Question

When do molecules become polar?

Answer 1

A molecule is polar when its structure contains polarity, i.e. significant charge separation between bound atoms. This is a necessary but not a sufficient condition.

Explanation:

A polar molecule contains functional groups that enforce charge separation.....Alcohols are the classic polar group.....and we may represent this polarity by a diagram such as `H_3C-stackrel(""^+delta)CH_2-stackrel(delta^-)O-stackrel(delta^+)H`.

While an INDIVIDUAL bond may be polar, it is the GEOMETRIC SUM of ALL the bond dipoles that determines MOLECULAR POLARITY. Methylene chloride, `"H"_2"CCl"_2`, and `"chloroform"`, `CHCl_3`, are thus BOTH polar molecules....but `"CCl"_4`, with FOUR `C-Cl` bonds is NON-POLAR because the vector sum of the four individual `stackrel(+delta)C-stackrel(delta-)Cl` dipoles is CLEARLY ZERO.....

Of course the intermolecular hydrogen bonding that results is a special case of bond polarity, but it might also occur in non-hydrogen bonding molecules such as `H_3stackrel(""^+delta)C-stackrel(delta^-)O-stackrel(""^+delta)CH_3` or `stackrel(""^(+)delta)CH_2stackrel(delta^-)X_2`.

This bond polarity gives rise to greater intermolecular interaction, and decreased volatility.