Carboxyl groups resonance

Carboxylic acids are weak acids and m the absence of electron attracting substituents have s of approximately 5 Carboxylic acids are much stronger acids than alcohols because of the electron withdrawing power of the carbonyl group (inductive effect) and its ability to delocalize negative charge m the carboxylate anion (resonance effect)... 

The negatively charged oxygen substituent is a powerful electron donor to the carbonyl group Resonance m carboxylate anions is more effective than resonance m carboxylic acids acyl chlorides anhydrides thioesters esters and amides... 

The effect of a carboxy group is illustrated by the reactivity of 2-bromopyridine-3- and 6-carboxylic acids (resonance and inductive activation, respectively) (cf. 166) to aqueous acid under conditions which do not give hydroxy-debromination of 2-bromopyridine and also by the hydroxy-dechlorination of 3-chloropyridine-4-car-boxylic acid. The intervention of intermolecular bifunctional autocatalysis by the carboxy group (cf. 237) is quite possible. In the amino-dechlorination (80°, 4 hr, petroleum ether) of 5-carbethoxy-4-chloropyrimidine there is opportunity for built-in solvation (167) in addition to electronic activation. This effect of the carboxylate ion, ester, and acid and its variation with charge on the nucleophile are discussed in Sections I,D,2,a, I,D,2,b, and II,B, 1. A 5-amidino group activates 2-methylsulfonylpyridine toward methanolic am-... 

In para-amino benzoic acid, there is another resonance structure right next to the six-sided ring. It is a carboxyl group, shown with a single bond between carbons, and a double bond between the carbon and the oxygen. This is also a place where the electron can bounce around between the three nuclei. 

Data for acetic acid and eight other monobasic acids lead to a value of 1.20 v.e. for the resonance energy of the carboxyl group relative to the OH... 

The superior coordinating capacity of phosphonate over carboxylate is illustrated in the 9Be NMR spectra in Fig. 25 (260). The similarity of the spectra obtained by reaction of BeS04 and methylphosphonic acid or phosphonacetic acid indicates that the carboxylate group is not bound to the beryllium under these experimental conditions. It should be noted that substitution of a water molecule by a phosphonate ligand causes the 9Be resonance to move upfield when it co-ordinates, as does the fluoride ion (271), the only other monodenate ligand... 

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... 

We can write a similar delocalization picture for the ortfio-substituted compounds, but this is countered by the opposing inductive effect close to the carboxyl. However, the steric effect, as described above, means large groups in the ortho position can force the carboxyl group out of the plane of the ring. This weakens the resonance effect, since delocalization is dependent upon coplanarity in the eonjugate system. 

Similarly, the C-1 resonance of an axial anomer is shielded relative to that of its eq.uatorial isomer. Also very distinctive are signals due to the carbon of a primary alcohol group (C-6, in the region of 6O-65 p.p.m.) and to the carboxyl group of an uronic acid moiety. Typically, as seen in Fig. 1, the carboxyl C=0 resonance is in the region of 1T5 p.p.m. although, as noted below (see Fig. 3), it is strongly pH dependent. 

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