Ch 1 · Aldehydes and ketones · AS & A Level Chemistry 9701
Topic 1 of 1 in Carbonyl compounds — AS & A Level Chemistry 9701 — 58 lessons.
Introduction
Aldehydes (-CHO) and ketones (mathrmC=O within a chain) share the carbonyl group but differ in reactivity because the carbonyl carbon of an aldehyde also carries a hydrogen atom, making aldehydes easier to oxidise. This lesson covers their preparation, addition and reduction reactions, and the tests used to identify and distinguish them.
Key ideas — Oxidation of alcohols to carbonyl compounds
Primary alcohols are oxidised by acidified potassium dichromate(VI) to aldehydes, distilled off as they form to prevent further oxidation. Secondary alcohols are oxidised under reflux by the same oxidant to ketones, which resist further oxidation. CH_3CH_2OH + [O] arrow CH_3CHO + H_2O CH_3CH(OH)CH_3 + [O] arrow CH_3COCH_3 + H_2O
Key ideas — Reduction of aldehydes and ketones
Aldehydes are reduced by NaBH_4 to primary alcohols, and ketones to secondary alcohols; this reverses the oxidation above, restoring the hydrogen atoms removed. CH_3CHO + 2[H] arrow CH_3CH_2OH
🧑🏫 Teacher's explanation
Think of reduction in organic chemistry as 'adding hydrogen' or 'removing oxygen'. If oxidation is like taking away pieces of a Lego structure, reduction is like putting them back on. In the context of carbonyls, the carbon atom has a double bond to oxygen (C=O). To turn this into an alcohol, we need to add hydrogen atoms to that specific carbon.
📖 Definitions
🔍 Worked example — Easy
Write the balanced equation for the reduction of propanal to propan-1-ol using [H].
🔍 Worked example — Standard
A sample of butanone (CH_3 extCOCH_3) is reduced using NaBH_4. Write the molecular formula of the resulting alcohol.
⚠️ Common errors
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🎯 Exam question
Key ideas — Nucleophilic addition of hydrogen cyanide
Hydrogen cyanide adds across the carbonyl double bond of an aldehyde or ketone (generated in situ from KCN and dilute acid) to form a hydroxynitrile, extending the carbon chain by one carbon: CH_3CHO + HCN arrow CH_3CH(OH)CN This is an example of nucleophilic addition, characteristic of the polar mathrmC=O bond.
🧑🏫 Teacher's explanation
Imagine a carbonyl group (C=O) as a magnet with a positive pole at the carbon and a negative pole at the oxygen. Because the oxygen is much more electronegative, it pulls electron density toward itself, leaving the carbon 'electron-deficient' or electrophilic. This makes the carbon a prime target for a 'nucleophile'—a species looking for a positive charge to share its electrons with.
📖 Definitions
🔍 Worked example — Easy
Calculate the molar mass of the product formed when propanal (ceCH3CH2CHO) reacts with HCN.
🔍 Worked example — Standard
A sample of 0.50 mol of ethanal (ceCH3CHO) reacts completely with excess HCN. Calculate the mass of the hydroxynitrile produced.
🔍 Worked example — Exam-style
In a mechanism, the CN^- ion attacks the carbonyl carbon. Explain why the π bond breaks and where the electrons go.
⚠️ Common errors
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🎯 Exam question
Key ideas — Mechanism of nucleophilic addition
The carbonyl carbon carries a partial positive charge because oxygen is more electronegative, mathrmC^delta+=O^delta-. The cyanide ion, :CN^-, uses a lone pair on carbon to attack this electrophilic carbon, and the pair of electrons in the mathrmC=O pi bond moves onto the oxygen, forming a tetrahedral alkoxide intermediate. This intermediate is then protonated by a hydrogen ion (from HCN or water) to give the neutral hydroxynitrile.
🧑🏫 Teacher's explanation
Imagine a carbonyl group as a magnet with a positive pole (the carbon) and a negative pole (the oxygen). Because oxygen is much more 'greedy' for electrons (electronegative), it pulls the electron density toward itself, leaving the carbon atom feeling 'electron-poor' or electrophilic.
📖 Definitions
🔍 Worked example — Easy
Identify the nucleophile and the electrophile in the reaction between propanal (ceCH3CH2CHO) and cyanide ion (ceCN^-)
🔍 Worked example — Standard
Draw the structural formula of the product formed when ethanal (ceCH3CHO) reacts with hydrogen cyanide (ceHCN), assuming the mechanism is nucleophilic addition.
🔍 Worked example — Exam-style
Calculate the molar mass of the hydroxynitrile formed from the reaction of propanal (ceC_3H_6O) with ceHCN. (Use C=12, H=1, N=14, O=16)
⚠️ Common errors
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🎯 Exam question
(a) Draw the mechanism for the nucleophilic addition of CN^- to propanal, showing all lone pairs and curly arrows. [3] (b) Explain why the reaction is faster with propanal than with a substituted ketone like propanone. [1] [4 marks]
Key ideas — Detecting carbonyl compounds with 2,4-DNPH
2,4-dinitrophenylhydrazine reacts with the carbonyl group of any aldehyde or ketone to give an orange precipitate, confirming a mathrmC=O group is present (without distinguishing aldehyde from ketone). The precipitate's melting point can be compared against known values to identify the specific carbonyl compound.
🧑🏫 Teacher's explanation
Imagine you are at a crowded party and you want to find everyone wearing a specific type of bright orange hat. You can't talk to everyone, so you throw a handful of orange confetti into the crowd. Anyone wearing that specific hat will catch the confetti and become part of a visible orange clump.
📖 Definitions
🔍 Worked example — Easy
A student reacts propanal with 2,4-DNPH. State the observation and the functional group identified.
🔍 Worked example — Standard
A sample contains 0.10 mol of butanone. Calculate the mass of the 2,4-dinitrophenylhydrazine derivative formed, assuming the reaction is C_4H_8O + C_6H_5N_2H arrow C_10H_15N_3O + H_2O. (M_r: Butanone = 72, 2,4-DNPH derivative = 197)
🔍 Worked example — Exam-style
A mixture contains an unknown aldehyde and an unknown ketone. Describe how 2,4-DNPH can be used to confirm the presence of a carbonyl group and how it fails to distinguish between the two.
⚠️ Common errors
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🎯 Exam question
(a) State the functional group present in X. [1] (b) Describe how you would use a different reagent to distinguish between propanal and propanone. [2] [3 marks]
Key ideas — Distinguishing aldehydes from ketones
Aldehydes are oxidised on warming with Fehling's solution (a blue copper(II) complex), giving a brick-red precipitate of copper(I) oxide; ketones give no colour change. Aldehydes similarly reduce Tollens' reagent (ammoniacal silver nitrate) to give a silver mirror; ketones give no reaction. The tri-iodomethane test, warming with iodine and aqueous sodium hydroxide, gives a pale yellow precipitate of CHI_3 for any compound containing a CH_3CO- group, such as ethanal or propanone, distinguishing them from carbonyl compounds without this group.
🧑🏫 Teacher's explanation
Think of a carbonyl group (C=O) as a chemical 'handle'. In aldehydes, one side of this handle is a tiny hydrogen atom, making it very easy for an oxidizing agent to 'grab' and pull away a hydrogen to turn it into a carboxylic acid. In ketones, both sides of the handle are bulky carbon chains, which physically block the oxidizing agent from attacking the carbon atom.
📖 Definitions
🔍 Worked example — Easy
Identify the product when ethanal (ceCH3CHO) is reacted with Tollens' reagent and state the observation.
🔍 Worked example — Standard
A sample of an unknown carbonyl compound gives a brick-red precipitate when warmed with Fehling's solution. If the compound has the molecular formula C_3H_6O, deduce its structure.
🔍 Worked example — Exam-style
Calculate the mass of CHI_3 produced when 0.10 mol of propanone (ceCH3COCH3) undergoes the iodoform test with excess I_2 and NaOH.
⚠️ Common errors
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🎯 Exam question
Worked example
Question. Propanal, CH_3CH_2CHO, reacts with hydrogen cyanide in the presence of a trace of alkali. (a) Name the mechanism and outline it in words, showing the intermediate and final product. [4] (b) State the role of hydrogen cyanide in this reaction. [1] (c) Name the organic product. [1]
🔍 Worked example — Easy
Calculate the molar mass of 2-hydroxybutanenitrile, CH_3CH_2CH(OH)CN. (Use A_r: C=12, H=1, O=16, N=14)
🔍 Worked example — Standard
A sample of propanal (CH_3CH_2CHO) is reacted with excess HCN. If 0.10 moles of propanal are used, how many moles of 2-hydroxybutanenitrile are produced, assuming 100\% yield?
🔍 Worked example — Exam-style
Explain why the reaction between an aldehyde and HCN requires a trace of alkali (e.g., KCN) to proceed at a reasonable rate.
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Practice
Answers