Back to: Organic Chemistry 200 Level
Welcome to class!
Hello there, future chemistry expert! I’m glad to see your curiosity still burning bright. In our last lesson on carboxylic acids, we discovered their structure, properties, and where they show up in our daily lives. Today, we’ll step into their chemical behaviour — the exciting ways they interact with other substances, and how these reactions shape both nature and industry.
Carboxylic Acids II
Acid–Base Reactions
Carboxylic acids are weak acids but still react readily with alkalis. When they meet a strong base like sodium hydroxide, they neutralise it to form a salt (carboxylate) and water:
CH₃COOH + NaOH → CH₃COONa + H₂O
With carbonates and hydrogencarbonates, the reaction produces a salt, water, and carbon dioxide:
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
That fizzing you might notice when vinegar meets baking soda is carbon dioxide escaping.
Reaction with Metals
Reactive metals such as sodium, potassium, and magnesium will displace hydrogen from the acid:
2CH₃COOH + 2Na → 2CH₃COONa + H₂
The hydrogen gas can be collected and tested with a lighted splint, producing a ‘pop’ sound.
Esterification
A fascinating transformation occurs when a carboxylic acid reacts with an alcohol in the presence of concentrated sulphuric acid: an ester is formed, along with water.
CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O
Esters are behind many artificial fruit flavours — think strawberry-flavoured drinks, pineapple sweets, or the fragrance in perfumes.
Reduction
With strong reducing agents like lithium aluminium hydride (LiAlH₄) in dry ether, carboxylic acids are reduced to primary alcohols:
R–COOH → R–CH₂OH
Decarboxylation
Heating a sodium salt of a carboxylic acid with soda lime (NaOH + CaO) removes the -COOH group as CO₂, leaving behind an alkane:
CH₃COONa + NaOH → CH₄ + Na₂CO₃
Laboratory Tests for Carboxylic Acids
Carbonate test: Effervescence with Na₂CO₃ or NaHCO₃ shows CO₂ release.
pH indicator: Turns blue litmus paper red.
Distinct odour: Sharp, vinegar-like smell for lower members.
Summary
- Carboxylic acids react with bases, metals, and carbonates to form salts.
- Esterification produces sweet-smelling esters used in flavours and perfumes.
- They can be reduced to primary alcohols.
- Decarboxylation removes the acid group to form alkanes.
- Simple lab tests confirm their presence.
Evaluation
- Write balanced equations for the reaction of ethanoic acid with:
- a) Sodium hydroxide
- b) Sodium carbonate
- State the conditions for esterification and name one use of esters.
- What type of alcohol is produced when a carboxylic acid is reduced with LiAlH₄?
- Explain what happens in a decarboxylation reaction.
Fantastic effort! You now understand how carboxylic acids react and why they’re so valuable in the lab and in industries. With Afrilearn, you’re steadily building the expertise that will make complex organic chemistry second nature to you.