Carboxylic Acids: A GCSE AQA Tutorial

Carboxylic acids are a family of organic compounds characterized by the presence of a carboxyl group (-COOH). This functional group consists of a carbon atom double-bonded to an oxygen atom and single-bonded to a hydroxyl group (-OH). Understanding their structure, properties, and reactions is crucial for GCSE AQA Chemistry.

1. Structure and Naming

The general formula for a carboxylic acid is R-COOH, where R represents an alkyl group (a chain of carbon and hydrogen atoms).

• Methanoic acid (HCOOH): The simplest carboxylic acid, also known as formic acid. It's found in ant stings.
• Ethanoic acid (CH?COOH): Commonly known as acetic acid, the main component of vinegar.
• Propanoic acid (CH?CH?COOH): A three-carbon carboxylic acid.
• Butanoic acid (CH?CH?CH?COOH): A four-carbon carboxylic acid with a distinctly unpleasant smell.

Naming follows a systematic approach:

1. Identify the longest carbon chain including the carboxyl carbon.
2. Number the carbon atoms, starting from the carboxyl carbon (this is always carbon 1).
3. Name any alkyl groups or other substituents attached to the chain.
4. Add the suffix "-oic acid".

Example: CH?CH(CH?)CH?COOH is 2-methylbutanoic acid.

2. Properties of Carboxylic Acids

• Acidity: Carboxylic acids are weak acids. They partially ionize in water to produce hydrogen ions (H?) and carboxylate ions (RCOO?). This is represented by the equilibrium:

RCOOH(aq) ? RCOO?(aq) + H?(aq)

• Solubility: Lower molecular weight carboxylic acids (e.g., methanoic acid, ethanoic acid) are soluble in water due to hydrogen bonding between the carboxyl group and water molecules. Larger carboxylic acids are less soluble.
• Boiling Points: Carboxylic acids have relatively high boiling points compared to other organic compounds of similar molecular weight. This is because of strong hydrogen bonding between the molecules.

3. Reactions of Carboxylic Acids

Carboxylic acids undergo several important reactions:

• Reaction with metals: Carboxylic acids react with reactive metals (like sodium, magnesium, zinc) to produce hydrogen gas and a metal carboxylate salt.

2CH?COOH(aq) + Mg(s) ? (CH?COO)?Mg(aq) + H?(g)

• Reaction with bases: Carboxylic acids react with bases (like sodium hydroxide) to form a salt and water. This is a neutralization reaction.

CH?COOH(aq) + NaOH(aq) ? CH?COONa(aq) + H?O(l)

• Esterification: Carboxylic acids react with alcohols in the presence of an acid catalyst (e.g., concentrated sulfuric acid) to produce esters and water. This is a condensation reaction.

CH?COOH(l) + CH?CH?OH(l) ? CH?COOCH?CH?(l) + H?O(l)

4. Practical Applications

Carboxylic acids and their derivatives have numerous applications, including:

• Ethanoic acid (vinegar): Used as a food preservative and in cleaning products.
• Methanoic acid (formic acid): Used in the textile industry and as an antimicrobial agent.
• Esters: Widely used as flavourings and fragrances.

This tutorial provides a foundational understanding of carboxylic acids for GCSE AQA Chemistry. Further exploration of specific reactions and applications will deepen your knowledge. Remember to practice examples and apply the concepts to different scenarios.