KINETICS AND MECHANISMS OF HOMOGENEOUS REACTIONS OF ALKANES IN SULFURIC-ACID IN THE PRESENCE OF CARBOCATIONS .6. DECOMPOSITION OF 2,2,4-TRIMETHYLPENTANE

The possibility of branched ion-chain reactions (BICR) is demonstrated . The product balances and isobutylene transformations are studied for the decomposition of isooctane (R8H) dissolved in 86 - 94 wt % sulfur ic acid, giving isobutane (R4H) and isobutylene. On the basis of recen tly obtained data, the mechanism of BICR with an ''equilibrium'' acid- type catalytic branching is proposed: R8H + R4+ + gammaH+ + (1 - gamma )HL --> (1 + gamma)R4+ + (1 - gamma)R4L + R4H. Isobutylene is found to exist in the completely latent form R4L; the chain carriers (tertbuty l cations R4+) are generated upon R8H oxidation with sulfuric acid; br anchings are caused by increase of [R4+] in the course of the reaction due to rapid and equilibrated protonation R4L + H+ reversible K R4+ HL, the chain termination occurs via the reverse reaction; and the br anching coefficient gamma = Kh0/(1 + Kh0), depending upon medium acidi ty h0. The kinetic model of BICR is developed under the conditions of equilibrium distribution of R8H and R4H between the gaseous phase and the solution in the reactor, which corresponds, in a first approximati on, to the experimental data (the existence of an induction period, in itiation of the reaction by sources of carbocations, S-shaped kinetics of isobutane accumulation and isooctane consumption, and the effect o f medium acidity). It is shown that at a conversion greater than 50%, the role of side reactions, square in R4L and responsible for C5 - C-7 alkane formation, becomes substantial; at Kh0 much greater than 1, th e limiting value of gamma = gamma(lim) is equal to 1. The possibility of acidic decomposition of HC(CH3)3-n(neo-C5H11)n hydrocarbons, the an alogs of R8H (n = 1), according to the BICR mechanism with gamma(lim) = n (where n = 2 or 3), is considered. In general, BICR with ''nonequi librium'' branchings are likely. The resemblance and distinctions betw een BICR with a branched radical-chain, a nonbranched ion-chain, and a utocatalytic reactions are considered.