The carboxylic acids are an important functional group in organic chemistry. They are distinguished by the presence of a carboxylic acid group consisting of a carbon atom double-bonded to an oxygen atom and also bonded to a hydroxyl group. This can be written as -COOH for simplicity. The most well known carboxylic acid is ethanoic acid, which is commonly known as vinegar.
Naming Carboxylic Acids
Carboxylic acids follow the standard naming procedure in organic chemistry, counting the carbon in the acid group as the first carbon. So methanoic acid is H-COOH, ethanoic acid is H3C-COOH, propanoic acid is H5C2-COOH, and so on. When the COOH group is attached to a benzene ring the resulting compound is called benzoic acid.
Properties of Carboxylic Acids
The physical properties of carboxylic acids are governed by their ability to form hydrogen bonds. Their melting and boiling points are higher that the alcohols of similar size because they form dimers in which two molecules are connected end-to-end by two hydrogen bonds between the hydroxyl group of one acid molecule (which has a slightly positive charge) and the oxygen of the other acid group (which is slightly negative).
These dimers mean that the molecules appear to be twice the size, and hence the melting and boiling points are much higher. The solubility in water of carboxylic acids reduces rapidly with size. Acids of up to four carbon atoms dissolve readily in water, but as the molecules get larger, the hydrophobic “tails” are more involved.
Acidity of Carboxylic Acids
Carboxylic acids are acidic because the hydrogen on the -COOH group is transferred to a water molecule in solution to make the acidic H3O+ ion. This is a reversible reaction and in ethanoic acid only about one percent of the molecules are ionised at any one time. This means that the acids are weak acids, and their pHs typically are not as low as the stronger acids like hydrochloric acid or sulfuric acid which are almost totally ionised in solution.
Reactions of Carboxylic Acids
Carboxylic acids react as typical acids with many substances, although their reactions are much slower and more limited. So, they react with the more reactive metals to form hydrogen and a salt. With metal hydroxides, they react in a typical neutralisation reaction, producing a salt and water, and with carbonates and hydrogen carbonates they react to form a salt and carbon dioxide.
Making Esters
An important group of organic chemicals made from carboxylic acids is the esters. These are formed by the reaction of a carboxylic acid with an alcohol, The two molecules react to produce water and a molecule with two “tails” and a -COO- link. So, a reaction of ethanoic acid and ethanol prouces water and an ester called ethylethanote (ethyl from ethanol and ethanoate from ethanoic acid).
The esters are an interesting group of compounds with strong (and often sweet) smells, including a pear drops smell.
Source:
Jim Clark, “Introducing Carboxylic Acids”, 2004, on Chemguide.co.uk, Accessed 5th April 2010
Simon Davies - Simon Davies has lived a varied life so far After completing a BSc in Chemistry at the University of Bristol, UK, he worked as a ...
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