The retro‐aldol mechanism in the pyrolysis kinetics of primary, secondary, and tertiary β‐hydroxy ketones in the gas phase

Abstract

The pyrolysis kinetics of primary, secondary, and tertiary p-hydroxy ketones have been studied in static seasoned vessels over the pressure range of 21-152 torr and the temperature range of 190"-260°C. These eliminations are homogeneous, unimolecular, and follow a first-order rate law. The rate coefficients are expressed by the following equations: for l-hydroxy-3-butanone, log k l ( s -' ) = (12.18 2 0.39) -(150.0 ? 3.9) k J mol-' (2.303RT)-'; for 4-hydroxy-2-pentanone, log k l ( s -' ) = (11.64 ? 0.28) -(142.1 ? 2.7) k J mol-' (2.303RT)-'; and for 4-hydroxy-4-methyl-2-pentanone, log k 1 k -l ) = (11.36 ? 0.52) -(133.4 -t 4.9) k J mol-' (2.303RT)-'. The acid nature of the hydroxyl hydrogen is not determinant in rate enhancement, but important in assistance during elimination. However, methyl substitution at the hydroxyl carbon causes a small but significant increase in rates and, thus, appears to be the limiting factor in a retroaldol type of mechanism in these decompositions. 0

A computational study of the thermolysis of β‐hydroxy ketones in gas phase and in m‐xylene solution

“…Experimental and calculated values of the activation parameters and the rate constants are collected in Table . There is a reasonable agreement between the experimental and the calculated values, the tertiary : secondary : primary alcohol rate constant ratio being calculated, at T = 503.15 K, as 5.9:4.7:1.0 in m ‐xylene solution and 44.1:5.0:1.0 in the gas phase, compared with the experimental values, 3.7:1.3:1.0 and 13.5:3.2:1.0, respectively.…”

“…, the lowest barrier in gas phase and in m ‐xylene solution corresponds to the decomposition of III (tertiary alcohol). These results agree with the experimental evidence (the small effect of the methyl group at C 4 ) …”

“…Similar results were obtained for the thermal decomposition of β‐hydroxy ketones in dilute m ‐xylene solution . A small effect of the methyl group was observed in both cases, the tertiary : secondary : primary alcohol rate constant ratio being calculated, at T = 503.15 K, as 3.7:1.3:1.0 in m ‐xylene solution, and 13.5:3.2:1.0 in the gas phase …”

“…b-Hydroxy ketones decompose thermally [1][2][3] to give mixtures of aldehydes and/or ketones in a reaction, which is the reverse of the aldol condensation ( Fig. 1).…”

Experimental and computational thermochemical study of 3-hydroxypropanenitrile

“…Experimental and/or theoretical studies of the thermolysis reaction of b-hydroxy derivatives [4][5][6][7][8][9][10][11][12][13][14][15], and specially b-hydroxynitriles [5,9] have been carried out for some of us. In a recent work [16], we have published the results obtained from a calorimetric and computational study of the thermochemistry of 3-buten-1-ol and 3-butyn-1-ol concluding that the weak intramolecular hydrogen bond from the OH hydrogen to the p-bond charge cloud that exists in both compounds has not significant influence on the enthalpy of formation of these species.…”