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questions :Physical properties of carboxylic acid
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[Member (365WT)]answers [Chinese ]Time :2019-07-02
Among the saturated monocarboxylic acids, formic acid, acetic acid, and propionic acid have strong sourness and irritation. It has a foul smell of 4 to 9 C atoms and is an oily liquid. Containing 10 C or more is a paraffin-like solid with low volatility and no odor.

This is due to the presence of hydrogen bonds between the formic acid molecules. According to methods such as electron diffraction, low-grade acids exist in the form of dimers even in steam due to the presence of hydrogen bonds. The intermolecular hydrogen bond energy of formic acid is 30 KJ/mol, and the hydrogen bond between ethanol molecules is 25 KJ/mol.
The melting point of a linear saturated monocarboxylic acid varies in a zigzag pattern with an increase in the number of C atoms in the molecule, and the melting point of an acid having an even number of C atoms is higher than the melting point of an acid of two adjacent C atoms, which is due to an even number of C atoms. In the chain, the methyl and carboxyl groups at the chain end are on both sides of the chain, and in the chain of odd C atoms, on the same side of the C chain, the former has higher symmetry, which allows the lattice of the carboxylic acid to be arranged more closely. They have a greater attraction between them and have a higher melting point.

The carboxyl group is a hydrophilic group and can form a hydrogen bond with water, so the lower carboxylic acid can be miscible with water at any ratio; as the relative molecular mass increases, the hydrophobic group (hydrocarbon group) becomes larger and larger, and the solubility in water becomes more and more small.
X-ray studies on long-chain fatty acids have shown that the C chains in these molecules are arranged in a zigzag pattern, and the two molecules are linked by hydrogen bonds, and the associated bimolecules are arranged in layers one by one. In the middle of the layer, the carboxyl groups are associated with each other, and the gravitational force is strong. The layer between the layers is adjacent to each other with weak gravitational hydrocarbon groups, which are easy to slide with each other, which is also the reason for the lubricity of higher fatty acids.

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