hydrogen bond(carboxylic acid)

http://upload.wikimedia.org/wikipedia/commons/thumb/c/c9/Carboxylic_acid_dimers.png/320px-Carboxylic_acid_dimers.png

Experimentally, based on the bond angles and isolation of dimeric structures of light carboxylic acids, it is believed that the lone pair on the carbonyl oxygen would form hydrogen bonding with the electron deficient hydroxyl hydrogen on another carboxylic acid molecule. Because of sharing of electron density of this carbonyl oxygen, the electron density from the H-O bond within the same carboxyl group is shifted toward it and becomes electron deficient such that the hydrogen tends to share electron from the carbonyl oxygen of the other molecule. The result is formation of two intermolecular hydrogen bonds within a hexagonal ring as illustrated in the picture from the hyperlink above. i.e. one hydrogen bond per molecule.

Even with molecules containing more than one acid group e.g. benzene-dicarboxylic or -tricarboxylic acids, 3 hydrogen bonds per carboxylic acid group seems impossible.

ON AVERAGE two hydrogen bonds only.

Personally I think 3 H-bonds are possible, but would be overally less stable than the 2 H-bond state (since some molecules would not be able to form 2 H-bonds, while the 3rd H-bond is not as strong according to Leo).