[anilin]

Resonance effect reduces basicity  (increases acidity). I'd like to know why amidines (eg  acetamidine pka 12.4) and guanidines (pka13.6) are stronger bases than amines even though they both produce resonance stabilization.
Thanks

[Vidya]

High basicity of acetamidine is due to protonation of =NH to give very stable amidinium cation 
same reason is applicable on Guanidine.Protonation of =N gives highly stable guanidinium cation  in conjugation with double .
I hope my point is clear.

[anilin]

High basicity of acetamidine is due to protonation of =NH to give very stable amidinium cation 
same reason is applicable on Guanidine.Protonation of =N gives highly stable guanidinium cation  in conjugation with double .
I hope my point is clear.

Thanks for your explanation but we get protonation in amine as well. Isn’t it because of the resonance stabilization in amidinium cation and guanidinium cation? Can we say resonance stabilization in a conjugated acid increase acidity of its base?

[Vidya]

Yes that is the reason I said highly stable guanidinium cation  in conjugation with double .No such conjugation is possible in simple amines.