Of dibenzalacetones

A large excess of acetone is taken, so as to diminish. the formation of dibenzalacetone. 

Ether Formation. It is reported that the reduction products of certain a,j8-unsaturated ketones, for example, dibenzalacetone (XL), contained sonfe of the isopropyl ethers pf the carbinols.6 Carbinols of this type are especially susceptible to ether formation often recrystallization from an alcohol is sufficient to give the ether. However, ether formation is by no means the usual reaction with unsaturated ketones. Indeed, in most cases no ether was noted in the products. Even in the case of dibenzalacetone, the normal product, i.e., the carbinol, has been obtained in 58% yield.36 9,9-Dimethylanthrone-10 (XLI) gave 64% of a material corresponding in analysis to the isopropyl ether (XLII).17 Similar observations of ether formation with a-halogen ketones will be discussed later. There appears to be no way of predicting with any degree of certainty when ether formation is likely, but it is not a common side reaction. 

Mix 0.05 mole of benzaldehyde with the theoretical quantity of acetone, add one-half the mixture to a solution of 5 g of sodium hydroxide dissolved in 50 mLofwaterand40 mL ofethanol at room temperature (<25°C). After 15 min add the remainder of the aldehyde-ketone mixture and rinse the container with a little ethanol to complete the transfer. After one-half hour, during which time the mixture is swirled frequently, collect the product by suction filtration on a Buchner funnel. Break the suction and carefully pour 100 mL of water on the product. Reapply the vacuum. Repeat this process three times in order to remove all traces of sodium hydroxide. Finally, press the product as dry as possible on the filter using a cork, then press it between sheets of filter paper to remove as much water as possible. Save a small sample for melting point determination and then recrystallize the product from ethanol using about 10 mL of ethanol for each 4 g of dibenzalacetone. Pure dibenzalacetone melts at IIO-IITC, and the yield after recrystallization should be about 4 g. 

Write formulas to show the possible geometric isomers of dibenzalacetone. Which isomer would you expect to be most stable Why ... 

FIG. 1 H nmr spectrum of dibenzalacetone (250 MHz). (An expanded spectrum appears above the normal spectrum.)... 

From the H nmr spectrum of dibenzalacetone (Fig. 1) can you deduce what geometric isomer(s) is (are) formed ... 

The aldol reaction (aldol condensation) is one of the fundamental reactions of organic chemistry, since it leads to the formation of a new carbon-carbon bond and is broadly applicable. A condensation reaction is one in which two molecules are joined with the concomitant expulsion of a small stable molecule, usually water or an alcohol. The aldol reaction may be used to condense various combinations of aldehydes and ketones. The mixed aldol condensation of an aldehyde having no a-hydrogen atom with a ketone is specifically known as the Claisen-Schmidt reaction. This variation of the aldol condensation is illustrated here in the synthesis of dibenzalacetone. 

In a 500 ml. wide-mouthed reagent bottle place a cold solution of 25 g. of sodium hydroxide in 250 ml. of water and 200 ml. of alcohol (1) equip the bottle with a mechanical stirrer and surround it with a bath of water. Maintain the temperature of the solution at 20-25°, stir vigorously and add one-half of a previously prepared mixture of 26-5 g. (25 -5 ml.) of purebenzaldehyde (Section IV,115) and 7 -3 g. (9-3 ml.) of A.R. acetone. A flocculent precipitate forms in 2-3 minutes. After 15 minutes add the remainder of the benzaldehyde - acetone mixture. Continue the stirring for a further 30 minutes. Filter at the pump and wash with cold water to eliminate the alkali as completely as possible. Dry the solid at room temperature upon filter paper to constant weight 27 g. of crude dibenzalacetone, m.p. 105-107°, are obtained. Recrystallise from hot ethyl acetate (2-5 ml. per gram) or from hot rectified spirit. The recovery of pure dibenzalacetone, m.p. 112°, is about 80 per cent. 

Sufficient alcohol is used to dissolve the benzaldehyde rapidly, as well as to retain the benzalacetone in solution until it has had time to react with the second molecule of aldehyde. Lower concentrations of base slow up the formation of the dibenzalacetone and thus favor side reactions which tend to yield a sticky product. Higher concentrations of base give added difficulty in washing. These concentrations were suggested by, and are approximately the same as, those used in the preparation of benzalacetophenone described in Org. Syn. 7, r. 

Dibenzalacetone has been prepared by condensing benzal-dehyde with acetone using as condensing agents dry hydrogen chloride,1 io per cent sodium hydroxide solution,2 and glacial acetic acid with sulfuric acid.3 It has also been obtained by condensing benzalacetone with benzaldehyde in the presence of dilute sodium hydroxide.4 Straus and Ecker 5 were the first to record the use of ethyl acetate for crystallization. 

Both the cis- and the trans-disubstituted spiranes resulted, in different ratios, depending on the reaction conditions. Clearly, the trans spiranes are chiral. The first conjugate addition to the Michael acceptors 75a-c is intermolecular in nature and defines the sense of chirality at the first chiral center. Subsequent intramolecular ring closure to the spiranes 76 defines the cis or trans configuration of the product. When cyclohexane-1,3-dione (74a) was reacted with dibenzalacetone (75a) in the presence of ca 5 mol% (—)-quinine (3a, Scheme 4.3), a 2.5 1 trans/cis mixture resulted, with the trans isomer 76 having optical purity of ca 30% (Scheme 4.37) [61] (the absolute configuration of the predominant enantiomer was not assigned). 

The crossed condensation of an aromatic aldehyde with a ketone usually gives a high yield of the unsaturated ketone directly. Acetone is condensed with either one or two molecules of benzaldehyde to give ben-zalacetone (68%) or dibenzalacetone (94%), respectively. Alkyl stytyl ketones, CjHs CH = C(R)COR, have been prepared from benzalde-hyde and higher ketones in the presence of hydrochloric acid or alkali hydroxide. Substituents on the phenyl group include methyl, hydroxyl, methoxyl, and nitro groups. A survey of condensa-... 

The reaction of an aldehyde with a ketone employing sodium hydroxide as the base is an example of a mixed aldol condensation reaction, the Claisen-Schmidt reaction. Dibenzalacetone is readily prepared by condensation of acetone with two equivalents of benzaldehyde. The aldehyde carbonyl is more reactive than that of the ketone and therefore reacts rapidly with the anion of the ketone to give a /3-hydroxyketone, which easily undergoes base-catalyzed dehydration. Depending on the relative quantities of the reactants, the reaction can give either mono- or dibenzalacetone. 

In the present experiment sufficient ethanol is present as solvent to readily dissolve the starting material, benzaldehyde, and also the intermediate, benzalacetone. The benzalacetone, once formed, can then easily react with another mole of benzaldehyde to give the product, dibenzalacetone. The detailed mechanism for the formation of benzalacetone is ... 

Depending upon reaction conditions, dibenzalacetone and ethyl malonate can be made to yield any of three products by Michael addition. 

Place in a small flask 1 cc. of acetone, 4 cc. of water, 4 cc. of benzaldehyde, 20 cc. of alcohol, and 5 cc. of a 10 per cent solution of sodium hydroxide. Boil gently for 5 minutes. Cool and shake. Filter off the crystals and wash them with 20 cc. of cold alcohol. Recrystallize from 20 cc. of boiling alcohol. Let the solution cool. Filter and wash with 10 cc. of cold alcohol. Dry on a porous plate and determine the melting-point of the crystals. Dibenzalacetone crystallizes in yellow plates and melts at 111°-112°. 

A feature of the book is the introduction of directions for the preparation of certain compounds on a very small scale. Students often acquire the habit of careless work in the laboratory practice in organic chemistry. Preparation-work on the small scale serves to counteract this effect and to develop a technique that is valuable. Such work is often necessary in the identification of unknown compounds when a small amount only of the substance is available. In many cases a crystalline derivative whose melting-point can be determined, can be prepared in a pure condition from but two or three drops of a substance. Among the examples of work of this kind which are given are the preparation of acetanilide from acetic acid, glyceryl tribenzoate from glycerol, dinitrobenzene from benzene, and dibenzalacetone from acetone. In order to facilitate such work, a section in the first chapter is devoted to a consideration of the technique used in the manipulation of small quantities of substances. 

In sharp contrast to the predominant formation of 1 1 adducts, reactions of 1,3-dienes with various disilanes in the presence of phosphine-free palladium catalyst provide bis-silylative dimerization products 63 in high yields (Eq. 30) [58,59]. In comparison with the Pd(OAc)2 and PdCl2(ArCN)2 catalysts originally found [58], the use of Pd(dba)2 (dba dibenzalacetone) in DMF was found to be more effective in enabling the reaction to proceed at room temperature [59]. Worthy of note is that head-to-head coupling products 63 having carbon-carbon double bonds with E-geometry are exclusively formed in these reactions. 

---