Dipole Moment

$$\text{Dipole Moment Order: } \text{CH}_3\text{Cl} > \text{CH}_3\text{F} > \text{CH}_3\text{Br} > \text{CH}_3\text{I}$$

$$\text{CH}_3\text{Cl } (1.860\text{ D}) > \text{CH}_3\text{F } (1.847\text{ D}) > \text{CH}_3\text{Br } (1.830\text{ D}) > \text{CH}_3\text{I } (1.636\text{ D})$$

Concept: The dipole moment ($$\mu$$) of a bond depends on two factors: the partial charge separation (q) and the bond length (d), given by the formula:

$$\mu = q \times d$$

• Fluorine vs. Chlorine Anomaly: Fluorine is the most electronegative halogen, which creates a higher magnitude of partial charge (q) in the $$\text{C--F}$$ bond than in the $$\text{C--Cl}$$ bond. However, due to the extremely small atomic size of fluorine, the $$\text{C--F}$$ bond length (d) is significantly shorter than the $$\text{C--Cl}$$ bond length.
• Dominating Factor: The increase in bond distance (d) from $$\text{C--F}$$ to $$\text{C--Cl}$$ outweighs the small decrease in charge separation (q). As a result, methyl chloride ($$\text{CH}_3\text{Cl}$$) has a higher net dipole moment than methyl fluoride ($$\text{CH}_3\text{F}$$).
• $$\text{CH}_3\text{Cl} \to \text{CH}_3\text{I}$$ Trend: Moving further down to $$\text{CH}_3\text{Br}$$ and $$\text{CH}_3\text{I}$$, the electronegativity drops drastically, reducing the charge separation (q) enough that the dipole moment decreases systematically despite increasing bond length.