is independent of pH (range 4-5). Measurements were not performed in the range 5 < pH < 9 since in this region conductivity studies are dilllcult to interpret due to the buffering effect of nitrophenol (PK.9 - 7). Electron spin resonance measurementso2) show that the OH radical, in addition to attachment to position 4 of Qnitrophenol, adds to position 2 of the aromatic ring to yield the 4nitro-l&Shydroxycyclohexadienyl radical (equation (2)):
finding is in agreement with the observation”’ of 2-hydroxy-4-nitrophenol as the main stable product from the reaction of OH with 4-nitrophenol. However, it is still an open question whether the decay of II or II’ leads to 2-hydroxy4 nitrophenol in a one step disproportionation or involves more complicated reactions. This
At pH 4-S the conductivity changes 10 p s after the pulse are suggested to arise from deprotonation of radical II’ to yield radical II (equation (3)). A contribution of HN02 is not expected within this time range since the rate of eliiination of
nltrophenolsin aqueous solution
15
HNt& from I’ is expected to be of the same order of magnitude as that from radical I (k, = 2.3 x lo) s-l, see section (a)). Furthermore the observed G(H*) is much larger than G(N0;) (the.difIerence between the observed G(H’) and G(OH) corresponds to the measured G(N02-), see below). The PK. for dissociation of radical II’ is estimated to be 44 since the yield of H’ does not decrease in going from pH 5 to pH 4. At pH > 9 radical II is formed directly from +nitrophenolate so that, as described in section (a), no conductivity changes occur when radical II is formed in alkaline solutions. As already mentioned, on the basis of optical measurements it was suggested”’ that the OH radical adds to the NO2 group. It was further concluded”’ that the pK,, value of the OH adduct is 5.3. This value can be excluded on the basis of the conductivity results described in the present work.
The difference between G(OH) and G(H’) at pH 4-5 lops after the pulse corresponds to 0.6. Within experimental error, this value is equal to G(N&-) as determined at the same pH values from YO-y irradiated solutions (Table 1). It is therefore suggested that at pH 4-5 radical II and radical I’ are the stable protolytic forms of the radicals. This is reason&k since the phenolic Oand the nitro group are conjugated in radical II but not in radical I (equations (3) and (4)). (c) pK. values of hydroxpcycloheradienyl radicals The pK values of OH adducts of phenols
containing electron withdrawing groups were measured using the conductivity technique. The PK. value for the OH adduct of 4-nitrophenol (radical II’, see section (b)) is low (~4) in comparison with the PK., values of the OH adducts of Qcyanophenol, Qhydroxybenzaldehyde and 4hydroxyacetophenone which were determined to be larger than 6. The PK. values of Qnitro and 4-cyanophenol difIer by only 1 unit. From the observed larger difference in the PK. values of the OH adducts it is concluded that the relative abilities of the nitro and the cyan0 group to stabilixe a negative charge depend on whether or not the anion is radical in nature. It is of interest to note
P. O’NFZLLer 01.
16
that the PK. value of the nitrobenzene radical anion (PK. = 3.2”“) is much lower than that of the radical anion of cyanobenzene (PK. = 7.2(“1 which indicates that also in this case the negative charge of the radical anion is stabilized much more e5ciently by the NO* group than by the CN group. (d) Elimination
of
OH-
from
hydroxycyclo-
hexadienyl radicals The experimental observation of 2-hydroxy4
nitrophenol as a stable product is surprising, since OH adducts of phenolic compounds are known”“s”6’ to eliminate water to yield phenoxyl radicals. If the pH of the solution is L PK. of the OH adduct, formation of phenoxyl radicals proceeds by elimination of OH- from the ionized OH adducts.‘6’ However, as judged by the absence of conductivity changes at pH r 9 except those due to production of NO*-, the 4nitro-1,2dihydroxycyclohexadienyl radical (radical II) does not observably eliminate OH-. The first order rate constant for this process is estimated to be eld s-’ which is to be compared with the value r lad s-’ for elimination of OH- from OH adducts of phenolate”” and methoxyphenolates.’ The low value for the rate constant for elimination of OHfrom radical II is suggested to be due to strong withdrawal of electron density from the conjugated system of radical II by the N02 group. In order to see whether the rate of OH- elimination from OH adducts of other phenolates is influenced by electron withdrawing substituents to a comparable degree, OH adducts of ionized 4 cyanophenol, Qhydroxybenzaldehyde and 4 hydroxyacetophenone were prepared by reaction of OH with these phenolates and investigated using optical and conductometric detection. At pH values above the p&values of the phenols (pK,,‘s - 8) the OH adducts of 4cyanopheno1, 4 4hydroxyacetohydroxybenzaldehyde and phenone eliminate OH- to yield phenoxyl radicals with rate constants of 3, 4 and 7 x 10 s-‘, respectively. Thus, the rate of elimination of OHfrom the ionized OH adduct is reduced by electron withdrawing substituents as compared to electron donating subs&tents. However, the rate of OHelimination from the OH adduct of nitrophenolate is much lower than expected on the basis of the Hammett u value for N01, as compared to those for CN, CHO and COCH,. Deviation from a Hammett relationship has also been observed”” with respect to the rate of reaction of SO4” with nitrobenzene as compared with cyanobenzene and other substituted benzenes. This shows that not
only the PK. values but also the reactivity of nitroaromatic radicals show an unusual behaviour. (e) Selectivity of OH addition
As seen in Table 1 the yield of nitrite on reaction of OH with the nitrophenols depends on the relative positions of the OH(O-) and N02 groups in the substrates. This dependence is suggested to be due to the predominant addition of the electrophilic”’ OH radical to those ring positions which are activated by the electron donating OH or O- group. Since the yield of nitrite from 4 nitrophenolate corresponds to - 33% and that”’ of 2-hydroxy4nitrophenoi to -55% of the OH radical yield, the OH radical adds predominantly to positions ortho and pam to the phenolate group as observed in the reaction of OH with methoxyphenols” and methoxybenzenes.‘n The low yield of nitrite from 3-nitrophenolate is in agreement with this concept. However, in the case of 2nitrophenolate the yield of nitrite is lower than expected. One possible explanation may be steric hindrance of OH addition to the orrho position carrying the nitro group. On protonation of the phenolate group in the case of 2- and Cnitrophenolate the yield of nitrite is reduced by a factor of -2 (Table 1). This is suggested to be due to the smaller ortho-para directive effect of the OH group in comparison with that of the O- group. Acknowledgement-We assistance.
thank G. Meyer for technical
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in aqueous solution
17
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