The table below lists pKₐ values (25 °C, aqueous solution) for the conjugate acids of three structurally related amines. One entry is missing.
Amine
Structure
pKₐ (conjugate acid)
A
CH₃CH₂NH₂
10.7
B
(CH₃)₂CHNH₂
10.9
Y
(CH₃)₃CNH₂
?
Assume ideal behavior and that pKₐ + pK_b = 14.00 for each conjugate acid–base pair.
(a) Calculate pK_b for amines A and B. Show your work.
(b) Using only the pKₐ data provided for A and B, determine which of the two is the stronger base in water. Explain your reasoning.
(c) Examine the structure of amine Y. Relative to A and B, predict whether Y will be the strongest, weakest, or intermediate base in water. Estimate a reasonable pKₐ value for its conjugate acid and briefly justify the estimate with structural arguments.
(d) Evaluate whether simple inductive effects alone are sufficient to account for the entire trend across A, B, and Y. Support your claim with at least one piece of evidence from your earlier answers.