Carboxylic acid (RCOOH) is an organic compound class that comprises carbon, oxygen, and a hydroxyl group, which is commonly referred to as carboxyl group (COOH). This carboxyl group is appended to the end of the compound. The carbon has a double bond with oxygen (C=O) and a single bond with the hydroxyl group (-OH). Carboxylic acids are commonly found in alcohols, oils, waxes, and fats. They make up several medicines, household items, and foods.

Composition and Strength

Carboxylic acid. Image credit: Flickr
Carboxylic acid. Image credit: Flickr

The name “carboxyl” can be attributed to the association between the carbonyl and hydroxyl group. The acid is polar in nature, meaning it has an imbalanced electrical charge. And it’s considered a weak acid, which explains its strong odor. For example, vinegar’s strong pungent smell can be attributed to its acetic acid, a type of carboxylic acid. In fact, vomit has butyric acid (another carboxylic acid variant) and therefore the strong smell.    

Carboxylic acid is a strong acid, but not as much when compared to mineral acids such as sulfuric acid, hydrochloric acid, and nitric acid. Carboxylic acid is widespread in nature, typically merging with different functional groups. Thanks to its carbonyl group, it accepts hydrogen bonds, and the presence of the hydroxyl group means the acid can donate a hydrogen bond. The acid is used widely for making other organic compounds. For instance, it can produce esters after reacting with alcohol.

Different Carboxylic Acids

There are different carboxylic acid types. For example, the fatty acid is a type of carboxylic acid, which has a lengthy chain of hydrogen and carbon atoms bonding to a carboxyl group. The other carboxylic acids are methanoic acid (HCOOH), ethanoic acid (CH3COOH), propanoic acid (CH3CH2COOH), pentanoic acid (C4H9COOH), butanoic acid (C3H7COOH), and hexanoic acid (C5H11COOH). Some other more comprehensible examples are citric acid (found in lemons, oranges, and also certain soft drinks), tartaric acid (found in baking powder), salicylic acid (found in aspirin), and ascorbic acid (in vitamin C). All carboxylic acids end as “-ic acid”.

Based on its type, a carboxylic acid can be linear (unbranched) or non-linear (branched). It can vary in length/size, consisting of several hydrogen and carbon atoms. For instance, the chemical structure of ethanoic acid and propanoic acid are not the same. 

Making Carboxylic Acid

Carboxylic acid can be made in laboratories by oxidizing aldehydes and primary alcohols; and also via hydrolysis (chemically breaking down a compound using water) of esters, amides, and nitriles. The oxidization happens with the help of potassium dichromate solution and dilute sulfuric acid. The oxidization causes the potassium dichromate to turn green from orange. The dichromate ion is the most important element of this process, and therefore the potassium dichromate solution can be substituted with sodium dichromate.

The primary alcohol can be oxidized into carboxylic acid in a couple of stages: first into aldehyde and later into the acid. If you’re starting with the aldehyde for oxidization into carboxylic acid, you’re skipping the first step. Generally, it’s not common to start the oxidization process with an aldehyde. The process usually begins with the primary alcohol.

Hydrolyzing Nitriles to Make Carboxylic Acid

A nitrile can be converted into carboxylic acid in two ways: acid hydrolysis and alkaline hydrolysis – both the methods entail causing a chemical reaction between water and the triple bond of carbon-nitrogen.

In acid hydrolysis, nitrile is heated in tandem with a dilute acid – for instance, dilute hydrochloric acid. In alkaline hydrolysis, the dilute acid is replaced with an alkali – for instance, sodium hydroxide solution. However, the result of alkaline hydrolysis would be ethanoate ions (sodium ethanoate when the sodium hydroxide solution is used). These ions have to be converted into carboxylic acid. For this conversion, hydrogen ions from hydrochloric acid must be added to make the mixture acidic.

Boiling Point and Solubility

Compared to alcohols, hydrocarbons, ketones or aldehydes that have an almost identical molecular weight, carboxylic acid has a higher boiling point. For example, formic acid (a simple carboxylic acid) boils when set to 101 degree Celsius or 214 degrees Fahrenheit. This is much higher than ethanol or ethyl alcohol’s boiling point, which is at a much lower 78.5 degree Celsius or 173 degree Fahrenheit. The acid has a high boiling point, thanks to the dimeric association comprising hydrogen bonding. For instance, water has a high boiling point because of its strong hydrogen bond attraction. Carboxylic acid has a higher boiling temperature than water since additional energy is needed for vaporizing its robust structures.

Carboxylic acid’s reaction in water is not much different from how alcohol, ketones and aldehydes react with water. In other words, carboxylic acid easily dissolves in water. This solubility is courtesy the potassium, sodium and ammonium salts present in carboxylic acid. The acid can be converted into salt by introducing a strong base such as potassium hydroxide or sodium hydroxide.

Applications

Carboxylic acid’s variants have multiple applications. The acid can be used for making polymers, solvents, food additives and pharmaceuticals.

  • Formic acid (simplest carboxylic acid) can be used as a disinfectant and also in textile treatment.
  • Acetic acid is used extensively to make esters and cellulose plastics. In fact, aspirin is also made from acetic acid.
  • Stearic and palmitic acid are key in the manufacturing of cosmetics, soaps, candles, and pharmaceuticals. Stearic acid is also used for manufacturing rubber.
  • Oleic acid is used to make detergents, soaps and textiles.