Phthalic anhydrides pyrolysis

Cyclohexadiene has been prepared by dehydration of cyclohexen-3-ol,3 by pyrolysis at 540° of the diacetate of cyclohexane-1,2-diol,4 by dehydrobromination with quinoline of 3-hromocyclohexene,6 by treating the ethyl ether of cyclohexen-3-ol with potassium bisulfatc,6 7 by heating cyclohexene oxide with phthalic anhydride,8 by treating cyclohexane-1,2-diol with concentrated sulfuric acid,9 by treatment of 1,2-dibromocyclo-hexane with tributylamine,10 with sodium hydroxide in ethylene glycol,10 and with quinoline,6 and by treatment of 3,6-dibromo-cyclohexene with sodium.6... 

The pyrolysis of a number of compounds at temperatures around 600— 800° and at pressures of the order of 10 2 mm. has been shown to give rise to benzyne. These compounds include for example indanetrione 29>, and phthalic anhydride 30 38>. The dimerisation of benzyne to yield biphenylene has been used preparatively 31 33>, an(j the pyrolysis of tetrafluorophthalic anhydride 34>, and tetrachlorophthalic anhydride 3i-33) gave the corresponding octahalobiphenylenes. In the case of the pyrolysis of tetrachlorophthalic anhydride some hexachlorobenzene is also formed, while the pyrolysis of tetrabromophthalic anhydride results in the formation of hexabromobenzene but no octabromobiphenylene. The disproportionation of tetrabromobenzyne to form carbon and bromine is a function of the high temperature involved and, as we shall see later, both tetrabromo- and tetraiodo-benzyne behave normally in solution. 

Pyrolysis of diethyl phthalate in a flow reactor at 700 °C yielded the following products ethanol, ethylene, benzene, naphthalene, phthalic anhydride, and 2-phenylnaphthalene (Bove and Arrigo, 1985). 

Pyrolysis of phthalic anhydride in the presence of polyvinyl chloride at 600 °C for 10 min gave the following compounds biphenyl, fluorene, benzophenone, 9-fluorenone, o-terphenyl, 9-phen-ylfluorene, and three unidentified compounds (Bove and Dalven, 1984). 

In a study of the formation and reactions of arynes at high temperatures. Fields and Meyerson plyrolyzed a thiophene solution of phthalic anhydride at 690° by mass spectrometry and gas chromatography they found benzene, naphthalene, benzo[b]thiophene, phenylthiophenes, bithienyls, thienothiophene 1, and naphthothiophene in the pyrolysis products. Pyrolysis of thiophene itself produced benzo-[Z lthiophene, thienothiophene 1, phenylthiophene, and bithienyl. The... 

Accordingly, a 0-1 molar solution of phthalic anhydride in benzene (100 ml.) was pyrolyzed at 690° under nitrogen at a steady rate of 30 ml/hr (Fields and Meyerson, 1965). The pyrolysis tube was Vycor, filled with Vycor beads contact time was 11 5 sec, which gave a 58% conversion of the phthalic anhydride. Acetylene was steadily evolved, along with carbon dioxide and carbon monoxide in a 1 1 ratio these were identified in the gas stream by mass-spectral analysis of samples taken at regular intervals. The benzene was distilled off and the products boiling over 180° (2 g) were analyzed by mass spectrometry on a Consolidated Model... 

In addition to naphthalene and biphenyl, about 1 % each of biphenylene and triphenylene was found in the hydrocarbon portion of the phthalic anhydride pyrolysate. Neither was found in the benzene pyrolysis. These results indicated that phthalic anhydride gave benzyne on pyrolysis, and the benzyne subsequently reacted with benzene ... 

Following the discovery of benzyne and biphenylene formation by pyrolysis of phthalic anhydride (Fields and Meyerson, 1965), two separate groups announced isolation of substituted biphenylenes by pyrolysis of substituted phthalic anhydrides. Brown et al. (1966) obtained chloro-and methyl-substituted biphenylenes by pyrolysis at 700-720° and 0-1 to 1 5 torr. The yields were (Brown et al., 1967) ... 

To confirm reaction scheme (4) for the formation of naphthalene and biphenyl, phthalic anhydride was allowed to react with benzene-dx (Fields and Meyerson, 1966c). Barring an appreciable isotope effect, two-thirds of the naphthalene should contain a deuterium atom and one-third only protium. Biphenyl arises in two ways insertion of benzyne into a C—H bond of benzene and pyrolysis of benzene. Biphenyl from benzyne insertion should form with retention of the deuterium atom biphenyl from benzene pyrolysis should—again ignoring any isotope effect—be d0, dlt and dz on the statistical basis of losing two, one, or zero deuterium atoms from a total of twelve protiums and deuteriums in the over-all reaction of two benzene molecules. The amount of biphenyl-... 

Produots from Pyrolysis of Phthalic Anhydride in Benzene-dj... 

The reaotion of benzyne from the pyrolysis of phthalic anhydride with chlorobenzene was expected to give chlorobiphenyls by insertion and naphthalene and chloronaphthalenes by 1,2- and 1,4-addition and rearomatization with respective loss of chloroacetylene and acetylene. 

With the discovery of benzyne formation by pyrolysis of phthalic anhydride, a new field was opened for the investigation of aryne reactions at high temperatures. A first concern was to determine the generality of aryne formation from aromatic acid anhydrides. Such syntheses could be of considerable significance because of the enormous quantities of aromatic mono- and polyanhydrides available from petroleum aromatics by oxidation. 

Thiophyne formation in this way requires that benzene be formed benzene was found among the pyrolysis products from phthalic anhydride and thiophene, neither of which originally contained any benzene. 

The questions raised by these products led to a study of the pyrolysis products of thiophene itself. Thiophene, 0-4 mole, pyrolyzed alone under the identical conditions as with phthalic anhydride, gave 0-76 g of product (thiophene-free) that analyzed (relative intensities in the low-voltage mass spectrum) ... 

As benzothiophene was a product of both the reaction of benzyne with thiophene and the pyrolysis of thiophene alone (Fields and Meyerson, 1966d), we investigated the reaction of benzyne with benzothiophene. Pyrolysis of a mixture of phthalic anhydride and benzothiophene gave the products shown in Table 11. Anthracene and dibenzothiophene probably arose via 1,2-, and phenanthrene via 1,4-addition to the thiophene ring ... 

The striking similarity in the nature and some of the relative concentrations of the products in the three reactions strongly suggests common mechanisms and intermediates for their formation. Phthalic anhydride gives benzyne upon pyrolysis the parallel behavior of acetylene suggests that it also forms benzyne at high temperatures. 

This evidence that benzyne is at least one of the intermediates in acetylene pyrolysis has many implications. However, as the ratios of products from acetylene and hexafluorobenzene differ appreciably from those obtained from phthalic anhydride, it might be best at this point to call acetylene a benzynoid precursor. Additional data will be needed and are being accumulated to determine to what extent acetylene reactions proceed at high temperatures through a benzyne intermediate (Fields and Meyerson, 1967a). 

The procedure described is essentially the same as that of Buck-ley and Scaife.3 The yield has been increased from 55.5% up to 72% by using 1.3 mol eq of phthalic anhydride and by carefully controlling the pressure and cooling the receiving flask. Although 2-nitropropene has previously been prepared by pyrolysis of 2-nitro-1-propyl benzoate in 72% overall yield from 2-nitro-l-propanol,4 the present method is preferred for its preparation since the procedure is much simpler and the product is directly obtainable from 2-nitro-l-propanol without first preparing its ester. It is also applicable to the preparation of 1-nitro-l-propene (58%),5-6 2-nitro-1 -butene (82%),7 and 2-nitro-2-butene (60%).6,7 In general, aliphatic nitroolefins have the tendency to polymerize readily with alkali. 

In view of evidence that the ion at m/e 76 (C6H4) was linear (Momigny et al., 1962), it was accepted that the parallel drawn between yield of biphenylene on pyrolysis and the intensity of the ion at m/e 76 could not imply a cyclic structure for the ion (R. F. C. Brown etal., 19G7). However, other workers have remarked that formulation of the CeH ion as cyclic benzyne was consistent with the behaviour of phthalic anhydride on pyrolysis (Budzikiewicz et al., 1967c). 

Pyrolysis of phthalic anhydride yielded biphenylene (Fields and Meyerson, 1968) and the mass spectrum of phthalic anhydride contained a peak at m/e 76, corresponding to benzyne . However, although... 

Thermoset plastics have also been pyrolysed with a view to obtain chemicals for recycling into the petrochemical industry. Pyrolysis of a polyester/styrene copolymer resin composite produced a wax which consisted of 96 wt% of phthalic anhydride and an oil composed of 26 wt% styrene. The phthalic anhydride is used as a modifying agent in polyester resin manufacture and can also be used as a cross-linking agent for epoxy resins. Phthalic anhydride is a characteristic early degradation product of unsaturated thermoset polyesters derived from orf/io-phthalic acid [56, 57]. Kaminsky et al. [9] investigated the pyrolysis of polyester at 768°C in a fiuidized-bed reactor and reported 18.1 wt% conversion to benzene. 

Methoxyl, cyano, and nitro groups may also be present in the molecule during pyrolysis. Examples are y-methoxycrotononitrile (83%), acrylonitrile (64%), 1-cyano-l, 3-butadiene (70%), and a series of conjugated nitroalkenes. The esters of /3-nitro alcohols are pyrolyzed at 195° or merely refluxed with sodium bicarbonate in dilute aqueous methanol. By the latter procedure the yields of conjugated nitro alkenes are. 90-95%. Direct dehydration by heating the nitro alcohols to 180° with phthalic anhydride has also been used. ... 

Pyrrolo[l,2-a]indolequinones were prepared also by cyclization reactions. One of them involves copper-catalyzed pyrolysis of 2-azido-5-methoxy-6-methyl-3-(2,4-pentadienyl)-l,4-benzoquinone to give compound 45 (87JOC3956). The second pyrrole ring was also formed from a 2-hydroxy-propyl side chain of an indolo-4,7-dione (80JOC5057). Benzo analogs were prepared in a Friedel-Crafts reaction from either phthalic anhydride (67TL765) or 2-pyrrolidinylcarbonyl chloride (86H2797). 

Benzyne (from pyrolysis of phthalic anhydride) reacts with thiophen at 690°C to give mainly naphthalene, benzo [b] thiophen (54), 2- and 3-phenyl-thiophens, and bithienyl.57 The bithienyl (probably the 2,2 -isomer) is also formed from thiophen alone under the same conditions. The phenylthiophens arise by insertion of benzyne into C—H bonds of thiophen. The source of naphthalene is most likely to be via a Diels-Alder reaction (Scheme 7, X = H), and Fields and Meyerson assume a mechanism involving initially 1,2-cyclo-addition of benzyne to a C—S bond of thiophen to account for the formation of 54 (Scheme 8).2,57,58 However, there is no evidence to support such a route, which seems unconvincing, not least because benzo [b]thiepin (53)... 

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