Question

Which of the following is the correct melting point order of dichlorobenzene isomers?

• A. 1,2-Dichlorobenzene<1,3-Dichlorobenzene<1,4-Dichlorobenzene
• B. 1,4-Dichlorobenzene<1,2-Dichlorobenzene<1,3-Dichlorobenzene
• C. 1,2-Dichlorobenzene<1,4-Dichlorobenzene<1,3-Dichlorobenzene
• D. 1,3-Dichlorobenzene<1,2-Dichlorobenzene<1,4-Dichlorobenzene

Hint:

More symmetrical molecules pack better in the solid phase and generally have higher melting points.

Answer 1

The Correct Option is A

The melting point of dichlorobenzene isomers depends on the symmetry of the molecules:

• 1,4-Dichlorobenzene is the most symmetrical, leading to better packing in the solid state and hence the highest melting point.
• 1,3-Dichlorobenzene has intermediate symmetry.
• 1,2-Dichlorobenzene has the least symmetry, resulting in poor packing and the lowest melting point.