Table 3.

Effect of Changing Ring Substituents, or the Substitution of C-4 of the Ring by N, on the Equilibrium Constant for Addition of Glycine to Benzaldehyde to Form the Corresponding Imine.^a

aldehyde	(Kadd)Xb	$\frac{{(K_{\text{add}})}_{\mathrm{X}}}{{(K_{\text{add}})}_{\mathrm{H}}}$	ΔΔGXc (kcal/mol)	${(K_{\text{add}})}_{\mathrm{X}}^{+}$ d	$\frac{{(K_{\text{add}})}_{\mathrm{X}}^{+}}{{(K_{\text{add}})}_{\mathrm{H}}^{+}}$	ΔΔGXe (kcal/mol)	p(Ka)X-IMDf	Δ(pKa)Xg
	44	1.0	0	0.0033	1.0	0	6.29	0
	1.76	0.040	1.9	72	22,000	−5.9	12.13	5.8
	0.49	0.011	2.7	0.29	88	−2.7	10.16	3.9
	1.5h	0.034	2.0	1 × 10−4h	0.030	2.1	6.19h	−0.1
	500	11.4	−1.4	0.002	0.61	0.3	4.95	−1.3

^aData from Table 1 for reactions in D₂O, unless noted otherwise.

^bEquilibrium constants for addition of glycine anion to the aldehyde in the first Table row to form the corresponding unprotonated imine.

^cThe effect of the substituent X on the change in Gibbs energy for formation of the imine from the free aldehyde and amino acid.

^dEquilibrium constants for addition of glycine zwitterion to the aldehyde in the first Table row to form the corresponding iminium ion.

^eThe effect of the substituent X on the change in Gibbs energy for formation of the iminium ion from the free aldehyde and amino acid.

^fApparent acidity constants for the substituted iminium ions.

^gThe effect of the substituent X on the pK_a for ionization of the iminium ion.

^hData from ref 9 for reactions in D₂O at 25 °C and I = 1.0 (KCl).