Question

Which of the following is correct order of the property mentioned?

• A. o-dichlorobenzene > m-dichlorobenzene > p-dichlorobenzene (Boiling Point)
• B. p-dichlorobenzene > m-dichlorobenzene > o-dichlorobenzene (Melting Point)
• C. $CH_3CI > CH_3F > CH_3Br> CH_3I$ (Dipole moment)
• D. tert-butyl bromide > iso-butyl bromide > butyl bromide (Boiling point)

Answer 1

Answer: (C)

$CH_3CI > CH_3F > CH_3Br> CH_3I$ (Dipole moment)

Answer 2

The problem asks us to identify the correct order of the mentioned properties for different sets of compounds. We will analyze each option individually.

1. o-dichlorobenzene > m-dichlorobenzene > p-dichlorobenzene (Boiling Point)

• Boiling point depends on the strength of intermolecular forces. For isomers, molecular weight is the same, so the primary factors are polarity (dipole-dipole interactions) and packing efficiency (London dispersion forces).
• Dipole Moments: o-dichlorobenzene has the largest dipole moment, followed by m-dichlorobenzene, and p-dichlorobenzene has a zero dipole moment due to cancellation of bond dipoles.
  • o-dichlorobenzene: High dipole moment (C-Cl bonds at 60°).
  • m-dichlorobenzene: Intermediate dipole moment (C-Cl bonds at 120°).
  • p-dichlorobenzene: Zero dipole moment (C-Cl bonds at 180°).
• Based on dipole moment, one might expect o > m > p. However, actual boiling points are:
  • o-dichlorobenzene: 180.5 °C
  • p-dichlorobenzene: 174 °C
  • m-dichlorobenzene: 173 °C
• The actual order is o-dichlorobenzene > p-dichlorobenzene > m-dichlorobenzene. The higher symmetry of p-dichlorobenzene allows for more efficient packing in the liquid state, leading to stronger London dispersion forces that slightly outweigh the dipole-dipole interactions of m-dichlorobenzene.
• Therefore, the given order is incorrect.

2. p-dichlorobenzene > m-dichlorobenzene > o-dichlorobenzene (Melting Point)

• Melting point depends on the efficiency of crystal packing in the solid state. More symmetrical molecules can pack more efficiently, leading to stronger intermolecular forces and higher melting points.
• Symmetry: p-dichlorobenzene is highly symmetrical, allowing for very efficient crystal packing. o-dichlorobenzene and m-dichlorobenzene are less symmetrical.
• Actual melting points are:
  • p-dichlorobenzene: 53.5 °C
  • o-dichlorobenzene: -17.2 °C
  • m-dichlorobenzene: -24.8 °C
• The actual order is p-dichlorobenzene > o-dichlorobenzene > m-dichlorobenzene.
• Therefore, the given order is incorrect.

3. $CH_3CI > CH_3F > CH_3Br> CH_3I$ (Dipole moment)

• Dipole moment ($\mu$) is a product of charge separation (q) and bond length (r), i.e., $\mu = q \times r$.
• Electronegativity (EN) difference (C-X): C-F (largest) > C-Cl > C-Br > C-I (smallest). This suggests C-F bond should be most polar.
• Bond Length (r): C-F (shortest) < C-Cl < C-Br < C-I (longest).
• The actual dipole moments (experimental values in Debye) are:
  • CH₃Cl: ~1.87 D
  • CH₃F: ~1.85 D
  • CH₃Br: ~1.80 D
  • CH₃I: ~1.64 D
• The slightly higher dipole moment of CH₃Cl compared to CH₃F is due to the longer C-Cl bond length compensating for the smaller electronegativity difference compared to C-F. The product (q x r) is maximized for CH₃Cl.
• The correct order is CH₃Cl > CH₃F > CH₃Br > CH₃I.
• Therefore, the given order is correct.

4. tert-butyl bromide > iso-butyl bromide > butyl bromide (Boiling point)

• Boiling point for isomers of similar molecular weight decreases with increasing branching. This is because increased branching leads to a more spherical shape, reducing the surface area available for intermolecular contact and thus weakening London dispersion forces.
• Structures and Branching:
  • Butyl bromide (n-butyl bromide): $CH_3CH_2CH_2CH_2Br$ (least branched, straight chain)
  • iso-butyl bromide: $(CH_3)_2CHCH_2Br$ (one branch)
  • tert-butyl bromide: $(CH_3)_3CBr$ (most branched, most spherical)
• Expected Boiling Point Order: n-butyl bromide > iso-butyl bromide > tert-butyl bromide.
  • n-butyl bromide BP: 101.6 °C
  • iso-butyl bromide BP: 91.5 °C
  • tert-butyl bromide BP: 73.2 °C
• Therefore, the given order is incorrect.

Based on the analysis, only option 3 is correct.