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Question

How would you compare the acidity of the most acidic proton in each of the following molecules?

  1. diphenylmethane
  2. toluene
  3. triphenylmethane

The answer says 3 is the most acidic, followed by 1, and finally 2 is the least acidic. Can someone please tell me how to determine this? I know the benzene group is electron donating group, but am not quite sure how to correlate this knowledge with determining the relative acidities.

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You should consider the stability of carbanion that is left behind, after removing $\ce{H+}$. One could make arguments for resonance stabilization of each of the carbanions. You could "push" the minus charge around the phenyl rings, and there is a general rule that the more "good" resonance structures that one can draw, the more stable the molecule.

It turns out that the tertiary carbanion in 2) is flanked by three phenyl groups (thus many good resonance structures compared to 1) and 3) ).

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