Problem 21 Solution
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The proton on the carbonic acid is the most acidic proton. Spartan calculations indicate that this proton has the greatest positive electrostatic potential suggesting that it is the most acidic proton. It is followed by methanol, and propyne is the least acidic of the three. This is consistent with the experimental values of pKa for these three compounds
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propyne |
methanol |
carbonic acid |
Furthermore, the conjugate base of this compound exhibits the greatest resonance stabilization.
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The conjugate base of propyne is a carbanion. This is the least stable of the three congugate bases. Relative to oxygen, carbon is not good at stabilizing the negative charge.
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The conjugate base of methanol is an alkoxide. Which is much more stable than a carbanion. It does not exhibit the resonance stabilization of a carboxylate anion because the negative charge resides on a single oxygen atom.
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In a carboxylate anion, the negative charge is
distributed over two oxygen atoms as shown by the two resonance
structures. This makes the attached proton the most acidic.
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Last modified 24 November, 2003 by zetelsjh@jmu.edu
