Acetaldehyde in aqueous solution

It is proposed to reduce the concentration of acetaldehyde in aqueous solution from 50 per cent to 5 per cent by mass, by extraction with solvent S at 293 K. If a countercurrent multiple-contact process is adopted and 0.025 kg/s of the solution is treated with an equal quantity of solvent, determine the number of theoretical stages required and the mass flowrate and concentration of the extract from the first stage. 

The complex ion catalyzes various types of organic reactions including oxidation of ethylene to acetaldehyde in aqueous solution (the Wacker Process) ... 

The Wacker process (Eq. 1) was developed nearly 50 years ago [1-3] and represents one of the most successful examples of homogeneous catalysis in industry [4-9]. This palladium-catalyzed method for the oxidation of ethylene to acetaldehyde in aqueous solution employs a copper cocatalyst to facilitate aerobic oxidation of Pd° (Scheme 1). Despite the success of this process, certain features of the reaction have Umited the development of related aerobic oxidation reactions. Many organic molecules are only sparingly sol-... 

Schuchmann MN, von Sonntag C (1988) The rapid hydration of the acetyl radical. A pulse radiolysis study of acetaldehyde in aqueous solution. J Am Chem Soc 110 5698-5701 Schuchmann MN, von Sonntag C, Tsay YH, Kruger C (1981) Crystal structure and the radiation-induced free radical chain- reaction of 2-deoxy-p-D-erythro-pentopyranose in the solid state. Z Naturforsch 36b 726-731... 

Schuchmann MN, von Sonntag C. (1988) The rapid hydration of the acetyl radical. A pulse radiolysis study of acetaldehyde in aqueous solution. /Aw Chem Soc 110 5698-5701. 

Hirshberg and Farkas photolyzed acetaldehyde in aqueous solution by the full light of a mercury arc at room temperature and at aldehyde concentrations of 10 -10 M. They concluded that the main product of the photolysis was aldol, formed in an association reaction of an excited and a normal aldehyde molecule. 

PROBLEM 16.9 Why does acetaldehyde in aqueous solution form an excellent yield of cyanohydrin even though acetaldehyde is substantially hydrated in water K 1) Might it not be argued that, because about one-half of the aldehyde is present in hydrated form at equilibrium, no more than about 50% cyanohydrin can be formed. Why is this argument wrong Hint. See Rgure 16.34. 

Acetaldehyde, CH3CHO, b.p. 21°, is generally used in aqueous solution, which has also a characteristic odour paraldehyde, (CH3CHO)3, is a liquid polymer, b.p. 124°, slightly soluble in water, odour similar to that of acetaldehyde, but less intense. Chloral, CCI3CHO, a liquid, is almost invariably encountered as the stable solid hydrate , CCl3CH(OH)2, m.p. 57 . Both have a characteristic odour the hydrate is readily soluble in water. 

The Hydrate and Enol Form. In aqueous solutions, acetaldehyde exists in equihbrium with the acetaldehyde hydrate [4433-56-17, (CH2CH(0H)2). The degree of hydration can be computed from an equation derived by BeU and Clunie (31). Hydration, the mean heat of which is —21.34 kJ/mol (—89.29 kcal/mol), has been attributed to hyperconjugation (32). The enol form, vinyl alcohol [557-75-5] (CH2=CHOH) exists in equihbrium with acetaldehyde to the extent of approximately 1 molecule per 30,000. Acetaldehyde enol has been acetylated with ketene [463-51-4] to form vinyl acetate [108-05-4] (33). 

In contrast to the hydrolysis of prochiral esters performed in aqueous solutions, the enzymatic acylation of prochiral diols is usually carried out in an inert organic solvent such as hexane, ether, toluene, or ethyl acetate. In order to increase the reaction rate and the degree of conversion, activated esters such as vinyl carboxylates are often used as acylating agents. The vinyl alcohol formed as a result of transesterification tautomerizes to acetaldehyde, making the reaction practically irreversible. The presence of a bulky substituent in the 2-position helps the enzyme to discriminate between enantiotopic faces as a result the enzymatic acylation of prochiral 2-benzoxy-l,3-propanediol (34) proceeds with excellent selectivity (ee > 96%) (49). In the case of the 2-methyl substituted diol (33) the selectivity is only moderate (50). 

Two examples are (1) the thermal, gas-phase decomposition of acetaldehyde at high temperatures and (2) the reaction of the hydrated 2-propylchromium ion with molecular oxygen in aqueous solution. The reactions and their rate laws are as follows ... 

R)-Benzoins and (/ )-2-hydroxypropiophcnonc derivatives are formed on a preparative scale by benzaldehyde lyase (BAL)-catalyzed C-C bond formation from aromatic aldehydes and acetaldehyde in aqueous buffer/DMSO solution with remarkable ease in high chemical yield and high optical purity (Eq. 8.112).303 Less-stable mixed benzoins were also generated via reductive coupling of benzoyl cyanide and carbonyl compounds by aqueous titanium(III) ions.304... 

Hydroxy-perhydropyrimido[2,l- ][l,4]oxazine 287 was obtained in 38% yield from 3-(2-hydroxyethyl)-l,3-propa-nediamine and glyoxal in aqueous solution. A small amount of the pentacyclic 319 also formed. Using acetaldehyde the 9-Me homologue formed in 56% yield, without the pentacyclic compound <1996H(43)1991>. 

For the reaction between hydrocyanic acid (HCN) and acetaldehyde (CH3CHO) in aqueous solution,... 

The reactions are accompanied by a considerable volume change, and a dilatometric method was employed by Bell and Higginson (1949), who added acetaldehyde-water mixtures (containing about equal quantities of MeCHO and MeCH(OH)2) to an excess of acetone, and thus measured kj, in presence of a large number of acid catalysts. The direct hydration of acetaldehyde in aqueous buffer solutions is inconveniently fast at room temperatures, but ( (j + A ) was measured dilatometrically at 0°C by Bell and Darwent (1950), who established the existence of general acid-base catalysis. 

There are three environments that we may consider for this reaction iyw-acetaldehyde oxime data in the condensed phase , the gas phase and in aqueous solution . The reaction exothermicities are 51.0, 56.5 and ca 53 kJmoD, respectively. The only other case where the values are known for all three phases is that of acetone, equation 28 ... 

At the lowest concentrations of 2-propanol, 10"2 M, there is reasonable agreement between calculated and experimental results. These results then are consistent with the assumption that acetone is the only oxidation product from the radical produced from 2-propanol in aqueous solution containing oxygen. It has similarly been shown (23) that acetaldehyde is the only organic oxidation product in the radiation-induced oxidation of aqueous ethanol. However, our experiments indicate that in... 

Recently, researchers at Catalytica proposed a new technology for ethylene oxidation (368). Typical compositions are aqueous ca. O.l mM Pd2+, 5-25 mM Cl , and ca. 0.30 M NavH(3+ -v)PVxMoi2 04o (preferably x = 2-3). The Pd2+ and chloride concentrations are only 1/100 those in the oridinary Wacker system. The solutions at pHO-l result in high reaction rates and stability of Pd2+, as shown in Fig. 65. The stability of Pd2+ is further improved by the presence of chloride ion in a concentration of about 0.01 M. In this system, the phosphomolybdate serves two functions in the Pd° reoxidation (l) It solubilizes high concentrations of Vs + in aqueous solution and (2) it accelerates the reoxidation of V4+ by dioxygen. Kinetics (the reaction is first-order in Pd2+ and in ethylene concentrations and zero-order in Vs + concentration) shows that the oxidation of ethylene to produce acetaldehyde is rate-determining. 

Woolley230 has found that a number of hydroxo metal complexes of the macrocycle (64) are active in the hvdration of C02 and acetaldehyde. Electronic spectroscopy231 and X-ray crystallography232 suggest coordination numbers in aqueous solution of five for [CoL]2+ and [ZnL]z+, and six for [NiL]2+ and [CuL]2+. For the [ZnL]2+ system, the p/Ca of the coordinated water molecule is 8.69 at 25 °C and the hydroxo complex is a reasonable catalyst for C02 hydration. The complex [Cu(glycylglycinate)OH] is also active in C02 hydration.233... 

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