Formaldehyde resonance forms

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. 

Condensation of phenol with formaldehyde is a base-catalyzed process in which one resonance form of the phenoxide ion attacks formaldehyde. The resulting trimethylol phenol is then crosslinked by heat, presumably by dehydration with the intermediate formation of benzylcarbocations. The resulting polymer is Bakelite. Since the cost of phenol is relatively high and... 

This oxonium ion may in some cases react in its resonance form, the carbonium ion. The propagation step could be by electrophilic attack of the electrophilic carbonyl atom of the methylene group on the oxygen atom of the carbonyl group of the highly polar formaldehyde. This mechanism is similar to that advanced for the polymerization of trioxane or tetrahydrofuran. A further refinement of the mechanism takes into account the likely possibility that this oxonium ion... 

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.)... 

PROBLEM 1.15 There is a third resonance form for formaldehyde, but it contributes very little to the structure and is usually ignored. Can you find it and explain why it is relatively unimportant ... 

The phosphonium salt is then deprotonated by triethylamine and undergoes the Wittig reac-tion with formaldehyde. Deprotonation with the modestly basic triethylamine is possible because the anion is stabilized by resonance forms involving both the electron-withdrawing groups on the benzene ring ... 

Hydroxypyrroles. Pyrroles with nitrogen-substituted side chains containing hydroxyl groups are best prepared by the Paal-Knorr cyclization. Pyrroles with hydroxyl groups on carbon side chains can be made by reduction of the appropriate carbonyl compound with hydrides, by Grignard synthesis, or by iasertion of ethylene oxide or formaldehyde. For example, pyrrole plus formaldehyde gives 2-hydroxymethylpyrrole [27472-36-2] (24). The hydroxymethylpyrroles do not act as normal primary alcohols because of resonance stabilization of carbonium ions formed by loss of water. 

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... 

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>. 

In the addition of hydride donors to aldehydes (other than formaldehyde) the tetrahedral intermediate is a primary alkoxide. In the addition to ketones it is a secondary alkoxide. When a primary alkoxide is formed, the steric hindrance is smaller. Also, when the ( —() double bond of an aldehyde is broken due to the formation of the CH(OM) group of an alkoxide, less resonance stabilization of the C=0 double bond by the flanking alkyl group is lost than when the analogous transformation occurs in a ketone (c.f. Table 7.1). For these two reasons aldehydes react faster with hydride donors than ketones. With a moderately reactive hydride donor such as NaBH4 at low temperature one can even chemoselectively reduce an aldehyde in the presence of a ketone (Figure 8.2, left). 

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. 

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