Acetaldehyde Specifications

Commercial acetaldehyde has the following typical specifications assay, 99% min color, water-white acidity, 0.5% max (acetic acid) specific gravity, 0.790 at 20°C bp, 20.8°C at 101.3 kPa (1 atm). It is shipped in steel dmms and tank cars bearing the ICC red label. In the Hquid state, it is noncorrosive to most metals however, acetaldehyde oxidizes readily, particularly in the vapor state, to acetic acid. Precautions to be observed in the handling of acetaldehyde have been pubHshed (103). 

Specifications and Analytical Methods. Vinyl ethers are usually specified as 98% minimum purity, as determined by gas chromatography. The principal impurities are the parent alcohols, limited to 1.0% maximum for methyl vinyl ether and 0.5% maximum for ethyl vinyl ether. Water (by Kad-Fischer titration) ranges from 0.1% maximum for methyl vinyl ether to 0.5% maximum for ethyl vinyl ether. Acetaldehyde ranges from 0.1% maximum in ethyl vinyl ether to 0.5% maximum in butyl vinyl ether. 

Acrolein is produced according to the specifications in Table 3. Acetaldehyde and acetone are the principal carbonyl impurities in freshly distilled acrolein. Acrolein dimer accumulates at 0.50% in 30 days at 25°C. Analysis by two gas chromatographic methods with thermal conductivity detectors can determine all significant impurities in acrolein. The analysis with Porapak Q, 175—300 p.m (50—80 mesh), programmed from 60 to 250°C at 10°C/min, does not separate acetone, propionaldehyde, and propylene oxide from acrolein. These separations are made with 20% Tergitol E-35 on 250—350 p.m (45—60 mesh) Chromosorb W, kept at 40°C until acrolein elutes and then programmed rapidly to 190°C to elute the remaining components. 

Interest ia the toxicity of aldehydes has focused primarily on specific compounds, particularly formaldehyde, acetaldehyde, and acroleia (13). Litde evidence exists to suggest that occupational levels of exposure to aldehydes would result ia mutations, although some aldehydes are clearly mutagenic ia some test systems. There are, however, acute effects of aldehydes. 

The fermentation-derived food-grade product is sold in 50, 80, and 88% concentrations the other grades are available in 50 and 88% concentrations. The food-grade product meets the Vood Chemicals Codex III and the pharmaceutical grade meets the FCC and the United States Pharmacopoeia XK specifications (7). Other lactic acid derivatives such as salts and esters are also available in weU-estabhshed product specifications. Standard analytical methods such as titration and Hquid chromatography can be used to determine lactic acid, and other gravimetric and specific tests are used to detect impurities for the product specifications. A standard titration method neutralizes the acid with sodium hydroxide and then back-titrates the acid. An older standard quantitative method for determination of lactic acid was based on oxidation by potassium permanganate to acetaldehyde, which is absorbed in sodium bisulfite and titrated iodometricaHy. 

Figure 21.2a shows a sample/reference half-cell pair for measurement of the standard reduction potential of the acetaldehyde/ethanol couple. Because electrons flow toward the reference half-cell and away from the sample half-cell, the standard reduction potential is negative, specifically —0.197 V. In contrast, the fumarate/succinate couple and the Fe /Fe couple both cause electrons to flow from the reference half-cell to the sample half-cell that is, reduction occurs spontaneously in each system, and the reduction potentials of both are thus positive. The standard reduction potential for the Fe /Fe half-cell is much larger than that for the fumarate/ succinate half-cell, with values of + 0.771 V and +0.031 V, respectively. For each half-cell, a half-cell reaction describes the reaction taking place. For the fumarate/succinate half-cell coupled to a H Hg reference half-cell, the reaction occurring is indeed a reduction of fumarate. 

Biological catalysts are called enzymes. Nearly every step of the breakdown of a complex molecule to a series of smaller ones, within living cells, is catalyzed by specific enzymes. For instance, when acetaldehyde is reduced in yeast... 

The phosphonatase (hydrolase) pathway is less widely used, typically for the degradation of 2-aminoethylphosphonate via phosphonoacetaldehyde to acetaldehyde, and the degradation of phosphonoacetate that involves a specific hydrolase (Kulakova et al. 2001). 

These enzymes catalyse the non-hydrolytic cleavage of bonds in a substrate to remove specific functional groups. Examples include decarboxylases, which remove carboxylic acid groups as carbon dioxide, dehydrases, which remove water, and aldolases. The decarboxylation of pyruvic acid (10.60) to form acetaldehyde (10.61) takes place in the presence of pyruvic decarboxylase (Scheme 10.13), which requires the presence of thiamine pyrophosphate and magnesium ions for activity. 

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). 

H2 production technologies based on natural gas. Operating the reaction at relatively lower temperature, between 300 and 450 °C could minimize the CO formation because the equilibria for WGS and CO oxidation reactions are thermodynamically more favorable at lower temperatures. In order to achieve this goal, highly selective catalysts that are specific for reforming via acetaldehyde formation rather than ethanol decomposition to CH4 and/or ethylene are required. The success in the development of ethanol-based H2 production technology therefore relies on the development of a highly active, selective and stable catalyst. 

The specific compounds that are responsible for the "pea" flavor have not been identified. Bengtsson and Bosund (52) suggested that acetaldehyde, hexanal and ethanol were important, while Murray et al. (53) isolated three methoxypyrazines that have very low taste or recognition thresholds and might, therefore, be of major significance in pea flavor. 

Poly(vinyl alcohol) (PVA) is a polymer of great interest because of its many desirable characteristics specifically for various pharmaceutical, biomedical, and separation applications. PVA has a relatively simple chemical structure with a pendant hydroxyl group (figure la). The monomer, vinyl alcohol, does not exist in a stable form, rearranging to its tautomer, acetaldehyde. Therefore, PVA is produced by the polymerization of vinyl acetate to poly(vinyl acetate) (PVAc) followed by the hydrolysis to PVA (figure 2). Once the hydrolysis reaction is not complete, there are PVA with different degrees of hydrolysis (figure lb). For practical purposes, PVA is always a co-polymer of vinyl alcohol and vinyl acetate [1]. 

Ethanal (acetaldehyde), the product of ADH and CYP action on ethanol is the substrate for isoforms of ALDH. Like ADH, ALDH isoenzymes are not entirely substrate specific and will act on aliphatic and aromatic aldehydes to generate the corresponding carboxylic acid. There are nine genes encoding ALDH isoforms and all are subject to polymorphism and the enzyme products of ALDH gene expression are found widely in... 

Because the initial oxygen concentration determines the relative abundance of specific abstracting radicals, ethanol oxidation, like methanol oxidation, shows a variation in the relative concentration of intermediate species according to the overall stoichiometry. The ratio of acetaldehyde to ethene increases for lean mixtures. 

The aldehydes, specifically formaldehyde and acetaldehyde, are rnidway in this spectrum. 

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