Acetaldehyde hydration rate constant

The hydration reaction has been extensively studied because it is the mechanistic prototype for many reactions at carbonyl centers that involve more complex molecules. For acetaldehyde, the half-life of the exchange reaction is on the order of one minute under neutral conditions but is considerably faster in acidic or basic media. The second-order rate constant for acid-catalyzed hydration of acetaldehyde is on the order of 500 M s . Acid catalysis involves either protonation or hydrogen bonding at the carbonyl oxygen. 

The following data give the dissociation constants for several acids that catalyze hydration of acetaldehyde. Also given are the rate constants for the hydration reaction catalyzed by each acid. Treat the data according to the Bronsted equation, and comment on the mechanistic significance of the result. 

Rate constants for the acid-catalyzed dehydration of acetaldehyde hydrate"... 

Correlation of the rate constants for the acid-catalyzed dehydration of acetaldehyde hydrate by the Brdnsted catalysis law. Data are from Table 10-6 and Ref. 19. 

Calculations support a cooperative mechanism for the hydration of formaldehyde, acetaldehyde, acetone, and cyclohexanone in water. The results are supported by determination of the rate constant for the neutral hydration of acetone, using labelled acetone and water. Conclusions include ... 

Consider two aldehydes at neutral pH, formaldehyde and acetaldehyde. The hydration/ dehydration (pseudo-) first-order rate constants and the nondimensional Henry s law constants are summarized below. Since in the following discussion you are interested in orders of magnitude only, you assume that aqueous molecular diffusiv-ities of all involved species are the same as the value for C02, (DIW = 2 x 10 5 cm2s 1) and that the corresponding values in air are the same as the value for water vapor (Dwateri = 0.26 cm2s 1). This allows us (as a rough estimate) to calculate v,w and v,a directly from Eqs. 20-15 and 20-16, respectively. 

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... 

The usual means of finding general catalysis is to measure reaction rate with various concentrations of the general acids or bases but a constant concentration of H30 +. Since the pH depends only on the ratio of [HA] to [A-] and not on the absolute concentrations, this requirement may be satisfied by the use of buffers. Catalytic rate constants have been measured for a number of acids and bases in aldehyde hydration-dehydration, notably by Bell and co-workers.10 For formaldehyde, a = 0.24, /3 = 0.40 earlier work11 gave for acetaldehyde a = 0.54, /3 = 0.45 and for symmetrical dichloroacetone a = 0.27, /3 = 0.50. 

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). 

A comparison between the saturation-transfer method with exchange broadening is possible in a system where no causes other than chemical exchange are expected to contribute to the line-broadening of the eacr-changlng species. Such a system is the acid catalyzed exchange reaction between acetaldehyde and its hydrate. Indeed, as can be seen from Table 2, the specific rate constants calculated from the two methods are the same within the experimental error, nnd furthermore the ratio of the forwards and reverse rate constants agrees in the two methods with K 1.15 whldi was measured Independently from the ratio of the areas under the peaks of the two species. 

Table 2. Rate constant of the acid catalysed reversible hydration of acetaldehyde.

Figure 5. Dehydration rate constant of acetaldehyde hydrate as a function of azide Ions concentration. Enzyme concentration was 3.4z10 4m. Acetaldehyde and Its hydrate total concentration 0.4M In 0.02M phosphate buffer, meter reading 7.5. The tenqierature was 32 C. I/Tq Is the rate constant In the absence of the enzyme.

Table 11.9 Second-order rate constants (kobs) of catalysed hydration of acetaldehyde [128]...

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