This is a general revision of the kinetics of iodine addition, w i t h a note on the equilibria in The rate of iodine addition over the concentration range that can be measured these reactions. is given by the expression the first term being chiefly operative in solvents such a s chlorobenzene, carbon tetrachloride, and carbon disulphide, and the second term in isobutyl ether, acetic acid, and nitrobenzene solutions. With certain aromatic compounds such as styrene and its derivatives iodine addition is accompanied by polymensation.AN earlier investigation on the kinetics of iodine addition (Bythell and Robertson, I., 1938, 173) established that these reactions, like the corresponding bromine and iodine chloride additions, showed third-order kinetics, with a small temperature coefficient in acetic acid and nitrobenzene solutions. Erroneous theoretical conclusions, however, were made from the measurements in carbon tetrachloride solution, chiefly as a result of confining the observations to a single compound, ally1 alcohol, the behaviour of which is now found to be anomalous. In the meantime several communications on the kinetics of iodine addition have appeared. Bhattacharyya and Rao ( J . Indian Chem. Soc., 1941, 18, 253), using 2-pentene and phenylacetylene as reactants, found third-order kinetics for iodine addition in acetic acid and ethyl alcohol solutions. Ghosh et al. (ibid., p. 245) state that iodine addition in carbon tetrachloride, carbon disulphide, and benzene solutions proceeds by a fourth-order mechanism. These observers, following Groh and Szelestey (2. anmg. Chem., 1927, 162, 333), who had previously investigated erucic acid in carbon tetrachloride and carbon disulphide and found f ourth-order kinetics, employed an elaborate formula, derived from the integration of the expression, -d[IJ/dt = k,[A][IJ3 -h,[AII][I2]2, and obtaining constant K, coefficients for p-amylene and pinene, concluded that the reaction proceeds, A + I,We, on the other hand, have followed the simpler procedure of adjusting the concentrations with one reactant A in excess, to reduce the effect of the reverse reaction, and have evaluated the reaction order by measurements a t different initial concentrations. To obtain the relative contributions of A and I, to the total reaction order, use was made of the formula : A11 +-21,. t,/t, = (c,/c,)"-* (G'"")":where t , and t, are the times of an equal fractional change of iodine addition, with concentrations cI and c2 for I,, and c' and c" for A, n being the order for I, and n' for A. The proposed formula holds only for the initial stages of the reaction, to -10% iodine addition, over which range, as is found experimentally, the yo addition-time curves are approximately straight lines.Employing this formula, we have established that in polar solvents, e.g., acetic acid, isobutyl ether, chloroform, chlorobenzene, and nitrobenzene, the reaction order for the organic compound (A) is unity ; in the non-polar solvents carbon tetrachloride and carbon disulphide, on the other hand, the v...
