Thermodynamics of hydrogen bonding in N-monosubstituted amides by nuclear magnetic resonance spectroscopy

Includes bibliographical references.High-resolution Nuclear Magnetic Resonance (NMR) studies of hydrogen bonding in N-monosubstituted amides at varying temperatures were carried out. Two special cases are considered, self-associated amides in (1) an inert solvent and (2) a hydrogen bonding solvent. The solutes studied are the N-monosubstituted acetamides, CH₃CONHR, where R = CH₃, CH(CH₃)₂, and C(CH₃)₃. In order to study the self-association of N-monosubstituted amides by NMR, two equilibrium constants, K₁₂ for the monomer⇌dimer equilibrium, and K̅ for n-mer+ monomer⇌(n+1)-mer equilibria, which are related to the concentration of the solution and the observed chemical shift of the solute, were used. From a series of chemical shift measurements of the hydrogen bonding proton at varying concentrations, the two equilibrium constants, K₁₂ and K̅ were obtained. Consequently the thermodynamic constants for hydrogen bonding in the self-associated amides were determined from the equilibrium constants over a range of temperatures, 20-60° C. An IBM-360/50 computer at Northern Illinois University was used for evaluating the equilibrium constants, K₁₂ and K̅, and thermodynamic constants, ΔH°, ΔS°, and ΔF°. The equilibrium constants for N-methylacetamide, N-isopropylacetamide, N-t-butylacetamide in the strongly hydrogen bonding solvent dioxane at 30° C are found to be 3.9, 3.5, and 3.0 (m.f.)⁻¹ for the monomer⇌dimer equilibrium, and 10.6, 8.0, and 4.6 (m.f.)⁻¹ for the higher species equilibria, respectively, revealing that K̅ is approximately equal to twice K₁₂. It is also found that K̅ is much larger than K₁₂ for the N-monosubstituted amide in the inert solvent carbon tetrachloride. The equilibrium constants found for N-methylacetamide, N-idopropylacetamide, and N-t-butylacetamide in the inert solvent carbon tetrachloride at 30° C are 15, 10, and 6 (m.f.)⁻¹ for the monomer⇌dimer equilibrium, and 240, 160, and 60 (m.f.)⁻¹ for the higher species equilibria, respectively. In both dioxane and carbon tetrachloride solutions of N-methylacetamide, N-isopropylacetamide, and N-t-butylacetsunide, both K₁₂ and K̅ decrease as the bulk of the N-alkyl substituent increases. The enthalpy changes on hydrogen bonding in N-monosubstituted amides were deterraineds (1) in dioxane solvent, ΔH₁₂° = -1.1 and ΔH̅° = -1.2 Kcal/mole for N-methylacetamide; ΔH₁₂° = -0.8 and ΔH̅° = -0.9 Kcal/mole for N-isopropylacetamide; ΔH₁₂° = -0.7 and ΔH̅° = -0.8 Kcal/mole for N-t-butylacetamide; and (2) in carbon tetrachloride solvent, ΔH₁₂° = -3.7 and ΔH̅° = -4.5 Kcal/moke for N-methylacetamide, ΔH₁₂° = -3.0 and ΔH̅° = -4.3 Kcal/mole for N-isopropylacetamide; ΔH₁₂° = -2.4 and ΔH̅° = -3.3 Kcal/mole for N-t-butylacetamide. The entropy and free energy changes for the hydrogen bonding in N-monosubstituted amides were also determined. In both dioxane and carbon tetrachloride solutions of N-methylacetamide, N-isopropylacetamide, and N-t-butylacetamide, -ΔH° and -ΔF° decrease as the bulk of the N-alkyl substituent increases.M.S. (Master of Science