Aldehydes hydration constants

The rate of attack of water upon the tri-/>-anisylmethyl cation is unaffected by binding of this cation to anionic micelles of sodium dodecyl sulfate (SDS) (Bunton and Huang, 1972) and equilibrium constants for aldehyde hydration are only slightly reduced by binding to micelles (Albrizzio and Cordes, 1979). These observations are also consistent with substrate binding at a wet micellar surface rather than in the interior of the micelle. 

TABLE 8.3 Intrinsic (H) and Effective (H ) Henry s Law Constants and Hydration Constants (Khydr) at 298 K for Some Aldehydes of Atmospheric Interest ... 

Water adds to the carbonyl group of aldehydes and ketones to yield hydrates (Equation 8.4). For ketones and aryl aldehydes, equilibrium constants of 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. 

While checking a sample of 2,5-anhydromannose-6-P for fructose-6-P by incubating it with phosphofructokinase and MgATP, we discovered that this aldehyde, which is sterically hindered from forming an internal hemiacetal, induced an ATPase activity (6). Since aldehyde hydration shows a large inverse equilibrium isotope effect of 0.73 when the hydrogen on the carbonyl carbon is replaced by deuterium (7,8), 2,5-anhydroman-nose-6-P-l-d will be 60% hydrated, compared to 52% hydration of the unlabeled aldehyde. If the free aldehyde were the activator, 48% of the unlabeled and 40% of the deuterated compound would be active, and a normal deuterium isotope effect of 0.48/0.40 = 1.2 would be seen on V/K (the apparent first order rate constant) for the activator, while if the hydrate were the active form, an inverse isotope effect of 0.52/0.60 = 0.87 would be seen. The observed value of 1.23 0.03 showed that the free aldehyde and not the hydrate was the activator (6). 

The feasibility of some of these radical pathways has been examined using Marcus theory to obtain rate constants for comparison with the experimental data (Eberson, 1984). For some relevant anions, including hydroxide, methoxide, t-butoxide, the anion of benzaldehyde hydrate and di-2-propyl-amide, the necessary E°(RO-/RO) values are available or can be estimated with sufficient accuracy. For the reaction of t-butoxide with benzophenone in THF, or the benzaldehyde hydrate anion with benzaldehyde in aqueous dioxan, direct electron transfers between the anion and the neutral are not feasible the calculated rate constants are orders of magnitude too low to be compatible with the observed reduction rates. Any radicals observed in these reactions must arise by some other more complex mechanism. The behaviour of an aromatic aldehyde hydrate dianion has not been examined in this way, but MNDO calculation (Rzepa and Miller, 1985) suggests that such a species could easily transfer either a single electron or a hydrogen atom to an accepting aldehyde. 

The simplest of the aldehydes is formaldehyde, whose oxidation by Ce(IV) in 2.0 M perchlorate media has been studied by Husain (1977). It is presumed that formaldehyde exists as a ketohydrate in acid solution (the hydration constant is lO M ). Michaelis-Menten kinetics describe the results, indicating the formation of a precursor complex. A detailed mechanism permits a calculation of the equilibrium quotients for formation of the Ce(IV)-formohydrate complex and the ionization of a... 

Table 10.2 A shows the association constants for the hydration of several carbonyl structures. For ketones and aryl aldehydes, the constants are less than unity, favoring the carbonyl. However, aliphatic aldehydes, carbonyl structures with electron withdrawing groups, and carbonyls in strained rings have equilibrium constants greater than unity. In...

Table 17 3 compares the equilibrium constants for hydration of some simple aldehydes and ketones The position of equilibrium depends on what groups are attached to C=0 and how they affect its steric and electronic environment Both effects con tribute but the electronic effect controls A hydr more than the steric effect... 

Equilibrium Constants (AChydr) and Relative Rates of Hydration of Some Aldehydes and Ketones... 

Electronic and steric effects operate m the same direction Both cause the equilib rium constants for hydration of aldehydes to be greater than those of ketones... 

The exceptions are formaldehyde, which is nearly completely hydrated in aqueous solution, and aldehydes and ketones with highly electronegative substituents, such as trichloroacetaldehyde and hexafluoroacetone. The data given in Table 8.1 illustrate that the equilibrium constant for hydration decreases with increasing alkyl substitution. 

Althoi h the equilibrium constant for hydration is unfavorable, the equilibrium between an aldehyde or ketone and its hydrate is established rapidly and can be detected by isotopic exchange, using water labeled with 0, for example ... 

Simple carbonyl compounds, such as ketones and aldehydes, can hydrate by the addition of water, as shown in (1). The equilibrium constant for... 

Pyridinecarboxaldehyde, 3. Possible hydration of the aldehyde group makes the aqueous solution chemistry of 3 potentially more complex and interesting than the other compounds. Hydration is less extensive with 3 than 4-pyridinecarboxaldehyde but upon protonation, about 80% will exist as the hydrate (gem-diol). The calculated distribution of species as a function of pH is given in Figure 4 based on the equilibrium constants determined by Laviron (9). 

Table 8.3 shows the solubilities of some potentially important aldehydes in the form of the Henry s law constant (H) and the effective Henry s law constant (// ) (Betterton and Hoffmann, 1988 Olson and Hoffmann, 1989). These aldehydes not only dissolve in aqueous solutions but also hydrate to form gera-diols (Buschmann et al., 1980, 1982) ... 

Because of this hydration, the total solubility, i.e., effective Henry s law constant, is larger than expected based on physical solubility alone. The data in Table 8.3 show that most aldehydes have quite large effective Henry s law constants (// ), the exceptions being acetaldehyde and benzaldehyde. As a result of these high solubilities, significant concentrations can occur in fogs and clouds and hence be available to complex with S(IV). 

When treating the overall transformation kinetics of an organic compound as we have done for the hydrolysis of benzyl chloride (Eq. 12-11), we assume that the reverse reaction (i.e., the formation of benzyl chloride from benzyl alcohol) can be neglected. For many of the reactions discussed in the following chapters we will make this assumption either because the reverse reaction has an extremely small rate constant (i.e., the reaction is practically irreversible), or because the concentration ) of the reactant(s) are very large as compared to the concentration(s) of the product(s). There are, however, situations in which the reverse reaction has to be taken into account. We have already encountered such a reaction in Illustrative Example 12.1. To demonstrate how to handle the reaction kinetics in such a case, we use the hydration of an aldehyde to yield a diol (Fig. 12.3). This example will also illustrate how the equilibrium reaction constant, Kn is related to the kinetic rate constants, kY and k2, of the forward and reverse reaction. 

Figure 20.12 Air-water exchange of an aldehyde A converting to a diol D by a hydration/dehydration reaction. Since the diol D cannot leave the water, the slope of its concentration at the air/water interface is zero. For simplicity, the scales of A and D are chosen such that the equilibrium constant of hydration, K and the Henry s law constant of the aldehyde, KAa/vl, are 1. The dashed straight line marked [A] onre>ct,ve he>Ps t0 picture the modification due to the reactivity of A.

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. 

It appears that, as the length of the chain of the aldehydo form increases, the extent of hydration decreases. For glyceraldehyde, the aldehydrol aldehyde ratio23 in deuterium oxide at 24° is 17.5 1 for d-erythrose it is 10 1. For the tetraacetate of aMehydo-L-arabinose in 7 3 oxolane-deuterium oxide, the ratio is 9 1 whereas, for the corresponding L-fucose derivative, which differs only in the presence of an additional carbon atom at the other end of the chain, it is 3 1. The chemical shifts and the coupling constants indicate that these two compounds, surprisingly, adopt different conformations.80... 

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