For flash vacuum pyrolysis experiments on a preparative scale the apparatus shown in Figure 1 is used.² It consists of a vertically-mounted assembly of fused silica and Pyrex tubing 3.2 cm in diameter. The high temperature section A, 45 cm in length, is made from fused silica. It carries inner 29/32 ground glass joints. The upper evaporation section B, 25 cm in length, is made of Pyrex. It is fitted with two outer 29/32 ground glass joints (Note 1). Both tubes are loosely filled with Raschig rings cut from thin-walled, fused silica tubing 1 cm in diameter. The evaporation zone is heated by glass fiber heating tape (180 × 1 cm) controlled by a variac transformer. Heat for the pyrolysis section is provided by an electric tube furnace (Note 2). The temperature is measured with a thermocouple fixed on the surface of the silica tube in the middle of the hot reaction zone. The substance to be pyrolyzed is added to the evaporator under reduced pressure and at a constant drop rate from a 250-mL, pressure-equalizing dropping funnel C (Note 3). A 250-mL, long-necked, round-bottomed flask D cooled to −78°C serves as a receiver. (Caution! Flask D (Note 4) must be checked by visual inspection at regular intervals. If condensed product accumulates in the entrance neck, the cooling bath level must be lowered to prevent clogging. Flask D must always be cooled to the neck of the flask. Otherwise material might be transferred from the receiver into trap E, which might then become plugged). Flask D is connected by a wide glass tube (Note 4) to a cooling trap E kept at liquid nitrogen temperature (Note 5).

At a pyrolysis oven temperature of 630°C, an evaporator temperature of 200°C, and a pressure of 12 mm, 2-toluic acid chloride (240 g, 1.55 mol) (Note 6) is added in about 2.5 hr at a constant rate of 40–60 drops/min (Note 7). The pressure should be monitored in the addition funnel C during the pyrolysis. A sudden rise in pressure indicates that a restriction is building up between the pyrolysis reactor and the pump. In this case addition of reactant must be stopped. Ten minutes after the addition is complete the apparatus is flushed with nitrogen. The dark brown, oily condensate in the receiver is dissolved in 500 mL of acetone and diluted with 250 mL of cold water (Note 8) and (Note 9). The homogeneous solution is stirred at room temperature for 1 hr. It then is separated into two phases by the addition of 500 mL of 2 N sodium carbonate solution and 250 mL of ether. The aqueous layers are extracted twice with 250-mL portions of ether. The ether extracts are washed with 2 N sodium carbonate solution and with saturated sodium chloride solution. The combined ether layers are dried over sodium sulfate. Removal of the solvent and distillation of the residue yields 65 g of a yellow oil, bp 87–90°C/13 mm, with the following composition: Benzocyclobutenone (97%), benzyl chloride (3%) (Note 10). Fractionation at 12 mm through a 25-cm spinning band column (Note 11) gives 58 g of benzocyclobutenone (>99%) as a colorless oil, bp 84–86°C/12 mm (Note 12) and (Note 13). The product crystallizes at 0°C and then melts at 10–15°C (Note 14).

Most of the acetone is removed from the combined aqueous layers by distillation. Upon acidification with concentrated hydrochloric acid a thick precipitate forms. Filtration and drying leads to 106 g of almost colorless 2-toluic acid, mp 103–107°C. This material is of sufficient purity for conversion into the acid chloride and for subsequent flash vacuum pyrolysis.