How do we distinguish aldehydes from ketones, and what reactions does the carbonyl group undergo?
Aldehydes and ketones, oxidation of aldehydes, reduction with sodium tetrahydridoborate, nucleophilic addition of hydrogen cyanide, and the tests for carbonyl compounds (2,4-DNPH, Tollens' reagent and Fehling's solution).
An OCR H432 module 6 answer on carbonyl compounds: aldehydes and ketones, oxidation of aldehydes, reduction with sodium tetrahydridoborate, nucleophilic addition of hydrogen cyanide, and the 2,4-DNPH, Tollens and Fehling tests.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this topic is asking
OCR specification point 6.1.2 wants you to describe aldehydes and ketones, the oxidation of aldehydes (but not ketones), reduction by sodium tetrahydridoborate, the nucleophilic addition of hydrogen cyanide, and the tests that detect a carbonyl group (2,4-DNPH) and distinguish aldehydes from ketones (Tollens' reagent and Fehling's solution). The carbonyl group links alcohols to carboxylic acids.
Aldehydes and ketones
Oxidation and reduction
Nucleophilic addition of hydrogen cyanide
The tests
Examples in context
Example 1. Identifying an unknown in the lab. A practical chemist uses 2,4-DNPH to confirm a carbonyl, then takes the melting point of the orange derivative and compares it with data tables to identify the specific aldehyde or ketone.
Example 2. The blue-to-red Fehling's test in food science. Fehling's solution is used to detect reducing sugars (which contain an aldehyde group); a brick-red precipitate on warming shows the sugar is present, a direct application of aldehyde oxidation.
Try this
Q1. Name the reagent that gives an orange precipitate with both aldehydes and ketones. [1 mark]
- Cue. 2,4-dinitrophenylhydrazine (2,4-DNPH).
Q2. State the product when butanal is reduced by sodium tetrahydridoborate. [1 mark]
- Cue. Butan-1-ol (a primary alcohol).
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20204 marksA student has two unlabelled bottles, one containing propanal and one containing propanone. (a) Name a reagent that confirms both contain a carbonyl group, and the observation. (b) Name a reagent that distinguishes them, and the observation with each.Show worked answer →
(a) 2,4-dinitrophenylhydrazine (2,4-DNPH, Brady's reagent); both give an orange (or yellow-orange) precipitate, confirming a carbonyl group (1)(1).
(b) Tollens' reagent (ammoniacal silver nitrate), warmed: propanal (an aldehyde) gives a silver mirror as it is oxidised (1); propanone (a ketone) gives no change because it is not oxidised (1).
Markers reward 2,4-DNPH with the orange precipitate for both, and Tollens' giving a silver mirror with the aldehyde but no reaction with the ketone.
OCR 20224 marksPropanone reacts with hydrogen cyanide. (a) Name the mechanism and the product. (b) Outline the mechanism, showing the curly arrows.Show worked answer →
(a) Nucleophilic addition; the product is 2-hydroxy-2-methylpropanenitrile (a hydroxynitrile) (1).
(b) The cyanide ion (nucleophile) attacks the carbonyl carbon (curly arrow from the lone pair on to the carbon), and the pi bond breaks onto the oxygen (curly arrow from the double bond to O), forming a negatively charged intermediate (1); the oxygen then accepts a proton (from HCN or water) to give the group (1)(1).
Markers reward the mechanism name and product, the curly arrow from cyanide to the carbonyl carbon, the breaking of the pi bond onto oxygen, and protonation to the hydroxynitrile.
Related dot points
- The delocalised model of benzene and the evidence for it, the stability of the ring, and the electrophilic substitution reactions of benzene (nitration, halogenation and Friedel-Crafts acylation).
An OCR H432 module 6 answer on aromatic compounds: the delocalised model of benzene and the evidence for it, its stability, and the electrophilic substitution reactions of nitration, halogenation and Friedel-Crafts acylation.
- Acidity of carboxylic acids and their reactions with metals, alkalis and carbonates; esterification and ester hydrolysis (acid and alkaline); and the reactions of acyl chlorides and acid anhydrides.
An OCR H432 module 6 answer on carboxylic acids and esters: acidity and reactions with metals, alkalis and carbonates, esterification and acid and alkaline ester hydrolysis, and the reactions of acyl chlorides and acid anhydrides.
- Amines and their basicity and preparation, amino acids and the amide link, optical isomerism and chirality, condensation polymers (polyesters and polyamides), carbon-carbon bond formation using nitriles, and multi-step organic synthesis.
An OCR H432 module 6 answer on nitrogen compounds and polymers: amine basicity and preparation, amino acids and the amide link, optical isomerism, condensation polymers (polyesters and polyamides), carbon-carbon bond formation with nitriles, and organic synthesis.
- Chromatography (thin-layer with Rf values and gas chromatography with retention times), carbon-13 and proton NMR spectroscopy (chemical shift, integration and the n+1 splitting rule with TMS reference), and combining analytical techniques to identify structures.
An OCR H432 module 6 answer on chromatography and NMR: thin-layer and gas chromatography, carbon-13 and proton NMR with chemical shift, integration and the n+1 splitting rule, and combining analytical techniques to deduce structures.
- Classification and properties of alcohols (hydrogen bonding), combustion, oxidation by acidified dichromate (primary to aldehyde and carboxylic acid, secondary to ketone, tertiary not oxidised), dehydration to alkenes, and substitution to haloalkanes.
An OCR H432 module 4 answer on alcohols: classification and hydrogen bonding, combustion, oxidation by acidified potassium dichromate to aldehydes, ketones and carboxylic acids, dehydration to alkenes, and conversion to haloalkanes.
Sources & how we know this
- OCR A-Level Chemistry A (H432) specification — OCR (2015)