A co-axial Cylindrical capacitor is shown in the Figure(i) has dielectric with relative permittivity $\varepsilon_{r1} = 2$. When one – fourth portion of the dielectric is replaced with another dielectric of relative permittivity $\varepsilon_{r2}$, as shown in the Figure(ii), the capacitance is doubled. The value of  $\varepsilon_{r2}$ is _________.