Kinetics of Direct Iodination of Fluorene-5-carboxylic Acid in the Presence of a Sulfuric–Nitric Acid Mixture

A. N. Novikov, V. T. Slyusarchuk

(Presented by the scientific-methodological seminar of KhTF)

B. V. Tronov and A. N. Novikov [1–4] proposed a method for the direct iodination of aromatic compounds in the presence of a sulfuric–nitric acid mixture. They obtained iodo derivatives of aromatic compounds containing various functional groups in the nucleus, which had previously been considered difficult to access. The method is simple, inexpensive, single-stage, and is becoming widespread. For its further improvement it is necessary to study the reaction mechanism. The latter is impossible without establishing the kinetic regularities of the reaction.

This article presents the results of a study of the kinetics of the reaction of direct iodination of fluorene-5-carboxylic acid in the presence of a sulfuric–nitric acid mixture.

Experimental Part

In studying the kinetics, the sampling method was used. The reaction rate was monitored by the change in iodine concentration in the sample. The reaction in the sample was stopped by diluting it threefold with water. Iodine was extracted from the sample with carbon tetrachloride according to the developed procedure. The iodine concentration in carbon tetrachloride was determined from the measured value of optical density using a calibration curve constructed from a series of standard solutions.

The partial order and the rate constant with respect to iodine \((K')\) were determined by the method of fitting the equation. The initial amounts of the starting substances were: 25 ml of a 0.05 mol/l acetic-acid solution of iodine, 25 ml of a 0.1 mol/l acetic-acid solution of fluorene-5-carboxylic acid, 0.01 mole of sulfuric acid, and 0.002 mole of nitric acid. The temperature in the thermostat was 45°C. Samples were taken every 30 min, 5 ml each.

After dilution, iodine was extracted from the sample with two portions of carbon tetrachloride of 10 ml each, shaking vigorously for 2 minutes. The extracts were transferred to a 25 ml volumetric flask, brought to the mark with carbon tetrachloride, and the optical density was determined. Good agreement of the experimental results with the first-order equation was observed. Figure 1 presents the dependence of \(\ln \dfrac{a}{a-x}\) on time. The numerical value of \(K_I\) was determined from the kinetic equation in integral form for first order:

\[ \ln \frac{a}{a-x} = Kt, \]

$K' = 4.52 \cdot 10^{-3}\ \text{min}^{-1}$. At different times the value of $K'$ is well preserved. But when the initial concentrations were changed, nonconstancy of $K'$ was observed. Consequently, $K'$ depends on the ratio of the initial concentrations of iodine and the sulfuric–nitric acid mixture. To eliminate the influence of other components participating in the reaction, we used Ostwald’s excess-dilution method, taking all reagents except iodine in considerable excess. In this case it proved possible to exclude sulfuric acid from the reaction mixture. In an excess of nitric acid the

Fig. 1. Dependence of $\ln \dfrac{a}{a-x}$ on time

reaction proceeded at a conveniently measurable rate. To determine $K$ we used the half-conversion time ($\eta_{1/2}$). For a first-order reaction there is a relationship between $\eta_{1/2}$ and the rate constant

\[ \eta_{1/2}=\frac{0.693}{K}. \tag{2} \]

As is seen from equation (2), $\eta_{1/2}$ does not depend on the initial concentration of iodine. We carried out experiments with different initial iodine concentrations: $0.02\ \text{mol/l}$, $0.01\ \text{mol/l}$, $0.015\ \text{mol/l}$. From the plot of iodine concentration versus time (Fig. 2), $\eta_{1/2}$ was determined and, by formula (2), $K_1 = 5.66 \cdot 10^{-3}\ \text{min}^{-1}$ was calculated. As is seen from Fig. 2, $\eta_{1/2}$ does not depend on the initial iodine concentration. This is true for a first-order reaction.

To determine the dependence of the reaction rate on the concentrations of fluorene carboxylic acid and nitric acid, the isolation method was used. As applied to the reaction under study, one may write:

\[ I_2^{\,n_1}+\phi\text{kk}^{\,n_2}+\mathrm{HNO_3}^{\,n_3}\rightarrow \text{reaction products}; \]

\[ \phi\text{kk}—\text{fluorenecarboxylic acid}. \]

\[ -\frac{d I_2}{dt}=k[I_2]^{n_1}\cdot[\phi\text{kk}]^{n_2}\cdot[\mathrm{HNO_3}]^{n_3}. \]

When

\[ [\phi\text{kk}]_0 \gg [I_2]_0\ \text{and}\ [\mathrm{HNO_3}]_0 \gg [I_2]_0, \]

Fig. 2. Dependence of \(\eta^{1}_{1/2}\) on the initial iodine concentration

then

\[ -\frac{d I_2}{dt}=K_I [I_2]^{n_1}, \quad \text{where} \quad K_I=K\cdot[\mathrm{FCA}]_0^{\,n_2}\cdot[\mathrm{HNO}_3]^{n_3}. \]

Taking the logarithm of the last equation gives:

\[ \lg K_I=\lg K+n_2\lg[\mathrm{FCA}]_0+n_3[\mathrm{HNO}_3]_0. \tag{3} \]

The zero in brackets denotes the excess initial concentration. Varying \([\mathrm{FCA}]_0\) over a wide interval, we observed the constancy of \(K_I=5.66\cdot10^{-3}\ \mathrm{min}^{-1}\). At all values of \([\mathrm{FCA}]_0\), the half-conversion period is equal to 120 minutes, i.e., under the conditions of this method the reaction rate does not depend on the concentration of fluorene-5-carboxylic acid. Consequently, the order with respect to fluorene-5-carboxylic acid is zero, and in equation (3) the effect of the term \(n_2\ln[\mathrm{FCA}]_0\) is eliminated; then:

\[ \lg K_I=\lg K+n_3\lg[\mathrm{HNO}_3]_0. \tag{4} \]

On the graph of the dependence of \(\lg K_I\) on \(\lg[\mathrm{HNO}_3]_0\) (Fig. 3), the slope of the experimental curve to the abscissa axis is equal to \(n_3\), and the intercept cut off on the ordinate axis is \(\lg K\), where \(K\) is the reaction rate constant. According to the data of the graphical dependence, \(n_3=1.75\); \(\lg K=-2\).

The reaction rate does not depend on the concentration of the aromatic compound, i.e., the rate-limiting stage of the process is such that the aromatic compound does not participate in it:

\[ v=K[I_2]\cdot[\mathrm{HNO}_3]^{1.75}. \tag{5} \]

From equation (5) it is evident that the rate-limiting stage involves the interaction of iodine and nitric acid with the formation of the iodinating agent.

Fig. 3. Dependence of $\lg K_I$ on $\lg [\mathrm{HNO_3}]_0$

Conclusions

1. The partial reaction orders for the direct iodination of fluorene-5-carboxylic acid have been determined with respect to iodine (first), fluorene-carboxylic acid (zero), and nitric acid (1.75).

2. A kinetic equation has been derived, according to which the rate-limiting stage of the process is described by the kinetic equation:

\[ v = K[I_2]\cdot[\mathrm{HNO_3}]^{1.75}. \]

Literature

1. B. V. Tronov, A. N. Novikov. Izv. VSh, 3, 872, (1960).
2. A. N. Novikov, T. A. Khalimova. Author’s certificate 1162518, Bull. Invent., 10, (1964).
3. A. N. Novikov, T. A. Khalimova. ZhVKhO im. Mendeleeva, 7, 234, (1962).
4. V. T. Slyusarchuk, A. N. Novikov. ZhOrKh, 3, 7, 1323, (1967).