Crotonaldehyde

Crotonaldehyde is used to synthesize butyraldehyde to produce IMP (used as a curative with polyurethane elastomers). 

Crotonaldehyde is used to purify lubricating oil. Crotonaldehyde is also used to produce tear gas. It is used in the tanning of leather and in the production of insecticides. 

Acetaldehyde condenses in the presence of a little sodium sulphite or sodium hydroxide solution to aldol. The latter eliminates water upon distillation at atmospheric pressure, but more efficiently in the presence of a trace of iodine, which acts as a catalyst, to yield crotonaldehyde  

CHsCHO + HCH2CHO --------- CH3CH(OH)CH2CHO------ CH3CH=CHCHO 

Crotonaldehyde can be produced so cheaply on a commercial scale that its preparation in the laboratory is rarely undertaken. It is, however, included here as an illustration of the preparation of an unsaturated aliphatic aldehyde. 

If acetaldehyde is warmed with a concentrated solution of an alkali hydroxide, it is converted into a resinous product resulting from repeated aldol condensations between aldol, crotonaldehyde and acetaldehyde. 

EPA Classified Toxic Waste, RCRA Waste Number U053 DOT Label Flammable Liquid, UN 1143 

Formula C3H5CHO MW 70.09 CAS [4170-30-3] ([123-73-9] for the trans-isomer, tranx-2-butenal) 

Synonyms 2-butenal propylene aldehyde /5-methyl acrolein crotonic aldehyde 

Crotonaldehyde is used in the manufacture of butyl alcohol, butyraldehyde, and in several organic synthesis. 

Colorless liquid turning pale yellow on contact with air or light pungent suffocating 

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Normal butyl alcohol, propyl carbinol, n-butanol, 1-buianol, CH3CH2CH2CH2OH. B.p. 117 C. Manufactured by reduction of crotonaldehyde (2-buienal) with H2 and a metallic catalyst. Forms esters with acids and is oxidized first to butanal and then to butanoic acid. U.S. production 1978 300 000 tonnes. 

The reaction is readily illustrated by the formation of crystalline sorbic acid by the condensation of crotonaldehyde and malonic acid in hot pyridine solution ... 

Add 4 g. of malonic acid to 4 ml. of pyridine, and then add 3 1 ml. of crotonaldehyde. Boil the mixture gently under reflux over an asbestos-covered gauze, using a small Bunsen flame, for 40 minutes and then cool it in ice-water. Meanwhile add 2 ml. of concentrated sulphuric acid carefully with shaking to 4 ml. of water, cool the diluted acid, and add it with shaking to the chilled reaction-mixture. Sorbic acid readily crystallises from the solution. Filter the sorbic acid at the pump, wash it with a small quantity of cold water and then recrystallise it from water (ca, 25 ml.). The colourless crystals, m.p. 132-133°, weigh ro-i-2 g. 

Acetaldehyde (and other aldehydes containing at least one hydrogen atom in the a position) when treated with a small quantity of dilute sodium hydr oxide solution or other basic catalyst gives a good yield of aldol (p hydroxy-n-but3Taldehyde) (I), which readily loses water, either by heating the isolated aldol alone or with a trace of mineral acid, to form crotonaldehyde (II) ... 

The commercial method consists in the oxidation of crotonaldehyde, which is itself prepared from acetaldehyde (see Section 111,141) ... 

The reaction probably proceeds as follows. Crotonaldehyde is first formed by oondensation of the depolymerised acetaldehyde in the presence of acid ... 

The aniline then reacts with the ap-unsaturated aldehyde by 1 4-addition the addition product, under the influence of strong acid, cyclises to form 1 2-dihydroquinaldine. The latter is dehydrogenated by the condensation products of aniline with acetaldehyde and with crotonaldehyde simultaneously produced ( .c., ethylideneaniline and crotonylideneaniline) these anils act as hydrogen acceptors and are thereby converted into ethylaniline and n-butyl-aniline respectively. 

The synthesis of spiro compounds from ketones and methoxyethynyl propenyl ketone exemplifies some regioselectivities of the Michael addition. The electrophilic triple bond is attacked first, next comes the 1-propenyl group. The conjugated keto group is usually least reactive. The ethynyl starting material has been obtained from the addition of the methoxyethynyl anion to the carbonyl group of crotonaldehyde (G. Stork, 1962 B, 1964A). 

Figure 6.15 The infrared vibrational spectrum of crotonaldehyde. The parts marked (a), (b) and (c) refer to a 10 per cent (by volume) solution in CCI4, a 1 per cent solution in CCI4, and a thin liquid film, respectively. [Reproduced, with permission, from Bowles, A. J., George, W. O. and Maddams, W. F J. Chem. Soc. (B), 810, 1969]...

Figure 6.17 The laser Raman vibrational spectmm of liquid crotonaldehyde. [Reproduced, with permission, from Durig, J. R., Brown, S. C., Kalasinsky, V F. and George, W. O., Spectrochim. Acta, 32A, 807, 1976. Copyright 1976 Pergamon Press]...

Table 6.4 Fundamental vibration wavenumbers of crotonaldehyde obtained from the infrared and Raman spectra...

F1= F1 F10 (s-lranx-crotonaldehyde) infrared spectmm, 159ff Raman spectrum, 159ff 14 9X (cyanotetraacetylene) interstellar, 120... 

Reactions with Aldehydes and Ketones. The base-catalyzed self-addition of acetaldehyde leads to formation of the dimer, acetaldol [107-89-1/, which can be hydrogenated to form 1,3-butanediol [107-88-0] or dehydrated to form crotonaldehyde [4170-30-3]. Crotonaldehyde can also be made directiy by the vapor-phase condensation of acetaldehyde over a catalyst (53). 

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