Cu+ is unstable in aqueous solutions. This is because in aqueous solution Cu+ undergoes disproportionation to form a more stable Cu2+ ion.
2Cu+(aq)→Cu2+(aq)+Cu(s)
The higher stability of Cu2+ ion in aqueous solution may be attributed to its greater negative ΔHydH∘ than that of Cu+. It compensates the second ionisation enthalpy of Cu involved in the formation of Cu2+ ions.