When they possess an electron withdrawing group and an electron donating group at opposite ends of the resonance structure, they have a large shift in charge distribution across the molecule, which causes the solvent molecules to reorient to a less energetic arrangement, called solvent relaxation.
12.
Electron donating groups are generally ortho / para directors for electrophilic aromatic substitutions, while electron withdrawing groups are generally meta directors with the exception of the halogens which are also ortho / para directors as they have lone pairs of electrons that are shared with the aromatic ring.
