Acetaldehyde, condensation

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

Reactions with Ammonia and Amines. Acetaldehyde readily adds ammonia to form acetaldehyde—ammonia. Diethyl amine [109-87-7] is obtained when acetaldehyde is added to a saturated aqueous or alcohoHc solution of ammonia and the mixture is heated to 50—75°C in the presence of a nickel catalyst and hydrogen at 1.2 MPa (12 atm). Pyridine [110-86-1] and pyridine derivatives are made from paraldehyde and aqueous ammonia in the presence of a catalyst at elevated temperatures (62) acetaldehyde may also be used but the yields of pyridine are generally lower than when paraldehyde is the starting material. The vapor-phase reaction of formaldehyde, acetaldehyde, and ammonia at 360°C over oxide catalyst was studied a 49% yield of pyridine and picolines was obtained using an activated siHca—alumina catalyst (63). Brown polymers result when acetaldehyde reacts with ammonia or amines at a pH of 6—7 and temperature of 3—25°C (64). Primary amines and acetaldehyde condense to give Schiff bases CH2CH=NR. The Schiff base reverts to the starting materials in the presence of acids. 

Union Carbide abandoned the ketene—crotonaldehyde route in 1953 in favor of the oxidation of 2,4-hexadienal made by acetaldehyde condensation. A silver compound used as the catalyst prevented peroxidation of the ethylenic bonds (39,40). Thein plant operated until 1970. 

Chain transfer also occurs to the emulsifying agents, leading to their permanent iacorporation iato the product. Chain transfer to aldehydes, which may be formed as a result of the hydrolysis of the vinyl acetate monomer, tends to lower the molecular weight and slow the polymerisation rate because of the lower activity of the radical that is formed. Thus, the presence of acetaldehyde condensates as a poly(vinyl alcohol) impurity strongly retards polymerisation (91). Some of the initiators such as lauryl peroxide are also chain-transfer agents and lower the molecular weight of the product. 

The earhest commercial route to -butyraldehyde was a multistep process starting with ethanol, which was consecutively dehydrogenated to acetaldehyde, condensed to crotonaldehyde, and reduced to butyraldehyde. In the late 1960s, production of -butyraldehyde (and isobutyraldehyde) in Europe and the United States switched over largely to the Oxo reaction of propylene. 

In some parts of the world, as in Russia, fermented alcohol can serve as a cheap source for hutadiene. The reaction occurs in the vapor phase under normal or reduced pressures over a zinc oxide/alumina or magnesia catalyst promoted with chromium or cohalt. Acetaldehyde has been suggested as an intermediate two moles of acetaldehyde condense and form crotonaldehyde, which reacts with ethyl alcohol to give butadiene and acetaldehyde. 

Another specific and important aspect to consider is the possibility that an environmentally heterogeneous photocatalyst can lead to the undesirable formation of reaction intermediates which are more toxic than the starting reagents. For instance, the Ti02-based photodegradation of ethanol, a relatively innocuous air pollutant, occurs through its transformation into the more toxic acetaldehyde. Condensation reactions can also lead to the formation of traces of methyl formate, ethyl formate, or methyl acetate. Catalyst design is therefore important to increase the overall oxidation rate to ensure complete mineralization (formation of C02 and H20). 

Halcon has developed a new non-noble metal catalyst for methanol reductive carbonylation (32). It is formed under more moderate conditions (1200 psi, 120 C) and permits a selective reaction at only 1200-1800 psi of reaction pressure. Under these conditions, the catalyst s activity is comparable with noble metal catalyzed carbonylations. The conversion rate is 1.5-3.0 mol/l.hr. and acetaldehyde selectivity is 85%. In concentrated solutions, a considerable portion of product acetaldehyde (20-40%) is converted to its acetal. The acetal can be readily hydrolyzed back to acetaldehyde at 100-150 without catalyst (33). Acetal formation is actually beneficial through prevention of undesirable acetaldehyde condensation reactions. 

Condensation of DAMN with aldehydes is not always straightforward. At pH 6.8, 3 mol acetaldehyde condense with 1 mol DAMN to give the tricyclic heterocycle 15 (75JOC2678). 

In the case of ketoximes and acetylene in an aqueous alkaline medium the following condensation processes leading finally to pyridines are conceivable. (i) Dimerization of ketoximes with elimination of hydroxylamine (analog of crotonic condensation) (Scheme SO), (ii) Acetaldehyde condensation with the oxime of a,/3-un saturated ketone 107. 

Other reactions are the acetaldehyde condensation to butanal (Eq. 7) followed by reduction to alcohol (Eq. 8) or reaction with formaldehyde to form 2-methy I b u tanal, which is reduced to the corresponding alcohol (Eq. 9). Other reactive combinations give condensation products, provided that one of the reactants has hydrogens on the carbon adjacent to the carbon bearing the hydroxyl group, so that the carbanion intermediate can form... 

In both processes the aqueous crude aldehyde is concentrated and byproducts are removed in a two-step distillation. Both processes give 94% yields of aldehyde, along with small amounts of 2-chloroethanol, ethyl chloride, acetic acid, chloroacetaldehydes and acetaldehyde condensation products. The Wack-er-Hoechst process currently accounts for 85% of the worldwide production capacity for acetaldehyde. 

Thus the gross differences in the rate laws for acetone and acetaldehyde condensation arise not from differences in reaction mechanism, but rather from differences in the relative rates of attack by enolate ion on reactant. In principle, at sufficiently low acetaldehyde concentrations, the rate law for the acetaldehyde should approach that for acetone. 

Saucier, C., Guerra, C., Pianet, I., Laguerres, M., Glories, Y. (1997). (-l-)-Catechin-acetaldehyde condensation products in relation to wine-ageing. Phytochemistry 46, 229-234. 

Very slowly. The temperature in the fractionating column is not allowed to rise much above room temperature, and therefore any paraldehyde that boils off with the acetaldehyde condenses and falls back into the flask. When the distillation is complete, note whether any liquid is left in the flask. The acetaldehyde is kept at a temperature of 5-10°, and used as required. 

In the 1990s, BP re-examined the iridium-catalyzed methanol carbonylation chemistry first discovered by Paulik and Roth and later defined in more detail by Forster [20]. The thrust of this research was to identify an improved methanol carbonylation process using Ir as an alternative to Rh. This re-examination by BP led to the development of a low-water iridium-catalyzed process called Cativa [20]. Several advantages were identified in this process over the Rh-catalyzed high-water Monsanto technology. In particular, the Ir catalyst provides high carbonylation rates at low water concentrations with excellent catalyst stability (less prone to precipitation). The catalyst system does not require high levels of iodide salts to stabilize the catalyst. Fewer by-products are formed, such as propionic acid and acetaldehyde condensation products which can lead to low levels of unsaturated aldehydes and heavy alkyl iodides. Also, CO efficiency is improved. 

Two stereoisomeric macrocyclic resorcinol-acetaldehyde condensation products,... 

The incorporation of acetaldehyde derived bridges between anthocyanins and flavan-3-ols via acetaldehyde condensation reactions has been well described in fermented beverages such as red wine. The presence of acetaldehyde in alcoholic solutions is attributed to either oxidatative products of ethanol or microbial byproducts. In the case of cranbeny fruit and spray dried juice neither product was subjected to yeast fermentation. It becomes a concern then that die observed anthocyanin-pigments may be an arti ct of harvest, storage, juice processing or analytic techniques. 

Pentaerythritol is made by reacting formaldehyde with acetaldehyde, condensing them together with the help of calcium hydroxide. 

Hogberg, A.G.S. Two stereoisomeric macrocyclic resorcinol-acetaldehyde condensation products. J. Org. Chem. 1980. 45. 4498. 

Phenylacetaldehyde (120 g, 1 mol) and 0.5 mol of another aldehyde component were dissolved in 350 mL saturated solution of ammonia in absolute ethanol. The solution was sealed in a 1-L stainless-steel bomb and heated at 225-230°C for 6 h with constant rocking. The pressure was 1000 psi. The bomb was then cooled and opened the contents was transferred to a distilling flask. As much ethanol as possible was distilled on the steam bath at atmospheric pressure. The residue was subjected to a preliminary distillation in which everything boiling up to 280°C (ImmHg) was collected except in the case of the acetaldehyde condensation, in which a preliminary distillation with superheated steam at 200°C was resorted to (omission of this preliminary treatment led to difficulties in the subsequent extraction step). The distillate was taken up in ether and extracted 15 times with 2 N HCl. The residual ether layer was discarded, and the aqueous layer plus a third layer—which invariably formed—were made basic with an excess of concentrated ammonium hydroxide. The bases liberated were extracted with ether and dried over potassium carbonate the solution was then concentrated. 

It is believed that this reaction involves the following consecutive steps dehydrogenation of ethanol to acetaldehyde, condensation of acetaldehyde into acetaldol, dehydration of aldol product to crotonaldehyde, and deoxygenation of crotonaldehyde. This mechanism is illustrated here. 

By-products Methane, Acetaldehyde, Ethanol, CO2 Propionic acid. Acetaldehyde Condensation products Negligible... 

Saucier C, Guerra C, Laguerre M, Glories Y (1997) (-t)-catechin-acetaldehyde condensation products in relation with wine-ageing. Phytochemistry 46 229-234... 

---