Question

1. A hydrogen atom in the organic base pyridine, CsHsN, can be substituted by various atoms or groups to give XCsH4N, where X is an atom such as Cl or a group such as CH3. The following table gives K, values for the conjugate acids of a variety of substituted pyridines. Atom or Group X (aq) + HCl(aq) → (aq) + Cl-(aq) NO K, of Conjugate Acid 5.9 x 10-2 1.5 x 10-4 6.8 x 10-6 1.0 x 10-6 н. substituted pyridine conjugate acid CH a. Suppose each conjugate acid is dissolved in sufficient water to give a 0.050 M solution. Which solution would have the highest pH? The lowest pH? Calculate the pH of the solution with the lowest pH. Show your work. b. Which of the substituted pyridines is the strongest Brønsted base? Which is the weakest? Calculate the Kb of the strongest base. If you have a 0.010 M solution of this base, what is the pH of the solution? Show your work. c. Do the substituted pyridines act as Lewis acids or bases? Draw a Lewis structure of the chlorinated base (you can abbreviate the ring as in the drawing above) and circle the part of the molecule that would act as the Lewis acid or base. Explain the trend in basicity you see for the chlorinated pyridine relative to the unsubstituted pyridine (where X = H), using concepts such as electronegativity or resonance.

Answer 1

pH = 1 (pka - log c) More pka, more pH pka = -logka = CH group when attached will have highest pH and No group when attached will have lowest pH For No pH = 1 (=log (5.9x10²) = log 0.05). pH = 1.265 when X is eleton donating group, then it will increase election density and it will behave as good Bronsted base => CH₂ = x Strongest Base run No = X . Weakent Base Kak = kw = K = Kw = 10t4 - 108 pH = 14 - pon = 14 - ( 2 (pky-logc)]

- pr = 14 - ( 2 (-log 108 - log 0.01) - pH = 9 They act as Lewis Base o Lewis Base cool c.. e ch: lewis structure Chlorine has strong -I effect, therefore it will decrease electon density and hence it will be least basic as compared to simple pysidine (When x=H).