Which of the following are arranged in the decreasing order of dipole moment?
Answer & explanation
Correct answer: option 2
The correct answer is option 2. \(CH_3Cl, CH_3F, CH_3Br\).
The dipole moment of a molecule is determined by two factors: the electronegativity difference between the atoms and the distance between the atoms.
In the case of \(CH_3F\) and \(CH_3Cl\), the electronegativity difference between the carbon atom and the halogen atom is greater for \(CH_3F\) (4.0 - 2.5 = 1.5) than for \(CH_3Cl\) (3.0 - 2.5 = 0.5). However, the \(C-F\) bond length in \(CH_3F\) is shorter than the \(C-Cl\) bond length in \(CH3Cl\) (139 pm vs. 178 pm).
The shorter bond length in \(CH_3F\) means that the negative charge on the fluorine atom is concentrated in a smaller space, which should result in a larger dipole moment, but because of the smaller C-F bond length which outweights the effect of greater electronegativity, the dipole moment in case of \(CH_3F\) will be lower than \(CH_3Cl\).