Among the saturated monocarboxylic acids, formic acid, acetic acid, and propionic acid have a strong sour taste and irritation. It contains 4~9 C atoms and has a putrid odor and is an oily liquid. It contains more than 10 C's as a paraffin-like solid, with very low volatility and no odor.
This is due to the presence of hydrogen bonds between the molecules of formic acid. According to methods such as electron diffraction, lower acids exist in the form of dimers even in vapor due to the presence of hydrogen bonds. The intermolecular hydrogen bond bond of formic acid is 30KJ/mol, while the intermolecular hydrogen bond of ethanol is 25KJ/mol.
The melting point of the linear saturated monocarboxylic acid changes zigzag with the increase of the number of C atoms in the molecule, and the melting point of the even C atomic acid is higher than that of the two adjacent odd C atomic acids, which is due to the fact that in the chain containing even C atoms, the chain end methyl group and carboxyl group are divided on both sides of the chain, while in the odd C atomic chain, it is on the same side of the C chain, the former has higher symmetry, which can make the crystal lattice of carboxylic acid more tightly arranged, and they have a greater attraction and a higher melting point.
The carboxyl group is a hydrophilic group and can form hydrogen bonds with water, so the lower carboxylic acids can be miscible with water in any ratio; As the relative molecular mass increases, the hydrophobic group (hydrocarbon group) becomes larger and the solubility in water becomes smaller.
X-ray studies of long-chain fatty acids have proved that the C chains in these molecules are arranged in a zigzag shape, and the two intermolecular carboxyl groups are associated with hydrogen bonds, and the associated bimolecules are arranged regularly layer by layer, and in the middle of each layer is a carboxyl group that is associated with each other, and the gravitational attraction is very strong, and the layers are adjacent to each other with weak gravitational alchemies, which are easy to slide between each other, which is also the reason why higher fatty acids have lubricity.