9701 · 33.1
Carboxylic acids flashcards
Revision flashcards for Cambridge 9701 Carboxylic acids (syllabus 33.1). Flip, recall, then mark a real past-paper question.
Card
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.
Card
Why do carboxylic acids have higher boiling points than alcohols of similar molar mass?
Carboxylic acids can form dimers through hydrogen bonding. Each molecule forms two hydrogen bonds with a neighbour, requiring more energy to separate the molecules and vaporise the liquid.
Card
What is a dimer?
A molecule made of two identical smaller molecules (monomers) linked together. In the case of carboxylic acids, two molecules are held together by two hydrogen bonds.
Card
Are carboxylic acids strong or weak acids? Explain.
They are weak acids. They only partially dissociate (ionise) in aqueous solution to release H⁺ ions, establishing an equilibrium. `RCOOH(aq) ⇌ RCOO⁻(aq) + H⁺(aq)`.
Card
What is the standard test for a carboxylic acid?
Add a small amount of sodium carbonate (Na₂CO₃) or sodium hydrogencarbonate (NaHCO₃). Effervescence (fizzing) will be observed as carbon dioxide gas is produced. This test distinguishes them from less acidic phenols.
Card
What are the products of the reaction between ethanoic acid and magnesium?
Magnesium ethanoate and hydrogen gas. `Mg(s) + 2CH₃COOH(aq) → (CH₃COO)₂Mg(aq) + H₂(g)`.
Card
What type of reaction occurs between a carboxylic acid and an alkali like NaOH?
A neutralisation reaction, forming a salt and water. For example, `CH₃COOH + NaOH → CH₃COONa + H₂O`.
Card
What reagent is used to reduce a carboxylic acid to a primary alcohol?
Lithium tetrahydridoaluminate, `LiAlH₄`, in dry ether, followed by aqueous workup. This is a powerful reducing agent.
Card
Why is `NaBH₄` (sodium tetrahydridoborate) not used to reduce carboxylic acids?
`NaBH₄` is a milder reducing agent and is not powerful enough to reduce the carboxyl group. It can, however, reduce aldehydes and ketones.
Card
How can ethanoic acid be formed from ethanol?
By oxidation using a strong oxidising agent, such as acidified potassium manganate(VII) (`KMnO₄/H⁺`) or acidified potassium dichromate(VI) (`K₂Cr₂O₇/H⁺`), under reflux.
Card
What is the name of the salt formed from propanoic acid and potassium hydroxide?
Potassium propanoate.