Acetaldehyde from decomposition

In about 2 hours the beads adhere together somewhat then paracet-aldehyde begins to collect at the bottom of the bottle. Water is added, 2—3 c.cs. at a time, at intervals during the formation. The yield is good, and there is practically no escape of acetylene or acetaldehyde from the apparatus. The action consists in the formation of a mercuric sulphate acetylene compound and its subsequent decomposition giving paracet-aldehyde. The passage of acetylene should be continued for about 2 days. The contents of the bottle are finally shaken up with ether, the ethereal solution separated, dried over anhydrous sodium sulphate, and distilled. Paracetaldehyde passes over as a colourless liquid, boiling point 124°. 

In marked contrast with all the other experiments, hydrogen atoms abstract the aldehydic hydrogen from acetaldehyde to form the acetyl radical, CHg. CO. The occurrence of this reaction is presumably due to the relatively weak CH bond (82 kcal mole ) (Benson, 1965) and the absence of an efficient addition reaction, in contrast to the olefins. A small amount of methyl radical is also observed and must arise from decomposition of the acetyl radical (reaction 29). 

Palmer 10 found that although ethanol decomposed to acetaldehyde in the presence of copper at 300° C. without the formation of secondary decomposition, this was not true when aldehyde alone was used. If hydrogen and acetaldehyde are passed over copper at 250° to 300° C. much of the aldehyde decomposes into secondary products. This anomaly, Palmer explains on the basis of the alcohol being selectively adsorbed by the catalyst surface so that the copper surface is covered with a layer of alcohol molecules which prevent the adsorption and consequent destruction of the aldehyde. The three steps in the dehydrogenation reaction were postulated to be (1) adsorption of alcohol, (2) activation of certain alcohol molecules, (3) evaporation of hydrogen and acetaldehyde from the catalyst surface. 

B. Excitation-Decomposition in Svbstitution Hoflf and Rowland (1957), in studying the reactions of tritium recoils with methanol, ethanol, and acetone, suggested the same reactions in order to account for most of their observed products in the liquid phase. In addition, they postulated an excitation-decomposition reaction as given in eqs. (10) and (11) in order to explain the formation of labeled acetaldehyde from ethanol. 

Sorbic acid is oxidized rapidly in the presence of molecular oxygen or peroxide compounds. The decomposition products indicate that the double bond farthest from the carboxyl group is oxidized (11). More complete oxidation leads to acetaldehyde, acetic acid, fumaraldehyde, fumaric acid, and polymeric products. Sorbic acid undergoes Diels-Alder reactions with many dienophiles and undergoes self-dimerization, which leads to eight possible isomeric Diels-Alder stmctures (12). 

Most diaziridines are not sensitive towards alkali. As an exception, diaziridines derived from 2-hydroxyketones are quickly decomposed by heating with aqueous alkali. Acetaldehyde, acetic acid and ammonia are formed from (162). This reaction is not a simple N—N cleavage effected intramolecularly by a deprotonated hydroxy group, since highly purified hydroxydiaziridine (162) is quite stable towards alkali. Addition of small amounts of hydroxybutanone results in fast decomposition. An assumed reaction path — Grob fragmentation of a hydroxyketone-diaziridine adduct (163) — is in accord with these observations (B-67MI50800). 

The decomposition of acetaldehyde has Eq. (8-6) as the rate-controlling step, this being the one (aside from initiation and termination) whose rate constant appears in the rate law. In the sequence of reactions (8-20)—(8-23), the same reasoning leads us to conclude that the reaction between ROO and RM, Eq. (8-22), is rate-controlling. Interestingly, when Cu2+ is added as an inhibitor, rate control switches to the other propagating reaction, that between R and O2, in Eq. (8-21). The reason, of course, is that Cu2+ greatly lowers [R ] by virtue of the new termination step of reaction (8-30). 

Interestingly, the Fischer indole synthesis does not easily proceed from acetaldehyde to afford indole. Usually, indole-2-carboxylic acid is prepared from phenylhydrazine with a pyruvate ester followed by hydrolysis. Traditional methods for decarboxylation of indole-2-carboxylic acid to form indole are not environmentally benign. They include pyrolysis or heating with copper-bronze powder, copper(I) chloride, copper chromite, copper acetate or copper(II) oxide, in for example, heat-transfer oils, glycerol, quinoline or 2-benzylpyridine. Decomposition of the product during lengthy thermolysis or purification affects the yields. 

From the mechanism given in problem 7-8 for the decomposition of acetaldehyde, derive a rate law or set of independent rate laws, as appropriate, if H2 and C2Hs are major products (in addition to CH4 and CO). 

The energy of activation in such chain reactions can be evaluated from those of the individual steps in the process. For example, in decomposition of acetaldehyde, we have... 

The two intermediates depicted above differ fundamentally from each other. The COx-producing intermediate has a direct metal-carbon (M-R) bond whereas the C2-producing intermediate has a metal-oxygen-carbon (M-O-R) bond. From known organic decomposition pathways, the formation of selective oxidation products from the M-O-R intermediate is likely. An a-H elimination produces acetaldehyde and a P-H elimination produces ethylene. 

Butenes were subjected to photosensitized reaction with molecular oxygen in methanol. 1-Butene proved unreactive. A single hydroperoxide, l-butene-3-hydroperoxide, was produced from 2-butene and isolated by preparative gas chromatography, Thermal and catalyzed decomposition of pure hydroperoxide in benzene and other solvents did not result in formation of any acetaldehyde or propionaldehyde. The absence of these aldehydes suggests that they arise by an addition mechanism in the autoxidation of butenes where they are important products. l-Butene-3-hydroperoxide in the absence of catalyst is converted predominantly to methyl vinyl ketone and a smaller quantity of methyl vinyl carbinol —volatile products usually not detected in important quantities in the autoxidation of butene. 

Aliphatic amines have much less effect on the later reactions of the gas-phase oxidation of acetaldehyde and ethyl ether than if added at the start of reaction. There is no evidence that they catalyze decomposition of peroxides, but they appear to retard decomposition of peracetic acid. Amines have no marked effect on the rate of decomposition of tert-butyl peroxide and ethyl tert-butyl peroxide. The nature of products formed from the peroxides is not altered by the amine, but product distribution is changed. Rate constants at 153°C. for the reaction between methyl radicals and amines are calculated for a number of primary, secondary, and tertiary amines and are compared with the effectiveness of the amine as an inhibitor of gas-phase oxidation reactions. 

The decomposition of the diazonium sulfate in the presence of alcohol may take place with considerable violence, and it is necessary to watch the reaction carefully so as to be able to check it, if necessary, by the external application of cold water. Acetaldehyde is rapidly evolved, and some will generally escape from the condenser. It is therefore advisable to lead the escaping gases through water, not only in order to avoid possibility of fire, but to retain any nitrotoluene which may be entrained. 

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