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A-Level Reactions and tests for aldehydes and ketones
What the A-Level syllabus expects for Reactions and tests for aldehydes and ketones, and how to practise it.
What the syllabus expects
- Describe how aldehydes come from primary alcohols and ketones from secondary alcohols, and how each is reduced back again, with LiAlH4 or with H2(g) over a Ni catalyst.
- Describe how hydrogen cyanide, with a KCN catalyst, reacts with aldehydes and with ketones.
Scope: see also 11.3(o) - Describe how 2,4-dinitrophenylhydrazine (2,4-DNPH) reveals the presence of a carbonyl compound.
- Decide whether an unknown carbonyl compound is an aldehyde or a ketone from simple tests, such as warming with Fehling's and Tollens' reagents and how readily it oxidises.
- Infer a CH3CO- group in a carbonyl compound when warm alkaline aqueous iodine yields tri-iodomethane.
How it's examined
Questions on this topic most often ask you to describe, explain. About 2% of the past-paper style questions in Rae's bank for this subject sit in this topic.
Worked examples
Example 1 (4 marks)
BHB, acetoacetate (AcAc, CH3-CO-CH2-CO2H) and acetone (CH3-CO-CH3) are liver-made ketone bodies that fuel the brain during fasting or hard exercise. Describe a simple chemical test, with observations, to tell apart each pair: AcAc from acetone, and AcAc from BHB.
Show the worked answer
AcAc (CH3COCH2COOH) contains both a ketone C=O and a carboxylic acid group; acetone (CH3COCH3) is a ketone only; BHB (CH3CH(OH)CH2COOH) has an alcohol and a carboxylic acid but no ketone. AcAc vs acetone: add aqueous sodium hydrogencarbonate (or sodium carbonate). AcAc has a -COOH group and gives effervescence (bubbles of CO2); acetone has no acid group and gives no effervescence. AcAc vs BHB: add 2,4-dinitrophenylhydrazine (2,4-DNPH / Brady's reagent). AcAc has a carbonyl (ketone) group and gives an orange/yellow precipitate; BHB has no carbonyl (only -OH) and gives no precipitate. (NaHCO3 cannot separate this pair as both have -COOH.)
Example 2 (2 marks)
Propose a straightforward chemical test that would confirm that acetophenone is present.
Show the worked answer
Acetophenone, C6H5COCH3, is a methyl ketone (contains the CH3CO- group), so it gives a positive tri-iodomethane (iodoform) test. Warm the sample with aqueous iodine and sodium hydroxide (I2 / NaOH(aq)). A pale yellow precipitate of tri-iodomethane, CHI3 (with its characteristic antiseptic smell), is formed, confirming the CH3CO- group. (It gives a negative Tollens'/Fehling's test, confirming it is a ketone not an aldehyde.)
Example 3 (2 marks)
Referring to the mechanism, explain why only ketones possessing an alpha methyl group undergo the iodoform reaction, whereas acids such as ethanoic acid and esters such as CH3COOCH3, which also carry an alpha methyl group, do not react in the same way.
Show the worked answer
The iodoform reaction proceeds by base removing an alpha hydrogen to form a carbanion/enolate, which is then halogenated; this repeats until the CH3 group is fully triiodinated (CI3), after which OH- attacks the carbonyl carbon and the CI3- ion leaves, giving CHI3. This requires (i) alpha hydrogens on a CH3 group next to a C=O so the group can be triiodinated, and (ii) a carbonyl carbon that, once bearing CI3, is electrophilic enough to be attacked by OH- with CI3- expelled as a leaving group. In a methyl ketone, CH3-CO-R, the carbonyl carbon fits both requirements. Ethanoic acid (CH3COOH) is deprotonated by the base to give the carboxylate ion CH3COO-, whose negative charge greatly reduces the electrophilicity of the carbon and deactivates the alpha C-H, so it is not iodinated/attacked. In the ester CH3COOCH3, the carbonyl carbon already bears an -OCH3 group that donates electron density (and OCH3- is a poor leaving group here), so it is far less electrophilic and does not undergo the same triiodination/cleavage.
More worked questions on this topic
- Benzaldehyde, C6H5CHO, is an organic substance well known for its almond-like smell. It can rea (2 marks)
More A-Level H2 Chemistry topics
The make-up of the atom and how its electrons are arranged · How atoms bond and how that governs a substance's behaviour · Ideal gases and working with gas mixtures · Competing definitions of acids and bases · Trends in the elements across a period and down a group · The mole and reacting-quantity calculations · all of A-Level H2 Chemistry