Formaldehyde, stabilized electrons

The British Pharmacopoeia (1993) recognizes five methods for the sterilization of pharmaceutical products. These are (i) dry heat (ii) heating in an autoclave (steam sterilization) (iii) filtration (iv) ethylene oxide gas and (v) gamma or electron radiation. In addition, other approaches involving steam and formaldehyde and ultraviolet (UV) light have evolved for use in certain situations. For each method, the possible permutations of exposure conditions are numerous, but experience and product stability... 

The hydroxycarbene isomer (H)Co(CO)3(CHOH) was also examined. It yielded a complex with molecular electronic energy more than 60 kcal/mole higher on the energy scale. The hydroxycarbene complex is not likely to play a significant role in the catalytic cycle. It is of some interest to inquire why the 18e hydroxycarbene complex (H)(CO) Co(=CH0H) is less stable than the 16e isomer (H)(CO)3C0(CH2O). The results suggest that the formation of the carbonyl double bond makes the critical difference. The electronically delocalized structure (H)(CO)3Co+5-CH2 0" may provide some extra stabilization for the formally unbonded formaldehyde moiety. The resonance form is dipolar and could be further stabilized by polar solvents. 

Another method of stabilizing the surface of semiconductor electrodes relates to an electrolyte modification, using solution mediators that efficiently accept the electron from the semiconductor to subsequently reduce C02. In aqueous solution, Taniguchi et al. used a p-GaP photoelectrode in the presence of 15-crown-5 ether at a potential of-0.95 V (versus SCE) [119]. In this case, current efficiencies of 44%, 15%, and 4% were observed for methanol, formic acid, and formaldehyde, respectively. 

To be of use in microscopy or flow cytometry, this bond needs to be visualized (to the eye or to the photodetector) by the addition of a fluorescent tag. Visualization can be accomplished by one of two different methods. With direct staining, cells are incubated with a monoclonal antibody that has been previously conjugated to a fluorochrome (for example, fluorescein or phycoerythrin or any fluorochrome with appropriate absorption and emission spectra). This procedure is quick and direct it merely involves a half-hour incubation of cells with antibody (at 4°C), followed by several washes to remove weakly or nonspecifically bound antibodies. Cells thus treated are ready for flow analysis (although final fixation with 1% electron microscopic-grade formaldehyde will provide a measure of biological safety and long-term stability). 

The relative reluctance of CH2OH to accept an electron and form CH2OH- -> CH3OH is probably due to the stabilization of CH2OH in a planar form, which tends to shed an additional electron and form a carbonium, CH2OH+. The latter species converts to formaldehyde following a proton transfer to the solvent. 

The formaldehyde disproportionation has been examined by semi-empirical MO methods (Rzepa and Miller, 1985). With the MNDO procedure, transfer of hydride from hydrate mono-anion to formaldehyde is exothermic by 109 kJ mol-1, and the transition structure [29], corresponding to near symmetrical transfer of hydride, lies 72 kJ mol -1 above the separated reactants. Inclusion of two water molecules, to model solvation effects, stabilizes reactants and transition structures equally. Hydride transfer from the hydrate dianion was found to have a less symmetrical transition structure [30] not unexpected for a more exothermic reaction, but the calculated activation energy, 213 kJ mol-1, is unexpectedly high. Semi-classical primary kinetic isotope effects, kH/kD = 2.864 and 3.941 respectively, have been calculated. Pathways involving electron or atom transfers have also been examined, and these are predicted to be competitive with concerted hydride transfers in reactions of aromatic aldehydes. Experimental evidence for these alternatives is discussed later. 

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. 

The situation is quite different for n- r transitions. The lone electron pair is particularly well stabilized by polar and particularly by protic solvents so it becomes energetically more difficult to excite. Figure 2.45 shows the spectrum of N-nitrosodimethylamine in different solvents. Results of calculations indicate that the negative solvatochromism of carbonyl compounds can be explained on the basis of the structural changes due to the formation of hydrogen bonds (Taylor, 1982). Molecular dynamics simulations, however, indicate that the net blue shift is primarily due to electrostatic interactions (Blair et al., 1989). A large number of water molecules around the entire formaldehyde are responsible for the total blue shift the first solvation shell only accounts for one-third of the full shift. 

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