Thermolysis of α-pinene in supercritical lower alcohols

Abstract

Thermal isomerization of α pinene in supercritical solvents, viz., ethanol, methanol, and propan 1 ol, was carried out, and differences in the rate and selectivity of the process were revealed. In supercritical ethanol the reaction rate increases sharply and the selectivity remains unchanged with an increase in the temperature (from 290 to 390 °C) or pressure (from 90 to 270 atm). The main reaction products are limonene, isomeric alloocimenes, and pyronenes. The selectivity for limonene in propan 1 ol is higher than in other alcohols when the conver sion of α pinene not higher than 50%. In supercritical ethanol (430 °C, 120 atm, 140 s) limonene is more stable than α pinene (conversion 8%).Organic synthesis in supercritical media is a modern, dynamically developing branch of organic chemistry. 1-4 However, transformations of terpenoid hydrocarbons and their functional derivatives in supercritical media remain virtually unstudied. Only single examples for the catalytic hydrogenation of terpenoids in supercritical CO 2 5 and subcritical water 6 are known, as well as the esterification of some terpenoid alcohols. 7The purpose of the present work is to study transfor mations of α pinene in supercritical alcohols (methanol, ethanol, and propan 1 ol) and reveal a relationship of the temperature and pressure to the reaction rate and the influence of the residence time on the conversion. Experimental(+) α Pinene (>98%) and (+) limonene (>97%) were pur chased from Aldrich.Thermal isomerization under supercritical conditions was carried out in a laboratory setup using a tubular flow type reac tor 6 m long and 1.75 mm in diameter (the reactor volume was about 14 cm 3 ). A solution of the starting substance was supplied by a piston pump to the reactor from which the reaction mixture was transferred to a heat exchanger for subsequent cooling and further to a separator where samples for analysis were taken.The composition of the reaction products was analyzed by the GC MS method on a Hewlett Packard 5890/II gas chro matograph with an HP MSD 5971 quadrupole mass spectrom eter as a detector. An НР 5 quartz column (30 m × 0.25 mm) was used (copolymer of 5% diphenyl and 95% dimethylsiloxane as the stationary phase, the film thickness 0.25 µm); the carrier gas was helium with a constant flow rate of 1 mL min -1 ; the injector temperature was 280 °C; temperature program ming: 50 °C (2 min), 50-200 °C (4 °C min -1 ), 200-300 °C (20 °C min -1 ), 300 °C (20 min); the energy of ionizing electrons was 70 eV. Qualitative analysis was performed comparing reten tion indices of components and their complete mass spectra with the corresponding data for pure compounds if any and with the data of the Wiley7 library of mass spectrometric data (375000 mass spectra) and catalogue. 8 The percentage composi tion of mixtures was calculated from the surface areas of chro matographic peaks using no correction coefficients. Results and DiscussionThe behavior of α pinene in supercritical 96% ethanol was studied at T = 290-390 °C and P = 90-270 atm, which ...

Environmentally Benign Transformations of Monoterpenes and Monoterpenoids in Supercritical Fluids

“…Regardless of an alcohol used, limonene 3 turned out to be the major product of a-pinene 1 isomerization at 573 K and 100 atm (Scheme 1); smaller amounts of isomeric alloocimenes 4 and pyronenes 5 and 6 were also formed [19]. In general, the distribution of isomerization products was in close agreement with the identical reaction in the gas phase [21], but the transformations occurred considerably faster.…”

“…When transformations of a-pinene 1 were conducted in supercritical 96% ethanol under constant pressure, a rise in temperature from 563 to 663 K caused an increase in conversion from 17 to 99% [19]. The highest content of limonene 3 and alloocimenes 4 in the reaction mixture was observed at 623 K and was equal to 53 and 35%, respectively; whereas as one would expect, the highest content of a-and b-pyronenes 5 and 6 formed from alloocimenes 4 was reached at the highest temperatures and was equal to 26% at 663 K. An increase in pressure from 90 to 270 atm at constant temperature induced an abrupt rise in conversion of a-pinene 1 without significantly affecting the selectivity.…”

“…b-Pinene 2 turned out to be more thermally stable as compared to a-pinene 1; significant degree of its conversion upon thermolysis in supercritical ethanol was observed only at temperatures above 633 K [16]. The major isomerization products of b-pinene 2 included b-myrcene 7, limonene 3, and its isomer para-menth-1(7),8-diene 8 (Scheme 2).…”

“…The authors [16] studied the enantiomeric purity of the starting pinenes and limonene 3 formed using these monoterpenes showed that the transformation of apinene 1 into limonene 3 is accompanied by complete racemization; whereas thermolysis of b-pinene 2 into limonene 3 is a stereospecific process leading to an optically active product.…”

“…It is quite important that no formation of the products corresponding to the interaction between alcohols used as solvents and monoterpenes was observed regardless of the reaction temperature and pressure [16].…”