Table 1.

Interaction Energies Between–X and the Carbonyl Group at the Reacting Carboxylic Acid Derivative (ΔΔG_R) or the Product Oxygen Anion (ΔΔG_P), Estimated from Data Presented in Chart 1 and as Illustrated in Figure 2 for the –OEt Group.

Substituent	ΔΔG° b kcal/mol	ΔΔGPc kcal/mol	ΔΔGRd kcal/mol
-CH3	3.5	0.5	-3.0
-SEt	5.8	-3.7	-9.5
-OEt	12.1	-4.6	-16.7
-NH2	15.9	-1.8	-17.7
-O-	22.8	2.6	-20.2

^aThe values of (ΔΔG_R) and (ΔΔG_P) are calculated from the substituent effects on (pK_E) and (pK_a)_XH, respectively (Chart 1), using a factor of 1.36 to convert ΔpK to ΔΔG°.

^bThe substituent effect on the free energy for ionization of acetaldehyde to form the enolate, calculated from the values of (pK_a)_CH reported in Chart 1.

^cThe estimated polar interaction between the substituent and the oxygen anion at the enolate, calculated from the substituent effect on (pK_a)_OH for the enolate of acetaldehyde (Chart 1).

^dThe estimated resonance interaction between the substituent and the carbonyl group of the carbon acid, calculated using data given in Chart 1 and using eq 1.