Acetaldehyde, hydration equilibrium

For example, Figure 2.3 shows plots of the a constants of X vs. log p/T of aliphatic carboxylic acids (XCOaH) and vs. log k for the dehydration of acetaldehyde hydrate by XC02H. Deviations from Equations 2.18 and 2.19 occurwhen the rate of reaction or position of equilibrium becomes dependent on steric factors. For example, Taft studied the enthalpies of dissociation, A Hd, of the addition compounds formed between boron trimethyl and amines (X1X2X3N) and found that when the amine is ammonia or a straight-chain primary amine the dissociation conforms to Equation 2.20, in which 2 ° is the sum of the a values for the... 

In aqueous solutions, acetaldehyde exists in equilibrium with the acetaldehyde hydrate [CH3CH(OH)2], The enol form, vinyl alcohol (CH2=CHOH) exists in equilibrium with acetaldehyde to the extent of 0.003% (1 molecule in approximately 30,000) and can be acetylated with ketene (CH2=C=0) to form vinyl acetate (CH2=CHOCOCH3). 

The Hydrate and Enol Form. In aqueous solutions, acetaldehyde exists in equilibrium with the acetaldehyde hydrate [4433-56-1], (CH3CH(OH)2). The degree of hydration can be computed from an equation derived by Bell and Chime (31). Hydration, the mean heat of which is —21.34 lcJ/mol (—89.29 kcal/mol), has been attributed to hyperconjugation (32). The enol form, vinyl alcohol [557-75-5] (CH2=CHOH) exists in equilibrium 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 the general acid-catalyzed dehydration of acetaldehyde hydrate, Eigen (1965) has proposed a one-encounter mechanism (transition state 17), in which both the acidity and the basicity (conjugate base) of the catalysts are important (moderated by solvent). Bell (1966) has further discussed the occurrence of cyclic paths in carbonyl hydration. Reimann and Jencks (1966) have concluded from rate and equilibrium data on the addition of hydroxylamine to an aldehyde, that proton... 

The slope of the plot of log/cnA versus pA a is the same as that against -logAgq, because p/fns is constant and independent of the acid (HA). This relationship is very important as it is often very difficult to measure an equilibrium constant explicitly and changes would be even more difficult to determine accurately. It is usually a simple matter to determine dissociation constants accurately and moreover there are large databases of measured pAS, values in existence. The equilibria (Equations 31-33) sum to give the equilibrium of the initially formed intermediate (MeCHOH ) from HA and acetaldehyde hydrate. 

The nature of catalysis in homogeneous systems has been the subject of a considerable amount of research. A catalyst is any substance which affects the rate of reaction but is not consumed in the overall reaction. From thermodynamic principles we know that the equilibrium constant for the overall reaction must be independent of the mechanism, so that a catalyst for the forward reaction must also be one for the reverse reaction. In aqueous solution, a large number of reactions are catalyzed by acids and bases for our purposes we shall employ the Bronsted definition of acids and bases as proton donors and acceptors, respectively. Catalysis by acids and bases involves proton transfer either to or from the substrate. For example, the dehydration of acetaldehyde hydrate is subject to acid catalysis [20], probably by the mechanism (II). 

FIGURE 16.29 The magnirnde of the equilibrium constant (A) depends on the relative energies of the carbonyl compounds and their hydrates. We know that for acetaldehyde the energies of hydrate and aldehyde are comparable, because A" 1. Acetone is more stable than acetaldehyde and formaldehyde is less stable. Steric factors make the acetone hydrate less stable than the acetaldehyde hydrate. By contrast, the formaldehyde hydrate is more stable than the acetaldehyde hydrate. The equilibrium constants reflect these changes. 

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... 

The addition of water across carbon-carbon double bonds, a reaction thoroughly investigated by Lucas and Taft, requires strong activation and is catalyzed by hydrogen ions and hydroxyl ions. Addition of water across the 0= =0 bond of aldehydes has also been studied kinetically. Whereas chloral and formaldehyde are largely hydrated (at equilibrium in dilute aqueous solution), acetaldehyde and other... 

The occurrence of isotopic exchange of between water and carbonyl compounds has been observed to take place slowly with acetone (Cohn and Urey, 1938) and much more rapidly with acetaldehyde (Herbert and Lauder, 1938). This gives qualitative evidence for reversible hydration (since no other reasonable mechanism exists for isotopic exchange), but gives no quantitative information about the equilibrium position. Similarly, the fact that exchange occurs in the unhydrolysed ester during the hydrolysis of carboxylic esters (Bender, 1951) shows that the species RC(0H)20R is a stable intermediate rather than a transition state. 

In principal, all steps in these water-elimination reactions are reversible. Because in the reactions discussed above the final products are thermodynamically more stable, the equilibrium lies fully on the right side (i.e., the reverse rates are very slow). However, there are also cases where hydration of a radical is a fast process. For example, reaction (67) occurs at a rate of 2 x 104 s1 (Schuchmann MN and von Sonntag 1988), i.e., a million times faster than the rate of hydration of its non-radical parent, acetaldehyde. 

Thus, we need to prepare 180-labeled ethyl alcohol from the other designated starting materials, acetaldehyde and 180-enriched water. First, replace the oxygen of acetaldehyde with 180 by the hydration-dehydration equilibrium in the presence of 180-enriched water. 

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. 

An important example of this effect is the decrease in equilibrium constants for ketones as compared to aldehydes. The replacement of the aldehyde hydrogen of acetaldehyde (K = 1.3) with a methyl group, to produce acetone (K = 2 X 10-3), results in a decrease in the equilibrium constant for hydration by a factor of approximately 1000. The inductive effect of the electron-donating alkyl group also helps shift the equilibrium for ketones toward the reactant. 

The product is in fact a hemiacetal. Like hydrates, most hemiacetals are unstable with respect to their parent aldehydes and alcohols for example, the equilibrium constant for reaction of acetaldehyde with simple alcohols is about 0.5 as we saw in Chapter 13. 

There is an equilibrium between the two structures, but it generally lies very much in favor of the keto form. Thus, vinyl alcohol is formed initially by hydration of acetylene, but it is rapidly converted into an equilibrium mixture that is almost all acetaldehyde. 

It is occasionally possible to obtain an independent check on the correct value of 6 . Bell and Clunie (1952a) measured the u.v. absorption of aqueous acetaldehyde solutions in the temperature range 0°-54° C, and also studied the heat changes on dissolving acetaldehyde in water at 0° C and 25° C, a calorimeter with rapid response being used. Since the hydration reaction in pure water has a half-time of about 1 min at 25° C and 8 min at 0°C, the heat evolved during the first second could be identified with the physical heat of solution, AH . Calorimetric measurements were also made in very dilute alkali, in which the hydration reaction has a half-time < 0-01 sec the heat evolved in these experiments is —ol)AH , where a is the equilibrium degree of dissociation of... 

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