JEE MainChemistryHaloalkanes and Haloarenes
The melting points of three isomeric dichlorobenzenes A, B and C are given below: Isomer Melting Point (K) A 256 B 323 C 249 Identify the isomers A, B and C respectively.
Options
- AA = p -dichlorobenzene, B = o -dichlorobenzene, C = m -dichlorobenzene
- BA = m -dichlorobenzene, B = p -dichlorobenzene, C = o -dichlorobenzene
- CA = o -dichlorobenzene, B = p -dichlorobenzene, C = m -dichlorobenzene
- DA = o -dichlorobenzene, B = m -dichlorobenzene, C = p -dichlorobenzene
Correct answer
C. A = o -dichlorobenzene, B = p -dichlorobenzene, C = m -dichlorobenzene
Step-by-step solution
The melting point of isomeric dihalobenzenes depends on the symmetry of the molecule, which dictates how well it packs into the crystal lattice. The para-isomer is the most symmetrical and therefore has the most efficient packing in the solid state, leading to the highest melting point (323 K). Thus, B is p -dichlorobenzene. Among the remaining two isomers, the ortho-isomer has a slightly higher melting point (256 K) compared to the meta-isomer (249 K). Therefore, A is o -dichlorobenzene and C is m -dichlorobenzene