Intrinsic Acidities of α, β, γ Chlorosubstituted Aliphatic Acids from Gas Phase Equilibrian Mesurements

Measurements of the proton transfer equilibria: A1− + A2H = A2− with a pulsed electron beam high pressure mass spectrometer were extended to α, β, γ chlorosubstituted aliphatic acids. The equilibrium constants were used to evaluate ΔG0 for the proton transfer reactions. Assuming ΔG ≈ ΔH and using standard acids AH for which the difference between the bond dissociation energy D(A—H) and the electron affinity of A, EA(A) was known one could evaluate the corresponding difference for the newly measured acids and place them on an absolute acidity scale. The gas phase acidity was observed to increase in the order: acetic, propionic, butyric, γ-Cl butyric, β-Cl butyric, β-Cl propionic, α-Cl butyric, α-Cl propionic, α-Cl acetic. The gas phase acidities are compared with those observed in aqueous solution. The effects of the Cl substituent parallel those in solution but are much larger. The attenuation occurring in solution is attributed to weaker hydrogen bonding of the chloro stabilized acid anions to water molecules.