Aldehydes and ketonesEdexcel International A Level Chemistry: Flashcards
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What is the functional group of an aldehyde?
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- What is the functional group of an aldehyde?
- –CHO: a carbonyl group at the end of the carbon chain, bonded to at least one hydrogen.
- How does a ketone differ from an aldehyde in structure?
- A ketone's carbonyl carbon is bonded to two carbon atoms, within the chain; its name ends -one.
- Why can aldehydes and ketones not hydrogen bond with each other?
- They have no hydrogen bonded to oxygen (or N or F).
- Why are short-chain aldehydes and ketones soluble in water?
- The oxygen lone pairs form hydrogen bonds with the δ+ hydrogens of water.
- What is seen when an aldehyde is warmed with Tollens' reagent?
- A silver mirror forms as Ag⁺ is reduced to Ag.
- What is seen when an aldehyde is warmed with Fehling's solution?
- The blue solution gives a brick-red precipitate of copper(I) oxide.
- What is seen when an aldehyde is heated with acidified potassium dichromate(VI)?
- The orange solution turns green as it is reduced to chromium(III).
- What does LiAlH₄ in dry ether do to an aldehyde and to a ketone?
- Aldehyde to primary alcohol; ketone to secondary alcohol. It provides H⁻ ions.
- Why must LiAlH₄ be used in dry ether?
- It reacts violently with water.
- Name the mechanism of HCN with a carbonyl compound and the nucleophile.
- Nucleophilic addition; the nucleophile is CN⁻ (from KCN).
- Why does ethanal plus HCN give no optical activity?
- The planar C=O is attacked equally from both sides, giving a racemic mixture of the two isomers.
- What does 2,4-DNPH show, and how is the compound then identified?
- It shows a C=O group (orange precipitate). Recrystallise it, measure the melting temperature and compare with data values.
- What is seen in the iodoform test and which group does it show?
- A pale yellow precipitate of CHI₃; it shows a CH₃C=O group.
Exam questions on Aldehydes and ketones
- A chemist compares three compounds of similar relative molecular mass: butane (Mr 58, boiling temperature −1 °C), propanal (Mr 58, boiling temperature 49 °C) and propan-1-ol (Mr 60, boiling temperature 97 °C).Explain why propanal has a higher boiling temperature than butane, even though their relative molecular masses are the same.2 marks
- A technician has two unlabelled liquids, one is propanal and the other is propanone. She tests a sample of each with Tollens' reagent, with Fehling's solution and with acidified potassium dichromate(VI) solution, warming gently where necessary.Write an equation, using [O] to represent the oxidising agent, for the reaction of propanal with acidified potassium dichromate(VI), and state the colour change seen for propanal but not for propanone.2 marks
- Compound X has the molecular formula C₄H₈O. It gives an orange precipitate with 2,4-dinitrophenylhydrazine solution. It gives no reaction when warmed with Tollens' reagent. It gives a pale yellow precipitate when warmed with iodine and sodium hydroxide solution.Deduce the structure and name of compound X, explaining each observation.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).