Resonance effects on acidity

The resonance effect on acidity can be expressed in terms of the electron density (q0) at the oxygen atom as... 

D. Resonance Effects on Acidity—benzoic acids and phenols (review the solution to problem 20-45)... 

The resonance effect makes an acid stronger when the resonance energy introduced with the structural change is greater in the ion than in the acid. One of the most important resonance effects on acid strengths occurs when the structural change introduces a double or triple bond next to the atom from which the proton dissociates. The resulting structure is A—B—C H for the acid and... 

On the other hand, methyl substituents have a weak electron-donating effect opposing that of the aromatic ring. This also favours resonance in the non-ionized acid. There is only a modest effect on acidity, except when the methyl is in the ortho position, where the effect is closer to the carboxyl group. However, ortho substituents add a further dimension that is predominantly steric. Large groups in the ortho... 

Compound is a slightly stronger acid than compound . The CH tO group has both an inductive effect and a resonance effect on the acidity of . 

Studying substituent effects on acidity is the standard method of determining whether a group is donating or withdrawing by induction and resonance,... 

The study of gas-phase reactions has obviously been of much assistance in understanding solution-phase reactions. Sometimes, however, this relationship breaks down because the reactants give a different reaction in the gas phase than in the solution phase. Caserio and Kim present such an example in Chapter 5, a thorough study of gas-phase reactions (ion cyclotron resonance) of alcohol nucleophiles with protonated carboxylates, carbonates, and phosphates. The goal of this work is to understand solvent and counterion effects on acid-catalyzed esterification. The gas-phase reaction pathways, however, turn out to be different from those in solution phase. 

The relative acidities of carboxylic acids (and. by inference, the stabilities of their carboxylate ions) have been used to compare the electron-donating and electron-withdrawing properties of substituents. These studies are particularly valuable to distinguish between inductive and resonance effects on the stabilities of compounds and ions. Some examples ... 

ACIDITY OF CARBOXYLIC ACIDS Resonance Stabilization of the Carboxylate Ion Inductive Effect on Acidity Acidity of Aromatic Carboxylic Acids... 

As an example, experimental kinetic data on the hydrolysis of amides under basic conditions as well as under acid catalysis were correlated with quantitative data on charge distribution and the resonance effect [13]. Thus, the values on the free energy of activation, AG , for the acid catalyzed hydrolysis of amides could be modeled quite well by Eq. (5)... 

Utilization of resonance effects can facilitate unenhanced Raman measurement of surfaces and make the technique more versatile. For instance, a fluorescein derivative and another dye were used as resonantly Raman scattering labels for hydroxyl and carbonyl groups on glassy carbon surfaces. The labels were covalently bonded to the surface, their fluorescence was quenched by the carbon surface, and their resonance Raman spectra could be observed at surface coverages of approximately 1%. These labels enabled assess to changes in surface coverage by C-OH and C=0 with acidic or alkaline pretreatment [4.293]. 

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). 

SOMe the enhancement in the meta-position is almost as large as in the para-position. The authors go on to show the applicability of op (g) values to certain solution processes, particularly those in non-aqueous solvents, but including the dissociation of thiophenols in 48% ethanol, the results of Bordwell and Andersen80 to which reference has been made earlier (Section III.A.1). A separation of field/inductive and resonance effects is also essayed for the gas-phase acidities of the phenols, and SOMe and S02Me feature in the discussion. There is reference to a oR° value of + 0.07 for SOMe as an unpublished result of Adcock, Bromilow and Taft (cf. 0.00 from Ehrenson and coworkers65 and — 0.07 from Katritzky, Topsom and colleagues128.)... 

It is to be expected from chemical evidence that the replacement of hydrogen by an aliphatic radical would have some further inhibiting effect on the carboxyl resonance. It is found, however, that to within the experimental error of about 0.2 v.e. the resonance energy is the same for methyl and ethyl esters as for carboxylic acids. 

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