Resonance carboxylate anions

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)... 

Resonance description of electron delocalization in carboxylate anion... 

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... 

Another example of the effect of resonance is in the relative acidity of carboxylic acids as compared to alcohols. Carboxylic acids derived from saturated hydrocarbons have ipK values near 5, whereas saturated alcohols have pA values in the range 16-18. This implies that the carboxylate anion can accept negative charge more readily than an oxygen on a saturated carbon chain. This can be explained in terms of stabilization of the negative charge by resonance, ... 

INTRINSIC AND EXTRINSIC FLUORESCENCE. Intrinsic fluorescence refers to the fluorescence of the macromolecule itself, and in the case of proteins this typically involves emission from tyrosinyl and tryptopha-nyl residues, with the latter dominating if excitation is carried out at 280 nm. The distance for tyrosine-to-tryp-tophan resonance energy transfer is approximately 14 A, suggesting that this mode of tyrosine fluorescence quenching should occur efficiently in most proteins. Moreover, tyrosine fluorescence is quenched whenever nearby bases (such as carboxylate anions) accept the phenolic proton of tyrosine during the excited state lifetime. To examine tryptophan fluorescence only, one typically excites at 295 nm, where tyrosine weakly absorbs. [Note While the phenolate ion of tyrosine absorbs around 293 nm, its high pXa of 10-11 in proteins typically renders its concentration too low to be of practical concern.] The tryptophan emission is maximal at 340-350 nm, depending on the local environment around this intrinsic fluorophore. 

As already indicated for acrolein and for the five-membered heteroaromatic rings (e.g. furan Fig. 2.11), resonance may also be important between nonbonded electrons on a single atom and a 7T-bond system. For example, an unshared electron pair of oxygen greatly contributes to the stabilization of the carboxylate anion ... 

The length of a double bond between any two atoms (e.g., C=C) is almost exactly 0.020 nm less than that for a single bond between the same atoms. If there is resonance, hence only partial double bond character, the shortening is less. For example, the length of the C-C bond in benzene is 0.140 nm the C-O distances in the carboxylate anion are 0.126 nm. 

The stabilization energy of the carboxylate anion is substantially greater than that of the acid, because the anion is a resonance hybrid of two energetically equivalent structures, 2a and 2b, whereas the acid is represented by a hybrid of nonequivalent structures, 1a through 1c ... 

The rules for resonance stress that the greatest stabilization is expected when the contributing structures are equivalent (Section 6-5B). Therefore we can conclude that the resonance energy of a carboxylate anion should be... 

Nitro compounds are a very important class of nitrogen derivatives. The nitro group, —N02, like the carboxylate anion, is a hybrid of two equivalent resonance structures ... 

Ruterjans et al. (55) carried out proton magnetic resonance studies of RNase Ti and, based upon the chemical shifts of the C-2 proton of the histidine residues of the enzyme, suggested that histidine residues interact with carboxylate anions of amino acid residues. 

A very good analogy is a mule. A mule is a hybrid of a horse and a donkey. A mule is neither a horse nor a donkey but it has properties of each. The resonance hybrid of the carboxylate anion is a resonance hybrid of the contributing resonance forms and has properties of each. 

Although carboxylic acids exist in equilibrium with their resonance-stabilized carboxylate anions, it is important to understand that resonance stabilization alone will... 

Scheme 2.4 Rationalization of the carboxylate anion resonance forms using arrow pushing.

In studying the relationships between functional groups and proton acidities, we will first look at carboxylic acids. As illustrated in Scheme 2.2, carboxylic acids dissociate to form protons and carboxylate anions. Furthermore, as shown in Scheme 2.3, the carboxylate anion is stabilized through two resonance forms. It is this resonance stabilization that serves as the primary driving force behind the acidic nature of carboxylic acids. Further evidence of the relationship between resonance stabilization of anions and acidity can be seen when comparing the pKa values of carboxylic acids to the pKa values of alcohols. 

Carboxylic acids arc more acidic than alcohols because the negative charge of a carboxylate ion is stabilized by resonance. This resonance stabilization favors formation of carboxylate anions over alkoxide anions, and increases Ka for carboxylic acids. 

The pA i of oxalic acid is lower than that of a monocarboxylic acid because the carboxylate anion is stabilized both by resonance and by the electron-withdrawing inductive effect of the nearby second carboxylic acid group. 

The resonance stabilisation of phenolate and carboxylate anions is just sufficient for them to serve as leaving groups in a classical bimolecular nucleophilic sub-... 

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