Alkyl substituted phenoxyl decay in a hydrogen transfer equilibrium

Article abstract of DOI:10.1002/kin.20042

The kinetics of radical decay in the equilibrium: 2,4.6-tri-tert- butylphenoxyl radical 1 + 2,6-di-tert-butyl-4-methylphenol 2 = 2.4,6-tri-tert-butylphenol 3 + 2,6-di-tert-butyl-4-methylphenoxyl radical 4 was studied at 298 and 273 K by means of EPR spectroscopy. At 298 K second order prevails, whereas at 273 K the best fit was order 3/2. The extinction of 4 takes place in two steps: dimerization followed by disproportionation of the dimer, but the stable radical 1 enters in crossed dimerization with 4 to yield nonradical products. The mechanism ensures a constant |4|/|1| ratio along the decay. 2004 Wiley Periodicals, Inc.

Full text of DOI:10.1002/kin.20042

Alkyl Substituted Phenoxyl
Decay in a Hydrogen
Transfer Equilibrium
MARTA E. J. CORONEL
´
Departamento de Quımica, Universidad Nacional de Mar del Plata, Funes 3350, 7600 Mar del Plata, Argentina
Received 25 March 2004; accepted 20 July 2004
DOI 10.1002/kin.20042
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The kinetics of radical decay in the equilibrium: 2,4,6-tri-tert-butylphenoxyl radi-
cal 1 + 2,6-di-tert-butyl-4-methylphenol 2 = 2,4,6-tri-tert-butylphenol 3 + 2,6-di-tert-butyl-4-
methylphenoxyl radical 4 was studied at 298 and 273 K by means of EPR spectroscopy. At 298 K
second order prevails, whereas at 273 K the best fit was order 3/2. The extinction of 4 takes place
in two steps: dimerization followed by disproportionation of the dimer, but the stable radical 1
enters in crossed dimerization with 4 to yield nonradical products. The mechanism ensures a
constant [4]/[1] ratio along the decay. ꢀ 2004 Wiley Periodicals, Inc. Int J Chem Kinet 37: 1–4,
C
2005
of the dimer to give a quinone methide (Q) and 2 [3,4].
In the equilibrium of reaction (1) both radical decay
keeping constant the proportion [4]/[1] = 1.3 was ob-
served.
INTRODUCTION
In the course of measuring the equilibrium constant for
the reaction (1), the concentration of the phenoxyl rad-
icals 1 and 4 was measured by electron paramagnetic
resonance (EPR) spectroscopy [1,2]. Now, reaction (1)
has been studied at 273 and 298 K for the purpose of
investigating the kinetics of 1 and 4 phenoxyl radical
decay in the equilibrium.
Reaction (1) will be indicated
1 + 2 = 3 + 4
and the dimerizaton of 4 gives
(2)
(3)
(4)
4 + 4 = 42
followed by disproportionation:
42 → Q + 2
(1)
where 42 being the dimer of radical 4.
In the presence of 4, 1 undergoes crossed dimer-
ization and disproportionation of the reaction product
4 1:
The isolated radical 1 is stable. Otherwise, 4 is in-
volved in dimerization followed by disproportionation
4 + 1 = 4 1
4 1 → Q + 3
(5)
(6)
Correspondence to: Marta E. J. Coronel; e-mail: mcoronel@
bart.mdp.edu.ar.
2004 Wiley Periodicals, Inc.
c
ꢀ

2
CORONEL
EXPERIMENTAL
Reaction (1) was carried out in benzene–toluene (1:1).
The persistent radicals 1 and 4 were generated in
the EPR sampling tubes by addition of 20 ꢀl of
1.2 × 10−4 M benzene-toluene (1:1) solution of galvi-
noxylradical(G;2,6-di-tert-butyl-ꢁ-(3,5-di-tert-butyl-
4-oxohexa-2,5-dien-1-ylidene)-p-tolyloxy free radi-
cal; Aldrich) to 100 ꢀl mixed solutions of phenols 3
and 2 in the same solvent mixture. Samples (120 ꢀl)
were frozen and degassed by vacuum pumping. The
monitoring of the equilibrium was accomplished in the
thermostated cavity of a Bruker ER-200 X-band EPR
spectrometer, equippedwithaB-VT1000variabletem-
peratureunit, wheresampleswereletthaw. EPRspectra
were recorded after 1 min delay and the records were
repeated at constant intervals for one or two half-life
periods of time.
Phenols 2 and 3 were purchased from Aldrich and
their purity was checked by TLC and 1H-NMR. NMR
spectra (200 MHz, CDCl3) were recorded on a Bruker
AC-200 NMR spectrometer at the spectroscopy ser-
vice unit UMYMFOR (Conicet- FCEN, University of
Buenos Aires).
Allthesolvents(Merck, analyticalgrade)weredried
over potassium carbonate and distilled before use.
RESULTS AND DISCUSSION
Figure 1 EPR first derivative spectrum of (a) 4, (b) 1, and
(c) 1 + 4 in the equilibrium of reaction (1) at 273 K.
Thepersistenceoftheradicals4(Fig. 1a)and1(Fig. 1b)
makes it possible to monitor them by continuous wave
EPR. The central line in the signal of 1 is about the
center field of the records and it is free from contribu-
tions of 4 [5] (Fig. 1c). This allows the measurement of
[1] independently of [4] by the complete quenching of
G (10−7–10−6 M) in 0.2 M solutions of 3, which gives
[1]. On the other hand, [4] was calculated by measuring
the peak to peak amplitude of the fifth line (from left to
right, Fig. 1c) in the first derivative spectra taken from
the concentration given by its dimerization equilibrium
at 296 K [5–7].
fast equilibration of 1 and 4 in reaction (1) has taken
place, but before the much slower decay of dimer 42
has occurred to any extent.
The least-square fit affords
2k3k4/(k−3 + k4) = 7347.68(R = 0.9979)
(9)
Taking into account the previously reported values
for k3, 4.5 × 107 M−1 s−1 [8], and K3 = k3/k
=
3.7 × 105 M−1 [6], k4 was found to be 9.96 × 10−−3 3s−1
At 298 K, the decay of 4 approaches second-order
kinetics that agrees with the theoretical derivation:
and k = 122 s−1. Thus, the decay of 4 approaches
−3
second-order kinetics because k ꢁ k4 despite the
−3
disproportionation of the dimer is a unimolecular reac-
tion. The decline of 4 in the presence of 1 in the equi-
librium (1) also follows second-order kinetics (Fig. 2).
A set of reactions has been designed involving the par-
allel dimerization of 4 and the crossed dimerization of
1 and 4, both followed by disproportionation, as repre-
sented in Eqs. (3)–(6). That group of equations, includ-
ing Eq. (2), that accounts for the full reaction mech-
anism was solved using a noncommercial numerical
−d[4]/dt = 2[4]2[k3k4/(k−3 + k4)]
(7)
(8)
that followed by integration yields
1/[4] = [2k3k4t/(k−3 + k4)] + (1/[4]0)
Here, [4]0 represents the molar concentration of 4 im-
mediately after mixing the reactants, i.e. after the very

ALKYL SUBSTITUTED PHENOXYL DECAY IN A HYDROGEN TRANSFER EQUILIBRIUM
3
Figure 2 Second-order decay of radical 4 in the equilibrium of reaction (1) at 298 K, R = 0.9988.
integration program specially developed for this pur-
pose (Table I). Equations (3) and (5) might imply rate
constants k3 and k5 of similar order, but the equilibrium
of Eq. (5) should approach a half the value of that of
Eq. (3). The molecule 42 is a head-to-tail dimer [4];
it involves the attachment of the oxygen (head) of 4
on the C4 (tail) of another radical 4. But position C4
is much more hindered in 1; therefore, 1 must bind its
oxygen to the C4 of 4, that is, it has only one half of
the chances to react with 4.
considered variables in the overall scheme of equations
to be solved.
Reaction (1) is considered to be diffusion controlled
[6,9], and the rate constant k1 was calculated by means
of the Smoluchowsky–Einstein–Stokes equation [10]
kAB = (2/3)(RT/ꢂ)[2 + (rA/rB) + (rB/rA)]
∼
where rA
r are the hydrodynamical radii of the
B
=
molecules that encounter each other. The reactions de-
picted by Eqs. (3)–(6), when studied at 273 K, show
a change in the order of decay into 3/2 (Fig. 3). This
behaviour, which is not uncommon in the kinetics of
On the other hand, the rate of formation of 42 should
become diminished in the presence of 1 because of that
competition. Thus, both 42 and 4 1 formation must be
Figure 3 Order 3/2 decay of radical 4 in the equilibrium of reaction (1) at 273 K, R = 0.9986.

4
CORONEL
Table I Rate Constants for Differential Equations for
the Full Reaction Mechanism at 298 K
2. Coronel, M. E. J. Thermochim Acta 2003, 398, 33–
37.
3. Stebbins, R.; Sicilio, F. Tetrahedron 1970, 26, 291–
297.
4. Ingold, K. U. In Free Radicals, Kochi, K. J. (Ed.); Wiley:
New York, 1973; Vol. 1, Ch. 2.
5. Coronel, M. E. J.; Colussi, A. J. Int J Chem Kinet 1988,
20, 749–752.
6. Lucarini, M.; Pedulli, G. F.; Cipollone, M. J Org Chem
1994, 59, 5063–5070.
7. Wertz, J. E.; Bolton, J. R. Electron Spin Resonance.
Elementary Theory and Practical Applications;
McGraw-Hill: New York, 1972; Ch. 2.
8. Levin, P. P.; Khudyakov, I. V.; Kuzmin, V. A. Int J Chem
Kinet 1980, 12, 147–158.
9. Paulon, D. E.; Coronel, M. E. J. J Chem Soc, Perkin
Trans 2 1998, 885–888.
10. Brusa, M. A., Perissinotti, L. J.; Churio, M. S.; Colussi,
A. J. J Photochem Photobiol A 1996, 101, 105–
111.
Reaction
Rate Constant
1 + 2 → 3 + 4
3 + 4 → 1 + 2
4 + 4 → 42
1.0 × 1010 M−1 −1
1.0 × 1010 M−1 s−1
4.0 × 107 M−1 s−1
122 s−1
7.8 × 10−3 s−1
2 × 107 M−1 s−1
150 s−1
7.8 × 10−3 s−1
s
42 → 4 + 4
42 → Q + 2
4 + 1 → 4 1
4 1 → 4 + 1
4 1 → Q + 3
free radical reactions, has been examined previously
[11,12]. The full scheme and the rate constants for the
kinetics at 298 K are presented in Table I.
BIBLIOGRAPHY
11. Brokenshire, J. L.; Roberts, J. R.; Ingold, K. U. J Am
Chem Soc 1972, 94, 7040–7051.
1. Pedeconi, F. F.; Coronel, M. E. J. An Asoc Quim Argent
12. Weiner, S. A.; Mahoney, L. R. J Am Chem Soc 1972,
94, 5029–5036.
2000, 88, 53–58.