Thermal decomposition of isopropanol

Abstract

The pyrolysis of isopropanol has been investigated at temperatures in the range 721–801 K and pressures between 10 and 100 Torr, using a KCl-coated silica reaction vessel. Under these conditions the reactions occurring were essentially homogeneous and, for reaction times corresponding to < 30 % conversion, led to the formation of hydrogen and acetone, together with smaller amounts of water and propene; traces of methane, ethane, ethylene, and acetaldehyde were also detected. Product against time curves showed a marked fall-off in rates of formation of hydrogen, acetone, water and propene with time. Both the former and the latter pair of products were formed at similar initial rates and by reactions which were first order with respect to isopropanol.

Rate constants for the formation of hydrogen and acetone yielded the expression log(k_H2/s^–1)=(14.0 ± 0.6)–(57 700 ± 2000)/θ and for propene and water, the expression log(k_C3H6/s^–1)=(13.1 ± 1.0)–(58 200 ± 3600)/θ where θ= 2.303 RT/cal mol^–1(1 cal = 4.18 J).

These results together with earlier observations can be satisfactorily interpreted on the basis of a free radical chain mechanism initiated by the splitting of a C—C bond in isopropanol, and yielding equal amounts of hydrogen and acetone and of propane and water in the propagating steps.

A comparison of the results of this investigation with those obtained previously from the decomposition of 3-hydroxybut-1-ene strongly suggests that in the latter, the principal products butadiene and water are also formed by a free radical chain mechanism rather than a molecular process.