Conformational Effects on Gas-Phase Acidities of Isomeric C₃ and C₅ Alkanols

The competitive threshold collision-induced dissociation technique is used to examine conformational effects on the relative gas-phase acidities of selected alcohols. By use of HF and H₂O as reference acids in a local thermochemical network to obtain absolute acidities, the measured 0 K gas-phase acidities for the propanol and pentanol isomers are Δ_acid H₀(CH₃CH₂CH₂O-H) = 1563.9 ± 2.9 kJ/mol, Δ_acid H₀((CH₃)₂CHO-H) = 1568.2 ± 2.7 kJ/mol, Δ_acid H₀(CH₃(CH₂)₄O-H) = 1556.4 ± 2.9 kJ/mol, and Δ_acid H₀((CH₃)₂CHCH₂CH₂O-H) = 1556.5 ± 3.0 kJ/mol. Conformational stabilization during deprotonation results in the observed acidity differences between isomers, which can be compared with the "intrinsic" acidity strength defined as deprotonation of the extended all- anti staggered conformations without relaxation. The intrinsic acidities for the propanol and pentanol isomers are 1567 and 1562 kJ/mol, respectively. The difference in intrinsic and observed acidity is largely due to the result of a twisted geometry of the alkoxide ion, stabilized by electrostatic interaction between the electronegative terminal O atom and a H atom on the γ-carbon. These interactions are primarily due to internal rotation about the C_α-C_β bonds for n-propoxide and the primary pentoxides.