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MCQEasyJEE 2023Electronegativity & Dipole Moment

JEE Chemistry 2023 Question with Solution

The pair from the following pairs having both compounds with net non-zero dipole moment is:

  • A

    cis-butene, trans-butene

  • B

    Benzene, anisidine

  • C

    CH2Cl2\mathrm{CH_2Cl_2}, CHCl3\mathrm{CHCl_3}

  • D

    1,41,4-Dichlorobenzene, 1,31,3-Dichlorobenzene

Answer

Correct answer:C

Step-by-step solution

Standard Method

Given: We must identify the pair in which both compounds have net non-zero dipole moment.

Find: The correct option.

Comparison diagram showing four option pairs with molecular structures and indicated dipole moments, including butenes, benzene derivatives, chloromethanes, and dichlorobenzenes.

Use molecular symmetry to judge whether bond dipoles cancel.

  1. cis-Butene vs. trans-Butene
  • cis-Butene: has a non-zero dipole moment.
  • trans-Butene: has zero dipole moment because the arrangement is symmetrical and bond dipoles cancel.

So, this pair is not correct.

  1. Benzene vs. anisidine
  • Benzene: has zero dipole moment due to symmetrical structure.
  • Anisidine (Methoxybenzene): has non-zero dipole moment.

So, this pair is not correct.

  1. CH2Cl2\mathrm{CH_2Cl_2} vs. CHCl3\mathrm{CHCl_3}
  • CH2Cl2\mathrm{CH_2Cl_2} (Dichloromethane): has non-zero dipole moment.
  • CHCl3\mathrm{CHCl_3} (Chloroform): also has non-zero dipole moment.

So, both compounds in this pair have non-zero dipole moments.

  1. 1,41,4-Dichlorobenzene vs. 1,31,3-Dichlorobenzene
  • 1,41,4-Dichlorobenzene: has zero dipole moment due to symmetry.
  • 1,31,3-Dichlorobenzene: has non-zero dipole moment.

So, this pair is not correct.

Therefore, the correct option is C.

Note: The solution labels the correct option as B, but the accompanying explanation clearly shows that option (3) is the pair in which both compounds have non-zero dipole moments. Hence, the defensible answer is C.

Common mistakes

  • Assuming that the presence of polar bonds always gives a non-zero molecular dipole moment. This is wrong because molecular geometry can make bond dipoles cancel. Always check the overall symmetry of the molecule.

  • Confusing cis and trans isomers of butene. This is wrong because their spatial arrangements are different, so dipole cancellation is different. Compare the directions of the bond dipoles before concluding.

  • Treating benzene derivatives as always non-polar. This is wrong because substitution can break symmetry and create a net dipole moment. Examine whether the substituents are arranged symmetrically or asymmetrically.

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