Base Strengths of Substituted Tritylamines, N‐Alkylanilines, and Tribenzylamine in Aqueous Solution and the Gas Phase: Steric Effects Upon Solvation and Resonance Interactions

Abstract

The dissociation constants of the conjugate acids of N-tritylacetamide (1h; pK BH + = 3.81) and N-benzyl-N-methyl-4,4Ј,4ЈЈ-trimethoxytritylamine (4i;

Basicity of Amines in the Gas Phase – Analysis of the Base‐Strengthening Effect of an N‐Trityl Group by Use of a Triadic Formula

“…We suggest that this, or an appreciable proportion of it, corresponds to the difference in gas-phase base strength (for N-protonation) between TrNHAc and MeNHAc due to reduced resonance interaction between the lone pair on nitrogen and the acetyl group in TrNHAc. We were unable to be so precise in our estimate of the extent of the base-strengthening effect for O-protonation of the N-trityl group above the contribution attributable to its polarisability, but it lies between 3 and 12 kcal·mol -1 , [11] and almost certainly contributes towards the enhanced base strength of TrNHAc in aqueous solution.…”

“…A possible complication in comparisons between the solution and gas phase is that different standard states apply -activities of 1 mol·dm -3 in solution and 1 standard atm in the gas phase. [11,12] …”

“…[10] Comparison of reactions in solution and in the gas phase is a well established strategy for investigation of solvent effects, solvation phenomena and molecular structural effects upon reactivity, and we applied this to the peculiarities described above. [11] Base strengths in aqueous solution are expressed by Equation (1) and those in the gas phase by Equation (2), where GB is the gas-phase base strength (a free energy term) and PA the proton affinity (an enthalpy term). A possible complication in comparisons between the solution and gas phase is that different standard states apply -activities of 1 mol·dm -3 in solution and 1 standard atm in the gas phase.…”

“…A possible complication in comparisons between the solution and gas phase is that different standard states apply -activities of 1 mol·dm -3 in solution and 1 standard atm in the gas phase. [11,12] By conventional ICR mass spectrometry [13] we observed that the increase in base strength from PhNHMe to PhNHTr in the gas phase (δGB = 8.7 kcal·mol -1 ) is virtually identical to that between MeNH 2 and TrNH 2 (9.0 kcal·mol -1 ), which can only be due to the increase in polarisability between Me and Tr. Consequently, there is no base strengthening in N-tritylamines through steric inhibition of resonance in the gas phase, and we attributed the enhanced base strengths of RNHTr in solution to steric effects upon solvation.…”

“…The gas-phase base strength of tribenzylamine (Bn 3 N; Scheme 1) has been shown to be unexceptional (GB = 230.7 kcal·mol -1 ), so its anomalously low base strength in solution (pK BH + = 4.90) must also be a solvation effect. [11] Following the same strategy to investigate whether the much higher solution base strength of TrNHAc (pK BH + = 3.81) in relation to MeNHAc (pK BH + = -0.56) is a solvation effect, we measured their base strengths in the gas phase. By a combination of conventional ICR mass spectrometry [13] and B3LYP/6-31G(d) calculations with the aid of the Gaussian 98 program, [14] GB values for MeNHAc (191.1 kcal·mol -1 ) and TrNHAc (211.1 kcal·mol -1 ) for protonation on nitrogen were obtained (i.e., TrNHAc is the stronger base by 20.0 kcal·mol -1 ).…”