Why Is Benzoic Acid Slightly Soluble in Water?
Benzoic acid, chemical structure C6H5COOH, is the uniting of the water insoluble benzene molecule (with one hydrogen atom dropped) with a carboxylic acid group, (-COOH). It is this carboxylic acid group that gives benzoic acid its measure of water solubility. This has to do with “ionizability.” Water can attach to benzoic acid by hydrogen bonding. Beyond that, water molecules can stabilize the formation of the “benozate” ion.
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Primary Reason for Low Solubility
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The primary reason benzoic acid dissolves only slightly in cold water is that, even though the carboxylic acid group is polar, the bulk of the benzoic acid molecule is non-polar (water is polar). It is only the carboxylic group that is polar. In addition, there are no internal stabilizing structures that favor carboxylate, -COO(-), over carboxylic, -COOH.
Hydrogen Bonding
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When not in the presence of water, two molecules of benzoic acid may form what is called a dimer. In this instance, one molecule hydrogen-bonds to the second molecule.
In the presence of water, though short of ionization, water can hydrogen bond to benzoic acid. Thus
C6H5COOH + H2O ---> C6H5COO--H--OH2.
Such a hydrogen-bonded species may go to the point of ionization.
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Ionization
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Beyond hydrogen bond formation, full ionization can take place if there is some causative agent to force this. Bases can force ionization, but to a limited degree water produces ionization, according to the reaction equation
C6H5COOH + H2O ---> C6H5COO(-) + H3O(+)
Ionization guarantees water-solubility, since water is a polar solvent.
Heat Increases Solubility
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Adding heat greatly increases solubility because some of the increased energy sufficiently lengthens the hydrogen-bonds, so that ionization occurs. Ions are by definition polar, so the general truism, like dissolves like, indicates the ions will then dissolve in water.
Increasing Solubility
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In addition to temperature changes, there are other ways to increase or decrease the water-solubility of benzoic acid. The addition of a strong acid decreases the ionization through the “common ion” effect. Increasing the pH increases ionization of the benzoic acid, perhaps leading to reaction.
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