Bond Dipole vs. Molecular Dipole

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MCATMadness

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A molecule like dichloromethane clearly has two bond dipoles (C-Cl) but no molecular dipole (b/c the two bond dipoles cancel out), so wouldnt the molecule be symmetric and thus, have a dipole moment of 0?

in berk review, it says dichloromethane is asymmetric with a bond dipole. how do you explain this?

and since it depends on the arrangement of atoms about the central atom, how can you tell what configuration the mlc should be in and thus, if the bond dipoles cancel to yield a dipole moment of 0?

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Think of bond dipoles as vectors in 3D space, and the molecular dipole as the vector sum of those bond dipoles. If they sum to zero, then the molecule has no NET dipole moment and is non-polar, otherwise the molecule will be polar. To actually add those vectors in your head doesn't require actual math as far as MCAT is concerned, but you do need good spatial visualization.

Now for the example you mentioned, you should know that it's impossible for a tetrahedral arrangement to have no molecular dipole unless all for of the dipoles are equal in magnitude, which obvious can't be the case in CH2Cl2 cuz there are 2 C-H bonds and 2 C-Cl bonds, even if there is a plane of symmetry on the molecule. If you use point groups (ie inorganic stuff) then the PG would have to be Td on a tetrahedral to have no dipole, while the point group of dichloromethane is C2v.
 
Water has clearly two bond dipoles (H-O), and carbon dioxide also has clearly two bond dipoles (C-O). But one is a polar molecular, and the other isn't.. What gives..????
 
The dipoles in CO2 are equal and opposite since central atom is sp hybridized and thus the O=C=O bond angle is 180 deg, which makes the two C=O dipoles equal and opposite if you add them in space, and thus cancel out to form a NET dipole of zero.

Water on the other hand has a central atom that is sp3 hybridized so the H-O-H bond angles are close to 109 deg, and there's no way for two vectors of equal magnitude separated by 109 deg to cancel out to zero: thus water must be polar.
 
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So a molecular dipole moment depends not only on bond dipoles but molecular structure?

Exactly. Depends on the vector sum, like collegestud said.

CO2 is linear (carbon is sp hybridized) and the oxygen dipoles cancel out. Water is bent (oxygen is sp3 hybridized) and has a dipole.

CH2Cl2 has a dipole, but CCl4 does not.
 
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