Consider the following primary amines, each dissolved separately in water at 25 °C:
• CH₃CH₂NH₂ (ethylamine)
• CF₃CH₂NH₂
• CHF₂CH₂NH₂
The Hammett σ values for the substituents CH₃, CF₃, and CHF₂ on a saturated carbon adjacent to the reaction site are −0.15, +1.06, and +0.50, respectively.
(a) Using qualitative reasoning, rank the three amines in order of increasing base strength in water.
(b) The Hammett relationship ΔpKa=ρσ can be applied to estimate how substituents alter the pKₐ of an amine’s conjugate acid. Take ethylamine’s conjugate-acid pKₐ to be 10.7 and the reaction constant ρ for this series to be 1.0. Estimate the conjugate-acid pKₐ values for CF₃CH₂NH₂ and CHF₂CH₂NH₂.
(c) Compare your estimated pKₐ values from part (b) with the qualitative ranking from part (a). Discuss any consistency or discrepancy.
(d) One student claims that the electronegativity of fluorine is solely responsible for the dramatic basicity decrease in CF₃CH₂NH₂. Critique this claim, citing at least two distinct factors that contribute to the observed trend.