How do the carbonyl compounds react, and how can we distinguish an aldehyde from a ketone?
The carbonyl group, nucleophilic addition reactions (with HCN and with reducing agents), oxidation of aldehydes, and the tests that distinguish aldehydes from ketones.
An Eduqas A-Level Chemistry OA2.2 answer on the carbonyl group, nucleophilic addition with HCN and reducing agents, oxidation of aldehydes, and the tests distinguishing aldehydes from ketones.
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What this topic is asking
Eduqas topic OA2.2 covers the carbonyl compounds: the structure and polarity of the group, nucleophilic addition reactions (with hydrogen cyanide and with reducing agents), the oxidation of aldehydes (but not ketones), and the chemical tests that distinguish the two. The nucleophilic addition mechanism and the distinguishing tests are the central exam content.
The carbonyl group and nucleophilic addition
Reaction with hydrogen cyanide
Reduction to alcohols
A reducing agent such as sodium tetrahydridoborate(III) () adds hydrogen across the bond, reducing an aldehyde to a primary alcohol and a ketone to a secondary alcohol (the reverse of the AS oxidation). The reducing agent is written as in equations.
Oxidation and the distinguishing tests
Examples in context
- Example 1. The silver-mirror test in manufacturing
- Tollens' reagent deposits a thin, even silver layer when reduced by an aldehyde; the same reaction was historically used to silver the inside of vacuum flasks and mirrors.
- Example 2. Blood-glucose testing
- Benedict's (a Fehling-type) reagent is reduced by the aldehyde group of glucose to give a colour change from blue to brick-red, the chemical basis of early diabetic urine tests.
- Example 3. Hydroxynitriles as synthetic intermediates
- The nucleophilic addition of HCN to a carbonyl adds a carbon atom and an easily transformed nitrile group; the hydroxynitrile product can be hydrolysed to a hydroxy-acid, making this a key chain-lengthening step that also creates a new chiral centre.
Try this
Q1. Name the product of reducing propanone with . [1 mark]
- Cue. Propan-2-ol (a ketone is reduced to a secondary alcohol).
Q2. State the reagent and observation for a positive Fehling's test with an aldehyde. [2 marks]
- Cue. Warm with Fehling's (or Benedict's) solution; the blue solution forms a brick-red precipitate (copper(I) oxide).
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20194 marksPropanal reacts with hydrogen cyanide. (a) Name the type and mechanism of the reaction and the organic product. (b) Outline the mechanism using curly arrows.Show worked answer →
(a) Nucleophilic addition; the product is 2-hydroxybutanenitrile, (2).
(b) The cyanide ion (nucleophile) attacks the carbon of the carbonyl; a curly arrow goes from the lone pair on to the carbon, and one from the double bond to the oxygen, forming a negatively charged intermediate that is then protonated by HCN or water (2).
Eduqas 20214 marksDescribe a chemical test, including the reagent and the observation, that would distinguish between propanal and propanone.Show worked answer →
Use Tollens' reagent (ammoniacal silver nitrate) and warm (1). Propanal, an aldehyde, is oxidised and reduces the silver ions, forming a silver mirror on the test tube (1). Propanone, a ketone, is not oxidised and gives no silver mirror (1).
(Acidified potassium dichromate(VI) is an acceptable alternative: it turns orange to green with the aldehyde but stays orange with the ketone) (1).
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Sources & how we know this
- WJEC Eduqas GCE A Level Chemistry specification (from 2015) — WJEC Eduqas (2015)