Final Exam

December 21, 2000

8. R. P. Singh and J. M. Shreeve (p. 1787-1789) synthesized (C₆F₁₃)₂P(O)OH. Would you expect this acid to have a pK_a higher or lower than phosphoric acid? Briefly explain your reasoning.

Answer:

Phosphoric acid is P(O)(OH)₃ with pK_a1 estimated to be ~5 – 3 = 2, using Pauling's rules. Pauling's rules are based on electronegativity arguments, i.e., the X=O group is highly electronegative, thereby drawing electron density away from the rest of the molecule and making any OH bonds more acidic. The C₆F₁₃ group has a huge number of F atoms attached, thus could be expected to be even more electronegative than the oxo group. Thus, (C₆F₁₃)₂P(O)OH is predicted to be a stronger acid than phosphoric acid and have a lower pK_a.