Acetaldehyde hydrate formation

General acid Decomposition of acetaldehyde hydrate Hydrolysis of o-esters Formation of nitro componnd CH3CH(0H)2 = CH3CHO + H2O HC (0C2H5)3 -I- H2O = H COOC2H5 + 2C2H5OH CH2 N02 -I- acid = CH3-N02 + base ... 

FIGURE 19.65 Two reactions of acetaldehyde with hydroxide ion addition (hydrate formation) and enolate formation. 

Hydration. Water adds to the triple bond to yield acetaldehyde via the formation of the unstable enol (see Acetaldehyde). The reaction has been carried out on a commercial scale using a solution process with HgS04/H2S04 catalyst (27,28). The vapor-phase reaction has been reported at... 

The equilibrium constants for addition of alcohols to carbonyl compounds to give hemiacetals or hemiketals show the same response to structural features as the hydration reaction. Equilibrium constants for addition of metiianoHb acetaldehyde in both water and chloroform solution are near 0.8 A/ . The comparable value for addition of water is about 0.02 The overall equilibrium constant for formation of the dimethyl acetal of... 

Traditionally, ethanol has been made from ethylene by sulfation followed by hydrolysis of the ethyl sulfate so produced. This type of process has the disadvantages of severe corrosion problems, the requirement for sulfuric acid reconcentration, and loss of yield caused by ethyl ether formation. Recently a successful direct catalytic hydration of ethylene has been accomplished on a commercial scale. This process, developed by Veba-Chemie in Germany, uses a fixed bed catalytic reaction system. Although direct hydration plants have been operated by Shell Chemical and Texas Eastman, Veba claims technical and economic superiority because of new catalyst developments. Because of its economic superiority, it is now replacing the sulfuric acid based process and has been licensed to British Petroleum in the United Kingdom, Publicker Industries in the United States, and others. By including ethanol dehydrogenation facilities, Veba claims that acetaldehyde can be produced indirectly from ethylene by this combined process at costs competitive with the catalytic oxidation of ethylene. 

Chloral has three electronegative chlorine units attached to the a-carbon (CI3C-) to the aldehydes. The carbonyl carbon bears a partial positive charge so such electronegative elements destabilizes the carbonyl and favors the equilibrium towards significant formation of the hydrate product. For acetaldehyde, on the other hand, the equilibrium constant is 1. 

Catalytic conversions were experimentally studied in Russia toward the end of the nineteenth century, and especially in the twentieth century, and regularities were empirically established in a number of cases. The work of A. M. Butlerov (1878) on polymerization of olefins with sulfuric acid and boron trifluoride, hydration of acetylene to acetaldehyde over mercury salts by M. G. Kucherov (1881) and a number of catalytic reactions described by V. N. Ipatieff beginning with the turn of the century (139b) are widely known examples. S. V. Lebedev studied hydrogenation of olefins and polymerization of diolefins during the period 1908-13. Soon after World War I he developed a process for the conversion of ethanol to butadiene which is commercially used in Russia. This process has been cited as the first example of commercial application of a double catalyst. Lebedev also developed a method for the polymerization of butadiene to synthetic rubber over sodium as a catalyst. Other Russian chemists (I. A. Kondakov I. Ostromyslenskif) were previously or simultaneously active in rubber synthesis. Lebedev s students are now continuing research on catalytic formation of dienes. 

Higher aldehydes, for example acetaldehyde or n-butyraldehyde, have much less tendency to polymerize compared to formaldehyde [5, 6]. Reasons have been given in thermodynamic terms by referring to the lower enthalpy of polymerization (about —7 kcal mole" ) as compared to formaldehydes (—12 kcal mole" ), which results in ceiling temperatures of —40°C. In terms of reactivity, aliphatic aldehydes undergo hydration and hemiacetal formation to an extent of about 50%. 

If hydration of acetylene followed the same pattern as hydration of alkenes, we would expect addition of H— and —OH to the triple bond to yield the structure that we would call vinyl alcohol. But all attempts to prepare vinyl alcohol result— like hydration of acetylene—in the formation of acetaldehyde. 

Prior to 1916, acetaldehyde was manufactured by the oxidation of alcohol in the liquid phase with bichromate and sulfuric add.1 Since that time it has been ade quite largely by the hydration of acetylene in sulfuric acid solutions activated with mercury salts. However, the relatively low price of ethanol in America has made the formation of acetaldehyde by vapor phase dehydrogenation or limited oxidation of the alcohol attractive commercially. To this end several methods have been proposed for conducting the transformation industrially. Developments of processes employing vapor phase oxidation reactions have all been based largely on the prindples disclosed by the early work, a considerable portion of which had been undertaken purely for the purpose of research and not industrialization. 

Acetaldehyde to Acetic Acid. The formation of acetic acid furnishes an excellent example of liquid-phase oxidation with molecular oxygen. Acetic acid may be obtained by the direct oxidation of ethanol, but the concentrated acid is generally obtained by oxidation methods from acetaldehyde that may have been formed by the hydration of acetylene or the oxidation of ethanol. The oxidation usually occurs in acetic acid solution in the presence of a catalyst and at atmospheric or elevated pressures. Temperatures may range up to lOO C, depending upon conditions, but are usually lower. 

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