Carboxylic acid electrostatic potential maps

Table 25.1 shows the amino acids in the form in which they exist at a pH of 7 amine groups as positively charged ammonium ions, and carboxylic acid groups as negatively charged carboxylates. Their electrostatic potential maps compare their shape and charge distribution. 

Electronically, we find that strongly polarized acyl compounds react more readily than less polar ones. Thus, acid chlorides are the most reactive because the electronegative chlorine atom withdraws electrons from the carbonyl carbon, whereas amides are the least reactive. Although subtle, electrostatic potential maps of various carboxylic add derivatives indicate the differences by the relative blueness on the C-O carbons. Acyl phosphates are hard to place on this scale because they are not used in the laboratory, but in biological systems they appear to be somewhat more reactive than thioesters. 

Figure 27.1 A soap micelle solubilizing a grease particle in water. An electrostatic potential map of a fatty acid carboxylate shows how the negative charge is located in the head group.

Acid chloride, alcohols from, 804 alcoholysis of, 802-803 amides from, 803-804 amines from, 933-935 amjnolysis of, 803-804 carboxylic acids from, 802 electrostatic potential map of, 791... 

Electron delocalization in carboxylate ions is nicely illustrated with the aid of electrostatic 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. 

As their name implies, carboxylic acids are acidic—they dissodate slightly in aqueous solution to give H30+ and a carboxylate anion. Carboxylic acids are much weaker than inorganic acids like HC1 or H2S04, however. The Ka of acetic add, for example, is 1.78 X 10-5 (pKa = 4.75), meaning that only about 1% of acetic acid molecules dissociate in a 1.0 M aqueous solution. Note in the following electrostatic potential map of acetic add that the acidic -OH hydrogen is positively polarized (blue). 

Experimental evidence for the ecjuivalence of the two carboxylate oxygens comes from X-ray crystallographic studies on sodium formate. Both carbon-oxygen bonds are 127 pm in length, midway between the C=0 bond (120 pmi and C—O bond (134 pm) of formic acid. An electrostatic potential map of the formate ion also shows how the negative charge (red) is dispersed equally over both o.xygens. 

Use SpartanView to compare electrostatic potential maps of butanoate anion. 2-chlorobutanoate anion, and 4-chlorobutanoate anion. Which anion has the most negative oxygens, and which has the least Why Based on these data, predict the relative acidities of the corresponding carboxylic acids. 

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