Question

How can I identify dipoles in molecules?

Answer 1

You look for electronegativity differences between the atoms that are bonded to each other.

A bond dipole depends on electronegativity differences (`Δ"EN"`) between the atoms in the bond.

This causes the electrons in the bond to spend more time around one atom than the other.

For example, in `"HCl"`, the `"Cl"` is more electronegative than `"H"`, so the electrons spend more time around `"Cl"` than `"H"`.

We indicate a bond dipole by an arrow with a "+" at one end and pointing towards the negative end of the bond.

If `Δ"EN" < 0.5`, we usually say that the bond is ionic.

EXAMPLE

Identify the bond dipoles on chloromethane, `"CH"_3"C"l`.

Solution

The structure of chloromethane is

`Δ"EN"` for the `"C-H"` bond is 0.4. The `"C-H"` bond is nonpolar.

`Δ"EN"` for the `"C-Cl"` bond is 0.6. The `"C-Cl"` bond is polar.

The `"Cl"` atom is the negative end of the bond, so there is a `"C-Cl"` bond dipole.