Theory — Aldehydes & Ketones
Both aldehydes and ketones contain the carbonyl group — a carbon doubly bonded to oxygen (C=O). The carbonyl carbon is sp² hybridised, planar, and partially positive (δ+); the oxygen carries a partial negative charge (δ−). This dipole, combined with the empty π* orbital, makes the carbonyl carbon a very effective electrophile. Almost every reaction of aldehydes and ketones involves attack on this carbon by a nucleophile.
1. Naming
Aldehydes have the carbonyl on a terminal carbon (C1) with at least one H attached: R–CHO. The IUPAC suffix is -al: methanal (HCHO), ethanal (CH₃CHO), propanal, butanal, and so on. C1 of the parent chain is always the CHO carbon, so no locant is needed for the carbonyl. Aromatic aldehyde benzaldehyde (C₆H₅CHO) keeps its retained name. Other retained names: cinnamaldehyde, vanillin, citral.
Ketones have the carbonyl in the middle of the chain (between two C atoms): R–CO–R'. The IUPAC suffix is -one with a locant indicating the carbonyl carbon: propan-2-one (acetone), butan-2-one, pentan-3-one, cyclohexanone (no locant — symmetric), acetophenone (1-phenylethan-1-one). The locant is placed before the -one suffix in current IUPAC convention.
2. Structure and reactivity
The carbonyl C is sp² with three substituents in a plane (~120° bond angles); the C=O bond is shorter (1.22 Å) and stronger (~750 kJ/mol) than a typical C–C single bond. The high polarity (μ ≈ 2.7 D for acetone) makes the C electrophilic enough that even weak nucleophiles attack it readily, especially under acid catalysis (which protonates O, making C even more positive).
Two factors make aldehydes more reactive than ketones toward nucleophilic addition:
- Steric: aldehydes have only one R group (the other position is H), so the nucleophile has less hindrance approaching the C.
- Electronic: alkyl groups donate electron density to the carbonyl C (hyperconjugation), reducing its δ+ character. Two alkyl groups (ketone) reduce this more than one (aldehyde).
Order of reactivity (toward nucleophilic addition): HCHO > RCHO > R₂CO. Aromatic carbonyls (benzaldehyde, acetophenone) are less reactive than aliphatic because the aryl ring conjugates with C=O, delocalising the δ+.
3. Major reactions
(a) Reduction. NaBH₄ (mild) and LiAlH₄ (strong) both reduce aldehydes and ketones to alcohols. NaBH₄ is selective: it reduces aldehydes/ketones but leaves esters, carboxylic acids, and amides alone. LiAlH₄ reduces almost all carbonyl compounds and must be handled in dry, aprotic solvent under inert atmosphere (it reacts violently with water).
R-CO-R' + NaBH₄ / MeOH → R-CHOH-R' (2° alcohol)
Aldehyde → 1° alcohol; ketone → 2° alcohol
(b) Grignard addition. Grignard reagents (RMgX) add to carbonyls to give alcohols after aqueous workup. The carbon nucleophile (R⁻ equivalent) attacks the carbonyl C; the resulting alkoxide is then protonated. Reaction with HCHO gives 1° alcohol; with RCHO gives 2° alcohol; with R₂C=O gives 3° alcohol.
Then H₂O / H⁺ workup → R'-CHR-OH
RCHO + R'MgX → 2° alcohol after workup
(c) Cyanohydrin formation. HCN (or NaCN/H⁺) adds across the C=O to give a cyanohydrin — useful as a one-carbon-extension and as a precursor to α-hydroxy acids. Equilibrium-controlled; aldehydes give better yields than ketones.
(d) Hydration and acetal formation. Water adds reversibly to a carbonyl to give a gem-diol (the hydrate). For most aldehydes/ketones, the equilibrium favours the carbonyl. Exceptions: HCHO is >99% hydrate in water; chloral (CCl₃CHO) is fully hydrated; cyclopropanone is fully hydrated.
Two equivalents of alcohol with acid catalysis give an acetal: a key reaction because acetals are stable to base and to nucleophiles but are easily hydrolysed by aqueous acid. Acetals are widely used as protecting groups: convert C=O → acetal, do reactions elsewhere, then hydrolyse the acetal back to C=O.
Hydrolysis (reverse): R-CH(OR')₂ + H₂O / H⁺ → R-CHO + 2 R'-OH
Acetals are STABLE to base, LABILE to acid → ideal protecting group
(e) Imine formation. A primary amine (RNH₂) condenses with an aldehyde or ketone to give an imine (Schiff base, RCH=NR') with loss of water. Mechanism: nucleophilic addition of N to C=O → carbinolamine → loss of water (acid-catalysed) → imine. With secondary amines (R₂NH), the product is an enamine (R₂N-C=C-) instead of an imine, because there's no H on N to lose.
R-CO-CH₂R'' + R'₂NH ⇌ R'₂N-C(R)=CH-R'' + H₂O (ENAMINE — secondary amine)
1° amine → imine; 2° amine → enamine
(f) Wittig reaction. A phosphonium ylide (Ph₃P=CHR, prepared from Ph₃P + RCH₂X then strong base) reacts with an aldehyde or ketone to give an alkene with concurrent loss of triphenylphosphine oxide (Ph₃P=O). The position of the new C=C is precisely controlled — it forms between the original carbonyl C and the ylide C. Stabilised ylides (those with an EWG on C) tend to give E-alkenes; unstabilised ylides tend to give Z-alkenes (Schlosser variant gives E).
(g) Oxidation. Aldehydes are easily oxidised to carboxylic acids by mild oxidants (Tollens' reagent — Ag(NH₃)₂⁺/OH⁻, Fehling's — Cu²⁺/citrate/OH⁻) and stronger ones (Jones reagent CrO₃/H₂SO₄/acetone, KMnO₄). Ketones do NOT oxidise under any of these conditions — the carbonyl C has no H to lose. This is the basis of the Tollens' and Fehling's tests, which distinguish aldehydes from ketones.
4. Diagnostic tests (see Section IV)
| Test | Positive result | Detects | Notes |
|---|---|---|---|
| 2,4-DNP (Brady's) | Yellow-orange-red precipitate | Aldehydes & ketones | General carbonyl test; m.p. of derivative IDs the compound |
| Tollens' | Silver mirror on flask wall | Aldehydes ONLY | Ketones do not react |
| Fehling's | Brick-red Cu₂O precipitate | Aliphatic aldehydes | Aromatic aldehydes give weak/negative result |
| Iodoform | Yellow CHI₃ precipitate | Methyl ketones; CH₃CHO | R-CO-CH₃ pattern; also positive for RCH(OH)CH₃ |
| Schiff's | Magenta colour returns | Aldehydes only | Decolourised fuchsin; aldehyde restores colour |
5. α-Hydrogen acidity
The hydrogens on a carbon adjacent to a C=O (α-carbon) are unusually acidic. Why? Because deprotonation gives an enolate — a resonance-stabilised carbanion where the negative charge is delocalised onto oxygen. Typical pKa values:
- α-H of a simple aldehyde or ketone: pKa ≈ 17–20
- α-H of a 1,3-diketone (e.g., pentane-2,4-dione): pKa ≈ 9 — comparable to a phenol!
- For comparison: alcohol O–H ≈ 16, water = 15.7, terminal alkyne ≈ 25, alkane ≈ 50
This acidity means that bases as mild as NaOH can partially enolise α,β-functionalised carbonyls; with stronger bases (LDA, NaH), full enolisation is possible. Enolate chemistry — alkylation, aldol condensation, Michael addition — is the subject of "Carbonyl Chemistry II" and is not covered in this lab.
Instructions
This lab's Simulation section has four parts. Complete them in order.
Prerequisite: Ensure you have completed (or are familiar with) the Lab Skills & Safety lab, the Functional Group Tests lab, the Mechanisms lab, and the Alcohols lab. The carbonyl chemistry in this lab assumes you can recognise glassware, read an SDS, and predict mechanisms.
Simulation
Four interactive parts. Use the ↺ Reset Simulation button at any time to clear all answers and start over.
Eight carbonyl compounds. For each: (a) IUPAC name, (b) aldehyde or ketone, (c) hybridisation of the carbonyl carbon.
Six reactions of aldehydes & ketones. For each: read the prompt, click the reagent in the dispenser shelf to add it to the flask, then click the predicted product from the four options.
Eight conceptual problems on reactivity ordering, pKa values, and arrow-pushing. Choose the best answer for each.
Round 1 — SDS interpretation
Four key reagents used in carbonyl chemistry. Each has 4 questions.
Round 2 — Microscale diagnostic tests
Six unknown samples are presented. For each, run the indicated test and identify the functional group present based on the result.
Team Questions
Discuss with your team before answering. Type a brief response into each box.
Example Lab Notebook Entry
Use the format below as a template. Document each interaction in the simulation, then synthesise observations into a discussion.
Aldehydes & Ketones — Lab Notebook Entry
Submitted by: [Student Name]
Course: Organic Chemistry I · Section: 201-A · Date: April 25, 2026
Objective
To recognise aldehydes and ketones by structure and IUPAC name; to predict the products of common carbonyl reactions (reduction, Grignard addition, acetal formation, imine formation, oxidation, Wittig); to relate carbonyl reactivity to electronic and steric effects; and to identify unknown carbonyl compounds using diagnostic microscale tests (2,4-DNP, Tollens', Fehling's, iodoform, Schiff's, chromic acid).
Naming summary (Section I results)
| Structure shown | IUPAC name | Class | Hybridisation at C=O |
|---|---|---|---|
| CH₃CH₂CHO | Propanal | Aldehyde | sp² |
| CH₃COCH₂CH₃ | Butan-2-one | Ketone | sp² |
| C₆H₅CHO | Benzaldehyde | Aromatic aldehyde | sp² |
| C₆H₅COCH₃ | 1-Phenylethan-1-one (acetophenone) | Aromatic ketone | sp² |
| Cyclohexanone | Cyclohexanone | Cyclic ketone | sp² |
| (E)-PhCH=CHCHO | (2E)-3-Phenylprop-2-enal (cinnamaldehyde) | α,β-Unsaturated aldehyde | sp² (both C=C and C=O) |
| (E)-CH₃CH=CHCHO | (2E)-But-2-enal (crotonaldehyde) | α,β-Unsaturated aldehyde | sp² |
| HCHO | Methanal (formaldehyde) | Aldehyde | sp² |
Reaction bench observations (Section II results)
| Carbonyl | Reagent | Product (predicted) | Class of product |
|---|---|---|---|
| Propanal | NaBH₄ / MeOH | Propan-1-ol | 1° alcohol |
| Butan-2-one | CH₃MgBr, then H₂O / H⁺ | 2-Methylbutan-2-ol | 3° alcohol |
| Propanal | 2 equiv MeOH, H⁺ catalyst | Propanal dimethyl acetal | Acetal |
| Cyclohexanone | Methylamine (MeNH₂) | N-Methylcyclohexanimine | Imine |
| Butanal | Tollens' reagent (Ag(NH₃)₂⁺/OH⁻) | Butanoic acid + Ag mirror | Carboxylic acid |
| Cyclohexanone | Ph₃P=CH₂ (Wittig ylide) | Methylenecyclohexane | Alkene |
Microscale test results (Section IV, Round 2)
| Unknown | Test applied | Observation | Identified as |
|---|---|---|---|
| Sample 1 | 2,4-DNP | Yellow-orange precipitate | Aldehyde or ketone (general carbonyl) |
| Sample 2 | Tollens' | Silver mirror on flask wall | Aldehyde (specific) |
| Sample 3 | Fehling's | Brick-red precipitate | Aliphatic aldehyde |
| Sample 4 | Iodoform | Yellow CHI₃ precipitate | Methyl ketone (R-CO-CH₃) or CH₃CHO |
| Sample 5 | Schiff's | Magenta colour returns | Aldehyde |
| Sample 6 | 2,4-DNP only (Tollens' negative) | Orange precipitate; no silver mirror | Ketone |
Discussion
The simulation reinforced the centrality of the carbonyl group to organic synthesis: from a single C=O, six different functional-group transformations were possible (alcohol, larger alcohol, acetal, imine, carboxylic acid, alkene). The choice of reagent determines the outcome — reduction (NaBH₄, LiAlH₄, H₂/Pt) gives an alcohol; carbon nucleophiles (Grignard, organolithium, cyanide, Wittig) form new C–C bonds; nitrogen nucleophiles (amines, hydrazines) form C=N bonds; oxygen nucleophiles (alcohols) under acid catalysis form acetals.
The reactivity order RCHO > R₂C=O made physical sense: more alkyl substitution at the carbonyl C provides more steric hindrance for the incoming nucleophile and more electron density to the carbonyl C (reducing its δ+ character through hyperconjugation). Aromatic carbonyls were less reactive than aliphatic because the aryl ring conjugates with the C=O, delocalising the partial positive charge. This explained why benzaldehyde does not react with Fehling's solution but propanal does — the rate is too slow under the test conditions.
The diagnostic tests in Section IV were complementary, not redundant. 2,4-DNP detects all carbonyls (aldehyde or ketone); Tollens' and Fehling's distinguish aldehydes from ketones; iodoform identifies the methyl ketone substructure (or CH₃CHO); Schiff's confirms an aldehyde; chromic acid distinguishes 1° and 2° alcohols (positive) from tertiary alcohols (negative). A typical unknown identification therefore requires running multiple tests and interpreting the pattern of positives and negatives.
Section IV's SDS round emphasised that LiAlH₄ is among the most hazardous reagents commonly used in undergraduate labs — pyrophoric in moist air, violently reactive with water and protic solvents, and capable of igniting spontaneously. NaBH₄, while still a hydride reducing agent, is much milder and tolerates protic solvents. Formaldehyde (methanal) is an IARC Group 1 carcinogen with mandatory fume hood handling. Methyl vinyl ketone is highly toxic and a strong Michael acceptor. None of these reagents should be used without first reading the SDS.
Conclusion
Aldehydes and ketones share the C=O functional group but differ subtly in reactivity (RCHO > R₂CO) and dramatically in their response to oxidising agents (aldehydes oxidise to carboxylic acids; ketones do not). The combination of structure-determining information (IUPAC name, classification, hybridisation), reaction predictions (six major reactions), and diagnostic tests (six microscale tests) gives a complete framework for identifying and transforming carbonyl compounds in the laboratory.
References
1. Clayden, J.; Greeves, N.; Warren, S. Organic Chemistry, 2nd ed., Oxford University Press, 2012, Chs 6, 11, 12, 26–28.
2. McMurry, J. Organic Chemistry, 9th ed., Cengage, 2016, Chs 19–22.
3. IUPAC. Recommendations on Organic Nomenclature, 2013.
4. Sigma-Aldrich SDS for NaBH₄ (CAS 16940-66-2), LiAlH₄ (CAS 16853-85-3), formaldehyde 37% w/v aq. (CAS 50-00-0), and methyl vinyl ketone (CAS 78-94-4), accessed online March 2026.
Practice Questions
Work through each before peeking at the hint.