On the Mechanism of Dehalogenation of Organic Dihalogen Derivatives by Means of Iodide Salts

B. V. Tronov, L. A. Pershina

In manuals on the analysis of organic compounds [1], among the methods for studying organic halogen derivatives, the action of sodium iodide or potassium iodide is indicated. This method makes it possible to determine the position of the halogen atom in certain halogen derivatives. If chlorine or bromine atoms are located at neighboring carbon atoms, then under the action of iodide salts the corresponding iodine derivatives are formed, followed by the liberation of iodine with the formation of a double bond between the carbon atoms.

At the same time it is known that compounds with two iodine atoms at neighboring carbon atoms can quite well exist. For example, 1,2-diiodoethane with \(t_{\text{m.p.}}\) 81–82°; 1,2-diiodopropane; diiodo-1,2-propyl alcohol with \(t_{\text{m.p.}}\) 43° [2].

Thus, in the reaction of dihalogen derivatives with iodide salts, either direct formation of a double bond must occur in the reaction process itself, or iodine must be split off from an intermediately formed diiodide.

There are indications in the literature that iodine can be split off under the influence of the iodide ion of iodide salts [3, 4]. However, we have found no experimental evidence for this proposition.

In the present work the aim was set to test by direct experiment the possibility of iodine splitting off under the influence of iodide salts.

Experiments carried out with diiodo-1,2-propyl alcohol showed that diiodopropyl alcohol itself is not a very stable compound, which in alcoholic solution decomposes over the course of several days, liberating iodine.

The addition of sodium iodide accelerates the decomposition hundreds of times; moreover, the liberated iodine molecule with the iodide ion gives the complex anion \(J_3\). Evidently the energy of formation of this anion is what causes the splitting off of iodine from diiodo-organic compounds.

Experimental Part

As the object of investigation we chose allyl alcohol, which quite readily adds iodine in violet solutions. The iodination product, 1,2-diiodopropyl alcohol, has \(t_{\text{m.p.}} = 43^\circ\) after purification with alkali and washing with water. We introduce small changes into the procedure for obtaining 1,2-diiodopropyl alcohol: the reaction

proceeds best in a solvent (chloroform) without formation of oxidation products and their polymerization.

Deiodination of the product obtained was carried out in an ethyl alcohol solution at \(t = 25^\circ\mathrm{C}\) with various amounts of sodium iodide.

The results of the experiments are given in Table 1.

Table 1

Deiodination in the presence of the ready-made \( \mathrm{NaJ}_3 \) complex (Table 2) and in acetone solution (Table 3) was also studied.

Table 2

Table 3

Conclusions

1. The decomposition of diiodopropyl alcohol is greatly accelerated in the presence of sodium iodide.

2. The following reaction mechanism is proposed:

\[ \mathrm{CH_2J - CHJ - CH_2OH + NaJ \rightarrow CH_2 = CH - CH_2OH + NaJ_3} \]

The energy of formation of \(\mathrm{NaJ_3}\) is the reason for the cleavage of iodine from organoiodine compounds. The formation of polyiodide ions \(J'_5\), \(J'_7\), and \(J'_9\) is not of great significance here.

1. In acetone solution, in the presence of \(\mathrm{NaJ}\), the reaction proceeds considerably faster.

The \(\mathrm{NaJ_3}\) complex weakly accelerates the iodine-cleavage reaction.

References

1. R. Shriner and R. Fuson. Systematic Qualitative Analysis of Organic Compounds, IL, 1950.

2. R. Dillon, J. Amer. Chem. Soc., 54, 932, 1932.

3. R. Dillon, J. William, Ioung, I. Hovard, J. Amer. Chem. Soc., 52, 1953, 1930.

4. A. Slator, J. Chem. Soc., 20, 221, 1904.

5. H. Finkelstein, Ber. Deutsch. Chem. Gesellschaft, 43, 1528, 1910.