What Type of Compound Is Propanoic Acid?
Propanoic, or propionic acid, is a simple compound often used as a preservative in the feed and food industries. The manufacture of dyes, pharmaceutical compounds, agrochemicals, plastics and artificial flavors are other processes that make use of propanoic acid sometimes as well. Based on the function group it contains, propanoic acid belongs to a class of compounds called carboxylic acids.
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Structure
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Propanoic acid has the following molecular formula: C3H6O2. The three carbons form a chain; the carbon at the end of the chain has an oxygen atom double-bonded to it and an -OH group attached to it as well. This group of atoms or functional group is collectively called a carboxyl group, and compounds like propanoic acid are called carboxylic acids because they are relatively acidic. Acetic acid, also known as vinegar, is another example of a carboxylic acid.
Properties
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Propanoic acid is about 0.99 times as dense as water at room temperature. One mole of this acid weighs 74.08 grams, and its melting point is at about minus 20 degrees Celsius. In the absence of water, it is a clear liquid, but it is highly miscible in water and, hence, is usually sold in solution. It boils at about 141 degrees Celsius and has a flash point (the temperature at which it can form a flammable vapor) of 52 degrees Celsius.
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Features
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Like other carboxylic acids, propanoic acid is relatively acidic. That's because of the structure of the functional group. When the -OH part of the carboxyl group gives up its hydrogen ion, the oxygen atom is left with a negative charge. This charge, however, is not stuck on this oxygen atom only; rather, it is delocalized over the two oxygen atoms. This outcome is more stable (i.e., lower in energy) than if the negative charge were localized on the one atom only, and thus the conjugate base (the molecule after it's lost the hydrogen ion) of a carboxylic acid is more stable than the conjugate base of an alcohol.
Synthesis
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Propanoic acid can be produced in different ways. For one, propanal can be oxidized to yield the carboxylic acid. Just like other carboxylic acids, it could be prepared by adding hot base or acid to propanenitrile followed by an acid-water workup, or by carbonating the Grignard reagent formed from magnesium and 1-bromopropane. It can also be made from propanoic anhydride by hydrolysis or alcoholysis; the hydrolysis would form two molecules of the acid, while the alcoholysis would form an ester and the acid.
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