9844 J. Phys. Chem. A, Vol. 101, No. 51, 1997
Choure et al.
mechanism proceeds by a direct electron transfer from the
benzene ring to SO4•-. It has been shown by us in our earlier
work9,10 with halobenzenes (F+ ) -1.2) and halotoluenes (F+
) -1.6) that the reaction may occur both by addition/elimination
(reaction 4) and by outersphere electron-transfer (reaction 5).
The lifetime of SO4•- adduct to benzenes is known24 to be e100
ns. With the electron-donating substituents -CH3 and -OH,
the hydrolysis rate of the SO4•- adducts should be still higher,
and the reaction must be complete within the pulse width (50
ns) of our experiment.
group. Scheme 3 depicts the sequence of oxygen addition
reactions with 2-cresol, as an example, wherein the formation
of 2,3-DHT occurs from the peroxyl radical 3. The other
peroxyl radical 4 formed from the OH adduct 2 is of 1,4-type,
resulting in other products. The lack of formation of 2,4- and
2,5-DHT in 2-cresol is because of the fact that the radical sites
in 1,3-type OH adducts (structures 5-8) are not located at the
tertiary center.
Conclusions
The transients absorbing at 290 and around 400 nm in the
reaction of SO4•- with cresols are assigned to the radical cation
•
•-
The intermediates in the reactions of OH, O•-, and SO4
based on the similarity of the spectra reported10 by us in the
with cresols are formed from addition, H-abstraction, and
electron transfer. Oxygen is relatively more reactive with OH
adducts of cresols than those of chlorotoluenes. The addition
of oxygen to 1,3-type OH adducts of cresols occur at the carbon
substituted by the OH group. This study demonstrates the
usefulness of radiation chemical methods in obtaining informa-
tion on peroxyl radical chemistry of chlorotoluenes and cresols.
•-
SO4 reaction in the case of 3- and 4-chloroanisoles (λmax
)
290 and 450 nm), though the second peak in these systems is
at a higher wavelength (450 nm). This assignment is substanti-
ated by the observed faster decay of the transient absorption at
•-
290 and 390 nm in 4-cresol when SO4 reaction was carried
out at pH ) 9 (Figure 3D). As pointed earlier (cf. section 3B
(iii) SO4•- reaction), this decay was manifest by an increase in
absorption in the region 320-340 nm whose rate was estimated
as 1.2 × 105 s-1 from laser flash photolysis of 4-cresol in neutral
solutions. This transformation reaction must, then, be due to
the formation of substituted phenoxyl radical by deprotonation
of the radical cation (reaction 6, Scheme 1). Furthermore, some
other radiation products in addition to phenolic products were
formed in SO4•- reaction with cresols under γ-radiolysis. This
is evident from the HPLC chromatogram depicted in Figure
6G for 3-cresol, where the formation of quinone type products
(peaks l in Figure 5G) eluting after the phenolic products should
result from the oxidation of phenoxyl radical (reaction 7).
Acknowledgment. The authors wish to thank Prof. M. S.
Wadia and Dr. M. G. Kulkarni, Department of Chemistry,
University of Pune, for their useful suggestions in the discussion
of the results. The financial support from the Board of Research
in Nuclear Sciences, Department of Atomic Energy, Government
of India, for carrying out this project (37/28/95-R & D-II) is
gratefully acknowledged. One of us (M.M.M.B) is thankful to
the University of Aden for the award of a fellowship during
the course of this work.
The phenolic yields formed in the isomers of chlorotoluenes
and cresols are in accord with the usual activation and
deactivation effects of -Cl and -OH groups. For instance,
the lack of formation of 2,6-DHT and reduction in the yield of
2,4-DHT in the case of 2-cresol relative to the corresponding
phenols formed in chlorotoluene is due to the deactivation of
meta position of the -OH group. Similar deactivation of the
meta and activation of the para positions was responsible for
reduction in the yield of 2,4- and enhancement in 3,4-DHT
yields in the case of 4-cresol. An exception to this is the
behavior of 3-cresol, where a lower yield of 3,6-DHT and lack
of formation of phenols formed from the attack of -OH at the
para position were noticed.
References and Notes
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(ii) Oxygenated Solutions of Chlorotoluenes and Cresols. It
•
is known1-3 that the phenolic products can be formed by HO2
elimination from only 1,3-cyclohexadienyl peroxyl radicals,
whereas 1,4-type radicals result in endoperoxidic products. The
relatively larger phenolic yields observed in the case of cresols
(8) Merga, G.; Schuchmann, H.-P.; Rao B. S. M.; von Sonntag, C. J.
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•
suggest that the rates of HO2 elimination and other product
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forming reactions are higher than in the case of 3- and
4-chlorotoluenes. This finding is further substantiated by the
larger values of the stability constants, K (Table 2), observed
in the case of cresols. The lower yields of phenolic products
in 3- and 4-chlorotoluenes are possibly due to relatively higher
kr and lower kp values. In other words, the reverse reaction of
the equilibrium probably predominates over the product forma-
tion in these compounds. This means that a major fraction of
1,3-type structures are ultimately converted into 1,4-cyclohexa-
dienyl type structures.
An interesting observation is that only those dihydroxytolu-
enes with -OH groups ortho to each other were formed in
oxygenated solutions of cresols. The formation of such isomers
of cresols is possible only from 1,3-type peroxyl radicals when
the radical site is localized at the carbon carrying the -OH
(14) Roder, M.; Emmi, S. S.; Wojnorovits, L.; Foldiak, G. 20th Miller
Conference on Radiation Chemistry, Windermere, U.K., 1997, Abs. 12.
(15) Vinchurkar, M. S.; Rao, B. S M.; Mohan, H.; Mittal, J. P.; Schmidt,
K. H.; Jonah, C. D. J. Phys. Chem. 1997, 101, 2953.