Orientation in the addition of HD molecules to buta-1,3-diene over ZnO catalyst. A method of judging heterolytic and homolytic dissociation of hydrogen in catalysis

Abstract

Selective 1,2-addition of HD molecules to butadiene on ZnO catalyst yielded 75% of [3-²H₁]but-l-ene and 25% of [4-²H₁]but-1-ene. Although the adsorption of HD on ZnO prefers [graphic omitted] conformation at room temperature and the reverse conformation [graphic omitted] at –40 °C, reaction of HD at these two temperatures gave an identical ratio of [3-²H₁]/[4-²H₁]= 3. The dissociation of HD is in general expressed by [graphic omitted] In the case of homolytic dissociation H(I) and H(II) or D(I) and D(II) should be identical, i.e. β= 1, but in heterolytic dissociation it may not be unity. Provided that the hydrogen molecular identity is maintained in the hydrogenated products, the following equations were derived for n- and sec-butenyl intermediates: [3-²H₁]/[4-²H₁]=[²H₀](1 +αβ)/[²H₂](α+β) for sec-butenyl and [3-²H₁]/[4-²H₁]=[²H₂](α+β)/[²H₀](1 +αβ) for n-butenyl, where α is a relative rate constant of the reaction of H(I) and H(II) or D(I) and D(II) with butadiene. Accordingly, in homolytic dissociation the simple relations [3-²H₁]/[4-²H₁]=[²H₀]/[²H₂] or [²H₂]/[²H₀] should be established for homolytic dissociation, because α= 1 and β= 1, whereas [3-²H₁]/[4-²H₁] will be equal to neither [¹H₀]/[²H₂] nor [²H₂]/[²H₀] in heterolytic dissociation. It was confirmed that the hydrogenation on MoS₂ is brought about by homolytic dissociation but that on ZnO it is caused by heterolytic dissociation. A plausible mechanism assuming π-allyl anionic intermediates, [graphic omitted] was proposed and it is exlainable in terms of first-order kinetics in hydrogen pressure and the temperature-independent orientation in the 1,2-addition of HD.