All the three classes of aliphatic amines form H-bonds with water. As the size of alkyl group increases, the solubility decreases due to a corresponding increase in the hydrophobic part (hydrocarbon part) of the molecule. On the other hand, aromatic amines are insoluble in water due to presence of larger hydrocarbon part (C6H5-group) which tends to retard the formation of H-bonds.
Now, among the given compounds, C6H5NH2 is insoluble in water due to presence of C6H5-group (hydrocarbon part). In (C2H5)2NH, two alkyl groups are present and in C2H5NH2 only one alkyl group is present, hence C2H5NH2 is more soluble in water than (C2H5)2NH.
Combining all these facts, solubility of the given three compounds increases in the order:
C6H5NH2 < (C2H5)2NH < C2H5NH2
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