An electric dipole of dipole moment p is placed in a uniform external electric field E. Then, the

Torque on dipole form,   $\tau =\mathbf{pE}\sin \theta$ $\left(\overrightarrow{\mathrm{\tau }}=\overrightarrow{\mathrm{P}}\times \overrightarrow{\mathrm{E}}\right)$

So, torque is maximum if p is perpendicular to E.

Potential energy,  $U=-p\mathbf{E}=-p\mathbf{E}\cos \theta$

 when  $\theta$  = 180⁰, i.e. p is anti-parallel to E, then  potential energy will be maximum.