Aldehydes, Ketones and Carboxylic Acids · Lesson 15 of 15
Chapter Summary and Practice
“Connect carbonyl structure, addition, enolate reactions, acid chemistry and synthetic reasoning across the chapter.”
• Choose a reaction family from functional-group connectivity. • Connect aldehydes, ketones, acids and derivatives in a reaction map. • Explain selectivity through polarity, resonance and conditions. • Solve mixed synthesis, identification and quantitative reasoning problems.
Three questions organise carbonyl chemistry
First ask what is directly bonded to the carbonyl carbon. An aldehyde has H, a ketone has two carbon groups, and an acid or derivative has an O-, N- or halogen-containing substituent. Next ask whether there is an enolisable α-H. Finally ask which other functional groups must survive the proposed conditions. These questions distinguish addition at C=O, enolate chemistry beside C=O, proton transfer at COOH and substitution at an acyl carbon.
Carbonyl polarity explains a shared first step: electron-rich reagents attack electron-poor carbon and open the π bond. The next event determines the family of outcome. Protonation gives an addition product; loss of water after nitrogen attack gives a C=N derivative; loss of an activated heteroatom group gives acyl substitution. A mechanism is therefore more informative than a list of reagent names because it explains which atoms stay and which leave.
| Site or structural feature | Characteristic chemistry | Necessary distinction |
|---|---|---|
| C=O of aldehyde/ketone | Nucleophilic addition | Steric and electronic effects alter rate and equilibrium |
| Carbonyl oxygen | Protonation and H-bond acceptance | A lone pair does not make the molecule an O–H donor |
| α-C–H | Enolate formation and aldol reaction | The aldehydic H is not α-H |
| Aldehyde without α-H | Cannizzaro with concentrated alkali | No α-H alone is insufficient for ketones |
| COOH O–H | Acid–base reaction | pKa differs from solution pH |
| Acyl carbon of activated derivative | Addition–elimination | Leaving-group ability and activation matter |
| Aryl ring bearing CHO, COR or COOH | Usually meta-directed electrophilic substitution | Direct attachment is essential to the simple pattern |
A connected preparation and reaction map
Additional carbon-building paths complete the map. RX → RCN → RCOOH and RMgX + CO₂ → RCOOH add one carbon relative to R. Grignard addition to an aldehyde or ketone adds the reagent carbon group and gives an alcohol after work-up. Aldol joins the α-carbon of a donor to the carbonyl carbon of an acceptor. Soda lime instead removes the acid carbon; ideal Kolbe electrolysis removes two acid carbons and couples two remaining fragments. Counting before and after is an immediate check on a proposed route.
Reaction summaries must retain conditions
| Transformation | Typical conditions | What to check |
|---|---|---|
| Alcohol → aldehyde | Controlled oxidant such as PCC | Prevent acid overoxidation |
| Nitrile → aldehyde | Stephen or controlled DIBAL-H; hydrolysis | Partial, not complete, reduction |
| Arene → aryl ketone | Acyl chloride/anhydrous AlCl₃; work-up | Avoid strongly deactivated arene |
| Carbonyl → cyanohydrin | HCN with suitable cyanide catalysis | OH and CN on one carbon |
| Carbonyl → acetal/ketal | Alcohol or diol; dry acid; water removal | Reverse with aqueous acid |
| Carbonyl → N derivative | H₂N–Z; suitable mild acid | Product C=N–Z plus water |
| Carbonyl → alcohol | NaBH₄ or LiAlH₄; work-up | Reagent selectivity differs |
| Carbonyl → hydrocarbon | Zn(Hg)/HCl or NH₂NH₂/base/heat | Acid/base compatibility |
| Methyl ketone → carboxylate + CHI₃ | I₂/OH⁻ | One carbon goes into iodoform |
| Enolisable carbonyl → aldol | Dilute base; dehydration as appropriate | Donor and acceptor roles |
| Non-enolisable aldehyde → alcohol + carboxylate | Concentrated alkali | Disproportionation |
| Acid → α-halo acid | Cl₂ or Br₂/red P; hydrolysis | An α-H must be present |
Acidity and physical properties share structure but answer different questions
Carbonyl dipoles increase intermolecular attraction, and carbonyl oxygens accept hydrogen bonds from water. Acids add an O–H donor and can associate strongly. Increasing hydrocarbon size usually lowers water compatibility. These interactions explain physical trends; acid strength instead compares the relative stability and solvation of acid and conjugate base during proton transfer. Both depend on structure, but high boiling point is not a numerical measure of Ka.
Carboxylate distributes negative charge over two equivalent oxygen-centred contributors. Electron withdrawal generally strengthens the acid; donation generally weakens it. Distance weakens inductive effects, and aromatic position changes resonance contributions. The number of resonance drawings alone is not a stability calculation. Neutral acid resonance also must not be confused with equivalent carboxylate contributors.
An integrated example links preparation, test and reduction
Problem
Prepare acetophenone from benzene, identify it and convert it to ethylbenzene.
- 1.Friedel–Crafts acylation with CH₃COCl/AlCl₃ followed by work-up gives PhCOCH₃. The added two-carbon acyl fragment includes the carbonyl carbon.
- 2.A 2,4-DNP derivative supports an aldehyde/ketone; a negative ordinary Tollens’ result favours a ketone; a positive iodoform test supports CH₃CO. Haloform cleavage gives benzoate and CHI₃.
- 3.For ethylbenzene choose an appropriate carbonyl deoxygenation such as Clemmensen or Wolff–Kishner conditions. NaBH₄ alone instead gives 1-phenylethanol, retaining oxygen.
- 4.At each stage the aromatic ring and its six carbons remain intact. This one example connects a C–C bond-forming preparation, functional-group tests and choice of reduction endpoint.
Common errors can now be diagnosed structurally: putting OH at α instead of β in an aldol, losing the CN carbon during hydrolysis, calling every ketone iodoform-positive, using Grignard reagent in water, treating pKa as pH, or aromatising a saturated ring without a suitable reaction. Repair them by redrawing the reaction centre, accounting for electrons and atoms, and checking the whole reagent sequence before naming the product.
Quick Quiz
Quiz
Which question most directly distinguishes aldol eligibility from Cannizzaro in ordinary aldehydes?
Explanation: Aldol donation requires α-H; typical Cannizzaro substrates lack it.
Which route adds one carbon to R before giving RCOOH?
Explanation: CO₂ supplies the acid carbon.
Which conversion is nucleophilic acyl substitution?
Explanation: Esterification adds at acyl carbon then eliminates water, replacing an acyl OH group by OR.
Which reagent distinguishes pentan-2-one from pentan-3-one?
Explanation: Only pentan-2-one has the methyl-ketone pattern.
What does a one-unit lower pKa mean under comparable conditions?
Explanation: The negative base-10 logarithm reverses the strength order.
Practice Problems
- Name CH₃CH=CHCHO, CH₃COCH₂COCH₃ and HOOC(CH₂)₄COOH.
- Prepare ethanal from ethanol and from ethanenitrile.
- Predict products of propanone with HCN, hydroxylamine and ethylene glycol.
- Explain why propanal is normally more reactive than propanone in nucleophilic addition.
- Construct the ethanal aldol product, its dehydrated form and its fully reduced diol.
- List donor–acceptor roles for the two crossed ethanal/propanal condensation products.
- Balance benzaldehyde Cannizzaro reaction and identify both oxidation levels.
- Distinguish benzaldehyde, acetophenone and benzophenone with a short test sequence.
- Prepare phenylacetic acid from benzyl bromide and explain the extra carbon.
- Rank CH₃COOH, CH₂ClCOOH and CHCl₂COOH, and compare pKa 4.76 with 3.76.
- Explain the esterification mechanism and why water removal helps.
- Compare sodium propanoate with soda lime and with ideal Kolbe electrolysis.
- Design benzoic acid → benzaldehyde and benzoic acid → benzyl alcohol.
- Find the molecular formula from 69.77%C, 11.63%H, remaining O and molar mass 86, then use a positive iodoform test.
- Choose a route to 4-nitrobenzoic acid and explain why reversing nitration and oxidation changes the main isomer.
- Explain why a nitroaryl Grignard route and an unactivated phenol Fischer route need reconsideration.
Number from the principal group. The names are but-2-enal, pentane-2,4-dione and hexanedioic acid. The acid has six carbons including both COOH carbons; four intervening CH₂ groups are not the total chain count.
Key Takeaways
• Functional-group connectivity determines the carbonyl reaction family. • Nucleophilic addition, acyl substitution and enolate reactions have different bond changes. • Conditions control oxidation and reduction endpoints. • Aldol addition gives β-hydroxy products; condensation includes dehydration. • Cannizzaro gives alcohol and carboxylate without C–C coupling. • Acidity follows conjugate-base stability and compatible pKa comparisons. • Carbon accounting detects errors in synthesis and structure identification. • Every final product must preserve groups untouched by the stated reaction.
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Planning Syntheses and Determining Structures
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