Kinetics of Addition of HI to Isobutene and Vinyl Chloride

Abstract

The addition of HI to i‐butene above 200°C proceeds homogeneously and quantitatively to yield i‐butane+I₂ in accordance with the equation $$2\mathrm{HI}\mathrm{+i}\mathrm{-butene}\mathrm{\to i}\mathrm{-butane}+{\mathrm{I}}_2.$$ It is shown that the rate‐determining step is the formation of the intermediate t‐BuI which is present in vanishingly small stationary concentration $$\begin{array}{cc}\mathrm{HI}\mathrm{+i}\mathrm{-butene}\to {\displaystyle {lim}_{k_2}}t\mathrm{-BuI}& (\mathrm{slow}\mathrm{),}\\ \mathrm{\hspace{0.17em}\hspace{0.17em}t}\mathrm{-BuI}+\mathrm{HI}\mathrm{\to i}\mathrm{-BuH}+{\mathrm{I}}_2& (\mathrm{fast}\mathrm{).}\end{array}$$ The second‐order rate constant k₂ is given by (units of liter/mole‐sec) $$\log {\mathrm{k}}_2\text{=6.50−18 000/4.575T.}$$ From the known thermodynamic data the reverse, first‐order rate constant k₁ is given by (units of sec^—1) $$\log k_1\text{=12.52−36 400/4.575T.}$$ Comparisons with similar data for EtI and i‐PrI show that the substitution of Me groups for H lowers the E_act by about 6 kcal/Me group and also the A factor by about a factor of 5. This can be explained on the basis of a loss of internal rotational entropy in the transition state. Qualitative observations indicate that C₂H₃Cl adds HI at a rate intermediate between C₂H₄ and C₃H₆. The principle products are C₂H₅Cl+I₂ above 290°C, but sufficient C₂H₆+HCl appear to vitiate simple kinetic studies. Qualitative observation of the system EtCl+I₂ indicate an I‐atom‐catalyzed elimination of HCl with an E_act∼43 kcal/mole.