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I am attempting to answer a question which asks to explain the sequence of pKaH values for a series of substituted guanidines (NH2)2CNR where R varies.

The order is R = H , Ph , CH3CO , MeO , CN , NO2
pKaH = 14.5 , 10.8 , 8.3 , 7.5 , -0.4 , -0.9

I'm not quite sure how to explain this. I think pKaH refers to the pKa of the conjugate acid. I'm guessing CN and NO2 are both very electron withdrawing and allow H+ to be lost easily. However I thought that Ph was electron withdrawing as well, so I can't understand why it has such a high pKa. I'm also not really sure how the ketone and MeO groups would affect the pKa. From what I have found, methoxy is electron releasing so I am quite confused.

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