Ketones, slow combustion

Figure 6 shows the variation of peroxide concentration in methyl ethyl ketone slow combustion, and similar results, but with no peracid formed, have been found for acetone and diethyl ketone. The concentrations of the organic peroxy compounds run parallel to the rate of reaction, but the hydrogen peroxide concentration increases to a steady value. There thus seems little doubt that the degenerate branching intermediates at low temperatures are the alkyl hydroperoxides, and with methyl ethyl ketone, peracetic acid also. The tvfo types of cool flames given by methyl ethyl ketone may arise from the twin branching intermediates (1) observed in its combustion. 

The slow combustion reactions of acetone, methyl ethyl ketone, and diethyl ketone possess most of the features of hydrocarbon oxidation, but their mechanisms are simpler since the confusing effects of olefin formation are unimportant. Specifically, the low temperature combustion of acetone is simpler than that of propane, and the intermediate responsible for degenerate chain branching is methyl hydroperoxide. The Arrhenius parameters for its unimolecular decomposition can be derived by the theory previously developed by Knox. Analytical studies of the slow combustion of methyl ethyl ketone and diethyl ketone show many similarities to that of acetone. The reactions of methyl radicals with oxygen are considered in relation to their thermochemistry. Competition between them provides a simple explanation of the negative temperature coefficient and of cool flames. 

Figure 5. Effect of addition of formaldehyde on slow combustion of 40 mm. of diethyl ketone and 40 mm. of oxygen at 400°C. (1)...

Methyl ethyl ketone is unique, in that long and irreproducible induction periods were observed on occasion, reaction ensued only after 7 hours and then was completed within 10 minutes. During the long induction period the only detectable product was methanol. No convincing reason can be advanced to account for this anomalous behavior. The virtual absence of ethylene from the products of the low temperature slow combustion of methyl ethyl ketone strongly suggests that the low-temperature mechanism proceeds almost exclusively by further oxidation of the radicals produced by hydrogen abstraction from the parent ketone. 

One somewhat surprising minor product is 1 2-epoxypropane. Its formation parallels the formation of ethylene oxide and 1 1-epoxybutane in the slow combustion of acetone and diethyl ketone, respectively. It may perhaps be formed via... 

Biacetyl and acetylacetone have been studied by Salooja [47(a)] in a flow system. Biacetyl was rather reactive, appreciable reaction beginning at 350 °C and ignition occurring at about 530 °C under the experimental conditions employed acetylacetone began to react above 400 °C but ignited at 480 °C. Biacetyl was anomalous in that it did not appear to exhibit a zone of negative temperature coefficient of the rate of combustion, probably because no stable olefinic intermediates are formed in the oxidation process. Some measurements on the rate of slow combustion of methyl vinyl ketone have also been reported [47(b)]. 

The three compounds, acetoacetate, acetone, and 3-hydroxybutyrate, are known as ketone bodies.60b The inability of the animal body to form the glucose precursors, pyruvate or oxaloacetate, from acetyl units sometimes causes severe metabolic problems. The condition known as ketosis, in which excessive amounts of ketone bodies are present in the blood, develops when too much acetyl-CoA is produced and its combustion in the critic acid cycle is slow. Ketosis often develops in patients with Type I diabetes mellitus (Box 17-G), in anyone with high fevers, and during starvation. Ketosis is dangerous, if severe, because formation of ketone bodies produces hydrogen ions (Eq. 17-5) and acidifies the blood. Thousands of young persons with insulin-dependent diabetes die annually from ketoacidosis. 

The combustion of aliphatic ketones generally resembles that of hydrocarbons, the reactions being autocatalytic and possessing two regimes of slow oxidation, separated by a region of negative temperature coefficient of the rate. Cool flames are also observed under some circumstances. 

Properties Dimensional stability over temperature range from -40 to +71C. Attacked by nitric and sulfuric acids and by aldehydes, ketones, esters, and chlorinated hydrocarbons. Insoluble in alcohols, aliphatic hydrocarbons, and mineral and vegetable oils. Processed by conventional molding and extrusion methods. D 1.04 tensile strength about 6500 psi, flexural strength 10,000 psi, good electrical resistance, water absorption 0.3-0.4%. Combustible but slow-burning flame retardants may be added. Can be vacuum-metallized or electroplated. 

METIL TRICLOROSILANO (Spanish) (75-79-6) see methyl trichlorosilane. METIL VINIL CETONA (Spanish) (78-94-4) see methyl vinyl ketone. METOLACHLOR (51218-45-0) CijHjjClNOj Combustible liquid (flash point 374°F/190°C cc). Incompatible with strong acids, nitrates, oxidizers azo and diazo confounds. Moisture may cause slow decomposition. Thermal decomposition releases toxic nitrogen oxides. On small fires, use dry chemical powder (such as Purple-K-Powder), alcohol-resistant foam, or sand extinguishers. 

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