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Structure, Properties, and Nomenclature of Carboxylic Acids

Structure of Carboxylic Acids

In comparing the structures of formic acid and formaldehyde, it is to be found that apart from formic acid's acidic hydrogen, both have a planar structure that may be accounted for by carbonyl carbon's sp2 hybridization. As is expected, the strong carbon-oxygen double bond is shortened. Its length amounts to 1.23 Å.

Fig.1
Bond lengths in formic acid and formaldehyde.
Fig.2
Bond angles in formic acid and formaldehyde.

However, carboxylic acids have several particular structural features. In solution and in the liquid state of aggregation, simple carboxylic acids form dimers in which two carboxylic acid molecules are connected by two hydrogen bridges with a bond energy of about 7 kcal/mol.

Fig.3
Carboxylic acid dimer.

Furthermore, carboxylic acids have two conformers (local energy minima) regarding the orientation of the acidic hydrogen, whose spatial position may be altered by rotation of the carboxyl carbon-hydroxyl oxygen single bond. In one of the conformers, the acidic hydrogen points in the carbonyl oxygen's direction (syn conformer). When the acidic hydrogen points away from the carbonyl oxygen, the conformer is called anti. The energy difference between the two corresponding formic acid conformers in the gas phase amounts to 6 kcal/mol. The activation energy of the interconversion of the formic acid's conformers is 13 kcal/mol.

Fig.4
Syn and anti conformers of formic acid.

The formation of carboxylic acid dimers influences their boiling points. In comparison to the corresponding alcohols, the carboxylic acids' boiling points are considrably higher.

Tab.1
Melting and boiling points of some carboxylic acids.
Carboxylic acidsStructureMelting points (°C)Boiling points (°C)
Formic acid
Mouse
Fig.5
8.4100.7
Acetic acid
Mouse
Fig.6
16.6117.9
Propionic acid
Mouse
Fig.7
-20.8141
Acrylic acid
Mouse
Fig.8
13141.6
Benzoic acid
Mouse
Fig.9
122.1249

Carboxylic acids, esters, and carboxamides may be illustrated by two resonance structures, as the lone electron pairs of the oxygens, or the oxygen and nitrogen, in connection with the sp2 carboxyl carbon's p orbital represent a π system. Due to the lower electronegativity of nitrogen in comparison to oxygen, the zwitterionic structure in carboxamides is more stable than in carboxylic acids and esters. Thus, the actual structure of carboxamides resembles more the zwitterionic resonance structure than the carboxylic acids' and esters' actual structure.

Fig.10
Resonance structures of esters (1), carboxylic acids (2), and carboxamides (3).

The following illustrations depict the distribution of electron density on the molecular surface of methyl formate, formic acid and acetamide by the so-called electrostatic potential surfaces. Red coloured areas represent a more negative potential - that is, a higher electron density. Blue coloured areas depict a more positive potential, or, in other words, a lower electron density, while green coloured areas are relatively neutral. Such electrostatic potential surface illustrations are generated by semiemprical-quantum mechanical calculations. They may be useful, for instance, in identifying nucleophilic (high electron density, red) and electrophilic centers (low electron density, blue) of molecules.

Fig.11
Methyl formate
Fig.12
Formic acid
Fig.13
Acetamide

Exercises

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