Hello
the important bit to know is that transitions of electrons from bonding to antibonding orbitals in conjugated double bonds needs more energy or a shorter wavelength to be promoted from a low energy orbital to a higher energy orbital in contrast to electrons in non-bonding orbital where the electrons are higher in energy and need less energy (longer wavelength) to be promoted.
So my tip to be you is that you are looking for a lone pair in a sp or sp2 hybridized orbital as a result of pH 11 and secondly what is the chlorine (electronegativity) influence on this lone pair……..delocalization and secondly you will need to understand why a given compound has the pKa value it does.
hope it helps