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Which amongst the following compounds/species is least basic? [add image: guanidine-type structures with H2N-C(=O)-NH2, H2N-C(OH)=NH2+ (protonated), H2N-C(=NH)-NH2, H2N-C(=NH2+)-NH2]
A
Urea, $H_2N-C(=O)-NH_2$
B
Protonated urea (hydroxyl form), $H_2N-\overset{\oplus}{C}(-OH)-NH_2$
C
$H_2N-C(=NH)-NH_2$ (guanidine-type, neutral)
D
$H_2N-C(=NH_2^+)-NH_2$ (protonated guanidine)
Detailed Solution
Among these related structures, the species shown in option (2) — a protonated (cationic) form with a hydroxyl group — is the least stabilised by resonance/least favourable to form further by protonation, making it the least basic species among the given options.