Formaldehyde, resonance structures

There have been extensive investigations on the reaction mechanism. In most cases the reaction proceeds via initial nucleophilic addition of ammonia 2 to formaldehyde 1 to give adduct 5, which is converted into an iminium ion species 6 (note that a resonance structure—an aminocarbenium ion can be formulated) through protonation and subsequent loss of water. The iminium ion species 6 then reacts with the enol 7 of the CH-acidic substrate by overall loss of a proton ... 

The RAHB effect may be illustrated by the ubiquitous C=0- -H—N hydrogen bond of protein chemistry. As shown in Section 5.2.2, the simplest non-RAHB prototype for such bonding, the formaldehyde-ammonia complex (5.31c), has only a feeble H-bond (1.41 kcalmol-1). However, when the carbonyl and amine moieties are combined in the resonating amide group of, e.g., formamide, with strong contributions of covalent (I) and ionic (II) resonance structures,... 

We shall see that most of the reactions of simple carbonyl compounds, like formaldehyde, are a consequence of the presence of an electron-deficient carbon atom. This is accounted for in resonance theory by a contribution from the resonance structure with charge separation (see Section 7.1). The second example shows the so-called conjugate acid of acetone, formed to some extent by treating acetone with acid (see Section 7.1). Protonation in this way typically activates acetone towards reaction, and we... 

The complete active space valence bond (CASVB) method is an approach for interpreting complete active space self-consistent field (CASSCF) wave functions by means of valence bond resonance structures built on atom-like localized orbitals. The transformation from CASSCF to CASVB wave functions does not change the variational space, and thus it is done without loss of information on the total energy and wave function. In the present article, some applications of the CASVB method to chemical reactions are reviewed following a brief introduction to this method unimolecular dissociation reaction of formaldehyde, H2CO — H2+CO, and hydrogen exchange reactions, H2+X — H+HX (X=F, Cl, Br, and I). 

Aldehydes— The dipole moments of various aldehydes are given in Table XCIV, In formaldehyde resonance can occur only in the carbonyl bond, structures / and //, since the structure ///, with a positive charge in one of the hydrogen atoms, involves a divalent carbon atom. 

As described above, formaldehyde exists as trimethylene glycol in aqueous solution. Phenol reacts quickly with the alkali-hydroxyl group and produces resonance structural phenoxide ion, and trimethylene glycol is added to the O and P positions in the phenoxide ion. This quinoid-transition-state is stabilized by the movement of proton. The monomethylene-derivative produced in this way reacts further with formaldehyde and produces two types of dimethylol derivative and one type of trimethyl derivative. These reactions are expressed as second-order reactions ... 

The contribution of the resonance forms XXI, XXII, XXIII, and XXIV to the structure of the anions is frequently overlooked, yet many base-catalyzed condensation reactions of phenol and pyrrole undoubtedly proceed through these resonance structures at the moment reaction occurs. The condensation of phenol with aqueous formaldehyde, the Kolbc synthesis (p. 197), and the Reimer-Tiemann reaction (p. 202) are striking examples of reactions which occur through the seemingly less important carbanion structure of the resonance hybrid. (See p. 133.)... 

The preparation of this analytical reagent is described by Sawicki et al., who developed a sensitive test for formaldehyde and other water-soluble aliphatic aldehydes. Treatment of a drop of an aqueous solution of formaldehyde with excess basified reagent effects conversion to the azine (2). Ferric chloride then oxidizes (1) to (3). which condenses with the azine to form the brilliant blue cation (4. one of the resonance structures).. Spot plate, paper, silica gel. and column procedures described fur the detection and determination of aldehydes are particularly useful fur determination of formaldehyde In auto exhaust fbmes and polluted air. 

In contrast to aliphatic alcohols, which are mostly less acidic than phenol, phenol forms salts with aqueous alkali hydroxide solutions. At room temperature, phenol can be liberated from the salts even with carbon dioxide. At temperatures near the boiling point of phenol, it can displace carboxylic acids, e.g. acetic acid, from their salts, and then phenolates are formed. The contribution of ortho- and -quinonoid resonance structures allows electrophilic substitution reactions such as chlorination, sulphonation, nitration, nitrosation and mercuration. The introduction of two or three nitro groups into the benzene ring can only be achieved indirectly because of the sensitivity of phenol towards oxidation. Nitrosation in the para position can be carried out even at ice bath temperature. Phenol readily reacts with carbonyl compounds in the presence of acid or basic catalysts. Formaldehyde reacts with phenol to yield hydroxybenzyl alcohols, and synthetic resins on further reaction. Reaction of acetone with phenol yields bisphenol A [2,2-bis(4-hydroxyphenyl)propane]. 

The structure of the F3CO" ion is of particular interest because the CO bond length (121.4 pm) is very similar to that in formaldehyde (120.9 pm), which is usually taken as that appropriate for a CO double bond, apparently making carbon pentavalent in this ion (XTV). However, the CF bonds (139.4 pm) are considerably longer than in COF2 (131.7 pm) or CF4 (131.9 pm). This geometry is usually rationalized by writing resonance structures such as XV and XVI in which the octet... 

The more covaient bonds a structure has, the more stable it is. Consider the resonance structures for formaldehyde below. (Formaldehyde is a chemical used to preserve biological specimens.) Structure A has more covalent bonds, and therefore makes a larger contribution to the hybrid. In other words, the hybrid is more like structure A than... 

The reaction mechanism steps are depicted in Eq. (6)-(10). The formaldehyde reacts as a species generated from methylene glycol or oligomeric methylene glycol. The phenate ion is formed in a basic medium, and this hybrid of several resonance structures is an ideal nucleophile. 

Serine and Glycine. Serine can be converted to glycine by the loss of an active formaldehyde. This reaction is one of the most important suppliers of the Ci fragment. Two coenzymes are necessary, tetrahydrofolate (Chapt. VI-5), and pyridoxal phosphate. The elimination of the /3-C atom is a pyridoxal-catalyzed reaction involving the resonance structure mentioned before in Section 4. While this is taking place, the serine is also bound to tetrahydrofolate. The reaction is reversible Serine is also formed from glycine and active formaldehyde. 

Another way in which to gain structural information concerning the N-terminal residue of glycophorins A" and A is to study the N-terminal, mono[ C]methyl derivatives these are produced by using limited amounts of [ C]formaldehyde. There are distinct differences between the N, N -di[ C]methylamino and N -mono[ C]methylamino species (i) a significant, chemical-shift difference exists between the N-terminal dimethyl and monomethyl species (43 and 34 p.p.m.) (li) all of the C resonances of the N-terminal dimethyl species move upheld as the pH is increased (if they move at all), whereas all of the C resonances of the N-terminal, monomethyl species move downfield as the pH is increased and (in) A for the N-terminal monomethyl species tends to be much larger than that for the N-terminal dimethyl species. Point (in) would tend to indicate that it may be more advantageous to study the N-terminal monomethyl species. However, because of allowable protein concentrations, detection limits on available instruments, and technical difficulties, it has thus far... 

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. 

Proteins crosslinked by formaldehyde are important in photography, the leather industry and in bio-medical sciences. Due to the complex structure of the gelatin molecules (consisting of approximately 20 Afferent kinds of amino acids) and the very low crosslink density, it is not possible to detect crosslink resonances under normal conditions. In order to overcome this problem a 13C enriched formaldehyde is used. By comparison with the chemical shifts of model crosslink compounds it is concluded that the predominant crosslink is formed between the lysine and arginine components in gelatin. A possible mechanism for the reaction between these two amino acid components and the formaldehyde has been proposed 154>. 

Two 15N-enriched urea-formaldehyde resins with different crosslink density were studied by tfie solid state CP MAS 15N NMR. Despite at least six expected 15N chemical shifts arising from tertiary, secondary and primary amides in the different structural moieties, both resins exhibit only two major peaks. The lower field resonance is more pronounced in the highly cured resin, suggesting its origin in the tertiary amides. A DD experiment, which would confirm this assumption, does not result in clearly separated secondary and tertiary amides. Thus, from the analytical point of view, it seems that 13C NMR spectra are more useful than 15N NMR spectra, although 1SN resonance data provide a useful supplement 252). 

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