The mechanism of thermal eliminations. Part 25. Arrhenius data for pyrolysis of isochroman-3-one, benzyl methyl ether, 2-hydroxyethylbenzene, phenyl acetate, and 3,4-dihydro-2H-pyran

Abstract

The thermal decomposition of isochroman-3-one to benzocyclobutene and carbon dioxide, of benzyl methyl ether and 2-hydroxyethylbenzene to toluene and formaldehyde, and of phenyl acetate to phenol and ketene are each first-order unimolecular reactions. The Arrhenius parameters [E/kJ mol^–1, log (A/s^–1)] are 221.1, 14.305; 231.7, 12.92; 200.5, 11.21; and 221.8, 11.78, respectively. The frequency factors indicate that each reaction involves a cyclic semi-concerted mechanism; for phenyl acetate this is a four-centre process analogous to that involved in the formation of ketene from pyrolysis of either acetic acid, acetyl bromide, or acetamide. The higher log (A/s^–1) value for isochroman-3-one is typical of elimination from cyclic compounds, because their ground states are already constrained into the geometry required in the transition state. Chroman-1-one (dihydrocoumarin) is very much more stable than isochroman-3-one (and on decomposition gives mainly coumarin and benzo[b]furan, evidently due to the favourable gain in aromaticity). Decomposition of 3,4-dihydro-2H-pyran into ethene and propenal takes place according to the rate equation log k= 13.556 – 209.01/19.142 K, the rate coefficients being lower than the literature values which may therefore have been affected by surface catalysis. A 2-methyl group in the pyran produces a ca. 4.5-fold rate acceleration, very similar to that produced in the analogous vinyl ethers. The relationship between the elimination rates for a range of cyclic compounds and their open-chain counterparts is examined and it is found that their reactivities are closely similar. Hydrogen is shown to be transferred as an incipient anion in allyl ether pyrolysis, whereas in most eliminations it is transferred as an incipient cation.