Solutions, formaldehyde Equilibria

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. 

The difference in formaldehyde equilibrium concentration between homogeneous and heterogeneous polymerization is large enough to indicate a difference in the physical state of cationic chain ends in the dissolved and in the crystalline polymer. Thus, Model B is ruled out. In the homopolymerization of trioxane and in the heterogeneous copolymerization with small amounts of dioxolane the active centers of chains which have precipitated from the solution predominantly are directly on the crystal surface (Model A). According to Wunderlich (20, 21), this is the first case in addition polymerization where Model A—simultaneous polymerization and crystallization—has been proved experimentally. 

It may be easier to grasp the notion of shifts in equilibrium in a qualitative way using the Le Chatelier Principle. This states that, if a system at equilibrium is perturbed, the system will react in such a way as to minimize this imposed change. Thus, looking at the formaldehyde equilibrium (eqn. 3), any increase in HCHO in solution would be lessened by the tendency of the reaction to shift to the right, producing more methylene glycol. 

Here the electroactive species, O, is generated by a reaction that precedes the electron transfer at the electrode. An example of the CE scheme is the reduction of formaldehyde at mercury in aqueous solutions. Formaldehyde exists as a nonreducible hydrated form, H2C(0H)2, in equilibrium with the reducible form, H2C==0 ... 

However, a second mole of alcohol or hemiformal does not add at the ordinary pH of such solutions. The equilibrium constant for hemiformal formation depends on the nature of the R group of the alcohol. Using NMR spectroscopy, a group of alcohols including phenol has been examined in solution with formaldehyde (16,17). The spectra indicated the degree of hemiformal formation in the... 

In summary, the percentage of hydrate present in solution at equilibrium depends on both electronic and steric effects. Electron-donating substituents and bulky substituents (such as the methyl groups of acetone) decrease the percentage of hydrate present at equilibrium, whereas electron-withdrawing substituents and small substituents (the hydrogens of formaldehyde) increase the percent of hydration present at equilibrium. 

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 position of the equilibrium between a gem diol and an aldehyde or ketone depends on the structure of the carbonyl compound. The equilibrium generally favors the carbonyl compound for steric reasons, but the gem diol is favored for a few simple aldehydes. For example, an aqueous solution of formaldehyde consists of 99.9% gem diol and 0.1% aldehyde, whereas an aqueous solution of acetone consists of only about 0.1% gem diol and 99.9% ketone. 

The precatalysts, Pt(IV) or Pt(II) salts, were found to be reduced in a series of steps by excess P(CH20H)5 to give Pt[P(CH20H)5]4, which in aqueous solution exists in equilibrium with the five-coordinate cationic hydride [PtL4H][OH] [L = P(CH20H)3]. Since reaction mixtures are basic [rationalized by the formation of hemiacetals from P(CH20H)3 and formaldehyde], the major Pt species present during catalysis is ze-... 

The acrylate complex 10 was suggested to be the major solution species during catalysis, since the equilibrium in Scheme 5-11, Eq. (2) lies to the right (fQq > 100)-Phosphine exchange at Pt was observed by NMR, but no evidence for four-coordinate PtL, was obtained. These observations help to explain why the excess of phosphine present (both products and starting materials) does not poison the catalyst. Pringle proposed a mechanism similar to that for formaldehyde and acrylonitrile hydrophosphination, involving P-H oxidative addition, insertion of olefin into the M-H bond, and P-C reductive elimination (as in Schemes 5-3 and 5-5) [11,12]. 

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... 

Oxazolidines are five-membered cyclic ft-Mannich bases, some of which have, indeed, been examined as potential produgs of /l-amino alcohols of medicinal relevance such as ephedrines and /3-blockers. For example, 3,4-dime-thyl-5-phenyloxazolidine (11.106), the oxazolidine of ephedrine (11.107) undergoes hydrolysis to ephedrine and formaldehyde slowly at pH 1 and 12, but very rapidly in the neutral pH range (tm < 1 min at 37°) [135], Interestingly, the equilibrium reached between the reactants and products of hydrolysis was markedly pH- and concentration-dependent. However, despite its poor stability in aqueous solution, the oxazolidine was delivered through human skin significantly faster than ephedrine when applied as 1% aqueous solutions of pH 7 - 11. The lower basicity of the oxazolidine (pKa 5.5) compared to that of ephedrine (pKa 9.6) may explain the efficient skin permeation. 

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. ... 

Inspired by a bright report from Katritzky and coworkers on the use in Reformatsky reactions of a variety of benzotriazolyl aminals121, which in solution are in equilibrium with the corresponding imines, the formaldehyde aminal 61122 was reacted with lc, Zn and TMSC1 to give adducts 62 (equation 39), which can be successively converted into /3-lactams upon treatment with f-BuMgCl. 

The true regioselectivity of the process could not be evaluated since individual isomers 223 and 224 are capable of interconversion and exist in solution in dynamic equilibrium <1997RJ0524>. In contrast, the hydroxymethylation of 5-nitrotetrazole (37% formaldehyde, dilute H2SO4, 5-10 °C, 24 h) afforded a single isomer 224 (R = N02) in 67% yield <1997RJ01771>. 

Resorcinol differs from other phenols in that it reacts readily with formaldehyde under neutral conditions at ambient temperature. To make stable adhesives, which can be cured at the point of use, they are prepared with less than a stoichiometric amount of formaldehyde. About two thirds of a mole of formaldehyde for each mole of resorcinol will give a stable resinous condensation product. The resin is formed into a liquid of convenient solids content and viscosity. Such solutions have infinite stability when stored in closed containers. Glue mixes formed at the point of use from these solutions, on addition of paraformaldehyde-containing hardeners, will have a useful life of several hours due to two principal factors (1) the paraformaldehyde depolymerizes to supply monomeric formaldehyde at a slow rate, as determined by the pH (2) the availability of the formaldehyde is also controlled by the kind and amount of alcohol in the solvent. Formaldehyde reacts with the alcohol to form a hemiacetal. This reaction is reversible and forms an equilibrium which exerts further control on the availability of the formaldehyde. 

On the other hand, no difference in equilibrium concentration is expected between Models C and B. In the latter only the "dead chain segments are crystallized while the cationic active centers, at which depolymerization and polymerization of formaldehyde takes place, are in solution. 

When the equilibrium formaldehyde concentration is reached, polymer begins to precipitate. Further polymerization takes place in trioxane solution and, more importantly, at the surface of precipitated polymer. 

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