Formaldehyde hydration equilibrium

The net effect of this cyclohexadiene/phenyl ring insertion at the carbonyl group is to cause an increase in the overall equilibrium constant for the addition of solvent water, from A dd = 2.3 x 103 for hydration of formaldehyde154 to A dd = 4.0 x 107 for hydration of the p-quinone methide l,3 so that Kj = AT. dd/Aiadd = 1.7 x 104 for transfer of the elements of water from formaldehyde hydrate to 1 (Scheme 43). We have proposed that the relatively small driving force of 6 kcal/mol for this transfer of water from CH2(OH)2 to 1 represents the balance between larger opposing effects3 ... 

These stability effects are apparent in the equilibrium constants for hydration of ketones and aldehydes. Ketones have values of Keq of about 10-4 to 10-2. For most aldehydes, the equilibrium constant for hydration is close to 1. Formaldehyde, with no alkyl groups bonded to the carbonyl carbon, has a hydration equilibrium constant of about 40. Strongly electron-withdrawing substituents on the alkyl group of a ketone or aldehyde also destabilize the carbonyl group and favor the hydrate. Chloral (trichloroacetaldehyde) has an electron-withdrawing trichloromethyl group that favors the hydrate. Chloral forms a stable, crystalline hydrate that became famous in the movies as knockout drops or a Mickey Finn. 

Despite the favoring of formaldehyde hydrate by this equilibrium, there is sufficient free formaldehyde available for an ortho or para position of phenol to add to the highly electrophilic carbon of formaldehyde. As formaldehyde is consumed by this process, the equilibrium is displaced to the left providing further formaldehyde for reaction until all the phenol potential functionalities are taken up or all the formaldehyde is consumed. The structures of the phenol-formaldehyde polymers produced are difficult to study because the final product is infusible and insoluble. However, current thinking is that all possible monomer links can occur in a typical Bakelite sample (Eq. 21.30). 

Often the formaldehyde hydration is also included in the equilibrium (Deister et... 

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 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. 

As with resoles, we can use a three-phase model to discuss formation of a novolac. Whereas the resole is activated through the phenol, activation in novolacs occurs with protonation of the aldehyde as depicted in Scheme 12. The reader will note that the starting material for the methylolation has been depicted in hydrated form. The equilibrium level of dissolved formaldehyde gas in a 50% aqueous solution is on the order of one part in 10,000. Thus, the hydrated form is prevalent. Whereas protonation of the hydrate would be expected to promote dehydration, we do not mean to imply that the dehydrated cation is the primary reacting species, though it seems possible. 

The position of this equilibrium depends greatly on the substituents, X. Formaldehyde (X = H) exists mainly as the hydrate in aqueous solutions, while acetone (X = CH3) exists mainly in the carbonyl form (see table at right). 

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 reduction of formaldehyde at a mercury electrode is an example of a system in which a chemical reaction precedes the electrode reaction. Formaldehyde is present in aqueous solution as the hydrated form (as dihydroxy methane), which cannot be reduced at a mercury electrode. This form is in equilibrium with the carbonyl form... 

In principle the velocity of dehydration could be measured if a physical rather than a chemical method were available for removing the unhydrated carbonyl compound at a rate comparable to its hydration. It was claimed by Bieber and Triimpler (1947a) that this could be achieved by the removal of formaldehyde in a rapid gas stream, the rate of which appeared to be dependent on the pH of the solution. However, attempts to repeat their experiments have proved unsuccessful moreover, although they give no experimental details, calculation in terms of known kinetic and equilibrium constants shows that for a 1-ml liquid sample a gas flow of at least 30 litres/min would be required to produce an appreciable perturbation of equilibrium conditions (Bell and Evans, 1966). It is thus clear that this method has no practical application, at least to formaldehyde solutions. 

Cerium(IV) oxidations of organic substrates are often catalysed by transition metal ions. The oxidation of formaldehyde to formic acid by cerium(IV) has been shown to be catalysed by iridium(III). The observed kinetics can be explained in terms of an outer-sphere association of the oxidant, substrate, and catalyst in a pre-equilibrium, followed by electron transfer, to generate Ce "(S)Ir", where S is the hydrated form of formaldehyde H2C(OH)2- This is followed by electron transfer from S to Ir(IV) and loss of H+ to generate the H2C(0H)0 radical, which is then oxidized by Ce(IV) in a fast step to the products. Ir(III) catalyses the A -bromobenzamide oxidation of mandelic acid and A -bromosuccinimide oxidation of cycloheptanol in acidic solutions. ... 

The classical example is the reduction of formaldehyde [149], which exists in solution in equilibrium with its hydrated form, methyleneglycol. The latter species dominates in the equilibrium situation, but is not electroactive. So the preceding reaction is dehydration of the methylenegly c ol. 

For a given nucleophile the equilibrium lies farther on the product side the smaller the substituents R1 and R2 of the carbonyl compound are (Figure 9.1). Large substituents R1 and R2 inhibit the formation of addition products. This is because they come closer to each other in the addition product, where the bonds to R1 and R2 are closer than in the carbonyl compound, where these substituents are separated by a bond angle of about 120°. Formaldehyde is the sterically least hindered carbonyl compound. In H20 this aldehyde is present completely as dihydroxymethane, and anhydrous formaldehyde is exists completely as polymer. In contrast, acetone is so sterically hindered that it does not hydrate, oligomerize, or polymerize at all. 

Formaldehyde has a large equilibrium constant for hydrate formation because it has no bulky, electron-donating alkyl 2 X I03 groups. It is more than 99.9% in the hydrated form in... 

Since the suggestion of the sequential QM/MM hybrid method, Canuto, Coutinho and co-authors have applied this method with success in the study of several systems and properties shift of the electronic absorption spectrum of benzene [42], pyrimidine [51] and (3-carotene [47] in several solvents shift of the ortho-betaine in water [52] shift of the electronic absorption and emission spectrum of formaldehyde in water [53] and acetone in water [54] hydrogen interaction energy of pyridine [46] and guanine-cytosine in water [55] differential solvation of phenol and phenoxy radical in different solvents [56,57] hydrated electron [58] dipole polarizability of F in water [59] tautomeric equilibrium of 2-mercaptopyridine in water [60] NMR chemical shifts in liquid water [61] electron affinity and ionization potential of liquid water [62] and liquid ammonia [35] dipole polarizability of atomic liquids [63] etc. 

Formaldehyde, the least stable carbonyl compound, forms the largest percentage of hydrate. On the other hand, acetone and other ketones, which have two electron-donor R groups, form <1% of the hydrate at equilibrium. Other electronic factors come into play as well. 

The electroactive species A is generated by a reaction, generally an equilibrium displaced toward Z, that precedes the electron transfer step. Z is the reactant introduced in the cell or the predominant form of the reactant in the reaction medium. These reaction schemes were introduced to rationalize the various electrochemical phenomena observed during the reduction of certain aldehydes in aqueous solutions. Indeed, in water, formaldehyde, r-electron-poor heterocyclic aldehydes, and a few aldehydes with strongly electron-attracting groups exist as their nonreducible hydrated form in rapid equilibrium with the reducible carbonyl ... 

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