Acetaldehyde hydrate

The Hydrate and Enol Form. In aqueous solutions, acetaldehyde exists in equihbrium with the acetaldehyde hydrate [4433-56-17, (CH2CH(0H)2). The degree of hydration can be computed from an equation derived by BeU and Clunie (31). Hydration, the mean heat of which is —21.34 kJ/mol (—89.29 kcal/mol), has been attributed to hyperconjugation (32). The enol form, vinyl alcohol [557-75-5] (CH2=CHOH) exists in equihbrium 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 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. 

Sorensen, R Jencks, W. Acid- and base-catalyzed decomposition of acetaldehyde hydrate and hemiacetals in aqneons solntion. J. Am. Chem. Soc. 1987, 109, 4675 690. 

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 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). 

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...

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. 

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. 

Y. Pocker, Proc. Chem. Soc., London 1960, 17 Acetaldehyd-Hydrat. 

Blossom S, Pumford NR, Gilbert KM (2004) Activation and attenuation of apoptosis of CD4 T cells following in vivo exposure to two common environmental toxicants, trichloro-acetaldehyde hydrate and trichloroacetic acid. J Autoimmun, 23 211-220. 

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

The following data give the dissociation constants and rate of acetaldehyde hydration catalysis by each acid. Treat the data according to the Brpnsted equation and discuss the mechanistic significance of the results. 

Synonyms cas 302-17-0 knockout drops trichlor acetaldehyde hydrated trichloroethylidene glycol Chlorbenside... 

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). 

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 ... 

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 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.

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