Carboxylic acids resonance forms

It is interesting to note that the data in Figure 1 and Table I show that alcohols are better extractants for HDBP than are carboxylic acids. One might expect the reverse to be true because carboxylic acids probably form hydrogen-bonded complexes with HDBP which are similar in structure to the very stable HDBP dimer. Such structures have resonance stabilization and favorable hydrogen bond angles. However, one must consider the energy of association between the solvent molecules themselves. Association between solvent molecules must be broken in order for... 

Small silver nanoparticles are stabilized by ionic species that adsorb to their surface, much like the citrate ions prevent rapid agglomeration of gold nanoparticles. The ionic repnlsion experienced by the silver particles are likely due to carboxylic acids that form after the oxidation of aldehydes. Upon the addition of o er ionic species, the silver nanoparticles approach each other more easily and grow in size. Stndents observe a change in color from yellow to green as the plasmon resonance band shifts to longer wavelengths. 

The electron-withdrawing effect of the positively charged carbonyl carbon atom also increases the acidity of the a-hydrogen atoms of acid derivatives. Their acidity also depends on resonance and inductive effects of the attached substituents (Table 22.1). Therefore, derivatives of carboxylic acids can form enolates that undergo reactions that resemble the condensation reactions... 

The distinguishing characteristic of carboxylic acids is their acidity. Although weaker than mineral acids such as HC1, carboxylic acids dissociate much more readily than alcohols because the resultant carboxylate ions are stabilized by resonance between two equivalent forms. 

Carbonyl compounds are more acidic than alkanes for the same reason that carboxylic acids are more acidic than alcohols (Section 20.2). In both cases, the anions are stabilized by resonance. Enolate ions differ from carboxylate ions, however, in that their two resonance forms are not equivalent—the form with the negative charge on oxygen is lower in energy than the form with the charge on carbon. Nevertheless, the principle behind resonance stabilization is the same in both cases. 

The reaction is less selective than the related benzoylation reaction (/pMe = 30.2, cf. 626), thereby indicating a greater charge on the electrophile this is in complete agreement with the greater ease of nuclophilic substitution of sulphonic acids and derivatives compared to carboxylic acids and derivatives and may be rationalized from a consideration of resonance structures. The effect of substituents on the reactivity of the sulphonyl chloride follows from the effect of stabilizing the aryl-sulphonium ion formed in the ionisation step (81) or from the effect on the preequilibrium step (79). 

Because of the resonance stabilization possible in its deprotonated form, the 5-tetrazolyl moiety is actually nearly as acidic (pKa ca. 6) as many carboxylic acids. This has led to its inclusion in many drug series as a carboxyl surrogate. Apparently related in concept to indomethacin (26a), intrazole (26) is a nonsteroidal antiinflammatory agent which also inhibits platelet aggregation, and therefore is of potential value in keeping the contents of the... 

Vitamin C, also known as L-ascorbic acid, clearly appears to be of carbohydrate nature. Its most obvious functional group is the lactone ring system, and, although termed ascorbic acid, it is certainly not a carboxylic acid. Nevertheless, it shows acidic properties, since it is an enol, in fact an enediol. It is easy to predict which enol hydroxyl group is going to ionize more readily. It must be the one P to the carbonyl, ionization of which produces a conjugate base that is nicely resonance stabilized (see Section 4.3.5). Indeed, note that these resonance forms correspond to those of an enolate anion derived from a 1,3-dicarbonyl compound (see Section 10.1). Ionization of the a-hydroxyl provides less favourable resonance, and the remaining hydroxyls are typical non-acidic alcohols (see Section 4.3.3). Thus, the of vitamin C is 4.0, and is comparable to that of a carboxylic acid. 

The reaction involves the transfer of an electron from the alkali metal to naphthalene. The radical nature of the anion-radical has been established from electron spin resonance spectroscopy and the carbanion nature by their reaction with carbon dioxide to form the carboxylic acid derivative. The equilibrium in Eq. 5-65 depends on the electron affinity of the hydrocarbon and the donor properties of the solvent. Biphenyl is less useful than naphthalene since its equilibrium is far less toward the anion-radical than for naphthalene. Anthracene is also less useful even though it easily forms the anion-radical. The anthracene anion-radical is too stable to initiate polymerization. Polar solvents are needed to stabilize the anion-radical, primarily via solvation of the cation. Sodium naphthalene is formed quantitatively in tetrahy-drofuran (THF), but dilution with hydrocarbons results in precipitation of sodium and regeneration of naphthalene. For the less electropositive alkaline-earth metals, an even more polar solent than THF [e.g., hexamethylphosphoramide (HMPA)] is needed. 

Amides. Although similar to esters in terms of being a functional derivative of a carboxylic acid, amides, unlike esters, are relatively metabolically stable. In general, amides are stable to acid- and base-catalyzed hydrolysis. This stability is related to the overlapping electron clouds within the amide functionality and the corresponding multiple resonance forms. Amidases are enzymes that can catalyze the hydrolysis of amides. Nevertheless, amides are much more stable than esters. 

Structures III and IV assist ionisation of the C-X bond, whereas structure II facilitates nucleophilic addition and consequently a bimolecular displacement of X. The various derivatives of carboxylic acids form a series with varying degrees of resonance stabilisation decreasing in the following order ... 

Thiazolidine-4-carboxylic acid containing peptides exhibit an enhanced polar character when compared with most of the side-chain protected cysteine peptides. 139 The anomalously low nucleophilicity of its imino group (pATa = 6.24) in comparison to that of proline (pXa = 10.60) is due to the resonance stabilization of its unprotonated form 187188 This supposed resonance stabilization is supported by the X-ray crystal structure of 4-thiaproline.1 88 ... 

The Mukaryama reagent,18 N-methyl-2-chloropyridinium iodide (6). transforms carboxylic acid 5 into the amide 7 The acid Is first activated in situ in the form of pyridinium salt 17 by an SN reaction with the Mukoixama reagent (6). This activation is a result of preventing resonance stabilization of the C-O double bond in the positively charged aryl ester 17. 

When a carboxylic acid loses CO the reaction is called decarboxylation. Although the reaction is usually exothermic, the energy of activation is usually high, making the reaction difficult to carry out. The energy of activation is lowered when the flea r bon is a carbonyl because either the anion intermediate is stabilized by resonance or tire acid forms a more stable cyclic intermediate. (A carboxylic acid with a carbonyl p-carbon is called a fi-keto acui.)... 

Above the pKa, the carboxylate is anionic and the charge is resonance stabilized the group is therefore less electrophilic and does not have a good leaving group. Therefore, reactions with nucleophiles are significantly suppressed when compared to the protonated form. Carboxylic acids are not prone to oxidative degradation. 

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