In simple terms
A friendly intro before the formal notes — no formulas yet.
Carboxylic Acids: From Vinegar to Synthesis
Carboxylic acids contain the –COOH group, making them acidic and able to form strong hydrogen bonds. These properties dictate their unique physical characteristics and chemical reactivity.
Imagine two people holding hands with both of their hands (a dimer). This is a much stronger connection than a simple handshake (a single hydrogen bond in an alcohol), making it harder to pull them apart. This is why carboxylic acids have surprisingly high boiling points for their size.
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–COOH: dimer H-bonds, higher bp than similar alcohol. | Sim hint: Weak acid — partial dissociation.
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With carbonates: CO₂ evolved — test for acid. | Sim hint: RCOOH + Na₂CO₃ → salt + CO₂ + H₂O.
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Reduction with LiAlH₄ gives primary alcohol. | Sim hint: Not with NaBH₄ for acids at AS.
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Acyl chloride route for esters and amides. | Sim hint: More reactive than acid itself.
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Key formulas
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Full topic notes
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Structure and Physical Properties
The carboxyl group, a combination of a carbonyl () and a hydroxyl () group, is highly polar. This polarity allows short-chain carboxylic acids (up to C4) to be completely miscible with water, as they can form hydrogen bonds with water molecules. As the non-polar hydrocarbon chain length increases, solubility in water decreases.
A defining feature of carboxylic acids is their ability to form stable hydrogen-bonded dimers in the liquid state and in non-polar solvents. Each acid molecule can donate one hydrogen bond (from its group) and accept one hydrogen bond (at its group). This results in two molecules being held together by two hydrogen bonds, effectively doubling the particle size. Consequently, more energy is needed to overcome these strong intermolecular forces, leading to significantly higher boiling points compared to alcohols with similar relative molecular masses.
Acidity and Characteristic Reactions
Carboxylic acids are classified as weak acids because they only partially dissociate in water. The equilibrium lies to the left, meaning that at any given time, most of the acid molecules remain undissociated.
RCOOH(aq) \rightleftharpoons RCOO^{-}(aq) + H^{+}(aq)
Despite being weak, they are still acidic enough to display typical acid reactions. These reactions are crucial for identifying and distinguishing carboxylic acids.
Reaction with reactive metals: They react with metals above hydrogen in the reactivity series (e.g., Mg, Zn) to produce a salt and hydrogen gas.
Neutralisation with bases: They react with bases (e.g., NaOH) and metal oxides (e.g., CuO) in a neutralisation reaction to form a salt and water.
Reaction with carbonates: They react with carbonates (e.g., ) and hydrogencarbonates (e.g., ) to produce a salt, water, and carbon dioxide gas. The effervescence produced is the definitive test for a carboxylic acid, as weaker acids like phenol do not react with .
Formation and Reduction of Carboxylic Acids
Carboxylic acids can be synthesised by the vigorous oxidation of primary alcohols or aldehydes. This is typically achieved by heating the alcohol or aldehyde under reflux with a strong oxidising agent, such as acidified potassium dichromate(VI) () or potassium manganate(VII) (). The reflux conditions ensure that any intermediate aldehyde formed is fully oxidised to the carboxylic acid.
The reverse of this oxidation is reduction. Carboxylic acids can be reduced back to primary alcohols using a powerful reducing agent. The standard reagent for this transformation at A-Level is lithium tetrahydridoaluminate, .
It is crucial to remember the specific reagents and conditions. For reduction, you must state LiAlH₄ (or lithium tetrahydridoaluminate) and the condition of 'dry ether'. The reaction is followed by an aqueous workup step. Remember that the weaker reducing agent NaBH₄ cannot reduce carboxylic acids.
Worked examples
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A 25.0 cm³ sample of a solution of a monoprotic carboxylic acid, HX, was titrated with 0.100 mol dm⁻³ sodium hydroxide solution. 22.50 cm³ of the NaOH solution was required for complete neutralisation. Calculate the concentration of the carboxylic acid solution.
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Step 1: Write the balanced chemical equation. HX + NaOH → NaX + H₂O The stoichiometry is 1:1.
A 0.460 g sample of an unknown monoprotic carboxylic acid, CHCOOH, was dissolved in water and titrated against 0.200 mol dm⁻³ NaOH. The volume of NaOH required for neutralisation was 25.00 cm³. Identify the carboxylic acid.
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Step 1: Calculate the moles of NaOH used. Moles of NaOH = 0.200 mol dm⁻³ × (25.00 / 1000) dm³ = 0.00500 mol.
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Glossary
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Revision flashcards
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What is the functional group of a carboxylic acid?
The carboxyl group, written as –COOH. It consists of a carbonyl group (C=O) and a hydroxyl group (–OH) attached to the same carbon atom.
Key takeaways
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Reaction with reactive metals: They react with metals above hydrogen in the reactivity series (e.g., Mg, Zn) to produce a salt and hydrogen gas.
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Neutralisation with bases: They react with bases (e.g., NaOH) and metal oxides (e.g., CuO) in a neutralisation reaction to form a salt and water.
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Reaction with carbonates: They react with carbonates (e.g., ) and hydrogencarbonates (e.g., ) to produce a salt, water, and carbon dioxide gas. The effervescence produced is the definitive test for a carboxylic acid, as weaker acids like phenol do not react with .
Practice — then mark it
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Practice Questions: Carboxylic Acids
Practice Questions: Carboxylic Acids
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