Pyrolysis, diketene to ketene

Dihydroxy-4-butyro)actone, 46, 24 Diisopropylcarbodiimide, 46, 98 Diketene, pyrolysis to ketene, 46, SO... 

Pyridones from 1,3,5 triketones, 46,60 Pyrolysis, diketene to ketene, 46, SO a-PYRONE, 46,101 in Diels Alder reactions, 46, 104 4-Pyrones, 2,6 disubstituted, from 1,3,5 pentanetnones by acid cycliza tion, 46, 61... 

Since reaction of wood with acetic anhydride leads to the formation of acetic acid by-product, which must be removed from the wood, there has been some interest in the use of ketene gas for acetylation (Figure 4.4a). Ketene, for reaction with wood, is produced by pyrolysis of diketene. Provided that the wood contains no moisture, no acetic acid by-product is produced. However, ketene presents handling problems it is very toxic and explosive, and it also has a tendency to dimerize. A comprehensive series of studies of ketene-based acetylation has been performed in Latvia and this work has been reviewed by Morozovs etal. (2003). Hardwoods have been found to be more reactive to ketene than softwoods and the optimal temperature for reaction has been determined as 47 °C. Application of vacuum and treatment of wood with ammonia solution has been used to remove the excess ketene. The reaction of wood with liquid diketene was also studied, with a WPG of 35 % being obtained after reaction for 3 hours at 52 °C. 

Ketene can be generated conveniently by pyrolysis of acetone in a hot tube or over a hot wire in a ketene lamp, or by pyrolysis of diketene in a hot tube. Other methods of preparation have been summarized. It has been shown that diketene cracks quite cleanly to ketene, although some allene and carbon dioxide are formed at the same time. ... 

Anhydride manufactured by acetic acid pyrolysis sometimes contains ketene polymers, eg, acetylacetone, diketene, dehydroacetic acid, and particulate carbon, or soot, is occasionally encountered. Polymers of aHene, or its equilibrium mixture, methylacetylene—aHene, are reactive and refractory impurities, which if exposed to air, slowly autoxidize to dangerous peroxidic compounds. 

The manufacture of the highly pure ketene required for ketenization and acetylation reactions is based on the pyrolysis of diketene, a method which has been employed in industrial manufacture. Conversion of diketene to monomeric ketene is accompHshed on an industrial scale by passing diketene vapor through a tube heated to 350—600°C. Thus, a convenient and technically feasible process for producing ketene uncontaminated by methane, other hydrocarbons, and carbon oxides, is available. Based on the feasibiHty of this process, diketene can be considered a more stable form of the unstable ketene. 

Pyrolysis of diketene at temperatures greater than 400°C gives two molecules of ketene. This method has been used iadustriaHy. At present there is no method to convert diketene efftciendy iato aHene [463-49-0] and CO2, the thermodynamic products. 

The results of low-temperature matrix-isolation studies with 6 [41a] are quite consistent with the photochemical formation of cyclo-Cif, via 1,2-diketene intermediates [59] and subsequent loss of six CO molecules. When 6 was irradiated at A > 338 nm in a glass of 1,2-dichloroethane at 15 K, the strong cyclobut-3-ene-1,2-dione C=0 band at 1792 cm in the FT-IR spectrum disappeared quickly and a strong new band at 2115 cm appeared, which was assigned to 1,2-diketene substructures. Irradiation at A > 280 nm led to a gradual decrease in the intensity of the ketene absorption at 2115 cm and to the appearance of a weak new band at 2138 cm which was assigned to the CO molecules extruded photo-chemically from the 1,2-diketene intermediates. Attempts to isolate cyclo-Cig preparatively by flash vacuum pyrolysis of 6 or low-temperature photolysis of 6 in 2-methyltetrahydrofuran in NMR tubes at liquid-nitrogen temperature have not been successful. 

The two most convenient procedures for preparing ketene are the present one and the pyrolysis of acetone over a hot wire. The latter procedure can give ketene at a faster rate (0.45 mole per hr. versus 0.2 mole per hr.), but it takes considerable adjustment to get optimum conditions, and trouble is sometimes caused by the wire getting coated with carbon. Furthermore, because the efl ciency of a given wire coil varies with time, passing throu a maximum, frequent calibration of the apparatus is necessary. The present method is more reliable and is the method of choice, when diketene is available. 

Methylene-2-oxetanone, or diketene (37), does not decarboxylate on heating but cleaves to form ketene at 250 °C. A thermal rearrangement of 3-aryl-4-benzal-2-oxetanones (38) to form naphthalene derivatives appears to involve this type of pyrolysis, followed by a [ 2 + 4] cycloaddition (78JOC1146, 72LA(765)15>. 

Diketene (4-methyleneoxetane-2-one) is formed by dimerization of ketene which in turn is prepared by pyrolysis of acetone or acetic acid. The compound is an industrial intermediate. It ring-opens with ethanol to give ethyl acetoacetate the nucleophile attacks the C-atom of the carbonyl group. 

The pyrolysis of diketene produces ketene in quantitative yield, and the product is not contaminated with hydrocarbons, carbon monoxide, etc., which are present in ketene produced by the cracking of acetone. In other instances also it may be possible to prepare ketenes in the state of highest purity by utilization of the dimers (for the preparation of the dimers, seep. 127). 

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