In simple terms
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The Acid Test: Understanding Carboxylic Acids
Carboxylic acids contain the –COOH group, making them weak acids that can form dimers via hydrogen bonds. Their key reactions include forming salts with bases and esters with alcohols.
Think of a carboxylic acid's proton as a library book. It can be 'lent out' to a water molecule, but the library (the carboxylate ion) always wants it back, so it's only ever partially 'checked out' at any one time. This is unlike a strong acid, which is like giving the book away permanently.
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R–COOH: The carboxyl group consists of a carbonyl group (C=O) and a hydroxyl group (–OH) on the same carbon atom.
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Carboxylic acids are weak acids, meaning they only partially dissociate in water to release H⁺ ions, with a typical Ka value around 10⁻⁵.
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They undergo characteristic reactions, such as forming a salt and water with bases, or producing CO₂ gas with carbonates.
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Strong intermolecular hydrogen bonds lead to high boiling points, often forming dimers which effectively double their molecular mass.
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Full topic notes
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Structure and Nomenclature
The functional group of a carboxylic acid is the carboxyl group, written as . It combines a carbonyl group () and a hydroxyl group () on the same carbon atom. The carbon atom of the carboxyl group is always designated as carbon-1 when naming the compound.
Nomenclature: To name a carboxylic acid, find the longest carbon chain containing the carboxyl group. Replace the '-e' from the corresponding alkane name with the suffix '-oic acid'.
Examples: Methane becomes methanoic acid (), ethane becomes ethanoic acid (), propane becomes propanoic acid ().
Structure: The carboxyl group is planar due to the hybridisation of the carbonyl carbon, which influences its reactivity and physical properties.
Physical Properties: Boiling Points and Solubility
Carboxylic acids have significantly higher boiling points than alcohols, aldehydes, or alkanes of comparable relative molecular mass. This is due to their ability to form strong intermolecular hydrogen bonds. In fact, they can form dimers, where two molecules are held together by two hydrogen bonds. This effectively doubles the molecule's size, increasing the van der Waals forces and requiring more energy to separate the molecules into the gaseous phase.
The solubility of carboxylic acids in water depends on their chain length. Short-chain carboxylic acids (up to four carbons) are miscible with water because the polar carboxyl group can form hydrogen bonds with water molecules. As the length of the non-polar hydrocarbon chain increases, the molecule becomes less polar overall, and solubility in water decreases significantly.
Acidity of Carboxylic Acids
Carboxylic acids are weak Brønsted-Lowry acids. In aqueous solution, they partially dissociate to form a carboxylate anion and a hydrogen ion (), which is often represented as the hydronium ion (). The position of this equilibrium lies to the left, indicating that only a small fraction of the acid molecules are ionised at any given time.
RCOOH(aq) \rightleftharpoons RCOO^-(aq) + H^+(aq)
Weak Acid Behaviour: Their partial dissociation is why they have a pH greater than 1 for typical concentrations, unlike strong acids like HCl.
Stabilisation: The carboxylate anion () is stabilised by resonance, as the negative charge is delocalised across the two oxygen atoms. This makes the anion more stable than an alkoxide ion () from an alcohol, explaining why carboxylic acids are more acidic than alcohols.
Test for Acidity: A key test is the reaction with carbonates, which is characteristic of acids stronger than carbonic acid.
Characteristic Reactions
The chemistry of carboxylic acids is dominated by their acidity and the reactions of the carboxyl group. They undergo several important reactions that you must know.
Reaction with Metals: Reactive metals (e.g., Mg, Zn) react to form a metal carboxylate salt and hydrogen gas.
Reaction with Bases: They are neutralised by bases (e.g., NaOH) 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 observed is a definitive test for a carboxylic acid.
Worked examples
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A 25.0 cm³ sample of a solution of ethanoic acid, , was titrated with 0.150 mol dm⁻³ sodium hydroxide solution. 22.50 cm³ of the NaOH solution was required for complete neutralisation. Calculate the concentration of the ethanoic acid solution in mol dm⁻³.
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Write the balanced equation:
A 1.85 g sample of a pure straight-chain carboxylic acid, Y, was reacted with excess sodium carbonate. The reaction produced 480 cm³ of carbon dioxide gas, measured at room temperature and pressure (RTP). Determine the molecular formula of the carboxylic acid Y. (Molar volume of a gas at RTP = 24.0 dm³ mol⁻¹).
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Write a general balanced equation:
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What is the functional group of a carboxylic acid?
The carboxyl group, , which consists of a carbonyl group () and a hydroxyl group () attached to the same carbon atom.
Key takeaways
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Nomenclature: To name a carboxylic acid, find the longest carbon chain containing the carboxyl group. Replace the '-e' from the corresponding alkane name with the suffix '-oic acid'.
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Examples: Methane becomes methanoic acid (), ethane becomes ethanoic acid (), propane becomes propanoic acid ().
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Structure: The carboxyl group is planar due to the hybridisation of the carbonyl carbon, which influences its reactivity and physical properties.
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