If you’re preparing to take your A level organic chemistry exam, you’ll need to have a good understanding of carboxylic acids and their derivatives. Here’s an overview of the key points to help get your revision off to a great start.
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Carboxylic acids and derivatives: an introduction
Carboxylic acids occur widely in nature and often play an essential role in the functions of living organisms. Some important examples of carboxylic acids include amino acids and fatty acids, which make up life as we know it.
Carboxylic acids are also mass produced for industrial and commercial purposes. They’re crucial in the manufacture of polymers, pharmaceutical products, industrial solvents, and various food additives. Acetic acid, acrylic acid, citric acid, maleic acid, and propionic acid are just some of the carboxylic acids used in industry.
What are carboxylic acids and derivatives?
Carboxylic acids are classed as weak acids, meaning they do not completely dissociate into ions. They react easily with carbonates to form salt, carbon dioxide, and water. Below is an example of this type of reaction.
In this reaction, a carboxylic acid (ethanoic acid) reacts with a carbonate (sodium carbonate) to form a salt (sodium ethanoate), water, and carbon dioxide.
Carboxylic acids contain the carboxyl functional group, the formula for which is written as —COOH. This functional group has both a carbonyl group, —C=O, and a hydroxyl group, —OH.
The carboxyl functional group has an R group attached to it, which can be an alkyl, an alkenyl, an aryl or any other group. The general formula of carboxylic acids can be written as R−COOH.
Some derivatives of carboxylic acids include:
- Acyl phosphate
- Acyl chloride
- Acid anhydride
These derivatives can be synthesised through the chemical reactions between carboxylic acids and other substances. Esters, for instance, are derived from the reaction between carboxylic acids and alcohols.
How are carboxylic acids named?
While carboxylic acids are typically identified by their common or trivial names, the IUPAC (International Union of Pure and Applied Chemistry) naming system is also used to avoid confusion.
Most carboxylic acid names contain the suffix -ic. Common names have an equivalent IUPAC-recommended name that accurately and precisely describes the composition and position of functional groups. For instance, the IUPAC systematic name of butyric acid is butanoic acid.
More complex compounds that contain carboxylic acids are named according to the position of one of the parent carbon chains. The other substituents must also be named, such as in the case of 3-chloropropanoic acid. Meanwhile, the base conjugates of carboxylic acids commonly have the suffix -ate. The conjugate base of acetic acid, for example, is called acetate.
Here are some examples of straight-chained and saturated carboxylic acids with their trivial or common names. The name in parentheses is the acid’s IUPAC systematic name.
- Formic acid (methanoic acid) – commonly found in insect stings as a defensive weapon.
- Acetic acid (ethanoic acid) – this is essentially vinegar without the other organic and inorganic impurities.
- Propionic acid (propanoic acid) – often used as a preservative when storing grains. Propionic acid is also present in milk and other dairy products.
- Butyric acid (butanoic acid) – commonly found in butter as a byproduct of fermentation.
- Caprylic acid (octanoic acid) – a fatty acid often found in coconut oil and coconut milk.
- Lauric acid (dodecanoic acid) – another fatty acid present in coconut oil. It’s also used as an ingredient in hand wash soaps.
- Arachidic acid (icosanoic acid) – a type of fatty acid found in peanuts.
- Undecylic acid (undecanoic acid) – an organic antifungal agent.
How are carboxylic acid derivatives formed?
Carboxylic acid derivatives are formed through specific reactions between particular types of carboxylic acids and other substances. Below are the generalised molecular structures of the derivatives of carboxylic acids.
Here are some of the common reactions that form carboxylic acid derivatives.
- Esters – these are produced from the reaction of carboxylic acid with alcohol, otherwise known as esterification. Sulphuric acid is used as a catalyst. One example of the esterification process is the reaction between ethanol and ethanoic acid, as shown in the balanced equation.
- Amides – these are formed from the reactions between ammonia or amines and carboxylic acid chloride.
- Thioester – this is produced when a carboxylic acid reacts with a thiol group. The reaction produces thioester and water, as shown in the generalised chemical equation below.
- Acyl phosphate – this is a product of the chemical reaction between carboxylic acid and adenosine triphosphate or ATP. It occurs as an intermediary reaction in a biological system, which produces acyl-phosphate and ADP.
- Acyl chloride – this is synthesised when carboxylic acids and phosphorous pentachloride chemically react. During the reaction, the hydroxyl group is replaced by chlorine. The reaction also produces hydrochloric acid.
What are the physical properties of carboxylic acid derivatives?
As carboxylic acids and carboxylic acid derivatives have similar functional groups, they share many of the same physical properties. For example, both types of compounds have high boiling points. They’re also generally hydrophilic and polar, which makes them soluble in water.
All of the derivatives have an acyl group but they also have relative leaving group stability, as shown in the diagram below.
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