Acetaldehyde Semicarbazon

Acetaldehyde oxime, AA89 Acetaldehyde semicarbazone, AC08 Acetamide, AA90... 

The form and significance of those references are described on pages 5-6. To illustrate the use of this table we shall suppose that a full citation is wanted for the information given in this volume for acetaldehyde semicarbazone. In Table I that compound appears on pages 56-57 and is assigned code number AC08. The literature reference appears there as 036-0501+" in Column 1. ... 

Methylene-1,3-diselenole (108) is obtained in high yield from acetaldehyde semicarbazone using the selenadiazole route (Scheme 25, see also Scheme 49, Section 3.14.10.1.2). Theexocyclic methylene in (108) is quite reactive protonation gives a highly reactive 2-methyl-1,3-diselenolylium ion resulting in polymerization <88JOC3529>. However, the methylene carbon has nucleophilic character due to the contribution from the dipolar mesomeric structure (Scheme 24). 

Dibenzotetraselenafulvalene can be prepared by this method starting from 1,2,3-benzo-selenadiazole and subsequent coupling of the resulting benzo-l,3-diselenole-2-selone with triphenyl phosphine in 66% yield <83CC295>. Likewise, the parent 2-methylene-1,3-diselenole (27) can be obtained from acetaldehyde semicarbazone after ring closure and subsequent treatment of the 1,2,3-selenadiazole (185) with potassium t-butoxide (selenadiazole route, see Scheme 49) <87TL5635>. 

If we react equimolar amounts of acetaldehyde, benzaldehyde, and semicarbazide, the product initially formed will contain 99.5 % of acetaldehyde semicarbazone and only 0.5 % of benzaldehyde semicarbazone. Under these conditions, the reaction is subject to kinetic control, the products being determined by the rates at which they are formed since the backward reactions are much slower. If, however, the reaction mixture is allowed to stand, so that the backward reactions can take place and lead to equilibrium, the product now contains 90% of benzaldehyde semicarbazone and only 10% of acetaldehyde semicarbazone. The reaction is then said to be subject to thermodynamic control, the proportion of products being determined by their relative stabilities rather than their rates of formation. 

Aldehydes Alkanes Benedict s test Dichromate test 2,4-Dinitrophenylhydrazine (2,4-DNP) Fehling test Iodoform test Oxime Permanganate test Semicarbazone Tollen s test No test Positive for all aldehydes Positive for all aldehydes Positive for all aldehydes (and ketones) Positive for all aldehydes Positive only for acetaldehyde Positive for all aldehydes (and ketones) Positive for all aldehydes Positive for all aldehydes (and ketones) Positive for all aldehydes... 

The most convenient route to TSF was reported by Cava in 1987 [64]. The procedure utilizes a novel iodine-promoted coupling of 2-methylene-1,3-diselenole (28). The starting 1,2,3-selenadiazole (29) is easily obtained from the oxidation of the semicarbazone of acetaldehyde with selium dioxide. 

Der lonyliden-acetaldehyd (XXVIII) ist bereits in vielen Arbeiten beschrieben worden. Erstmals benutzten ihn Kuhn und Morris 46) zu ihrer Synthese des Vitamin A. Ausgangsprodukt ist der /3-Ionyliden-essigester (XXVI), der in das o-Toluidid iibergefiihrt wird. Mit Phosphor-pentachlorid entsteht hieraus das Chlorid XXVII, woraus durch anschlie-Bende Reduktion mit Chromochlorid und Spaltung der Aldehyd XXVIII erhalten wird, der als Semicarbazon charakterisiert werden konnte. 

Removal of the acetaldehyde by distillation shifts the equilibrium to the right The formation of semicarbazones (III) is a reaction common to all aldehydes. For preparative reasons we have only described reactions involving aromatic aldehydes, since these afford semicarbazones which crystallise very readily (eqn. 2) ... 

If the overall reaction followed the BEP principle, the rate of formation of a given semicarbazone should be greater, the more stable it is. In other words, the rate constants for the forward reaction should run parallel to the corresponding equilibrium constants K. This is not the case, as the results in Table 5.1 show. Thus acetaldehyde reacts nearly 200 times faster with semicarbazide than does benzaldehyde, but the equilibrium constant favors benzaldehyde semicarbazone by a factor of ten. 

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