Dehydration of acetaldehyde

In practice, one proceeds as follows. The value of bh >s determined for the reaction with a series of acids of similar structure, that is, for carboxylic acids or ammonium ions, etc. Limiting the data to a single catalyst type improves the fit. since the inclusion of data for a second ype of acid catalyst might define a close but not identical line. This means that Ga may be somewhat different for each catalyst type. A plot of log(kBH/p) versus log(A BH(7//i) is then constructed. This procedure most often results in a straight line, within the usual —10-15 percent precision found for LFERs. One straightforward example is provided by the acid-catalyzed dehydration of acetaldehyde hydrate,... 

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. 

A synthesis of 5//-pyrrolo[l,2-r] imidazole 105 has been developed via a chemoselective addition/dehydration of acetaldehyde on diiodo imidazole 103 giving the vinylic imidazole 104. This compound, treated under the metathesis condition in the presence of the second-generation Grubbs catalyst, gave the final product 105 (Scheme 11) <2003TL1379>. 

Figure 2.3 The divisions on the ordinate are 1.00 units of pK or log k apart. The relative positions of the lines with respect to the ordinate are arbitrary. A pK, aliphatic carboxylic acids (XC02H), water 25°C vs. a. B log k, catalysis of dehydration of acetaldehyde hydrate by XC02H, aqueous acetone, 25°C vs. a. From J. Shorter, Quart. Rev. (London), 24, 433 (1970). Reprinted by permission of J. Shorter and The Chemical Society.

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 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 Bronsted equation is a Class I free energy relationship and this may be shown by considering as an example the acid-catalysed dehydration of acetaldehyde hydrate (Equation 30). This reaction also provides a good example of an acid-catalysed reaction following a Bronsted equation (Figure 7). 

Figure 7 Dehydration of acetaldehyde hydrate catalysed by acid.C...

R.P. Bell and W.C.E. Higginson, The Catalysed Dehydration of Acetaldehyde Hydrate, and the Effect of Structure on the Velocity of Protolytic Reactions, Proc. Roy. Soc. London, 1949, A197, 141. 

The dehydration of acetaldehyde has been studied in the presence of various weak acids in aqueous acetone at 25°C. The following table gives the name of the weak acid, its acidity constant, K, and the rate constant for the catalyzed reaction, k. Analyze these data to obtain the parameters of the Bronsted relation. 

Dehydration of Acetaldehyde Hydrate—Deviations from the Brdnsted Relation... 

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). 

Bell and Higginson [20] have studied the acid catalysis of the dehydration of acetaldehyde in aqueous acetone at 25°C some of their results are presented in Table P8-6. 

When applied to reactions catalyzed by acids or bases, the Bronsted relationship has a slightly different connotation. Examples of these are the base-catalyzed halogenation of ketones and esters and the acid-catalyzed dehydration of acetaldehyde hydrate. For an acid-catalyzed reaction. 

Examples for reactions of fast protonations (Equation 2.29) are ester hydrolysis and alcoholysis, inversion of sucrose, and the hydrolysis of acetals. The mutarotation of glucose and the dehydration of acetaldehyde hydrate are examples of slow protonations described with Equation 2.30. 

Figure 4. Br0nsted plot for general acid catalysis of the acid-catalyzed dehydration of acetaldehyde hydrate using carboxylic acids and phenols as catalysts. (Reprinted from Ref. 21 with permission of the Royal Society.)...

The value of cyanohydrins as synthetic intermediates lies in the new functional groups into which they can be converted. First, the secondary or tertiary hydroxyl group of the cyanohydrin may undergo acid-catalyzed dehydration to form an unsaturated nitrile. For example, acid-catalyzed dehydration of acetaldehyde cyanohydrin gives acrylonitrile, the monomer from which polyacrylonitrile (Orion, Table 29.1) is made. 

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