Oxidation kinetics of fluorene (Fl) and its halogenated derivatives, namely, 2,7-dichlorofluorene (Fl-Cl), 2,7-dibromofluorene (Fl-Br) and 2,7-diiodofluorene (Fl-I), by permanganate ion in both perchloric and sulfuric acid media have been investigated using conventional spectrophotometric technique. In both acidic media, the reactions manifested first order kineticsin [permanganate] and less than unit order each in [reductants] and [acid]. Increasing ionic strength had no effect on the oxidation rates. Oxidation rates of fluorenes in perchloric acid were higher than those in sulfuric acid and the order of the oxidation rates was: Fl > Fl-I > Fl-Br > Fl-Cl. Final oxidation products were identified by GC/MS and FT-IR analyses in all cases as 9H-fluorenone derivatives. Reaction constants as well as activation parameters of the second order rate constants were also evaluated.
Fluorenes (FLs) are an exclusive family of aromatic hydrocarbons. They are amongst products from the burning of gasoline [
Potassium permanganate is considered as the most powerful multi-electron oxidant employed in the kinetic studies of oxidation of various compounds in different media [
Fluorene and its derivatives, 2,7-dichloroflourene, 2,7-dibromoflourene and 2,7- diiodoflourene were synthesized as reported [
Kinetic runs were carried out under pseudo-first order conditions where the concentration of fluorine derivatives, [S] >> [permanganate]. The reactions tem- perature (25˚C) was controlled within ±0.1˚C and the ionic strength was adjusted to 0.5 mol∙dm−3. Kinetics of the oxidation reactions were followed spectrophotometrically within the UV-Vis spectral range by recording the disappearance of permanganate absorbance with time at λ = 526 nm. These measurements were performed on a thermostatted Shimadzu UV-VIS-NIR-3600 double-beam spectrophotometer. Fluorene derivatives were confirmed by both spectroscopic and analytical tools. NMR was recorded on a Bruker Advance 400 MHz and GC-Mass spectra were recorded on a Shimadzu GCMS-QP1000 EX mass spectrometer at 70 eV. The observed-first order rate constants (kobs) were calculated as slopes of the plots of ln(absorbance) versus time plots, which were straight for about 75% of the oxidation reactions and the such rate constants were reproducible to within 3% - 4%.
Various sets of the reactions mixtures containing different ratios of permanganate to fluorine derivatives were mixed at [H+] = 0.3 and I = 0.5 mol∙dm−3 for about 24 hours. Estimation of the remaining permanganate indicate that stoichiometry of the reactions was 5:4 (fluorine:permanganate) as illustrated by the following equation.
Fluorenederivative 9H-fluorenone derivative
This stoichiometric equation is consistent with products characterization (Head-space GC/MS revealed [M+, 100%] at 180 related to the 9H-fluoren-9-one [M+, 100%] at 249 related to the 2,7-dichloro-9H-fluoren-9-one, [M+, 100%] at 338 related to the 2,7-dibromo-9H-fluoren-9-one and [M+, 100%] at 432 related to the 2,7-diiodo-9H-fluoren-9-one. The mass spectrometry fragmentation pattern for 2,7-dichloro-9H-fluoren-9-one (as an example) showed the following signals: m/z: 247.98 (100.0%), 249.98 (64.1%), 248.98 (14.1%), 251.97 (10.2%), 250.98 (9.1%), 252.98 (1.5%). Furthermore, FT-IR spectra for both 2,7-dichloro- 9H-fluorene (Fl-Cl) and its oxidation product 2,7-dichloro-9H-fluoren-9-one (as an example) are shown in
showed a very strong signal at 1712 cm−1 related to the (C=O) group which is absent in the 2,7-dichloro-9H-fluorene (Fl-Cl). In addition, the finger prints of the product were different from that of the corresponding substrate.
The orders of the oxidation reactions regarding to the reactants concentrations have been evaluated from the plots of log kobs versus log (conc.).
The order with respect to [MnO4−] was investigated by changing its concentration from 1.0 × 10−4 to 8.0 × 10−4 mol∙dm−3 at constant concentrations of other reactants. The order was found to be unity as first order plots were linear for
104 | 103 [S] (mol∙dm−3) | 102 [H+] (mol∙dm−3) | I (mol∙dm−3) | 105 kobs (s−1) | |||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Perchloric acid | Sulfuric acid | ||||||||||
Fl | Fl-Cl | Fl-Br | Fl-I | Fl | Fl-Cl | Fl-Br | Fl-I | ||||
1.0 | 5.0 | 0.2 | 0.5 | 166.2 | 101.2 | 113.9 | 129.2 | 139.3 | 86.2 | 98.7 | 121.0 |
2.0 | 5.0 | 0.2 | 0.5 | 165.7 | 98.3 | 117.4 | 135.6 | 135.7 | 87.9 | 97.8 | 115.2 |
4.0 | 5.0 | 0.2 | 0.5 | 168.1 | 99.7 | 116.9 | 133.1 | 137.2 | 88.0 | 99.3 | 117.6 |
6.0 | 5.0 | 0.2 | 0.5 | 172.1 | 100.7 | 115.2 | 132.2 | 141.1 | 85.9 | 103.2 | 118.1 |
8.0 | 5.0 | 0.2 | 0.5 | 169.3 | 97.3 | 118.2 | 134.1 | 133.2 | 89.2 | 98.2 | 116.2 |
4.0 | 1.0 | 0.2 | 0.5 | 79.8 | 39.2 | 48.4 | 59.7 | 50.1 | 32.8 | 36.3 | 41.7 |
4.0 | 3.0 | 0.2 | 0.5 | 127.3 | 74.1 | 85.3 | 97.2 | 96.9 | 62.3 | 73.1 | 87.2 |
4.0 | 5.0 | 0.2 | 0.5 | 168.1 | 99.7 | 116.9 | 133.1 | 137.2 | 88.0 | 99.3 | 117.6 |
4.0 | 7.0 | 0.2 | 0.5 | 201.0 | 120.2 | 139.9 | 161.0 | 175.9 | 108.7 | 125 | 145.0 |
4.0 | 9.0 | 0.2 | 0.5 | 232.4 | 141.5 | 166.2 | 189.8 | 204.9 | 128.2 | 152 | 175.4 |
4.0 | 5.0 | 0.1 | 0.5 | 105.3 | 63.1 | 70.3 | 81.9 | 81.6 | 48.5 | 55.8 | 68.9 |
4.0 | 5.0 | 0.2 | 0.5 | 139.9 | 84.7 | 95.3 | 111.2 | 112.0 | 71.8 | 83.3 | 95.2 |
4.0 | 5.0 | 0.3 | 0.5 | 168.1 | 99.7 | 116.9 | 133.1 | 137.2 | 88.0 | 99.3 | 117.6 |
4.0 | 5.0 | 0.4 | 0.5 | 195.3 | 115.0 | 134.9 | 158.0 | 159.3 | 102.1 | 115.8 | 134.3 |
4.0 | 5.0 | 0.5 | 0.5 | 216.0 | 124.9 | 148.0 | 181.0 | 185.0 | 113.9 | 132.2 | 155.0 |
4.0 | 5.0 | 0.2 | 0.5 | 168.1 | 99.7 | 116.9 | 133.1 | 137.2 | 88.0 | 99.3 | 117.6 |
4.0 | 5.0 | 0.2 | 0.6 | 169.7 | 98.3 | 119.4 | 135.6 | 135.7 | 87.9 | 101.2 | 115.2 |
4.0 | 5.0 | 0.2 | 0.7 | 171.2 | 102.7 | 116.9 | 134.4 | 139.2 | 91.2 | 99.3 | 121.6 |
4.0 | 5.0 | 0.2 | 0.8 | 172.1 | 105.7 | 121.2 | 137.2 | 141.1 | 85.9 | 103.2 | 118.1 |
4.0 | 5.0 | 0.2 | 0.9 | 167.3 | 97.4 | 118.1 | 134.1 | 139.2 | 87.2 | 104.3 | 122.2 |
Ex Experimental error ±3%.
about 75% of the oxidation reactions. Furthermore, the non-variation of the values of kobs at different initial
The values of kobs were measured at various concentrations of fluorine derivatives (S) at fixed other reactants concentrations. The values of kobs were found to increase with increase [S] as listed in
The orders of reactions with respect to [H+] were investigated by measuring the oxidation rates at various [H+] (0.1 - 0.5 mol∙dm−3) and at fixed other variables. The rate constants were increased as [H+] increased in both acidic media as listed in
At constant concentrations of the reactants and with other conditions constant,
the ionic strength of the reactions media was varied (0.5 - 0.9 mol∙dm?3) using sodium perchlorate in perchloric acid and sodium sulfate in sulfuric acid medium. The results listed in
To calculate the activation parameters, the observed rate constants were measured at four temperatures, namely 288, 298, 308 and 318 K, at fixed other variables. The observed rate constants were found to increase with raising temperature and the activation parameters of the second order rate constants, k2, (k2 = kobs/[S]) were evaluated using Arrhenius and Eyring plots (
Known quantities of acrylonitrile monomer were added to the reactions mixtures in both acidic media and were kept in an inert atmosphere for about 6 hours. When the reactions mixtures were diluted with methanol, progressive white precipitates were formed suggesting intervention of free radicals during these reactions.
It was reported [
Scheme 1. Mechanism of oxidations of fluorenes by permanganate ion in acid media.
Acid | Substrate | DS≠, J∙mol−1 K−1 | DH≠, kJ∙mol−1 | DG≠298, kJ∙mol−1 | Ea≠, kJ∙mol−1 |
---|---|---|---|---|---|
Perchloric | Fl | −123.22 | 30.01 | 66.73 | 28.61 |
Fl-Cl | −133.35 | 26.03 | 65.79 | 26.33 | |
Fl-Br | −149.01 | 29.51 | 73.91 | 29.52 | |
Fl-I | −113.34 | 31.47 | 65.25 | 29.92 | |
Sulfuric | Fl | −148.41 | 26.55 | 70.77 | 36.72 |
Fl-Cl | −131.07 | 23.33 | 62.39 | 33.50 | |
Fl-Br | −134.27 | 25.19 | 65.20 | 37.36 | |
Fl-I | −157.81 | 24.29 | 71.32 | 39.07 |
Experimental error ±4%.
oxidant. This was also evidenced [
In the light of the above aspects, the following reactions mechanism, illustrated in Scheme 1, can be suggested. The mechanism involves reaction of permanganic acid oxidant with fluorene derivative to form a complex (C) and the later decomposes to yield manganate (VI) and fluorene free radical. The free radical is attacked by Mn(VI) forming the secondary alcohol (fluorenol) as an intermediate product and manganese (V) intermediate. In a further fast step, Mn(V) reacts with the intermediate product fluorenol to give the ketone (fluorenone) as the final oxidation product and an intermediate Mn (III) species. This step is followed by other fast steps including reactions of fluorene derivatives with acid permanganate species to form also fluorenone and Mn (III) spe-
cies. The last step is the attack of Mn(III) species on another fluorene molecule to give fluorenone and Mn(II), satisfying the observed stoichiometry.
Owing to the suggested mechanism, rate of oxidation reaction can be expressed by the following rate law:
The relationship between the oxidation rate and the oxidant, substrate and hydrogen ion concentrations is deduced (See Appendix A) to give the following equation:
Under pseudo-first order condition,
From Equations (2) and (3),
Regarding to Equations (4) and (5), plots of 1/kobs versus 1/[S] at constant [H+] and 1/kobs versus 1/[H+] at constant [S] should be linear with positive intercepts on the 1/kobs axes as were obtained,
Acid | Substrate | Constant | ||
---|---|---|---|---|
103 k1, s−1 | 102 K1, dm3∙mol−1 | 10−3 K2, dm3∙mol−1 | ||
Pechloric | Fl | 2.51 | 29.41 | 5.24 |
Fl-Cl | 1.80 | 28.80 | 3.26 | |
Fl-Br | 2.12 | 28.32 | 3.31 | |
Fl-I | 2.43 | 29.53 | 3.18 | |
Sulfuric | Fl | 2.41 | 28.90 | 4.90 |
Fl-Cl | 1.73 | 27.61 | 3.15 | |
Fl-Br | 2.21 | 28.84 | 3.10 | |
Fl-I | 2.34 | 28.53 | 3.34 |
On the other hand, the negative values of entropy of activation (DS≠) listed in
Oxidations of fluorene derivatives by potassium permanganate in acidic medialed to formation of the corresponding ketones (9H-fluorenone derivatives) and the oxidations rate was: Fl > Fl-I > Fl-Br > Fl-Cl. Reaction constants as well as activation parameters were evaluated.
Jassas, R.S., Fawzy, A., Obied, R.J., Abourehab, M.A.S. and Ahmed, S.A. (2017) A Comparative Kinetic Study on the Efficacious Permanganate Oxi- dation of Fluorenes in Perchloric and Sulfuric Acid Media. Open Journal of Physical Chemistry, 7, 35-49. https://doi.org/10.4236/ojpc.2017.72004
From Equations (A1) and (A3),
The total concentration of
Because of [H+] was high,
Similarly
Substituting Equations (A7), (A8) and (A9) into Equation (A4) (and omitting “T” and “F” subscripts) gives:
Under pseudo-first order condition,
From Equations (A10) and (A11),
and with rearrangement of Equation (A12),