Resonance in carboxylate anions

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

Resonance description of electron delocalization in carboxylate anion... 

Enzymatic isomerization of ds-aconitate to trans-aeon itate apparently also involves proton abstraction,165 with resonance in the anion extending into the carboxylic acid group. Its mechanism may be directly related to that of the oxosteroid isomerase. However, there are other 1,3-proton shifts in which neither a carbonyl nor a carboxyl group is present in the substrate (Eqs. 13-55,13-56). 

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

The mechanism of amide hydrolysis in base has the usual two steps of the general mechanism for nucleophilic acyl substitution—addition of the nucleophUe followed by loss of a leaving group— plus an additional proton transfer. The initially formed carboxylic acid reacts further under basic conditions to form the resonance-stabilized carboxylate anion, and this drives the reaction to completion. Mechanism 22.10 is written for a 1° amide. 

Compounds dissolve in base because they form sodium salts that are soluble in the aqueous medium. The salts of some high-molecular-weight compounds are not soluble, however, and precipitate. The salts of the long-chain carboxylic acids, such as myristic acid palmitic acid Cj, and stearic acid Cjg, which form soaps, belong to this category. Some phenols also produce insoluble sodium salts, and often these are colored due to resonance in the anion. 

On the other hand, in the alkoxide ion, RO , the negative charge is not delocalized and is concentrated on the single ojq gen atom. This anion, therefore, is not as stable as the resonance stabilized carboxylate anion. The resonance stabilization promotes dissociation in the carbo llc acids making them stronger in relation to the organic acids where lack of resonance stabilization decreases dissociation. 

ANSWER The reaction of a carboxylic add with an alkoxide can t proceed by addition-elimination to give an ester because there is another much easier reaction available that reaction is simple removal of the carboxylic acid hydroxyl proton to give the resonance-stabilized carboxylate anion. They don t call these compounds adds for nothing The lesson in this problem is that you have to think simple. Look first for trivial reactions (loss of the proton) before proceeding on to more complicated processes (addition-elimination). 

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, ... 

Section 19.4 Carboxylic acids are weak acids and, in the absence of electron-attracting substituents, have pK "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 in the carboxylate anion (resonance effect). 

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. 

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. 

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