Final answer:
The isomer of dichlorobenzene with the highest boiling point is para-dichlorobenzene (p-dichlorobenzene) due to its symmetrical structure, which leads to stronger intermolecular forces and closer packing in the solid phase. The correct answer is option C.
Step-by-step explanation:
The isomer of dichlorobenzene with the highest boiling point among 1,2-dichlorobenzene (o-dichlorobenzene), meta-dichlorobenzene (m-dichlorobenzene), and para-dichlorobenzene (p-dichlorobenzene) is para-dichlorobenzene (p-dichlorobenzene). This is because para-dichlorobenzene molecules are symmetrical, which allows them to pack closely together in the solid phase, resulting in stronger intermolecular forces and, consequently, a higher boiling point. The other isomers, with less symmetrical molecular structures, do not pack as efficiently, resulting in slightly lower boiling points.