Acetaldehydes 2 molecules

Although the carbonyl condensation reaction appears different from the three processes already discussed, it s actually quite similar. A carbonyl condensation reaction is simply a combination of a nucleophilic addition step and an -substitution step. The initially formed enolate ion of one acetaldehyde molecule acts as a nucleophile and adds to the carbonyl group of another acetaldehyde molecule, as shown in Figure 5. 

O Base abstracts an acidic alpha hydrogen from one acetaldehyde molecule, yielding a resonance-stabilized enolate ion. 

These propagation reactions are circular. They consume a methane radical but also generate one. There is no net consumption of free radicals, so a single initiation reaction can cause an indefinite number of propagation reactions, each one of which does consume an acetaldehyde molecule. Ignoring any accumulation of methane radicals, the overall stoichiometry is given by the net sum of the propagation steps ... 

The formation mechanisms and the nature of chromophores in PET are still a matter of discussion. Postulated chromophores are polyenaldehydes from the aldol condensation of acetaldehyde [73] and polyenes from polyvinyl esters [69], as well as quinones [74, 75], Goodings [73] has proposed aldol condensation as forming poly conjugated species by subsequent reactions of acetaldehyde molecules (Figure 2.16). 

The details of the organic chemistry of the reaction of ethylene with PdCl2 (equation (1) above) are also known and are shown in Fig. 9.2. The palladium ion complexes with ethylene and water molecules and the water adds across the bond while still complexed to palladium. The palladium then serves as a hydrogen acceptor while the double bond reforms. Keto-enol tautomerism takes place, followed by release of an acetaldehyde molecule from the palladium. 

In studying the reaction of oxygen atoms with CH3CHO by using the photochemical method, at a pressure of 100 mm. Hg and with sensitization by mercury, Cvetanovi664 came to another conclusion, namely, that the reaction of oxygen atoms with acetaldehyde yielded mainly hydroxyl and the CH3CO radical. The hydroxyl formed reacted with an acetaldehyde molecule to form water, and acetyl yields diacetyl. The main reaction products were found to be water and biacetyl. 

Col liq when fresbly distilled at reduced press, bp 72° at 12mm, d 1.103 and spheat 0.737 caI/g/°C. May be prepd by the aldol condensation reaction(qv) of two acetaldehyde molecules in the presence of a small amt of an alkali. Other methods of prepn and props are given in Refs 1,2,3 4. Aldol is used in solvent mixts... 

At 800° abs. and 760 mm. pressure the number of molecules which react per c.c. per second is 7-3 x 1016. Assuming the molecular diameter of the acetaldehyde molecule to be 5 x 10-8 cm., the number of molecules, calculated from the formula 2-5 x V2 x no2un2. e ElRT (page 100), which might be expected to react per second is 5-4 x 1016. Thus the simple theory of activation is applicable. 

At higher concentrations of acetaldehyde, bimolecular trapping of the enolate in Scheme 4.7 will become faster, so, at some stage, this will compete effectively with the reprotonation of the enolate. When the bimolecular capture of enolate by another acetaldehyde molecule becomes much faster than the reprotonation of the enolate, i.e. when /c4[CH3CHO] k, 1 + k-2[H+] + /c 3[BH+], another limiting approximation to the complex rate equation predicted from the mechanism (Equation 4.17) is obtained, Equation 4.19 ... 

As demonstrated in Scheme 3, the ThDP-bound active aldehyde 6 as an acyl-donor may be added to a second aldehyde cosubstrate (acyl-acceptor) in an acyloin condensation-type reaction. This carboligase reaction was intensively investigated with acetaldehyde as an acyl-donor, which may be either condensed to a further acetaldehyde molecule yielding acetoin [1,26,27,29,63,153,154] or to a wide range of various aliphatic, aromatic and heterocyclic aldehydes [5,14,118,151,154-157,161]. 

The interpretation of these data emphasizes the chain nature of the reaction so that processes which are relatively important when there is no chain, e.g., the formation of carbon monoxide, are here less important and almost negligible. Two initiation reactions are chosen. The normal dissociation into radicals by reaction (87) may be reinforced by the direct reaction between triplet acetaldehyde molecules and oxygen... 

The enolate ion of acetaldehyde attacks the carbonyl group of another acetaldehyde molecule. Protonation gives the aldol product. 

Aldol condensations also take place under acidic conditions. The enol serves as a weak nucleophile to attack an activated (protonated) carbonyl group. As an example, consider the acid-catalyzed aldol condensation of acetaldehyde. The first step is formation of the enol by the acid-catalyzed keto-enol tautomerism, as discussed earlier. The enol attacks the protonated carbonyl of another acetaldehyde molecule. Loss of the enol proton gives the aldol product. 

That the enamine is in a chiral environment on PDC could also be deduced from an examination of the partial reaction of the enzyme in the reverse direction (see Scheme 1). One can add acetaldehyde to the enzyme, presumably creating HEThDP ( 4), followed by C2aH ionization to form the enamine (k 3), then react the enamine with a second acetaldehyde molecule and release the acetoin from the enzyme. Chen and Jordan showed that the acetoin so formed is optically active91, clearly indicating that addition of acetaldehyde to the enamine has a definite sidedness , i.e. there is a preferred approach of acetaldehyde for reaction with the enamine. 

The abstraction of a-hydrogen in carbonyl compounds such as acetaldehyde by sodium hydroxide is a reversible reaction and forms an enolate ion that undergoes addition to the carbonyl carbon of another acetaldehyde molecule to give the aldol 3.13. This is called the aldol condensation " and its mechanism is shown in Scheme 3.6. 

The decomposition of acetaldehyde, sensitized by biacetyl, was studied at 499 °C by Rice and Walters , and between 410 and 490 °C by Boyer et al. They found the initial rate to be proportional to the square root of the biacetyl concentration and to the first power of the aldehyde concentration. The chains are initiated by the radicals originating from the decomposition of the biacetyl molecule. The decomposition of acetaldehyde can be induced also by di-r-butyl peroxide (at 150-210 °C, about 10-50 molecules decompose per peroxide molecule added), as well as by ethylene oxide (around 450 °C each added ethylene oxide molecule brings about the decomposition of up to 300 acetaldehyde molecules). For the influence of added diethylether, vinyl ethyl ether, ethyl bromide, and ethyl iodide etc., see Steacie °. 

Iodine accelerates the decomposition of acetaldehyde In the steady-state range, the order is approximately 1.0 and 0.5 in aldehyde and iodine, respectively. The experimental results of Rollefson and Faull have been reinterpreted and added to by O Neal and Benson . The iodine-catalysed reaction is a free radical chain process initiated by the attack of an iodine atom on the acetaldehyde molecule. The proposed mechanism fits the experimental data very well. The thermal decomposition of acetaldehyde is catalysed also by other halogens and halogen compounds . 

Setser carried out rrkm quantum statistical calculations for the decomposition of acetaldehyde molecule into free radicals. He examined three models out of which two were of the loose type ones, fitting essentially the Gorin model , while the third one was a tighter complex. The calculations definitely show that the decomposition is in the pressure-dependent region even around 100 torr. 

In the above calculations, iodine was considered not to deactivate the excited acetaldehyde molecules. Steacie and Lossing , however, questioned this assumption, referring to the experiments of Buchanan et where more ethane... 

An estimate of (j) may be obtained by studying the temperature dependence of high temperatures and low light intensities, all the formyl radicals formed in primary process I can be expected to dissociate and to convert subsequently into hydrogen molecules. This seems to be the case, since Calvert et al observed that attains a limiting value at high temperatures, which can be taken as equal to i. On this basis, Calvert et determined the value of 0.81 for i at 3130 A, which is in sharp contrast with that obtained in the presence of iodine. The discrepancy seems to support the suggestion that iodine deactivates the excited acetaldehyde molecules. 

Some CH4 formation was observed even in the presence of a sufficient amount of NO (at 26 °C = 0.0005) this was almost certainly the consequence of the intramolecular rearrangement of the singlet excited acetaldehyde molecules. Decomposition into free radicals is, however, of more importance than the intra-... 

If it is assumed that the butene-2 does not interact with the singlet acetaldehyde molecules and that the triplet energy transfer to butene-2 is rapid enough, than it follows from the results that, at 3130 A (/) decomposition occurs almost exclusively from the triplet state and (h) the majority of the excited singlet molecules cross over to the ground state. Both conclusions are in disagreement with those of Parmenter and Noyes . There is, however, not sufficient evidence available to decide between the two different opinions. 

Step 2. The peroxy acid radicals abstract hydrogen from acetaldehyde molecules to yield peroxy acetic acid and new acetyl radicals ... 

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