Acid-catalysed aromatization of anthranyl derivatives. A kinetic and thermodynamic study

Article abstract of DOI:10.1039/b107121a

The acid-catalysed solvolysis reaction of 9-methoxy-9-methyl-9,10-dihydroanthracene (2-OMe) in 50 vol percent acetonitrile in water at 25°C provides the substitution product 9-hydroxy-9-methyl-9,10-dihydroanthracene (2-OH) and the elimination product 9-methylanthracene (3). The rate-ratio of substitution-to-elimination was measured as k_S/k_E = 0.93. The alcohol also undergoes acid-catalysed aromatization to give the thermodynamically favoured product 3. The reaction enthalpy of this dehydration reaction was measured as ΔH = -10.2 ± 1.0 kcal mol^-1. The corresponding reaction of the secondary alcohol 9-hydroxy-9,10-dihydroanthracene (1-OH) has a reaction enthalpy of ΔH = -12.3 ± 0.8 kcal mol^-1. Addition of azide ion (0.25 M) gives rise to a large fraction of azide adduct 2-N₃, which rapidly undergoes solvolysis. The "azide-clock" method yields rate constants for reaction of the carbocation to give alcohol and elimination product ofk_w = 2.2 × 10⁸ s^-1 and k_e = 2.4 x 10⁸ s^-1, respectively. The thermodynamic stability of the carbocation was estimated as pK_R = -9.1. The alkene 9-methylene-9,10-dihydroanthracene (4) undergoes a slow acid-catalysed aromatization to give 3; no competing formation of 2-OH was observed.

Full text of DOI:10.1039/b107121a

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Acid-catalysed aromatization of anthranyl derivatives.
A kinetic and thermodynamic study
Necmettin Pirinccioglu, Zhi Sheng Jia and Alf Thibblin*
2
Institute of Chemistry, University of Uppsala, Box 531, SE-751 21 Uppsala, Sweden
Received (in Cambridge, UK) 7th August 2001, Accepted 4th October 2001
First published as an Advance Article on the web 7th November 2001
The acid-catalysed solvolysis reaction of 9-methoxy-9-methyl-9,10-dihydroanthracene (2-OMe) in 50 vol%
acetonitrile in water at 25 ЊC provides the substitution product 9-hydroxy-9-methyl-9,10-dihydroanthracene (2-OH)
and the elimination product 9-methylanthracene (3). The rate-ratio of substitution-to-elimination was measured as
kS/kE = 0.93. The alcohol also undergoes acid-catalysed aromatization to give the thermodynamically favoured
product 3. The reaction enthalpy of this dehydration reaction was measured as ∆H = Ϫ10.2 1.0 kcal molϪ1. The
corresponding reaction of the secondary alcohol 9-hydroxy-9,10-dihydroanthracene (1-OH) has a reaction enthalpy
of ∆H = Ϫ12.3 0.8 kcal molϪ1. Addition of azide ion (0.25 M) gives rise to a large fraction of azide adduct 2-N3,
which rapidly undergoes solvolysis. The “azide-clock” method yields rate constants for reaction of the carbocation to
give alcohol and elimination product of kw = 2.2 × 108 sϪ1 and ke = 2.4 × 108 sϪ1, respectively. The thermodynamic
stability of the carbocation was estimated as pKR = Ϫ9.1. The alkene 9-methylene-9,10-dihydroanthracene (4)
undergoes a slow acid-catalysed aromatization to give 3; no competing formation of 2-OH was observed.
Introduction
We are interested in the mechanistic details of solvolysis
reactions, e.g. the factors that govern the competition between
substitution and elimination. Relatively stable, solvent-equi-
librated, carbocations generally yield predominantly substitution
in nucleophilic solvents such as water.1–3 The reason is usually
that the alcohol is thermodynamically more stable than the
alkene and this is reflected in the relative energies of the trans-
ition states of the two competing reactions of the carbocationic
intermediate.
Recently, results were reported on systems which provide
nearly exclusively the elimination product despite the inter-
mediacy of the resonance-stabilized benzallylic carbocations
(R؉).4,5 It was clearly shown that the very large predominance
for dehydronation is due to the large thermodynamic stabilities
of the aromatic products and not to an unusually slow reaction
of the carbocation with water or added nucleophiles. Very large
reaction enthalpies of ∆H = Ϫ23.7 kcal molϪ1 and ∆H = Ϫ21.7
kcal molϪ1, respectively, were reported for the acid-catalysed
dehydration reactions of the alcohols A-OH and B-OH
(Scheme 1).4,5
reported results for the reactions of A-OMe and B-OMe
(Scheme 1)4,5 do not support this hypothesis since the former
gives traces of alcohol (the rearranged isomer), and B-OMe,
which provides a thermodynamically less stable alkene, does
not give any traces of alcohol. In contrast, Leute and Winstein
reported more than 30 years ago that the solvolysis of 9-
acetoxy-9,10-dihydroanthracene in 60% acetone in water yields
24% of 9-hydroxy-9,10-dihydroanthracene and 76% of anthra-
cene under kinetic control.6 However, it is not clear whether the
alcohol originates from direct reaction of the initially formed
ion pair with solvent water within the solvent cage, or if the
product ratio reflects the relative kinetic barriers for addition of
water and dehydronation of the solvent-equilibrated carbo-
cation (Scheme 2). We now report on a study of the acid-
Scheme 2
catalysed solvolysis of the tertiary anthranyl derivatives 2-OH
and 2-OMe (Scheme 3) and the mechanistic details are dis-
cussed. These reactions are also of biochemical interest since
hydrates of aromatic hydrocarbons have been proposed to be
intermediates in the mammalian metabolism of the parent
aromatic hydrocarbons.7
Scheme 1
A major question about this type of aromatization reaction is
whether the alcohol formation competes more successfully if
the aromatization is thermodynamically less favoured. The
DOI: 10.1039/b107121a
J. Chem. Soc., Perkin Trans. 2, 2001, 2271–2275
This journal is © The Royal Society of Chemistry 2001
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Table 1 Rate constants for the acid-catalysed solvolysis of 9-hydroxy-9-methyl-9,10-dihydroanthracene (2-OH) and 9-methoxy-9-methyl-9,10-
dihydroanthracene (2-OMe) in 50 vol% acetonitrile in water at 25 ЊC
Substratea
Nucleophile
106 kobs/sϪ1
kH/MϪ1 sϪ1
kS/kE
kN [N3Ϫ]/kE
3
2-OHb
2-OHd
2-OMee
2-OMed
4f
48.7
44.0
85.3
75.0
7.8
0.185 (0.181)c
0.86
0.29
0.25 M NaN3
0.25 M NaN3
5.3
4.8
0.93
1.5
225 × 10Ϫ6
a Substrate concentration 0.1 mM. b 0.1 M MeOCH2CO2H buffer, pH 3.58. c Rate constant measured with microcalorimetry. d 0.25 M NaN3, 0.25 M
HN3 and 0.25 M NaClO4, pH 4.29. e 0.1 M HOAc buffer, pH 3.53. f 0.1 M CF3CO2H buffer, pH 1.46.
Scheme 3
Results
The acid-catalysed solvolysis reaction of 9-methoxy-9-methyl-
9,10-dihydroanthracene (2-OMe) in 50 vol% acetonitrile in
water at 25 ЊC provides the substitution product 9-hydroxy-9-
methyl-9,10-dihydroanthracene (2-OH) and the elimination
product 9-methylanthracene (3) (Scheme 3). The reaction of 2-
OH exclusively provides the elimination product 3. The much
slower acid-catalysed reaction of 9-methylene-9,10-dihydro-
anthracene (4) also exclusively gives the anthracene product 3.
The kinetics of the reactions were studied by a sampling high-
performance liquid chromatography procedure, or by following
the appearance of 9-methylanthracene as a function of time
by UV spectrophotometry. The measured rate constants are
presented in Table 1. The separate rate constants (Scheme 3)
were derived by a computer simulation of the measured relative
concentrations (mol%) versus time (see Experimental).
The rate of solvolysis of 2-OMe does not increase with an
increase in formate buffer concentration (up to 0.6 M with
0.50 M NaCl added) in 33% acetonitrile in water, which indi-
cates that the acid catalysis is of specific rather than of general
type. A similar behaviour was observed with 2-OH (Fig. 1), so
Fig. 1 The rate constant (kobs) for disappearance of 2-OH as a
function of formate buffer concentration in 33.3 vol% acetonitrile in
water at pH 3.12 containing 0.50 M NaCl.
there is no significant general base catalysis on the dehydron-
ation reaction of the carbocation intermediate.
Addition of azide ion gives rise to the azide substitution
product 2-N3, but there is no rate increase for 2-OMe with
increasing azide buffer concentration. We therefore conclude
intermediate (ke, Scheme 2) is obtained from the measured
rate constants assuming a diffusion rate constant of kN = 5 ×
3
109 MϪ1 sϪ1 [eqn. (2)].
that there is no bimolecular substitution reaction with this
kN /kHOH = {(kN [N3Ϫ])/kS}/([N3Ϫ]/[HOH])
(1)
(2)
(3)
3
3
strong nucleophile. The azide product 2-N3 is not stable but
reacts to give 2-OH and 3 with a rate constant of ∼240 × 10Ϫ6 sϪ1
.
ke = {kE/(kN [N3Ϫ])}(kN [N3Ϫ])
3
3
The nucleophilic selectivity toward azide and water is large. It
can be expressed as a ratio of second-order rate constants in
accord with eqn. (1). The selectivity cannot be obtained directly
from the product ratio because of the fast decomposition of
the azide product. Instead, kw is derived from ke employing eqn.
(3). The rate constant for dehydronation of the carbocation
kw = (kS/kE)ke
Calorimetric measurements
The acid-catalyzed reactions of 1-OH and 2-OH have also been
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Table 2 Heat of reactions for the dehydration of 1-OH and 2-OH, and of 1-hydroxy-1-methyl-1,4-dihydronaphthalene (B-OH), 1-hydroxy-1,4-
dihydronaphthalene (A-OH), and 2-hydroxy-1,2-dihydronaphthalene (C-OH) at 25 ЊC
Substrate
Solvent
P0 a/µW
∆H/kcal molϪ1
1-OHb
2-OHd
B-OHf
A-OHg
C-OHg
MeCN–H2O 1 : 1
MeCN–H2O 1 : 1
Glycerol–H2O 1 : 3
Glycerol–H2O 1 : 3
Glycerol–H2O 1 : 3
11
1.4
27.5
20.0
12.0
Ϫ12.3 0.8c
Ϫ10.2 1.0e
Ϫ21.7 0.9
Ϫ23.7 0.4
Ϫ18.4 0.2
a Heat flow at time zero. b Substrate concentration 1.0 mM; 0.1 M 2-methoxyacetate buffer, pH 3.58. c Standard deviation, average of 9 runs.
d Substrate concentration 0.27 mM; 0.1 M 2-methoxyacetate buffer, pH 3.58. e Standard deviation, average of 4 runs. f Ref. 5. g Ref. 4.
studied by heat-flow microcalorimetry. The reactions were run
in 50 vol% acetonitrile in water. The observed decrease in heat
flow after the equilibration period (see Experimental) reflects
the pseudo-first-order behaviour of the reaction. The measured
rate constants agree with those obtained by the spectrophoto-
metric technique (Table 1). The measured reaction enthalpies
are given in Table 2 which also includes the data measured
previously for the closely related naphthalene hydrates.
Discussion
The acid-catalysed solvolysis of 2-OMe is concluded to involve
rate-limiting ionization since there is no increase in the rate of
reaction in the presence of added azide ion. This is in contrast
to the structurally related substrate Ph2(Me)COMe which
undergoes a much slower acid-catalysed solvolysis reaction
owing to rate-limiting addition of water.8 Another contributing
factor to the higher reactivity of 2-OMe is the cyclic nature of
the carbocation,9 and probably the development of partial
Fig. 2 Schematic energy diagram showing activation barriers and
aromaticity in the transition state of the ionization step. The
thermodynamic parameters (in kcal molϪ1).
solvolysis of 2-OMe provides nearly as much elimination as
substitution (Table 1). This is not what is usually seen in solv-
olysis reactions proceeding via relatively stable carbocations.1–3
For example, the structurally related carbocation Ph2(Me)Cϩ
yields predominantly substitution under the same conditions.8
The reason is concluded to be the large thermodynamic stabil-
ity of the aromatic elimination product 9-methylanthracene (3,
Scheme 3) in accord with our previous studies of aromatization
reactions. Thus, the extremely large kE/kS ratio of approx-
imately 1600 measured for the solvolysis of A-OMe in 25 vol%
acetonitrile in water has been concluded to be the result of a
reduced activation barrier for dehydronation caused by partial
aromaticity in the carbocation and not to an unusually slow
addition of water.4
stability of a carbocation. The measured parameters kw
=
2.4 × 108 sϪ1 and kH = 0.185 MϪ1 sϪ1 yields pKR = Ϫ9.1 (∆G =
12.4 kcal molϪ1). This value is of the same order of magnitude
as that of the 1-(4-methoxyphenyl)ethyl carbocation (pKR
=
Ϫ8.6 in 50% aqueous trifluoroethanol), but which shows a
7000-fold larger substitution-to-elimination ratio.3
The reactivity of 2-OH is much less (173 times) than that
of 1-hydroxy-1-methyl-1,4-dihydronaphthalene (B-OH), but
the rate-ratio of the corresponding ethers is only 77 times. The
reason for this difference is that the reaction of the substrate
2-OH, in contrast to B-OH,5 exhibits significant return. Meas-
ured kinetic and thermodynamic parameters for these reactions
and for some other aromatization reactions are collected in
Table 3. The schematic energy diagram shown in Fig. 2 could be
used for predicting some relations between kinetic and thermo-
dynamic parameters. A more stable carbocation, corresponding
to a larger (less negative) pKR, should increase the reaction rates
for the ether and the alcohol in accord with the expected close
resemblance between the rate-limiting ionization transition
state and the carbocation. It should also increase the barrier for
addition of water (kw) to the cation. However, there is not a
good correlation between the reactivity, as expressed by the
second-order rate constant of the acid-catalysed reaction of the
methyl ether, and the pKR value (Table 3). The table shows only
a very limited number of reactions, both secondary and tertiary
substrates, and a reasonable correlation may exist for the two
separate classes of substrates.
The rate constant (ke) for dehydronation of the carbocation
to give 3 (Scheme 2) has been measured by the “azide-clock”
method assuming that the reaction with azide ion is diffusion-
controlled with a rate constant of 5 × 109 MϪ1 sϪ1 3,10
Flash-
.
photolysis experiments have shown that carbocations with kw >
1 × 105 sϪ1 react with azide ion in a diffusion-controlled process
in aqueous acetonitrile.10 The rate constant measured in this
manner with the ether 2-OMe is ke = 2.4 × 108 sϪ1. A similar
value, ke = 2.2 × 108 sϪ1, was measured with the alcohol 2-OH.
The rate constant (kw) for addition of water to the carbocation
could not be obtained in the usual way. It was derived from the
measured ke value (see Results). The value, kw = 2.2 × 108 sϪ1
is slightly larger than that measured for Ph2(Me)Cϩ, kw = 1.7 ×
108 sϪ1 8b
An azide molecule is 625 times more reactive than a
,
.
water molecule toward the carbocation.
The pKR for the pseudo acid–base equilibrium for cation
hydration [eqns. (4) and (5)] is a measure of the thermodynamic
The magnitude of the enthalpy of the aromatization reaction
should mainly reflect the stability of the aromatic alkene prod-
uct. A higher stability should be connected with a more stable
carbocation owing to more partial aromatization in the cation
and, accordingly, gives rise to larger ionization rates of the sub-
strates (i.e., increase kH). This effect is not easily seen in the
(4)
KR = kw/kH = [ROH][Hϩ]/[Rϩ]
(5)
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Table 3 Kinetic and thermodynamic parameters of some aromatization reactions at 25 ЊC
b
Substrate
∆H/kcal molϪ1
pKR
kH a/MϪ1 sϪ1
kS/kE
kw/sϪ1
ke/sϪ1
2-OH
Ϫ10.2
Ϫ12.3
Ϫ20.8
Ϫ23.7
Ϫ39.9
∼Ϫ5f
Ϫ9.1
0.185
0.28
32
0.9
0.9
<0.003
0.0006
2.2 × 108
1.3 × 107 g
<2 × 106
∼1 × 107
2.4 × 108
2.3 × 107 g
6 × 108
1-OH
Ϫ5.9g
>Ϫ4.8
∼Ϫ6.6
Ϫ2.4g
B-OHc
A-OHd
∼2.5
∼1.6 × 1010
1,2-benzene hydratee
1.7
4
225 × 10Ϫ6
(2.2 × 108)
(2.4 × 108)
a In 50 vol% acetonitrile in water. b Measured with the methyl ethers. c Ref. 5. d Ref. 4; data extrapolated from 25 vol% acetonitrile in water. e Ref. 15.
f Ref. 11. g In water, ref. 16.
limited amount of recorded data, but the predicted increase
in ke with increase in reaction enthalpy is in accord with the
experimental data (Table 3). The effect is not large for the
two structurally related compounds 1-methoxy-1-methyl-1,4-
dihydronaphthalene (B-OMe) and 2-OMe. Thus, a difference in
reaction enthalpy of more than 10 kcal molϪ1 corresponds to an
approximately three-fold increase in ke.
The absence of an effect of general bases on the reaction rate
of 2-OH is consistent with a very exothermic dehydronation
step (ke) with a very small amount of hydron transfer in the
transition state in accord with what has been found previously
for the naphthalene-producing solvolysis reactions.4
experiments were of reagent grade and used without further
purification.
Syntheses
9-Hydroxy-9-methyl-9,10-dihydroanthracene (2-OH) was pre-
pared from anthranone and methylmagnesium iodide. Methyl-
ation with methyl iodide in dichloromethane in the presence
of freshly prepared silver oxide afforded 9-methoxy-9-
methyl-9,10-dihydroanthracene (2-OMe). The substrates were
purified by recrystallization from pentane. Analysis by NMR
and HPLC confirmed high purity. 9-Methylene-9,10-dihydro-
anthracene (4) was prepared by lithium aluminium hydride
reduction of (9-anthranylmethyl)trimethylammonium chloride.12
Hydronation of the alkene 4 (Scheme 3) should yield the
same carbocation intermediate as the reactions of 2-OMe
and 2-OH. However, the second-order rate constant for this
reaction is approximately 800 times smaller than that of the
reaction of 2-OH (Table 1) and this explains why no trace of
2-OH is observed during the reaction. Any 2-OH formed is
expected to be converted quickly to 3 under these acidic
conditions. The enthalpy of the aromatization of 4 has been
measured as 5.1 and 3.4 kcal molϪ1 with ICR and calorimetry
Kinetics and product studies
HPLC procedure. The reaction solutions were prepared by mix-
ing acetonitrile or methanol with water at room temperature,
ca. 22 ЊC. The reactions were initiated by addition of a few
microlitres of the substrate dissolved in acetonitrile to a 2 ml
HPLC vial containing 1.2 ml of the thermostatted reaction
solution to give a final substrate concentration of 0.1–0.2 mM.
The reaction flask was sealed with a gas-tight PTFE septum
and placed in a thermostatted aluminium block in the HPLC
apparatus. At appropriate intervals, samples were automatically
injected onto the column and analysed. The rate constants for
the disappearance of the substrates were calculated from plots
of substrate peak area versus time by means of a non-linear
regression computer program. Very good pseudo-first-order
behaviour was observed for all the reactions studied.
The azide product 2-N3 was not isolated but identified by its
UV spectrum, which is similar to that of 2-OH. The relative
response factors of alcohol (2-OH) and 9-methylanthracene (3)
were determined in a separate experiment. The relative response
factor of 2-N3 at 219 nm was assumed to be the same as that of
2-OH. The separate rate constants for the reactions (Scheme 3)
were evaluated by computer simulation based upon the product
composition data, obtained from the HPLC peak areas and the
relative response factors and the phenomenological reaction
scheme (Scheme 3).
(CCl4), respectively, and calculated as 7.7 kcal molϪ1 11
.
Experimental
General procedures
NMR spectra were recorded at 25 ЊC with a Varian Unity
300 MHz spectrometer. Chemical shifts are indirectly refer-
enced to TMS via the solvent signal (chloroform-d1 7.26 and
77.0 ppm). The high-performance liquid chromatography
analyses were carried out with a Hewlett-Packard 1090 liquid
chromatograph equipped with a diode-array detector on an
Inertsil 5 ODS-2 (3 × 100 mm) reversed-phase column. The
mobile phase was a solution of acetonitrile in water. The reac-
tions were studied at constant temperature maintained with a
HETO 01 PT 623 thermostat bath kept at 25.00 0.03 ЊC. A
Kontron Uvicon 930 spectrophotometer equipped with an
automatic cell changer kept at constant temperature with water
from the thermostat bath was used for some of the kinetic
studies. The pH was measured using a Radiometer PHM82 pH
meter with an Ingold micro glass electrode. The pH values given
are those measured before mixing with the organic solvent.
The microcalorimetric experiments were carried out with a
dual channel calorimeter (Thermometric Thermal Activity
Monitor 2277). The signals were recorded both on a two-
channel potentiometric recorder and with a computer.
When starting from pure 2-OMe, the concentrations of
2-OMe, 2-OH, and 3 are described by the eqns. (6)–(8) (Scheme
4).13 An analogous procedure was employed for the reaction of
mol% 2-OMe = a eϪm t ϩ (100 Ϫ a)eϪm t
mol% 2-OH = b eϪm t Ϫ b eϪm t
(6)
(7)
1
2
1
2
mol% 3 = 100 Ϫ mol% 2-OMe Ϫ mol% 2-OH (8)
Materials
Merck silica gel 60 (240–400 mesh) was used for flash chrom-
atography. Diethyl ether and tetrahydrofuran were distilled
under nitrogen from sodium and benzophenone. Methanol and
acetonitrile were of HPLC quality and HPLC UV gradient
quality, respectively. The commercially available 9-methyl-
anthracene (3) (99%, Lancaster) was purified by recrystalliz-
ation from pentane. All other chemicals used for the kinetic
Scheme 4
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2-OH in the presence of azide ion. The reaction system with 2-
OMe as substrate and with azide ion added is more complex.
This system was simulated by neglecting the formation of the
small amounts of 2-OH from 2-OMe and 2-N3.
calculation of the reaction heat (∆H) according to eqn. (9),
where n is the amount of substrate (mol) in the reaction vial.
P0 = ∆H kobs
n
(9)
where
The estimated errors are considered as maximum errors
derived from maximum systematic errors and random errors.
The maximum errors of the directly measured quantities were
thus allowed to propagate as systematic errors into derived
quantities, e.g., reaction rate constants.
a = 100 (k12 ϩ k13 Ϫ m2)/(m1 Ϫ m2)
b = 100 k12/(m2 Ϫ m1)
m1 = [(k12 ϩ k13 ϩ k21 ϩ k23)2/4 Ϫ k12k23 Ϫ (k21 ϩ k23)k13]1/2
1/2(k12 ϩ k13 ϩ k21 ϩ k23)
ϩ
m2 = Ϫ[(k12 ϩ k13 ϩ k21 ϩ k23)2/4 Ϫ k12k23 Ϫ (k21 ϩ k23)k13]1/2
ϩ 1/2(k12 ϩ k13 ϩ k21 ϩ k23)
Acknowledgements
UV Spectrophotometric procedure. The reactions were run in
3 ml standard quartz cells using the above-mentioned equip-
ment. Addition of a few microlitres of a concentrated solution
of the substrate in acetonitrile to 2.5 ml of prethermostatted
reaction solution gave an initial concentration of the substrate
in the reaction flask of about 0.1 mM. The increase in absorb-
ance at 253 nm was followed as a function of time and the
pseudo-first-order rate constant calculated by a non-linear
regression computer program.
Microcalorimetric procedure. This technique has the advan-
tage that both kinetic data and reaction heats are obtained from
the same kinetic experiment. The reactions were run in parallel
in the two channels, both composed of a sample compartment
and a reference compartment. Glass vials (3 ml) were used as
reaction and reference vessels. All four vessels were filled at the
same time with 2.5 ml of premixed reaction solution (organic
solvent and aqueous buffer). After this step, 20 µl of substrate
in acetonitrile were added to the two reaction vessels while 20 µl
of pure acetonitrile were added to the reference vessels. The
vials were sealed with gas-tight PTFE septa and slowly intro-
duced into the compartments of the instrument for about 15
min of pre-thermostatting. They were then lowered further
down into the detection chambers. The recording of the first-
order heat-flow decay was started after a total equilibration
time of 30–45 min. The reactions were followed for at least ten
half-lives.
We thank the Swedish Natural Science Research Council for
supporting this work and Mr Daniel Herzog for preparation of
9-methylene-9,10-dihydroanthracene (4). Professor Rory More
O’Ferrall has kindly provided us with unpublished data for
9-hydroxy-9,10-dihydroanthracene (1-OH).16
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The microcalorimeter was statically calibrated after a kinetic
experiment using the reaction solutions. The cooling constant
of the instrument was found to be 140 s, i.e., no correction for
this parameter was necessary for calculation of the rate con-
stants of the reaction heat decay.14 The rate constants were
obtained from data of heat flow versus time by means of a non-
linear regression computer program. Very good first-order rate
constants (kobs) were measured. These agree well with those
measured in the kinetics using the UV-spectrophotometric
technique (Table 1).
11 J. E. Bartmess and S. S. Griffith, J. Am. Chem. Soc., 1990, 112, 2931.
12 M. Takagi, T. Hirabe, M. Nojima and S. Kusabayashi, J. Chem. Soc.,
Perkin Trans. 1, 1983, 1311.
13 R. A. Alberty and W. G. Miller, J. Chem. Phys., 1957, 26, 1231.
14 A. Thibblin, Chem. Scr., 1983, 22, 70.
15 Z. S. Jia, P. Brandt and A. Thibblin, J. Am. Chem. Soc., 2001, 123,
10147.
16 A. C. McCormack, C. A. McDonnell, R. A. More O’Ferrall, A. C.
O’Donoghue and S. N. Rao, J. Am. Chem. Soc., submitted.
The extrapolated heat flow at time zero (P0) was used for
J. Chem. Soc., Perkin Trans. 2, 2001, 2271–2275
2275