Which of the following methyl halide will have the highest value of dipole moment?

$CH_3-Cl$

The correct answer is Option (4) → $CH_3-Cl$

The dipole moment (μ) of a molecule depends on two main factors:

• Magnitude of charge separation (electronegativity difference)

• Distance between the charges (bond length)

Dipole moment is given by μ = q × r

where q = magnitude of charge

r = bond length

In methyl halides (CH₃–X), the C–X bond polarity increases with electronegativity of the halogen, while bond length increases down the group.

Electronegativity order: F > Cl > Br > I

Bond length order: C–F < C–Cl < C–Br < C–I

Although fluorine is the most electronegative, the C–F bond is very short, reducing the dipole moment slightly. Chlorine provides a better balance of electronegativity and bond length, resulting in the maximum dipole moment.

Approximate dipole moment values: CH₃Cl ≈ 1.87, D CH₃F ≈ 1.85 D, CH₃Br ≈ 1.79 D, CH₃I ≈ 1.64 D

Thus CH₃Cl has the highest dipole moment.

Option A: CH₃–I Iodine has low electronegativity and the bond is long, leading to a smaller dipole moment. Incorrect.

Option B: CH₃–Br Bromine is less electronegative than chlorine, so the dipole moment is lower than CH₃Cl. Incorrect.

Option C: CH₃–F Although fluorine is highly electronegative, the very short C–F bond reduces the overall dipole moment slightly compared to CH₃Cl. Incorrect.

Option D: CH₃–Cl Chlorine provides an optimal combination of electronegativity and bond length, giving the highest dipole moment among methyl halides. Correct.