CLASS 12 CHEMISTRY · CHAPTER 8 · STUDY NOTES
Aldehydes, Ketones and Carboxylic Acids
A comprehensive study of carbonyl and carboxyl compounds, covering their nomenclature, structure, preparation methods, physical properties, and chemical reactivity, including key named reactions like Aldol condensation and Cannizzaro reaction.
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Structure and Nature of the Carbonyl Group
Connect carbonyl geometry with polarity and molecular attractions.Why a carbonyl carbon attracts nucleophiles
- Carbonyl group
- A carbon–oxygen double-bond group whose carbon and three attached atoms are approximately coplanar.
Carbonyl carbon is sp² with three sigma-bond directions about 120° apart and pi density above/below the plane. Aldehyde has at least one H directly on carbonyl carbon, including two in methanal; ketone has two carbon neighbours. Oxygen also has two lone-pair regions.
The actual bond is polar: Cδ⁺=Oδ⁻. Neutral and C⁺–O⁻ drawings are resonance contributors rather than two molecules in equilibrium. Carbon attracts nucleophiles; oxygen can accept protonation. The source matched boiling-point comparison is butane 273 K, methoxyethane 281 K, propanal 322 K, acetone 329 K and propan-1-ol 370 K.
Let’s look a little closer.
Go deeper: Intermolecular attraction is different from bond polarity
The small carbonyl compounds can accept hydrogen bonds from water but lack an O–H donor for the alcohol-like self-network. Increasing hydrocarbon extent lowers water solubility. The measured boiling chart compares masses 58–60; it is not a ranking of all substances in five classes.
Pi-lobe pictures indicate electron-density regions, not tracks followed by electrons. Formal charges in a contributor are bookkeeping; partial charges describe the actual polar bond. Fragrance, solvent and polymer applications of carbonyls do not establish that an unfamiliar compound is edible or safe.
Take a moment. What do you think?
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Question
A sketch labels the carbonyl carbon δ⁻ and oxygen δ⁺. What should be repaired, and where does a nucleophile approach?
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Nomenclature of Aldehydes and Ketones
Reconstruct the chain and find alpha-H rather than guessing from a name.Read the name as a map of bonds
- Alpha carbon
- A carbon directly adjacent to the carbonyl carbon.
| Attachment | Naming idea |
|---|---|
| Terminal CHO in a chain | Count its carbon as position one; suffix -al |
| Internal chain C=O | Give the carbonyl its appropriate low locant; suffix -one |
| CHO outside a ring | Use a suitable carbaldehyde name |
| C=O carbon within a saturated ring | Cyclic ketone carbonyl position one |
Open-chain aldehyde names use -al, counting carbonyl carbon as position one. Ketones use -one with appropriate low carbonyl locants. Unsaturation and substituents require their own positions; cyclic ketone carbonyl carbon is position one. Common acetone is propanone.
A CHO group outside a ring uses carbaldehyde naming, while an internal ring C=O gives a cyclic ketone. Benzaldehyde is an accepted name for benzenecarbaldehyde. Alpha-H belongs to a neighbouring carbon, not the H attached directly to an aldehyde carbonyl.
Go deeper: Ring and side-chain reactions must remain separate
Aromatic aldehydes/ketones undergo ring electrophilic substitution with the carbonyl-containing substituent generally deactivating and meta-directing. This concerns ring attack, not carbonyl nucleophilic addition. Their conditions and bond targets differ.
An aldehydic H is not an alpha-H. Benzaldehyde has no ordinary alpha-carbon hydrogen for aldol enolate formation, even though ring hydrogens are present. Locate the neighbouring atom and count its H before using an alpha-H rule.
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Question
For CH₃CH₂COCH₂CH₃, which carbons are alpha and what is the systematic name?
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Preparation of Aldehydes
Choose a route that stops at aldehyde and preserves the intended carbons.Stop at the aldehyde without losing carbon
- Selective reduction
- A reduction controlled to obtain a chosen product rather than continuing through every reducible group.
Controlled primary-alcohol oxidation gives aldehyde; stronger continued oxidation gives acid. Catalytic alcohol dehydrogenation gives carbonyl plus H₂. Reductive ozonolysis cleaves an alkene into carbonyl fragments according to the substituents of each alkene carbon.
Rosenmund reduction uses Pd/BaSO₄ for acyl chloride to aldehyde. Stephen nitrile reduction gives an imine before hydrolysis; suitable DIBAL-H conditions can give aldehydes from nitriles or esters after work-up. Reagents and conditions distinguish these from complete reduction.
Go deeper: Carbon ownership explains aromatic routes
Etard oxidation, protected toluene oxidation and benzal-chloride hydrolysis retain the side-chain carbon as CHO. Gatterman–Koch formylation instead introduces carbon from CO into an aromatic framework. Count the new carbon rather than drawing OH or CH₃ replacement without it.
An alkene cleavage produces two carbonyl centres, not one carbonyl plus an unexplained lost fragment. Work-up controls whether aldehydes remain or oxidise further. Nitrile carbon becomes aldehyde carbon, while nitrogen leaves through the work-up; it does not simply transform into oxygen.
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Question
A six-carbon symmetrical alkene gives two identical three-carbon aldehydes under reductive ozonolysis. What carbon-accounting constraint must any proposed drawing satisfy?
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Preparation of Ketones
Build the two carbon attachments of a ketone through a suitable route.Build two carbon neighbours around C=O
- Acylation
- Introduction of an acyl group containing a carbonyl carbon into a molecule.
Secondary-alcohol oxidation or dehydrogenation retains the two carbon neighbours around the new ketone carbonyl. Appropriate alkyne hydration gives an enol that tautomerises; unsymmetrical internal alkynes may produce more than one ketone rather than one universally selective product.
Dialkylcadmium with acyl chloride gives ketone in the chapter account. Nitrile plus Grignard gives an imine salt followed by hydrolysis to ketone. Friedel–Crafts acylation places an acyl group on a suitable arene using Lewis-acid conditions.
Go deeper: Addition and hydrolysis have different atom sources
In nitrile/Grignard chemistry, the nitrile carbon becomes carbonyl carbon and the reagent’s organic group becomes its new carbon neighbour. Work-up supplies oxygen and releases nitrogen-containing material. The diagram must not imply nitrogen has been converted into oxygen.
Friedel–Crafts replaces a ring H with an acyl group while restoring aromaticity. Strongly deactivated rings do not behave like benzene, and Lewis-acid binding can interfere. Aromatic ketone formation is different from methyl addition directly to carbonyl oxygen.
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Question
An organomagnesium methyl group reacts with a nitrile carrying an ethyl group, then hydrolysis. Which groups flank the carbonyl in the ketone?
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Mechanism of Nucleophilic Addition
Separate simple addition from addition followed by elimination.Watch a planar carbon become tetrahedral
- Nucleophilic addition
- Bond formation at electrophilic carbonyl carbon accompanied by conversion of the pi bond toward an oxygen lone pair.
Nucleophilic attack changes approximately planar sp² carbon into a tetrahedral sp³ centre. Alkoxide protonation then gives the neutral addition product. Comparable aldehydes usually react more readily than ketones because of less steric hindrance and fewer donating alkyl groups; conjugation and substituents also matter.
CN⁻ adds through carbon to give a cyanohydrin after work-up. Hydrogensulphite gives a reversible addition compound useful for purification. Grignard contributes a carbon group. These reagents do not all make the same product or the same number of carbon atoms.
Go deeper: An OH/OR intermediate differs from a C=N product
Alcohol addition gives hemiacetal, followed under suitable acid conditions by acetal with two OR groups; aqueous acid can reverse it. Ketones and a diol can form cyclic ketals. Water removal/dry conditions favour formation, while water with acid favours hydrolysis.
H₂N–Z derivatives form C=N–Z after addition and dehydration: oxime, hydrazone, phenylhydrazone or semicarbazone depend on Z. The available amino nitrogen attacks; an amide-like nitrogen is resonance-deactivated. Too much acid protonates nucleophile and reduces its availability. 2,4-DNP derivatives are coloured solids useful in characterisation, not a unique compound identifier.
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Question
A product retains OH and has one OR on the former aldehyde carbon. Is it already an acetal?
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Oxidation and Reduction of Carbonyls
Use oxidation and reduction to identify the exact bond changes.Different reagents change different bonds
- Haloform reaction
- Cleavage of a suitable methyl-carbonyl structure to carboxylate and a trihalomethane under appropriate oxidising halogen/base conditions.
| Selected change | Product or observation | Condition distinction |
|---|---|---|
| Carbonyl reduction | Alcohol | NaBH₄/LiAlH₄ or suitable hydrogenation |
| Net deoxygenation | Hydrocarbon | Clemmensen acidic; Wolff–Kishner basic/hot |
| Aldehyde mild oxidation | Carboxylate | Alkaline Tollens/appropriate Fehling |
| Methyl-carbonyl haloform | Carboxylate + trihalomethane | Suitable halogen/base oxidation |
NaBH₄, LiAlH₄ or catalytic hydrogenation can reduce aldehyde to primary alcohol and ketone to secondary alcohol. Clemmensen and Wolff–Kishner instead remove carbonyl oxygen in the net hydrocarbon: their acidic versus basic/hot conditions differ.
Tollens oxidises aldehyde to carboxylate in alkaline solution with a silver mirror. Fehling gives Cu₂O with suitable aliphatic aldehydes; ordinary aromatic aldehydes do not respond in this chapter comparison. Ordinary ketones resist these mild oxidations, while vigorous oxidation can cleave their carbon skeleton.
Go deeper: Test observations require chemical context
Methyl ketones give haloform and carboxylate with one fewer carbon. In the iodine pathway CHI₃ is a yellow solid. Ethanal and suitable alcohols that oxidise to a methyl-carbonyl structure can also respond, so “positive means ketone” is incomplete.
A silver mirror and a coloured precipitate are qualitative endpoints, not calibrated measures of reaction rate. Multiple functional groups can complicate a compound’s response. Pair an observation with structure and conditions rather than treating one visual cue as a uniquely identifying barcode.
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Question
Butan-2-one gives a three-carbon carboxylate in a haloform pathway. Where is the fourth carbon?
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Aldol and Cannizzaro Reactions
Locate alpha-H and track the new carbon bond.Locate alpha-H before selecting a pathway
- Aldol addition
- Formation of a beta-hydroxy carbonyl compound when an enolate donor bonds to another carbonyl carbon.
Alpha-H removal gives an enolate with carbon- and oxygen-charge contributors. A donor alpha carbon forms a bond to an acceptor carbonyl carbon. Protonation gives a beta-hydroxy carbonyl; subsequent water elimination gives an alpha,beta-unsaturated product.
Two enolisable partners can each act as donor or acceptor, giving two self and two cross connectivity choices. They are not automatically formed in equal amounts. Conditions and partner choice can improve selectivity. Aldehydes without alpha-H may instead undergo Cannizzaro disproportionation with concentrated alkali.
Go deeper: Addition, condensation and disproportionation are different
The page-29 summary calls aldol products alpha-hydroxy, but the main explanation and structures on page 15 establish beta-hydroxy. These notes use beta consistently. Condensation specifically includes subsequent water loss; an initial aldol adduct has not yet eliminated it.
Cannizzaro uses two aldehydes without alpha-H: one becomes alcohol and the other carboxylate. Carbon oxidation number changes from +1 to −1 and +1 to +3, balancing reduction and oxidation. Hydride transfer is part of its mechanism, not a new donor–acceptor carbon bond as in aldol.
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Question
For a two-partner cross-aldol map, A has alpha-H but B lacks it. Which donor label must be removed?
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Carboxylic Acids: Nomenclature and Structure
Count the carboxyl carbon and distinguish acid from its conjugate base.One carboxyl group connects several ideas
- Carboxyl group
- A carbonyl carbon also bonded to hydroxyl oxygen, represented by COOH.
Carboxyl carbon is position one in an open-chain acid name and belongs in the parent carbon count. Methanoic, ethanoic and butanoic acid respectively have one, two and four carbons. Ring/multiple-carboxyl attachments need suitable carboxylic/dicarboxylic names with locants.
Carboxyl carbon and its attachments are approximately coplanar. Oxygen donation contributes to its electronic structure, making simple aldehyde-like addition an incomplete account of acid reactions. Deprotonation retains the skeleton but changes COOH into negatively charged carboxylate.
Go deeper: Dimers and water contacts explain physical properties
Carboxylic acids can associate through two O–H···O hydrogen bonds, particularly in vapour or aprotic solvent. Each molecule donates to the other’s carbonyl oxygen; the contacts do not create a covalent ring. This association helps explain high boiling points compared with similar-mass aldehydes/ketones.
Small acids mix well with water, while increasing hydrocarbon extent reduces solubility. Higher members have lower volatility and may be waxy solids; benzoic acid is poorly soluble in cold water. Physical state and solubility are not determined by the mere presence of a polar COOH group.
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Question
An acid dimer drawing shows two new solid O–O bonds. How should it be corrected?
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Acidity and Substituent Effects
Compare stabilisation and a logarithmic acid-strength measure.Compare conjugate-base stabilisation
- Carboxylate resonance
- Equivalent oxygen-charge contributors that describe charge delocalisation in a carboxylate ion.
Carboxylate has two equivalent oxygen contributors, with negative charge delocalised over electronegative oxygens. Phenoxide has non-equivalent contributors with some charge on carbon. The quality and equivalence of contributors matter more than counting their number.
pKa=−log₁₀Ka; lower means stronger acidity in matched conditions. The chapter comparison gives CF₃COOH 0.23, benzoic 4.19 and ethanoic 4.76, versus phenol about 10 and ethanol about 16. CF₃COOH is strongest in this set; it is not the strongest possible carboxylic acid.
Go deeper: Substituent distance and conjugation both matter
Withdrawing substituents stabilise the conjugate base and tend to strengthen the acid, while donating ones can weaken it. Inductive influence usually diminishes with distance. Aromatic position also controls available resonance pathways, so one substituent ranking is not universal across structures.
The source para-nitrobenzoic acid pKa 3.41, benzoic 4.19 and para-methoxy 4.46 form a useful matched comparison. The nitro/benzoic Ka ratio is 10^0.78≈6.03. The numbers do not directly give percentage ionisation without concentration and other equilibrium information.
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Question
Using pKa 4.19 and 4.46, how does benzoic acid Ka compare with the para-methoxy derivative?
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Chemical Reactions of Carboxylic Acids
Choose the route and account for the carboxyl carbon, oxygen and coproducts.Track the carboxyl carbon through each route
- Decarboxylation
- Removal of the carboxyl carbon from an organic skeleton through a specified reaction pathway.
Oxidation of primary alcohol or aldehyde retains the carbon chain. Nitrile hydrolysis goes through an amide and retains nitrile carbon as COOH; Grignard plus CO₂ introduces one carbon before acidification. Suitable alkylbenzene side-chain oxidation requires benzylic H and accounts for other side-chain carbons in coproducts. Ester, acyl-halide and anhydride hydrolysis provide other acid routes.
Acids react with metals to release H₂, hydroxide to make water, and carbonate/hydrogencarbonate to give CO₂. Acyl replacement gives acid chloride, ester, amide or anhydride under distinct conditions. Ammonia first gives an ammonium salt, then heating can give amide. Fischer esterification includes addition, proton transfers, water elimination and catalyst regeneration.
Go deeper: Carbon shortening differs from radical coupling
Soda-lime treatment of a sodium carboxylate gives a hydrocarbon with one fewer carbon, placing the removed carbon in carbonate. Kolbe electrolysis instead releases carboxyl carbons as CO₂ at the anode and couples two alkyl fragments. An anode sketch omits counterions and cathode products; it is not a full cell equation.
LiAlH₄/borane reduces acid to primary alcohol; ordinary NaBH₄ does not generally do this. HVZ alpha-halogenation needs alpha-H and suitable halogen/red-P conditions. Aromatic COOH deactivates and directs meta ring substitution. Fatty-acid salts provide a soap head/tail structure; adipic acid contributes to nylon manufacture. These uses do not establish present regulatory approval or food suitability.
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Question
A sodium propanoate carbon skeleton is shortened by soda lime. Identify the hydrocarbon carbon count and where the removed carbon appears.
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