[bj]

if we are to compare dipole moment of CH3Cl, CH2Cl2, CHCl3 and CCl4, decreasing order of dipole moment will be CH3Cl>CH2Cl2>CHCl3>CCl4
but how to calculate value of dipole moment?

[sjb]

if we are to compare dipole moment of CH3Cl, CH2Cl2, CHCl3 and CCl4, decreasing order of dipole moment will be CH3Cl>CH2Cl2>CHCl3>CCl4
but how to calculate value of dipole moment?

A simple model would be vector addition.

[Enthalpy]

But vector addition would predict a moment for CH₂Cl₂ bigger than for CH₃Cl, wouldn't it?
2*cos(109.5°/2)~1.15 > 1.
Vector addition would also predict the same moment for mono- and trichloro- since the tetrachloro- has none.
Quick web check: monochloro 1.9D, dichloro 1.6D, trichloro 1.2D.
So it needs some complementary explanation, like carbon getting less polarized by a second chlorine.

[sjb]

Vector addition should not show the same for methyl chloride and chloroform (unless by coincidence). Yes there are other things at play. I imagine the approach described at e.g. http://chemistry.illinoisstate.edu/standard/che460/handouts/460waterdipole.pdf may be a bit too complex and may be a posthoc rationalisation.

Thoughts?

[bj]

Sir, thank you for giving your valuable time to the this thread, but in these molecules carbon is SP3 hybridized and atoms are not in one plane.

I am trying to figure this out, in case molecule is not planar as in case of benzene how to calculate dipole moment,

in case, if actual dipole moments are not possible with out bond lengths then is there any rule that predicts which molecule will have higher dipole.

[Enthalpy]

The rationale behind monochloro and trichloro having the same magnitude of the vector sum:
- If the polarization and direction of the bonds didn't depend on the number of chlorides
- Tetrachloro has no dipole moment
- Trichloro is one dipole less than tetra
- Monochloro is one dipole more than methane
- Here we add or subtract one dipole from zero moment, hence no worry with the direction
- So mono and trichloro would have the same dipole moment.

Except that the polarization of a bond does depend on the number of chlorines, as the example set shows.