Understanding Dipole Moment Order in Methyl Halides

Dipole moment (μ) is a vector quantity that measures the separation of positive and negative charges in a molecule. It depends on both the magnitude of charge separation and the distance between charges. For methyl halides (CH₃X, where X = F, Cl, Br, I), the dipole moment is influenced by the electronegativity difference between carbon and the halogen atom, as well as bond length.

Step 1: Compare Electronegativity Differences

Electronegativity values: F (3.98) > Cl (3.16) > Br (2.96) > I (2.66). Carbon has electronegativity 2.55. The electronegativity difference (ΔEN) decreases as: F > Cl > Br > I. A larger ΔEN suggests a larger dipole moment if bond lengths were equal.

Step 2: Consider Bond Lengths

Bond lengths increase: C–F (1.39 Å) < C–Cl (1.79 Å) < C–Br (1.97 Å) < C–I (2.14 Å). Dipole moment μ = charge (δ) × distance (d), so longer bonds can contribute to a larger dipole moment.

Step 3: Analyze the Combined Effect

For CH₃F, high ΔEN but short bond length; for others, lower ΔEN but longer bond. Experimentally, dipole moments are: CH₃F (1.81 D) < CH₃Cl (1.87 D) > CH₃Br (1.81 D) > CH₃I (1.62 D). Thus, CH₃Cl has the highest due to optimal balance of EN difference and bond length.

Final Order

CH₃Cl > CH₃F > CH₃Br > CH₃I

So, the correct option is: ${\mathrm{CH}}_3\mathrm{Cl}>{\mathrm{CH}}_{3}F>{\mathrm{CH}}_3\mathrm{Br}>{\mathrm{CH}}_{3}I$

Related Topics

• Electronegativity and Polar Bonds
• Molecular Polarity
• Bond Length and Strength
• Intermolecular Forces

Key Formulae and Theory

Dipole Moment: $\mu =\delta \times d$, where δ is charge magnitude and d is distance.

Electronegativity Difference: $\mathrm{\Delta EN}=\mathrm{EN}\left(X\right)-\mathrm{EN}\left(C\right)$

Bond polarity increases with ΔEN, but dipole moment also depends on molecular geometry and bond length.