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MCQEasyJEE 2023Aryl Halides

JEE Chemistry 2023 Question with Solution

The correct order of melting point of dichlorobenzenes is:

  • A
    Three dichlorobenzene isomers arranged in an order sign with para-dichlorobenzene first, then meta-dichlorobenzene, then ortho-dichlorobenzene.
  • B
    Three dichlorobenzene isomers arranged in an order sign with ortho-dichlorobenzene first, then para-dichlorobenzene, then meta-dichlorobenzene.
  • C
    Three dichlorobenzene isomers arranged in an order sign with meta-dichlorobenzene first, then para-dichlorobenzene, then ortho-dichlorobenzene.
  • D
    Three dichlorobenzene isomers arranged in an order sign with para-dichlorobenzene first, then ortho-dichlorobenzene, then meta-dichlorobenzene.

Answer

Correct answer:D

Step-by-step solution

Standard Method

Given: We need the correct order of melting points of the three dichlorobenzene isomers.

Find: Which option gives the correct decreasing order of melting point.

Comparison of ortho, meta and para dichlorobenzene with boiling points 453, 446, 448 K and melting points 256, 249, 323 K, highlighting better crystal fitting of para isomer.

From the given comparison in the solution image:

  • ortho-dichlorobenzene: melting point = 256 K256 \, \text{K}
  • meta-dichlorobenzene: melting point = 249 K249 \, \text{K}
  • para-dichlorobenzene: melting point = 323 K323 \, \text{K}

Thus,

para>ortho>meta\text{para} > \text{ortho} > \text{meta}

The reason is that the para isomer is more symmetrical, so it packs better in the crystal lattice and therefore has a higher melting point.

Therefore, the correct option is D.

Why symmetry controls melting point

Given: Melting point depends strongly on how efficiently molecules pack in the solid state.

Find: The order of melting points for dichlorobenzene isomers.

A more symmetrical molecule generally shows better crystal packing and stronger lattice stability.

Among the three isomers:

  • para-dichlorobenzene is the most symmetrical
  • ortho-dichlorobenzene is less symmetrical than para
  • meta-dichlorobenzene packs least effectively among these three in the given comparison

Hence the melting-point order is:

para-dichlorobenzene>ortho-dichlorobenzene>meta-dichlorobenzene\text{para-dichlorobenzene} > \text{ortho-dichlorobenzene} > \text{meta-dichlorobenzene}

So, the correct choice is D.

Common mistakes

  • Confusing melting point with boiling point. The solution image lists both, but the question asks only about melting point. Always read the property asked before choosing the order.

  • Assuming the isomer with the largest dipole moment must have the highest melting point. Dipole moment can influence intermolecular forces, but solid-state packing efficiency and symmetry are more important for melting point here.

  • Ignoring molecular symmetry. para-dichlorobenzene is more symmetrical, so it fits better in the crystal lattice. Better crystal fitting raises melting point.

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