# Why there is a huge difference between the basicity of urea and guanidine?

The two molecules have very similar structures, but urea has a $$\mathrm{p}K_\mathrm{b}$$ of $$13.9,$$ while guanidine have a $$\mathrm{p}K_\mathrm{b}$$ of $$0.4.$$ If guanidine cation can be stabilized by resonance, then why can't urea cation do so?

• Probably this can be the answer- all three canonical forms of guanidine are equivalent, while of urea aren't. – Zenix Dec 22 '19 at 15:58
• For the same reason that ammonia is a much stronger base than water, even though their structures are very similar in the same sense. – Ivan Neretin Dec 22 '19 at 16:41

Urea and guanidine are structurally similar except that the carbonyl oxygen of urea has been replaced by an imine $$\ce{NH}$$ in guanidine. When it come to protonation, carbonyl oxygen of urea is always protonated over its 2 $$\ce{NH2}$$ groups (Ref.1 and 2). In guanidine, it is the imine $$\ce{NH}$$ protonated first. This preference in guanidine is not because a lone pair of $$\mathrm{sp^2}$$ nitrogen is more basic than that of $$\mathrm{sp^3}$$ nitrogen (actually it is other way around), but the lone pair of $$\mathrm{sp^2}$$ nitrogen is the most available pair for protonation over those of two $$\mathrm{sp^2}$$ nitrogens (lone pairs of two $$\mathrm{sp^2}$$ nitrogen atoms are contributing to resonance stabilization at the time). However, when protonated, guanidinium ion was highly stabilized by resonance as depicted in the diagram:
2. B. Valentine, T. E. St. Amour, D. Fiat, “A $$\ce{^{17}O}$$ NMR study of the protonation of urea,” Organic Magnetic Resonance 1984, 22(11), 697-700 (https://doi.org/10.1002/mrc.1270221105).