4.29b

[Emily Mei 1B]

The question asks out of 3 sample molecules which one has the largest dipole moment. How do we determine relative dipole magnitudes based on structure without numbers?

[Jessica Yang 1J]

I think that you can treat the the C-Cl dipole bonds are vectors. That way in 3, the dipole moments cancel each other out, because they have the same magnitude but are pointing in different directions. The C-Cl dipole bonds in 2 (120 degrees) have less overlap than those in 1 (60 degrees). I think this means that the magnitude of adding the two vectors in 2 would be less than than adding to the two C-Cl dipole bonds in 1 which are pointing in relatively the same direction. Therefore, 1 has the largest dipole moment.

Hopefully, someone can confirm my thinking or offer another explanation?

[Vincent Grospe 3C]

Two things to look at when comparing dipole moments of different molecules: first and most importantly, the resultant vector of all of the bond dipole moments (whether the dipole arrows cancel each other or all go in the same direction) based on the molecule’s structure as well as its bond angles and, secondly, the bond’s dipole moment (or electronegativity difference between the bonded atoms.

The first factor is crucial. No matter how electronegative the bonded atoms are, if the dipole moments all cancel each other then the overall dipole moment is zero or non-polar. And you can determine the second factor by finding at the electronegative difference between the two atoms, because the magnitude of the bond’s dipole moment is directly proportional to its electronegativity difference.

Edit: So yes, Jessica is right: 3 is non-polar, because the bond dipoles all cancel each other. And 1 has the largest dipole moment, because it has the largest resultant vector due to the bond’s angles being closer.