When do molecules become polar?

1 Answer
anor277
Aug 6, 2017

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.

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