Electron delocalization carboxylate ions

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... 

A molecule for which resonance forms can be written is more stable than any of the contributing resonance forms. Thus the carboxylate ion (a resonance hybrid) is more stable than either of the contributing resonance forms. The difference in energy between the energy of the molecule and the energy of the most stable resonance form is die resonance energy (RE) of die molecule. The resonance energy represents die stabilization of die molecule due to die delocalization of electrons. 

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. 

If, however, the CO2 group is bonded to a carbon that is adjacent to a carbonyl carbon, the CO2 group can be removed because the electrons left behind can be delocalized onto the carbonyl oxygen. Consequendy, 3-oxocarboxylate ions (carboxylate ions with a keto group at the 3-position) lose CO2 when they are heated. Loss of CO2 from a molecule is called decarboxylation. 

The acidic properties of carboxylic acids are due to the carbonyl group, which attracts electrons from the C-O and O-H bonds. The carboxylate ion formed, R-COO , is also stabilized by delocalization of electrons over the O-C-O grouping. 

The heretofore uncharacterized allophanate ion adopts a planar configuration that facilitates the formation of an intermolecular N-H- O hydrogen bond (Figure 8.26). The relative instability of the patent aUophanic acid is consistent with this hydrogen-bonded ring stracture, as protonation at the exocyclic carboxyl oxygen would result in an overall reduction of -electron delocalization. 

Another example of n delocalization occurs in the carboxylate ion, RCOO (e.g. the ethanoate ion). A lone pair of electrons in the 2p orbital of the oxygen atom overlaps and merges with the n orbital of the adjacent carbonyl group (>C=0). This is an alternative but equivalent description to the resonance model described earlier. Four n electrons (two from the carbonyl group and two from the lone pair) are delocalized over three atoms. The resulting molecular orbital is known as a three-cxntre delocalized n orbital (Figure 14-57). 

Notice that delocalized electrons result from a p orbital overlapping the p orbitals of two adjacent atoms. For example, in nitroethane, the p orbital of nitrogen overlaps the p orbital of each of two adjacent oxygens in the carboxylate ion, the p orbital of carbon overlaps the p orbital of each of two adjacent oxygens and in benzene, the p orbital of carbon overlaps the p orbital of each of two adjacent carbons. 

We also saw that the greater stability of Ihe carboxylate ion is attributable to two factors— inductive electron withdrawal and electron delocalization. That is, the double-bonded oxygen stabilizes the carboxylate ion by decreasing the electron density of the negatively charged oxygen by inductive electron withdrawal and by an increase in delocalization energy. 

Although both the carboxylic acid and the carboxylate ion have delocalized electrons, the delocalization energy of the carboxylate ion is greater than that of the carboxylic acid because the ion has two equivalent resonance contributors that are predicted to be relatively stable, whereas the carboxylic acid has only one (Section 8.6). Therefore, loss of a proton from a carboxylic acid is accompanied by an increase in delocalization energy— in other words, an increase in stability. 

Phenol is a weaker acid than a carboxylic acid because electron withdrawal by the sp carbon in the phenolate ion is not as great as electron withdrawal by the oxygen in the carboxylate ion. In addition, the increased delocalization energy when a proton is lost is not as great in a phenolate ion as in a carboxylate ion, where the negative charge is shared equally by two oxygens. 

We have also seen that delocalized electrons are electrons that are shared by more than two atoms. When electrons are shared by more than two atoms, we cannot use solid lines to represent the location of the electrons accurately. For example, in the carboxylate ion, a pair of electrons is shared by a carbon and two oxygens. We show the pair of delocalized electrons by a dotted line spread over the three atoms. We have seen that this stmc-ture is called a resonance hybrid. The resonance hybrid shows that the negative charge is shared by the two oxygens. 

Amides are carboxylic acid derivatives. The amide group is recognized by the nitrogen connected to the carbonyl group. Amides are neutral compounds the electrons are delocalized into the carbonyl (resonance) and thus, are not available to bond to a hydrogen ion. 

---