Nitrate, thermolysis

Although acetyl and other acyl nitrates are dangerously explosive, in solution they undergo smooth thermolysis to alkyl nitrates with one less carbon atom (Scheme 154) a number of methods are discussed for the preparation of acyl nitrates. Thermolysis is conducted at high temperature to minimize the alternative mode of decomposition, production of carboxylic acid anhydrides and dinitrogen pentoxide. 

Thompson points out that there is no evidence that adducts give other than acetates on thermolysis. The exocyclic methylene intermediate (iv) postulated by Robinson could arise by proton abstraction from a Wheland intermediate analogous to (vll) above, rather than from the adduct (in). Similarly its decomposition does not necessarily require the intermediacy of the adduct (v). The fact that i -methyl-4-nitromethylnaphthalene is the product even when the nitrating medium is nitric acid and nitromethane would then require no separate explanation. 

Notable examples of general synthetic procedures in Volume 47 include the synthesis of aromatic aldehydes (from dichloro-methyl methyl ether), aliphatic aldehydes (from alkyl halides and trimethylamine oxide and by oxidation of alcohols using dimethyl sulfoxide, dicyclohexylcarbodiimide, and pyridinum trifluoro-acetate the latter method is particularly useful since the conditions are so mild), carbethoxycycloalkanones (from sodium hydride, diethyl carbonate, and the cycloalkanone), m-dialkylbenzenes (from the />-isomer by isomerization with hydrogen fluoride and boron trifluoride), and the deamination of amines (by conversion to the nitrosoamide and thermolysis to the ester). Other general methods are represented by the synthesis of 1 J-difluoroolefins (from sodium chlorodifluoroacetate, triphenyl phosphine, and an aldehyde or ketone), the nitration of aromatic rings (with ni-tronium tetrafluoroborate), the reductive methylation of aromatic nitro compounds (with formaldehyde and hydrogen), the synthesis of dialkyl ketones (from carboxylic acids and iron powder), and the preparation of 1-substituted cyclopropanols (from the condensation of a 1,3-dichloro-2-propanol derivative and ethyl-... 

ArNHj remains (as it is a weak base), but the concentration is low enough to prevent as yet undiazotised amine undergoing a coupling reaction with the first formed ArNj p. 147). Aromatic diazonium chlorides, sulphates, nitrates, etc., are reasonably stable in aqueous solution at room temperature or below, but cannot readily be isolated without decomposition. Fluoroborates, ArN2 BF4 , are more stable (cf. stabilising effect of BF on other ion pairs, p. 136) and can be isolated in the dry solid state thermolysis of the dry solid is an important preparative method for ffuoroarenes ... 

Bachmann and Biermann reported the synthesis of nitroalkanes from the thermolysis of acyl nitrates. The thermolysis of nitrite and nitrate esters over an asbestos catalyst is also reported to yield nitroalkanes. ... 

The nitration of both 4-methyl- and 4-chloro-2,6-dinitrotoluenes (66 and 67) with mixed acid in acetic acid at subambient temperature allows the isolation of the nitramines, (68) and (69), respectively. Thermolysis of (68) and (69) in refluxing methylene chloride yields the corresponding diazophenols, (70) and (71), respectively. Scilly and co-workers isolated 2-diazo-4,6-dinitrophenol (DINOL) (53) from the thermolysis of fV,2,3,5-tetranitroaniline (73) in ethyl acetate at 60 °C. 

The increase in the rate of decomposition under the influence of added Cr203 and K2Cr207 is shown on Figs. 171 and 172 respectively.lt has been found that an increase of the rate occurred only at temperatures near to the melting point of the samples. Guiochon [17] also found that cobalt salts increase the rate of thermolysis of ammonium nitrate. Other mineral salts (of manganese, nickel and copper) have a similar but much weaker action. A large number of salts of other metals are without any noticeable action. 

The products of the thermolysis of 3-phenyl-5-(arylamino)-l,2,4-oxadiazoles and thiazoles have been accounted for by a radical mechanism.266 Flash vacuum pyrolysis of 1,3-dithiolane-1-oxides has led to thiocarbonyl compounds, but the transformation is not general.267 hi an ongoing study of silacyclobutane pyrolysis, CASSF(4,4), MR-CI and CASSCF(4,4)+MP2 calculations using the 3-21G and 6-31G basis sets have modelled the reaction between silenes and ethylene, suggesting a cyclic transition state from which silacyclobutane or a trcins-biradical are formed.268 An AMI study of the thermolysis of 1,3,3-trinitroazacyclobutane and its derivatives has identified gem-dinitro C—N bond homolysis as the initial reaction.269 Similar AMI analysis has determined the activation energy of die formation of NCh from methyl nitrate.270 Thermal decomposition of nitromethane in a shock tube (1050-1400 K, 0.2-40 atm) was studied spectrophotometrically, allowing determination of rate constants.271... 

The reaction of 3(5),4-bis(TMS)-pyrazole (320) with concentrated nitric acid or hydrogen chloride leads to 3(5),4-bis(TMS)-pyrazolium nitrate (337) or-chloride (338), respectively207. Thermolysis of 337gives the hitherto unknown pure TMS-nitrate (339) in nearly quantitative yield (Scheme 46). 

T.B. Brill, Thermal Decomposition of Energetic Materials 14. Selective Product Distributions Evidenced in Rapid, Real-Time Thermolysis of Nitrate Esters at Various Pressures, Combust. Flame, 66 (1986) 9-16. 

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