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pKaH of substituted guanidines
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.
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.
Yeah, its the pKa of the conjugate acid. Ph is electron withdrawing but it's not particularly powerful, compare the pKa of toluene with acetonitrile/nitromethane to see that it does fit then trend 
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