Acetate, active acetaldehyde

Mo is the essential element of effective catalysts for propene oxidation to acrolein and acrolein oxidation to acrylic acid, while V is an essential element for effective catalysis of acrolein oxidation to acrylic acid. Mo-V-Nb oxide catalysts are capable of activating propane even at 573 K, but yields products of acetic acid, acetaldehyde, and carbon oxides. The addition of Te or Sb to Mo-V-Nb oxides induces certain structural changes leading to the formation of acrylic acid. ... 

Fig. 2. Pathway of ethanol utilization and ethyl acetate or acetaldehyde production by Candida utilis TCA cycle activity inhibited under ircn-limited conditions. Acetyl-CoA synthetase forward inhibited by acetaldehyde accumulated when elevated levels cf ethanol ( 3.5J w/v) present in the medium.

Most people have two forms of the acetaldehyde dehydrogenase, a low K jyi mitochondrial form and a high K jy[ cytosolic form. In susceptible persons, the mitochondrial enzyme is less active due to the substitution of a single amino acid, and acetaldehyde is processed only by the cytosolic enzyme. Because this enzyme has a high K ]y[, less acetaldehyde is converted into acetate excess acetaldehyde escapes into the blood and accounts for the physiological effects. 

Most known thiamin diphosphate-dependent reactions (Table 14-2) can be derived from the five halfreactions, a through e, shown in Fig. 14-3. Each half-reaction is an a cleavage which leads to a thiamin- bound enamine (center. Fig. 14-3) The decarboxylation of an a-oxo acid to an aldehyde is represented by step h followed by fl in reverse. The most studied enzyme catalyzing a reaction of this type is yeast pyruvate decarboxylase, an enzyme essential to alcoholic fermentation (Fig. 10-3). There are two 250-kDa isoenzyme forms, one an tetramer and one with an (aP)2 quaternary structure. The isolation of a-hydroxyethylthiamin diphosphate from reaction mixtures of this enzyme with pyruvate provided important verification of the mechanisms of Eqs. 14-14,14-15. Other decarboxylases produce aldehydes in specialized metabolic pathways indolepyruvate decarboxylase in the biosynthesis of the plant hormone indole-3-acetate and ben-zoylformate decarboxylase in the mandelate pathway of bacterial metabolism (Chapter 25). Formation of a-ketols from a-oxo acids also starts with step h of Fig. 14-3 but is followed by condensation with another carbonyl compound in step c, in reverse. An example is decarboxylation of pyruvate and condensation of the resulting active acetaldehyde with a second pyruvate molecule to give l -a-acetolactate, a reaction catalyzed by acetohydroxy acid synthase (acetolactate synthase). Acetolactate is the precursor to valine and leucine. A similar ketol condensation, which is catalyzed by the same S5mthase, is... 

Platinum-iron on alumina catalysts were characterized by Mbssbauer spectroscopy (Section 4) and their activity tested. Iron in clusters with high Pt Fe ratios, about 5, and fully combined with platinum, was catalytically inert for the CO-H2 synthesis reaction, attributed to a decrease in the electron density of the iron as indicated by the Mbssbauer isomer shift. The direction of electron transfer was opposite to that proposed for alkali-metal promoted iron catalysts. At low Pt Fe ratio, 0.1, ferromagnetic iron as well as Fe " ions and PtFe clusters were produced and dominated the activity/selectivity pattern. Rhodium on silica catalysts produced C2-compounds containing oxygen, specifically acetic acid, acetaldehyde and ethanol, with methane as the other major product. The addition of iron moved the C2-product formation sharply in favour of ethanol and now methanol was also formed. ... 

Acetylthiamine pjrrophosphate appears to be yet another form of active acetate. It has been assigned a key role in the lipoic acid-Unked oxidative decarboxylation of pyruvate as the primary product of the oxidation of active acetaldehyde, i.e., 2-hydroxyethylthiamine pyrophosphate. It has been proposed that 2-acetylthiamine pyrophosphate is an intermediate in all oxidative transformations of pyruvate and that 2-succinylthiamine pyrophosphate plays a similar role in oxidation of a-ketoglutarate. Further evaluation of this proposal is anticipated in the near future. 

However, failing incoporations of C-labeled aeetate and sueeessful ones of Relabeled glycerol as well as pyruvate in hopanes and ubiquinones showed isopen-tenyldiphosphate (IPP) to originate not only from the acetate mevalonate pathway, but also from activated acetaldehyde (C2, by reaction of pyruvate and thiamine diphosphate) and glyceraldehyde-3-phosphate (C3) R. In this way, 1-deoxy-pentulose-5-phosphate is generated as the first unbranched C5 preeursor of IPP. 

Diacetyl can be produced by either homolactic or heterolactic pathways of sugar metabolism (via free pyruvate) or by utilization of citric acid (see Figs. 1-1 lA and 1-1 IB). In this case, citric acid is first converted to oxaloacetic and acetic acids. The former is then decarboxylated to pyruvate which undergoes a second decarboxylation and condensation with thiamine pyrophosphate (TPP) to yield active acetaldhyde, which reacts with another pyruvate to yield a-acetolactate which undergoes oxidative decarboxylation to yield diacetyl and its equilibrium products see Fig. 1-11 A. In the case of other LAB, the precursor, a-acetolactate is not produced. Here active acetaldehyde, produced as described above, reacts with acetyl CoA to yield diacetyl see Fig. 1-1 IB. 

Diacetyl may be synthesized by either homolactic or heterolactic pathways of sugar metabolism as well as by utilization of citric acid (Fig. 2.9). Citric acid is hrst converted to acetic acid and oxaloacetate the latter is then decarboxylated to pyruvate. Although earlier reports indicated that diacetyl synthesis by lactic acid bacteria does not proceed via a-acetolactate (Gottschalk, 1986), more recent evidence suggests that this pathway is active in lactic acid bacteria (Ramos et al., 1995). Here, pyruvate undergoes a second decarboxylation and condensation with thiamine pyrophosphate (TPP) to yield active acetaldehyde. This compound then reacts with another molecule of pyruvate to yield a-acetolactate, which, in... 

Because the presence of yeasts can represent a logistical bottleneck in postfermentation clarification, an alternative that has been studied is the use of immobilized microorganisms. Here, yeasts are trapped in calcium alginate beads or strands that are collectively packed into a synthetic mesh sleeve that is immersed into the juice/must. Relatively few yeasts (<10V mL) escape the encapsulation matrix (Yokotsuka et al., 1993) but yet conduct an active alcoholic fermentation. Yajima and Yokotsuka (2001) reported that concentrations of some undesirable volatile compounds (methanol, ethyl acetate, and acetaldehyde) were lower in wines made using Saccharomyces immobilized in double-layer beads. Immobilized yeasts... 

Historically, the application of MMO as catalysts for propane oxidation to acrylic acid began in the late 1970s with Mo-V-Nb mixed oxides, previously reported as a catalyst for ethane oxidation [54]. The results of propane oxidation over this catalyst show that propane could be activated at 300°C, but producing only acetic acid, acetaldehyde, and carbon oxides. However, the possibility to activate... 

It is interesting to note that only acetic add, acetaldehyde, and CO2 have been detected by HPLC from the outlet of the anode compartment of a DEFC with Pt/C catalyst [56], while depending on electrode potentials, acetaldehyde, acetic acid, CO2, and trace amounts of CH4 can be found in electrolysis half-cell. It is also found that acetaldehyde can be exclusively produced at a potential <0.35 V versus RHE on a Pt catalyst in a long-time electrolysis experiment no acetic acid was detected in the potential range [6], This implies that the alcohol product distribution depends on electric energy input In an acid electrolyte, Pt-based catalysts have shown better EOR activity than other platinum group metal (PGM)-based ones. However, Pt itself is readily poisoned by various Ci, C2 intermediate species. Binary and ternary Pt-based... 

This enzyme, sometimes also called the Schardinger enzyme, occurs in milk. It is capable of " oxidising" acetaldehyde to acetic acid, and also the purine bases xanthine and hypoxanthine to uric acid. The former reaction is not a simple direct oxidation and is assumed to take place as follows. The enzyme activates the hydrated form of the aldehyde so that it readily parts w ith two hydrogen atoms in the presence of a suitable hydrogen acceptor such as methylene-blue the latter being reduced to the colourless leuco-compound. The oxidation of certain substrates will not take place in the absence of such a hydrogen acceptor. 

Oxidation. Acetaldehyde is readily oxidised with oxygen or air to acetic acid, acetic anhydride, and peracetic acid (see Acetic acid and derivatives). The principal product depends on the reaction conditions. Acetic acid [64-19-7] may be produced commercially by the Hquid-phase oxidation of acetaldehyde at 65°C using cobalt or manganese acetate dissolved in acetic acid as a catalyst (34). Liquid-phase oxidation in the presence of mixed acetates of copper and cobalt yields acetic anhydride [108-24-7] (35). Peroxyacetic acid or a perester is beheved to be the precursor in both syntheses. There are two commercial processes for the production of peracetic acid [79-21 -0]. Low temperature oxidation of acetaldehyde in the presence of metal salts, ultraviolet irradiation, or osone yields acetaldehyde monoperacetate, which can be decomposed to peracetic acid and acetaldehyde (36). Peracetic acid can also be formed directiy by Hquid-phase oxidation at 5—50°C with a cobalt salt catalyst (37) (see Peroxides and peroxy compounds). Nitric acid oxidation of acetaldehyde yields glyoxal [107-22-2] (38,39). Oxidations of /)-xylene to terephthaHc acid [100-21-0] and of ethanol to acetic acid are activated by acetaldehyde (40,41). 

Acetic anhydride adds to acetaldehyde in the presence of dilute acid to form ethyUdene diacetate [542-10-9], boron fluoride also catalyzes the reaction (78). Ethyfldene diacetate decomposes to the anhydride and aldehyde at temperatures of 220—268°C and initial pressures of 14.6—21.3 kPa (110—160 mm Hg) (79), or upon heating to 150°C in the presence of a zinc chloride catalyst (80). Acetone (qv) [67-64-1] has been prepared in 90% yield by heating an aqueous solution of acetaldehyde to 410°C in the presence of a catalyst (81). Active methylene groups condense acetaldehyde. The reaction of isobutfyene/715-11-7] and aqueous solutions of acetaldehyde in the presence of 1—2% sulfuric acid yields alkyl-y -dioxanes 2,4,4,6-tetramethyl-y -dioxane [5182-37-6] is produced in yields up to 90% (82). 

Acetyl chlotide is reduced by vatious organometaUic compounds, eg, LiAlH (18). / fZ-Butyl alcohol lessens the activity of LiAlH to form lithium tti-/-butoxyalumium hydtide [17476-04-9] C22H2gA102Li, which can convert acetyl chlotide to acetaldehyde [75-07-0] (19). Triphenyl tin hydtide also reduces acetyl chlotide (20). Acetyl chlotide in the presence of Pt(II) or Rh(I) complexes, can cleave tetrahydrofuran [109-99-9] C HgO, to form chlorobutyl acetate [13398-04-4] in about 72% yield (21). Although catalytic hydrogenation of acetyl chlotide in the Rosenmund reaction is not very satisfactory, it is catalyticaHy possible to reduce acetic anhydride to ethylidene diacetate [542-10-9] in the presence of acetyl chlotide over palladium complexes (22). Rhodium trichloride, methyl iodide, and ttiphenylphosphine combine into a complex that is active in reducing acetyl chlotide (23). 

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. 

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). 

---