Cu⁺ ion is not stable in aqueous solution because

Many copper (I) compounds are unstable in aqueous solution and undergo disproportionation reaction

second ionization enthalpy of copper is less than the first ionization enthalpy

Hydration energy of ${\mathrm{Cu}}_{\left(\mathrm{aq}\right)}^{2+}$ is much more negative than that of ${\mathrm{Cu}}_{\left(\mathrm{aq}\right)}^{2+}$

large value of second ionization enthalpy of copper is compensated by much more negative hydration energy of ${\mathrm{Cu}}_{\left(\mathrm{aq}\right)}^{2+}$