Role of Urea and Thiourea as Hydrogen Donors

In the complex formation with the carboxylate anion, although 1,3-dimethylthiourea shows a 10-fold increase in stability over urea, the alkylguanidinium ion (pXa = 14) is found to be the strongest proton donor [18] (Table 9.3). 

On the other hand, inverse affinity between thiourea and urea is reported in the complexation with sulfonate, in which great improvement in the affinity is also observed in the nitro-substituted arylurea derivatives [16] (Table 9.4). 

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