organic chemistry - Why are amidines more basic than amines . . . Betrixaban (which has 1 amidine group) Pentamidine (which has 2 amidine groups) Guanethidine (which has 1 guanidine group) Metformin (a biguanide, that is, having 2 guanidine groups fused together) Altretamine (which has 3 guanidine groups fused together into a cyclic structure) Famotidine (which has 1 amidine group and 1 guanidine group)
Why are amidines so much more basic than amides? In the case of the amidine we can draw two important (the fact that they are equivalent resonance structures is why they both are important) resonance structures to describe the protonated product In the case of the protonated amide the resonance structure with the double bond to oxygen places a positive charge on oxygen
organic chemistry - Comparing basic strength order of amines . . . According to Ref 1, the 1,4,5,6-tetrahydropyrimidines (THPs) are extremely basic compounds, as might be expected from their amidine structure (e g , 1,8-Diazabicyclo[5 4 0]undec-7-ene or DBU) The parent THP, 1,4,5,6-tetrahydropyrimidine ( IV ) has a $\mathrm{p}K_\mathrm{a}$ of about $13$ at $\pu{20 ^\circ C}$ , which is even reacted with water
organic chemistry - Why is DBU the most popular base with an amidino . . . DBU is a popular amidine base because it can be deemed to be of lower nucleophilicity than cheaper amidines e g formamidine It has been sometimes erroneously labelled as "non-nucleophilic" but seeing as it reacts with carbon dioxide and sulfonyl halides and anhydrides, this cannot be supported
Basicity comparison of ammonia derivatives (and guanidine) The comparison of the basicity of amines can be quantitatively done by comparing the $\mathrm{p}K_\mathrm{a}$ values of their conjugate acids because $\mathrm{p}K_\mathrm{a} + \mathrm{p}K_\mathrm{b} = \mathrm{p}K_\mathrm{w} = 14$
Comparing basicity of imidazole and 2-imidazoline imidazole: Already aromatic 2-imidazoline: Aliphatic amidine In the conjugate acid of imidazole there is considerable loss of aromatic character because now the pyrrole like Nitrogen atom also gets positive formal charge on it in some of the resonating structures Such N atom now hesitates more to delocalise its electrons over entire ring