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Loading home…Aldehydes contain the –CHO group; the carbonyl carbon is bonded to at least one hydrogen.
Ketones contain a carbonyl group –CO– bonded to two carbon atoms.
Formaldehyde (methanal), HCHO, is the simplest aldehyde.
Acetone (propanone), CH₃COCH₃, is the simplest common ketone.
Both families contain the polar C=O group and undergo addition reactions.
Formalin is an aqueous solution of formaldehyde used as a preservative.
Tollens’ reagent is ammoniacal silver nitrate.
Aldehydes reduce it to give a silver mirror; typical ketones do not.
Fehling’s solution (Cu²⁺ complex) gives a red Cu₂O precipitate with aldehydes.
Simple ketones generally fail Fehling’s and Tollens’ tests.
Acetaldehyde gives a clear positive Tollens’ test.
Together these tests identify the easily oxidised –CHO group.
Aldehydes oxidise readily to carboxylic acids.
Example: CH₃CHO → CH₃COOH under oxidising conditions.
Ketones resist mild oxidation.
Strong oxidation of ketones can cleave the chain to mixtures of acids.
This difference is the basis of many distinguishing tests.
Primary alcohol oxidation can stop at aldehyde if carefully controlled.
Compounds with CH₃CO– (methyl ketones) give yellow iodoform with I₂/NaOH.
Acetaldehyde (CH₃CHO) also gives a positive iodoform test.
Acetone is a common methyl ketone that responds positively.
The yellow precipitate CHI₃ has a characteristic smell.
Benzophenone (no methyl ketone group) does not give iodoform.
The test links alcohols and carbonyls that can form the CH₃CO– unit.
HCN adds to the carbonyl to give a cyanohydrin.
CH₃CHO + HCN → CH₃CH(OH)CN.
Cyanohydrins are useful intermediates for hydroxy acids after hydrolysis.
Ammonia can form addition/condensation products with aldehydes.
Aldehyde–ammonia chemistry is historically important for methanal/ethanal.
These reactions illustrate nucleophilic addition to C=O.
Formaldehyde (formalin) preserves biological specimens and is used in resins.
It is a feedstock for polymers such as Bakelite-type phenolics.
Acetone is an excellent solvent for labs and industry.
It is used in nail-polish remover and in making other chemicals.
Both are bulk organic chemicals with wide applications.
Handle formaldehyde carefully because of toxicity and irritation.
Carbonyl compounds contain the C=O group.
Aldehydes have the carbonyl carbon attached to H and to R (or H).
Ketones have the carbonyl carbon attached to two carbon groups.
Carboxylic acids, esters, and amides also contain carbonyls but are separate families.
The carbonyl carbon is sp² hybridised and trigonal planar.
The C=O bond is polar: carbon is δ⁺, oxygen is δ⁻.
Nucleophiles attack the carbonyl carbon; electrophiles interact with oxygen.
Aldehydes are generally more reactive than ketones toward nucleophiles.
Steric hindrance and inductive effects of alkyl groups make ketones less reactive.
Formaldehyde is especially reactive among aldehydes.
Resonance and polarisation explain addition–elimination behaviour in acids/esters.
IR spectroscopy shows a strong C=O stretch near 1700 cm⁻¹ (advanced mention).
School classification focuses on aldehydes vs ketones first.
Write general formulae RCHO and RCOR′.
Name simple members: methanal, ethanal, propanone.
Formalin is about 37–40% aqueous methanal with methanol stabiliser often.
Acetone is miscible with water and many organic solvents.
Understanding polarity predicts solubility and boiling points vs hydrocarbons.
Exam answers should define, classify, and sketch the polar C=O.
Link structure to the characteristic addition reactions that follow.
This foundation supports tests, preparations, and synthesis conversions.
Both contain C=O but differ in what is attached to the carbonyl carbon.
Aldehydes have an oxidisable C–H on the carbonyl carbon.
Tollens’ test: aldehydes give silver mirror; ketones usually do not.
Fehling’s/Benedict’s: aliphatic aldehydes give red Cu₂O; ketones usually negative.
Schiff’s reagent restores pink/magenta colour with aldehydes.
Oxidation: aldehydes → acids; ketones need vigorous cleavage conditions.
Reduction of both can give alcohols (aldehyde → 1°; ketone → 2°).
Nucleophilic addition is faster for aldehydes than for comparable ketones.
Iodoform test is positive for methyl ketones and for acetaldehyde.
2,4-DNP gives orange precipitates with both (Brady’s test) for C=O detection.
Physical properties alone rarely distinguish them reliably.
Write observations, not only reagent names, in exam answers.
Note exceptions: some aromatic aldehydes behave differently with Fehling’s.
Always use freshly prepared Tollens’ reagent and follow safety rules.
Silver mirror tests need clean glassware.
Summarise a table: test / aldehyde result / ketone result.
Link tests to the ease of oxidation of –CHO.
Ketones’ resistance to mild oxidation is a key teaching point.
These distinctions are among the most examined organic practical ideas.
Practice with ethanal vs propanone examples.
Correct identification guides downstream synthesis choices.
Methanal is manufactured by oxidation of methanol over silver or other catalysts.
Laboratory oxidation of methanol can also yield formaldehyde under controlled conditions.
Ethanal can be prepared by oxidation of ethanol with acidified dichromate (distil product).
Hydration of ethyne with HgSO₄/H₂SO₄ historically gives ethanal.
Wacker-type oxidation of ethene is an industrial route to ethanal.
Acetone is obtained from cumene process as a co-product with phenol (industrial).
Dry distillation of calcium acetate yields acetone in classical lab demos.
(CH₃COO)₂Ca → CH₃COCH₃ + CaCO₃.
Dehydrogenation/oxidation of propan-2-ol also gives acetone.
State catalysts and conditions for each important route.
Industrial methods dominate modern supply of these bulk chemicals.
School syllabi often emphasise alcohol oxidation and calcium acetate methods.
Write balanced equations for the preparations named in your board list.
Purification may involve distillation of low-boiling carbonyls.
Formaldehyde is often stored as formalin rather than pure gas.
Acetone’s low boiling point requires careful recovery in labs.
Safety: vapours are flammable and formaldehyde is hazardous.
Link preparations to available feedstocks (methanol, ethanol, propene/cumene).
Mention that over-oxidation of ethanol yields acetic acid if aldehyde is not removed.
Exam answers should give at least one lab and one industrial flavour where possible.
Preparations set up the reaction chemistry discussed next.
Nucleophilic addition is the characteristic reaction of aldehydes and ketones.
HCN adds to give cyanohydrins.
NaHSO₃ adds to give crystalline bisulphite compounds (useful purification).
Grignard reagents add to give alcohols after hydrolysis.
Hydrazine, hydroxylamine, and phenylhydrazine form C=N condensation products.
These derivatives help characterise carbonyl compounds as solids.
Reduction with NaBH₄ or LiAlH₄ gives alcohols.
Catalytic hydrogenation also reduces C=O to CH–OH.
Oxidation converts aldehydes to acids; ketones resist mild oxidants.
Aldol condensation joins enolisable carbonyls to give β-hydroxy carbonyls.
Aldol products may dehydrate to conjugated enones on heating.
Cannizzaro reaction: non-enolisable aldehydes disproportionate in strong base.
Acetals form from aldehydes with alcohols under acid catalysis (protecting groups).
Ketones form ketals similarly under appropriate conditions.
Write mechanisms with curly arrows when required for addition.
Emphasise equilibrium aspects of bisulphite and acetal formation.
Exam answers should sample addition, redox, and condensation with equations.
Link NaBH₄ reduction of ketones to secondary alcohols in synthesis maps.
Mention that formaldehyde lacks an α-H and shows special reactions (Cannizzaro).
Condensation with ammonia derivatives is a classic practical theme.
Mastering these reactions unlocks multistep organic synthesis.
Distinguishing tests: Tollens’, Fehling’s, Schiff’s, and iodoform as appropriate.
2,4-DNP detects carbonyls generally by forming hydrazones.
Acetals are geminal diethers formed from aldehydes + alcohols (H⁺).
RCHO + 2R′OH ⇌ RCH(OR′)₂ + H₂O.
Acetals are stable to base but hydrolyse in acid — useful protecting groups.
HCN addition lengthens the carbon chain by one and introduces –CN.
Hydrolysis of cyanohydrins yields α-hydroxy acids.
NaHSO₃ addition products are often water-soluble crystalline solids.
They regenerate the carbonyl on treatment with acid or base.
Hydroxylamine forms oximes: R₂C=N–OH.
Hydrazine forms hydrazones; phenylhydrazine forms phenylhydrazones.
Melting points of these derivatives help identify unknowns.
Write equations for each named reagent.
State that HCN is toxic — school often discusses the reaction theoretically.
Bisulphite addition is richer for aldehydes than for many ketones.
Protecting-group logic foreshadows the organic synthesis chapter.
Exam answers should connect each reagent to a clear product type.
Include one sentence on why acetals protect aldehydes during other reactions.
Compare oxime/hydrazone formation as condensation with loss of water.
Practice naming simple oximes and cyanohydrins.
These tools appear repeatedly in practical and theory papers.