Kinetics of the oxidation of fluoren-9-ols by 4-nitro-1-chlorobenzotriazole

Article abstract of DOI:10.1007/s007060170066

The oxidation of fluoren-9-ols by 4-nitro-1-chlorobenzotriazole in aqueous acetic acid is a second order reaction with a reaction constant of -1.4. The oxidation exhibits a kinetic deuterium isotope effect indicating the cleavage of the 9-C-H bond in the rate-determining step. The title oxidation is compared with that by 1-chlorobenzotriazole.

Full text of DOI:10.1007/s007060170066

Monatshefte fuÈr Chemie 132, 799±804 (2001)
Kinetics of the Oxidation of Fluoren-9-ols
by 4-Nitro-1-chlorobenzotriazole
Ratinum Ulagi1, Perumal Kuselan2, and Chockalingam Karunakaran3;Ã
1
Department of Chemistry, LRG Government Arts College for Women, Tirupur-641604, India
2
Department of Chemistry, Presidency College, Chennai-600005, India
3
Department of Chemistry, Annamalai University, Annamalainagar-608002, India
Summary. The oxidation of ¯uoren-9-ols by 4-nitro-1-chlorobenzotriazole in aqueous acetic acid is a
second order reaction with a reaction constant of 1.4. The oxidation exhibits a kinetic deuterium
isotope effect indicating the cleavage of the 9-C±H bond in the rate-determining step. The title
oxidation is compared with that by 1-chlorobenzotriazole.
Keywords. Structure-reactivity relationship; Reactivity-selectivity principle.
Introduction
Chloramines, used as disinfectants and antiseptics, contain one or more chlorine
atoms covalently attached to nitrogen and behave both as oxidizing and chlorinating
agents. The organic chloramines resemble hypochlorite in their oxidative behavior
but show higher stability. N-Chlorobenzotriazole (CBT) is a well-known chlor-
amine, and kinetic studies with CBT are numerous [1, 2]. This paper presents the
kinetic behavior of 4-nitro-1-chlorobenzotriazole (NCBT); the experimental results
show that the reactivity is marginally enhanced by the introduction of the nitro
group without decreasing the selectivity.
Results and Discussion
Reaction order
The oxidation of ¯uoren-9-ol with NCBT, performed in aqueous acetic acid under
the pseudo-®rst-order conditions of [¯uoren-9-ol] ꢀ [NCBT], afforded linear plots
of log[NCBT] vs. time. The linearity of these plots, together with the constancy of
the pseudo-®rst-order rate constant k0 ˆ d[NCBT]/dt Á 1/[NCBT] at different [NCBT]0,
indicated a ®rst-order dependence of the reaction rate on [NCBT] (Table 1). Also,
in a kinetic run, the pseudo-®rst-order rate constant, calculated from [NCBT]
(determined iodometrically) at different reaction times using the integrated pseudo-
®rst-order rate equation, remained constant (data not presented). The reaction is of
Ã
Corresponding author

800
R. Ulagi et al.
Table 1. Oxidation of ¯uoren-9-ol by NCBT at 303 K
103‰NCBTŠ
102‰alcŠ
HOAc-H2O
104k0
102k2
0
3
0
3
1
1
1
%ꢁv=v†
s
mol Á dm
mol Á dm
dm3 Á mol Á s
1.00
2.00
3.00
4.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
0.50
1.50
2.00
2.50
3.00
3.50
4.00
1.00
1.00
1.00
70-30
70-30
70-30
70-30
70-30
70-30
70-30
70-30
70-30
70-30
70-30
50-50
60-40
80-20
1.7
1.7
1.7
1.7
0.87
2.6
3.5
4.4
5.2
6.1
6.9
1.08
1.32
2.0
1.7
1.7
1.7
1.7
1.7
1.7
1.7
®rst order with respect to ¯uoren-9-ol. The pseudo-®rst-order rate constant
increased with increasing concentration of ¯uoren-9-ol, and the plot of k0 vs.
[¯uoren-9-ol]0 is a straight line passing through the origin (correlation coef®cient:
6
1
1
0.9999, standard deviation: 3:7 Â 10 , slope: 0.017 dm3 Á mol Á s ). Also, the
second-order rate constant (k2), calculated from the pseudo-®rst-order rate constants
(k0 ˆ k2[¯uoren-9-ol]0), remained constant at different [¯uoren-9-ol]0. The reaction
rate did not vary considerably with solvent composition (the acetic acid content of
the reaction medium was varied from 50 to 80%; due to the poor solubility of NCBT
in aqueous acetic acid, the fraction of acetic acid could not be reduced below 50%).
Kinetic isotope effect
Investigation of the primary isotope effect led to the identi®cation of the bond being
cleaved in the rate-limiting step. The kinetic deuterium isotope effect was deter-
mined by measuring the oxidation rate of ¯uoren-9-ol-ꢀ-d; the reaction showed a
small but signi®cant isotope effect (kH=kD) of 1.9, providing a de®nite indication
that the corresponding bond is broken in the rate-determining step [3].
Structure-reactivity relationship
The effect of substituents on the oxidation was examined by measuring the reaction
rates of some 2- or 3-substituted ¯uoren-9-ols. In each case the reaction was found
to be ®rst order in NCBT and alcohol; plots of log[NCBT] vs. reaction time were
linear, and the second-order rate constant at different [¯uoren-9-ol]0 (0.005±
3
0.010 mol Á dm ) remained constant (data not given). It is important to note that all
substituents employed were electron withdrawing groups lowering the oxidation
rate (Table 2). The reaction could not be studied with electron donating substituents
(e.g. 2-methyl, 2-methoxy) as these compounds were susceptible to nuclear chlori-
nation by chloramine. The reaction rates were determined at 303±323 K, and the

Oxidation of Fluoren-9-ols
801
Table 2. Kinetic and thermodynamic parameters of the NCBT oxidation of ¯uoren-9-ols in aqueous
acetic acid
102k2a
Ea
ÁH#
ÁS#
Substituent
log A
1
1
1
1
dm3 Á mol Á s
1.7 (1.18)
kJ Á mol
kJ Á mol
J Á K Á mol
1
1
H
74
58
60
57
61
10.9
4.7
8.0
4.1
8.4
71
55
57
57
58
45
108
100
119
93
2-Br
2-Cl
2-NO2
3-Br
0.46 (0.34)
0.51 (0.38)
0.17 (0.115)
0.79 (0.54)
a
303 K; the values in the parentheses correspond to oxidation by CBT
energy of activation and the frequency factor were calculated from the Arrhenius
plots (logk2 vs. 1/T). The Eyring plots (log(k2/T) vs. 1/T) afforded the activation
enthalpy and the activation entropy (Table 2); the logarithmic form of the Eyring
equation is ln(k2/T) ˆ ln(k/h) (ÁH#=RT) ‡ (ÁS#=R) where k is the Boltzmann
constant, h is the Planck constant, and R is the gas constant. The oxidation conforms
to the isokinetic relationship [4, 5]: the double logarithmic plot of k2(323 K) vs.
k2(303 K) is a straight line with a slope of 1.17 (Fig. 1, Exner plot; r ˆ 0:996,
sd ˆ 0:04). The isokinetic temperature (ꢁ), calculated from the slope of the Exner
plot (b0), is 221 K; ꢁ ˆ T1T2ꢁb0 1†=ꢁb0T2 T1†. The isokinetic temperature is the
temperature at which all compounds of the reaction series react equally fast, i.e. at
this temperature the substituents have no in¯uence on the free energy of activation.
Transgressing this temperature results in the reversal of the order of reactivity [3].
The validity of the isokinetic relationship revealed the operation of a common
mechanism in the NCBT oxidation of ¯uoren-9-ols. Also, the isokinetic temperature
is far away from the experimental temperature range, and hence the effect of
substituents on the reaction rate is signi®cant. These results suggested the appli-
cability of the Hammett equation to the title oxidation. The rate data at all
temperatures studied conform to the usual Hammett equation, and the reaction
Fig. 1. Exner plot

802
R. Ulagi et al.
Fig. 2. Hammett plot at 303 K
constant at 303 K is 1.4; plots of logk2 vs. ꢂ yielded straight lines (e.g. Fig. 2,
Hammett plot; r ˆ 0:9996, sd ˆ 0:01, 303 K). The negative ꢃ-value indicates that
the transition state of the reaction has considerable positive charge development.
Mechanism
The oxidation is a second-order reaction and exhibits a kinetic deuterium isotope
effect. Hence, a possible mechanism is the slow and rate-determining abstraction of
the ꢀ-hydrogen as a hydride ion by chloramine. If the chloronium ion were to be the
reactive species, the oxidation should be inhibited by the reaction product
benzotriazole [6], and ± contrary to the experimental results ± the data should not ®t
to the pseudo-®rst-order plots and the pseudo-®rst-order rate constant calculated
with [NCBT]0 and [NCBT] at different reaction times using the integrated ®rst-order
rate equation should not remain constant (typical kinetic data not given). The linear
Hammett plot with a negative reaction constant also supports this mechanism. The
kinetic deuterium isotope effect of the title reaction is small but signi®cant [3]. Low
kH=kD values may result if the CÁ Á ÁH(D)Á Á ÁCl arrangement in the transition state is
not linear [3] which is probable the case as the reaction centre is ¯anked by two
bulky benzene rings, therefore giving rise to a sterically crowded transition state.
Reactivity-selectivity principle
The oxidation of ¯uoren-9-ols by CBT in aqueous acetic acid is a second-order
reaction [2]. It conforms to the Hammett equation with a reaction constant of 1.5
at 303 K. Table 2 reveals that oxidation by NCBT is faster than by CBT. Introduction
of a nitro group, a strongly electron withdrawing substituent, makes the chlorine
side of the haloamine more electron-de®cient, thus increasing its electrophilicity.
Highly reactive systems are often less selective and, if amenable to correlation,
show lower susceptibility than similar less reactive systems [3]. For example, the
value of the reaction constant (susceptibility factor) for electrophilic aromatic
substitution decreases with increasing electrophilicity (reactivity) of the reagent, i.e.

Oxidation of Fluoren-9-ols
803
the reagent becomes less capable of discriminating between two differently
substituted benzene derivatives. The reaction constants of aromatic chlorination and
bromination at 298 K in acetic acid are 10 and 12.1, respectively [3]. The
oxidation of ¯uoren-9-ols by NCBT and CBT does not conform to the reactivity-
selectivity relationship. Although the reactivity of the chloramine is increased by the
introduction of the nitro group, the selectivity does not decrease; the reaction
constants of oxidations by NCBT and CBT do not differ signi®cantly.
Recently, Exner [7] has rede®ned and reexamined the reactivity-selectivity
principle (RSP) on an experimental basis, showing that the original narrow
de®nition of RSP is invalid, and formulated the principle in terms of simple
mathematical expressions, involving only rate constants and avoiding any ꢂ
constants. With a given reaction series, four types of behavior are possible: (i) a
valid RSP when the selectivity decreases with reactivity; (ii) anti-RSP when the
reverse is true; (iii) indifferent behavior when the change in selectivity is negligible;
(iv) a cross-over when RSP is valid in one part of the series and invalid in the other.
When two reagents (fast F and slow S) react with a series of similar substrates
(i ˆ 1; 2; . . . ; n), the RSP requires a linear relationship between the two series as
given by log kFi ˆ a ‡ b logkSi ‡ 2i, where 2i is the error of the equation; the
slope b must be positive. The mean difference Á is de®ned as Á ˆ
ꢁÆ log kFi Æ log kSi†=n. Exner's analysis of a hundred reaction series has shown
that (i) for a valid RSP (RSP(‡)) the slope b is signi®cantly less than unity and Á is
not too small, (ii) b is greater than unity for anti-RSP (RSP( )), (iii) for an
indifferent behavior, the slope b is not signi®cantly different from unity (0.95±1.05;
equal selectivity of the two reagents) (RSP(?)), and (iv) for a cross-over RSP Á is
too small (RSP(Æ)). The rates of oxidation of ¯uoren-9-ols by NCBT and CBT were
analyzed by the Exner method; the double logarithmic plot of k2ꢁNCBT† vs.
k2ꢁCBT† is a straight line (Fig. 3, RSP plot; r ˆ 0:998, sd ˆ 0:02) with a slope of
1.00 and an intercept of 0.16. The mean difference (Á) is 0.15. Since the slope b is
not signi®cantly different from unity, the RSP correlation shows indifferent
behavior, i.e., no change in selectivity (RSP(?)).
Fig. 3. RSP plot at 303 K

804
R. Ulagi et al.: Oxidation of Fluoren-9-ols
Conclusions
NCBT and CBT exhibit identical kinetic behavior. Introduction of a nitro group at
position 4 of CBT enhances the reactivity marginally without affecting the
selectivity.
Experimental
Materials
4-Nitro-1-chlorobenzotriazole (NCBT) was prepared by the addition of a cold aqueous solution of
NaOCl to 4-nitrobenzotriazole (obtained by the nitration of benzotriazole [8]) in acetic acid. NCBT
precipitated upon dilution; it was dissolved in CH2Cl2, dried over anhydrous MgSO4, concentrated,
and recrystallized from CH2Cl2-hexane; m.p.: 149ꢂC. The purity was checked by an iodometric assay
(99%). Fluoren-9-ol was obtained by NaBH4 reduction of ¯uoren-9-one and recrystallized from ether-
hexane. 2-Bromo¯uoren-9-ol, 3-bromo¯uoren-9-ol, 2-chloro¯uoren-9-ol, and 2-nitro¯uoren-9-ol were
prepared and recrystallized as reported earlier [2].
Isotopic substitution
Fluoren-9-ol-ꢀ-d was prepared as reported elsewhere [2]. The deuterated alcohol was checked for
1
deuterium incorporation by H NMR and MS and was found to contain ꢃ99% D.
Kinetics
A solution of NCBT in aqueous acetic acid was prepared and standardized iodometrically. The
reaction, under the condition that [¯uoren-9-ols] ꢀ [NCBT], was initiated by the addition of NCBT to
¯uoren-9-ols, both in aqueous acetic acid. The progress of the oxidation was monitored iodo-
metrically for not less than two half-lives, and the pseudo-®rst-order rate constant k0 calculated by the
method of least squares was reproducible within Æ 5%.
Product analysis
Oxidation of ¯uoren-9-ol with NCBT gave the corresponding ketone which was converted into its
2,4-dinitrophenylhydrazone; m.p.: 248ꢂC (Ref. [2]: m.p.: 248ꢂC), yield: 70%. The overall reaction
can be represented as
C6H3O2N4Cl ‡ C13H10O ! C6H4O2N4 ‡ C13H8O ‡ HCl
References
[1] Ganapathy K, Gurumurthy R, Mohan N, Sivagnanam G (1987) Monatsh Chem 118: 583
[2] Kuselan P, Venkatasubramanian N (1982) Indian J Chem 21B: 1095
[3] Klumpp GW (1982) Reactivity in Organic Chemistry. Wiley, New York
[4] Linert W (1994) Chem Soc Rev 23: 429
[5] Linert W, Jameson RF (1989) Chem Soc Rev 18: 477
[6] Karunakaran C, Ismail J, Mudimannan S (1994) React Kinet Catal Lett 53: 191
[7] Exner O (1993) J Chem Soc Perkin Trans 2, 973
[8] Elder®eld RC (1961) Heterocyclic Compounds, vol 7. Wiley, New York
Received October 23, 2000. Accepted (revised) January 22, 2001