Aldol, from acetaldehyde

The corrosion of metal surfaces and the precipitation of a metal sulfide by an aqueous acid solution can be prevented by an aldol-amine adduct. Aldol (from acetaldehyde) CH3CH(OH)CH2CHO has been utilized as a H2S scavenger that prevents the precipitation of metal sulfides from aqueous acid solutions. However, when the aldol or an aqueous solution of the aldol is stored, the solution separates quickly into two layers, with all of the aldol concentrated in the bottom layer. The bottom layer is not redispersible in the top layer or in water or acid. In addition, the aldol in the bottom layer has very little activity as a sulfide scavenger. Thus the use of aldol as a H2S scavenger in aqueous acid solutions can result in unsatisfactory results [245,247]. However, the aldol can be reacted with an amine, such as monoethanoleamine (=aminoethanol), to form an aldol-amine adduct to overcome these difficulties. The amine utilized to prepare the aldol-amine adduct must be a primary amine. The aldol-amine adduct preferentially reacts with sulfide ions when they are dissolved in the... 

The -butyraldehyde may be obtained from acetaldehyde [75-07-0] by aldol addition followed by hydrogenation, or from propylene by the 0x0 process. This latter process is predominantly favored (Eig. 7). 

The reaction can be stopped at this point, but more often a second mole of formaldehyde is permitted to reduce the newly formed aldol to a 1,3-diol, in a crossed Cannizzaro reaction (19-60). If the aldehyde or ketone has several a-hydrogens, they can all be replaced. An important use of the reaction is to prepare pentaerythritol from acetaldehyde ... 

It is often said that the property of acidity is manifest only in the presence of a base, and NMR studies of probe molecules became common following studies of amines by Ellis [4] and Maciel [5, 6] and phosphines by Lunsford [7] in the early to mid 80s. More recently, the maturation of variable temperature MAS NMR has permitted the study of reactive probe molecules which are revealing not only in themselves but also in the intermediates and products that they form on the solid acid. We carried out detailed studies of aldol reactions in zeolites beginning with the early 1993 report of the synthesis of crotonaldehyde from acetaldehyde in HZSM-5 [8] and continuing through investigations of acetone, cyclopentanone [9] and propanal [10], The formation of mesityl oxide 1, from dimerization and dehydration of... 

The formation of an aldol by the union of two molecules of aldehyde in presence of alkalies is an important example under this heading. The formation of acetaldol from acetaldehyde is as follows ... 

The Cannizzaro reaction is by no means confined to aromatic aldehydes. Formaldehyde undergoes the same change, yielding formic acid and methyl alcohol. That the aliphatic aldehydes from acetaldehyde upwards do not undergo the reaction is due to the fact that the aldol condensation (mentioned above), in virtue of its much greater velocity, takes precedence over the Cannizzaro reaction. 

The namesake aldol reaction is the formation of an addition compound, aldol, from two molecules of acetaldehyde, when this aldehyde is treated with aqueous sodium hydroxide. The terminology aldol comes from the functional groups in the product, aldehyde and alcohol. 

In contrast to transketolase and the DHAP-dependent aldolases, deoxyribose aldolase (DERA) catalyzes the aldol reaction with the simple aldehyde, acetaldehyde. In vivo it catalyzes the formation of 2-deoxyribose-5-phosphate, the building block of DNA, from acetaldehyde and D-glyceraldehyde-3-phosphate, but in vitro it can catalyze the aldol reaction of acetaldehyde with other non-phosphorylated aldehydes. The example shown in Scheme 6.28 involves a tandem aldol reaction... 

It may be stated at this point that the presence of a /3-hydroxy-butyrate fat in certain organisms is a matter of general biochemical importance. Usually /3-hydroxybutyric acid and the acetone bodies are derived from n-butyric acid directly. The unambiguous formation of jS-hydroxybutyric acid anhydrides from carbohydrates opens up new vistas its formation from acetaldehyde, and from pyruvic acid, through aldol intermediates can be understood without difficulty. Kirrmann s reaction, to which little attention has been paid, is at the same time an example of an oxygen shift, leading from hydroxyaldehydes to fatty acids. 

Another oxo plant, now being constructed, will make butyl compounds (88). These may be the source of butyl alcohol, butyl acetate, butyric acid for the manufacture of cellulose acetate butyrate and other products, butyraldehyde for polyvinyl butyral, and the eight-carbon compounds including 2-ethylhexanol. All these will add to the present production of the same compounds made by the older methods from acetaldehyde via aldol condensation. 

The silatropic ene pathway, that is, direct silyl transfer from an silyl enol ether to an aldehyde, may be involved as a possible mechanism in the Mukaiyama aldol-type reaction. Indeed, ab initio calculations show that the silatropic ene pathway involving the cyclic (boat and chair) transition states for the BH3-promoted aldol reaction of the trihydrosilyl enol ether derived from acetaldehyde with formaldehyde is favored [60], Recently, we have reported the possible intervention of a silatropic ene pathway in the catalytic asymmetric aldol-type reaction of silyl enol ethers of thioesters [61 ]. Chlorine- and amine-containing products thus obtained are useful intermediates for the synthesis of carnitine and GABOB (Scheme 8C.26) [62],... 

In a reaction which is mechanistically related to the Skraup reaction an a,/ -unsaturated carbonyl compound, generated by way of an acid-catalysed aldol condensation, reacts with a primary aromatic amine in the presence of acid to yield a quinoline derivative (Doebner-Miller reaction). For example, when aniline is heated with paraldehyde (which depolymerises to acetaldehyde during the reaction) in the presence of hydrochloric acid the final product is 2-methyl-quinoline (101) (quinaldine, Expt 8.40). Retrosynthetic analysis for the 1,2-dihydroquinoline reveals crotonaldedhyde as the unsaturated carbonyl component which is in turn formed from acetaldehyde (see Section 5.18.2, p. 799). 

Extensions of the proline-catalyzed aldol reaction Recently interesting extensions of the enantioselective proline-catalyzed aldol reaction have been reported. An enan-tioselective proline-catalyzed self-aldolization of acetaldehyde was observed by Barbas and co-workers (Scheme 6.21) [77]. Starting from acetaldehyde, the valuable building block 5 -h ydroxy-( 2E)-hexcnal, (S)-43, was obtained as a product with up to 90% ee, although the yield did not exceed 13%, irrespective of the reaction conditions. This reaction requires a small amount catalyst only (ca. 2.5 mol%). 

Pd-containing aluminophosphate molecular sieves have been used to carry out crossed aldol condensations between an aldehyde and a ketone by using a 0.5 % Pd/ MnAPSO-31 catalyst in a vapour-phase fixed bed reactor.[14] Thanks to the excess of the ketone with respect to the aldehyde (4 1), it is possible to get high selectivity to the desired product, i.e. 70 % of heptan-2-one from n-butyraldehyde and acetone and 89 % of pentan-2-one from acetaldehyde and acetone, the major by-product being, in both cases, MIBK from acetone self-condensation. 

For the thiophen mentioned above, all the carbon atoms stemmed from cysteine, carbohydrate apparently not having a role. On the other hand, theoretically, thiophens can be derived from furans simply by reaction with H2S. Belitz and Grosch251 postulate derivation from 2-mercaptoethanal (Strecker aldehyde from cysteine) and acrolein or butenal (aldol condensation product from acetaldehyde). Vemin and Parkanyi216... 

Although known as an explosive since 1894, PETN was used very little until after World War I when the ingredients to make the starting material became commercially available. The symmetrical, solid alcohol starting material, pentaerythritol, is made from acetaldehyde and formaldehyde, which react by aldol condensation under basic catalysis followed by a crossed Cannizzaro disproportionation to produce the alcohol and formate salt. Although the reaction takes place in a single mixture, it is shown below in two steps for clarity. 

During investigations with RibA it had been realized by serendipity that the initial product 52 from catalyzed self-aldolization of acetaldehyde again serves as a suitable acceptor for the sequential addition of a second donor molecule to... 

Aldol Condensation.— In discussing the addition products formed from acetaldehyde (p. 116), it was stated that it forms a condensation product with a second molecule of itself. The product is aldol, the reaction being known as the aldol condensation. 

Practice Problem 23.2 What is the structure of the enone obtained from aldol condensatioDj acetaldehyde ... 

The mechanism of many condensation reactions has been shown to comprise consecutive ADDITION and elimination reactions, as in the base-catalyzed formation of fEj-but-2-enal (crotonaldehyde) from acetaldehyde, via 3-hydroxybutanal (aldol). The overall reaction in this example is known as the aldol condensation. 

Reduction. The familiar preparation of pentaerythritol from acetaldehyde and formaldehyde involves aldolization and crossed Cannizzaro reduction. In the same way, cyclohexanone reacts with 5 moles of formaldehyde to give the pentaol (3). The reaction is conducted by adding calcium oxide (1,25 moles) to a stirred mixture O OH... 

CAS 104-76-7. CH3(CH2)3CHC2H5CH2OH. Properties Colorless liquid. D 0.83 (20C), bp 183.5C, fp -76C, vap press 0.36 (20C), refr index 1.4300 (20C), bulk d 6.9 lb/gal (20C), flash p 178F (81.1C). Miscible with most organic solvents, slightly soluble in water. Combustible. Derivation (a) Oxo process from propylene and synthesis gas (b) aldolization of acetaldehyde or butyraldehyde, followed by hydrogenation (c) from fermentation alcohol. 

The synthesis of 2 4-picolines from acetaldehyde and ammonia is assumed to be a complex multistep reaction, comprising condensation (Aldol or Michel type) cyclisation and dehydrogenation. The plausible route for the formation of 2 4-picoline may be assumed as follows (Fig.5)... 

Water is removed with especial ease from alcohols whose hydroxyl group is in a /9-position to an oxo, alkoxycarbonyl, nitrile, or nitro group. For example, on aldolization of acetaldehyde the acid present in the reaction medium suffices for dehydration of the product to the unsaturated carbonyl compound, i.e., to crotonaldehyde. One hour s boiling with dilute sulfuric acid suffices for preparation of 3-hepten-2-one from 4-hydroxy-2-heptanone 39... 

Reaction of a primary aromatic amine (such as aniline) with a carhonyl compound such as acetaldehyde, in the presence of acid, also generates a 2-alkylquinoline in the Doebner-Miller reaction. Reaction of aniline with acetaldehyde gave 262, but a subsequent aldol condensation with the imine derived from acetaldehyde (sec. 9.4.F.i) led to 263.1 Friedel-Crafts cyclization was followed by loss of aniline to give 264, and aromatization gave 2-methylquinoline (265). 

Especially in the range of Cg-Ci3 the 0x0 route holds the dominating position since the other routes, like the aldol route starting from acetaldehyde, the hydrogenation of fatty acid esters or the Ziegler alcohol route, can hardly compete pricewise [342, 899]. 

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