Carboxylates electron delocalization

Electron delocalization in carboxylate ions is nicely illustrated with the aid of elec trostatic potential maps As Figure 19 4 shows the electrostatic potential is different for the two different oxygens of acetic acid but is the same for the two equivalent oxygens of acetate ion... 

Notice too that the carbonyl oxygen of the carboxylic acid is protonated m the first step and not the hydroxyl oxygen The species formed by protonation of the car bonyl oxygen is more stable because it is stabilized by electron delocalization The pos itive charge is shared equally by both oxygens... 

Resonance description of electron delocalization in carboxylate anion... 

The electron-donating effect originates from the lone pair electrons on oxygen, with overlap into the Jt electron system. This electron donation will stabilize the non-ionized acid via electron delocalization, but would destabilize the conjugate base by creating a double charge in the carboxylate system. The net result is lower acidity. 

Ruthenium(III), d5 Ru111 is often associated with classical-type ligands, e.g. ammine, water, halides. They are octahedral low spin t2/ species with one unpaired electron and generally sub-stitutionally inert. The electronic structure of polynuclear carboxylates and mixed valence Ru" 1" complexes of the type [Ru(NH3)5]2L5+ has been the subject of much interest particularly with respect to the degree of unpaired electron delocalization within these molecules. 

The expansion of the concept to encompass cyclic electron delocalization or homoaromaticity occurred in the late 1950s. In 1956 Applequist and Roberts pointed out that the cyclobutenyl cation resembles the cyclopropenium cation . Doering and colleagues suggested that the cycloheptatriene carboxylic acids could be regarded as planar pseudoaromatic type structures with a homoconjugative interaction between C(l) and C(6) . Based on the results of solvolytic studies on the bicyclo[3.1. OJhexyl system, Winstein set out the general concept of homoaromaticity in 1959 ... 

Hiberty, P. C. Byrman, C. P. Role of x-electron delocalization in the enhanced acidity of carboxylic acids and enols relative to alcohols, 7. Am. Chem. Soc. 1995, 777, 9875-9880. 

The facile reduction of the -COOH group by BHj THF or BH3 SMej has been employed for chemoselective reductions of the carboxyl group in the presence of ester or lactone functionalities using a stoichiometric quantity of the borane. The carbonyl group in triacylboranes resembles the reactivity of an aldehyde or a ketone more than of an ester (ester resonance) due to electron delocalization from the acyl oxygen into the p orbital of boron. 

Crystal and molecular structure of pyrrole-2-carboxylic acid Ji-electron delocalization of its dimers-DFT and MP2 calculations102... 

The catalytic strategy is familiar from our discussion of PLP-dependent reactions reaction via a Schiff base, probable medium control of the decarboxylation, and desolvation of the carboxyl group on binding to the enzyme. What is most surprising is that pyruvate, with its very small electron sink, works as efficiently as PLP, which allows for more extensive electron delocalization. The specialness of PLP in enzymic catalysis must lie in other factors. 

There are large differences in the reactivity of the various carboxylic acid derivatives, such as amides, esters, and acyl chlorides. One important factor is the resonance stabilization provided by the heteroatom substituent, which is in the order N > O > Cl. Electron delocalization reduces the electrophilicity of the carbonyl group and the corresponding stabilization is lost in the tetrahedral intermediate. 

There are two factors that cause the conjugate base of a carboxylic acid to be more stable than the conjugate base of an alcohol. First, a carboxylate ion has a doubly bonded oxygen in place of two hydrogens of the alkoxide ion. Inductive electron withdrawal by this electronegative oxygen decreases the electron density of the ion. Second, the electron density is further decreased by electron delocalization. 

Electron delocalization can affect the nature of the product formed in a reaction and the of a compound. A carboxylic acid and a phenol are more acidic than an alcohol such as ethanol, and a protonated aniline is more acidic than a protonated amine because electron withdrawal stabilizes their conjugate bases and the loss of a proton is accompanied by an increase in resonance energy. 

Second, the weaker the basicity of Y, the smaller is the contribution from the resonance contributor with a positive charge on Y (Section 17.2) the less the carboxylic acid derivative is stabilized by electron delocalization, the more reactive it will be. 

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