Aldehydes, Ketones and Carboxylic Acids

🎯 Key Points

• Aldehydes have ≥1 H on carbonyl carbon; ketones have 2 alkyl/aryl groups - aldehydes are generally more reactive to nucleophilic addition (less steric hindrance, more electrophilic carbon)
• Tollens'/Fehling's test: positive ONLY for aldehydes (silver mirror / brick-red precipitate respectively)
• Iodoform test: positive for any CH₃CO- group (methyl ketones) or ethanol/acetaldehyde-type structures, not just aldehydes generally
• Cannizzaro reaction needs NO alpha-H (non-enolisable aldehyde) - self redox: one molecule oxidised to acid, another reduced to alcohol
• Carboxylic acids are stronger acids than phenols/alcohols due to greater resonance stabilisation of the carboxylate ion (charge spread over 2 oxygens equally)
• Clemmensen (Zn(Hg)/HCl) and Wolff-Kishner (N₂H₄/KOH) both reduce C=O fully to CH₂, useful when other reducible groups must survive each respective acidic/basic condition

Aldehydes (R-CHO) and Ketones (R-CO-R')

Contain the carbonyl group (C=O). Aldehydes have at least one H on the carbonyl carbon; ketones have two alkyl or aryl groups.

Distinguishing Tests

• Tollens' test (Ag mirror): positive for aldehydes only
• Fehling's test (brick-red ppt): positive for aldehydes only (not aromatic)
• Iodoform test (yellow ppt with I₂/NaOH): positive for CH₃CO- group (methyl ketones and acetaldehyde)

The carbonyl carbon of an aldehyde or ketone is electron-poor (δ+) because oxygen draws the π electrons toward itself. A nucleophile attacks this carbon, the C=O π bond breaks onto oxygen, and a tetrahedral alkoxide intermediate forms — the first step of nucleophilic addition. Image: Unknown author, CC BY 4.0, via Wikimedia Commons.

Carboxylic Acids (R-COOH)

• Stronger acids than alcohols and phenols due to resonance in carboxylate ion (RCOO⁻)
• Reactions: esterification, amide formation, reduction (LiAlH₄ to 1° alcohol)
• Derivatives: acid chlorides (RCOCl), acid anhydrides, esters (RCOOR'), amides (RCONH₂)
• Decarboxylation: RCOOH → RH + CO₂ (heating with soda lime)

Nomenclature

• Aldehydes: replace -e of alkane with -al (methanal/HCHO, ethanal/CH₃CHO); -CHO carbon is always C-1
• Ketones: replace -e with -one, number to give the C=O the lowest locant (propanone, pentan-2-one)
• Carboxylic acids: replace -e with -oic acid (methanoic/formic, ethanoic/acetic acid); -COOH carbon is C-1
• Common names retained: acetaldehyde, acetone, benzaldehyde, acetophenone, formic/acetic/benzoic acid

Preparation of Aldehydes and Ketones

• Rosenmund reduction: acyl chloride + H₂ over Pd/BaSO₄ → aldehyde (BaSO₄ poisons the catalyst to stop over-reduction to alcohol)
• Stephen reaction: nitrile + SnCl₂/HCl then H₃O⁺ → aldehyde (via imine); alternatively DIBAL-H reduces nitriles/esters to aldehydes
• Gattermann-Koch reaction: benzene + CO + HCl (anhyd. AlCl₃/CuCl) → benzaldehyde
• Etard reaction: toluene + CrO₂Cl₂ → benzaldehyde (via chromium complex, then hydrolysis)
• Ozonolysis of alkenes and hydration of alkynes (Markovnikov, dil. H₂SO₄/HgSO₄ → ketone; ethyne → acetaldehyde)
• Ketones from acyl chlorides with dialkylcadmium (R₂Cd), or aromatic ketones by Friedel-Crafts acylation

Aldol and Cross-Aldol Condensation

• Aldol: aldehydes/ketones with α-hydrogen self-condense in dilute base to a β-hydroxy carbonyl (aldol), which on heating dehydrates to an α,β-unsaturated carbonyl
• Cross-aldol: between two different carbonyl compounds both bearing α-H gives a mixture of four products (of limited use)
• A useful cross-aldol uses one partner with NO α-H (e.g. benzaldehyde, HCHO) so it can only act as the electrophile, giving a clean product
• Contrast with Cannizzaro: only for aldehydes with NO α-H

Preparation and Special Reactions of Carboxylic Acids

• Preparation: oxidation of 1° alcohols/aldehydes, hydrolysis of nitriles/esters/amides, from Grignard reagent + CO₂ (dry ice) then H₃O⁺, oxidation of alkylbenzenes to benzoic acid
• Hell-Volhard-Zelinsky (HVZ) reaction: carboxylic acid with α-H + Cl₂/Br₂ in presence of red P → α-halo carboxylic acid (a key route to α-amino/α-hydroxy acids)
• Formation of derivatives: acid chlorides (SOCl₂/PCl₅), anhydrides, esters (Fischer esterification), amides
• Reduction: LiAlH₄ or B₂H₆ reduces -COOH to 1° alcohol (not reduced by NaBH₄)
• HVZ does not work on formic acid or aromatic acids like benzoic acid (no α-H)

🚀 JEE Advanced Edge

Relative reactivity of carbonyl compounds to nucleophilic addition: HCHO > other aldehydes > ketones, governed by both steric factors (smaller groups = less hindrance to nucleophile approach) and electronic factors (alkyl groups donate electron density via +I effect, reducing the carbonyl carbon's electrophilicity). Aromatic aldehydes/ketones are LESS reactive than aliphatic ones because the ring donates electron density into the carbonyl via resonance.

Acidity order of substituted acetic acids: Cl-CH₂-COOH > CH₃COOH, and trichloroacetic acid > dichloroacetic acid > chloroacetic acid > acetic acid - each additional electron-withdrawing Cl atom further stabilises the carboxylate anion via induction, making the acid progressively stronger.

Worked problem (Cannizzaro): Benzaldehyde (C₆H₅CHO, no alpha-H) is treated with concentrated NaOH. Identify the products. Approach: Since benzaldehyde has no alpha hydrogen, it cannot undergo Aldol condensation, so it undergoes Cannizzaro instead: 2 C₆H₅CHO + NaOH → C₆H₅COONa (sodium benzoate) + C₆H₅CH₂OH (benzyl alcohol) - one molecule is oxidised, the other reduced, in a 1:1 ratio.

Nucleophilic Addition Reactions and Reactivity Order

• The carbonyl carbon is electrophilic (partial positive) and is attacked by a nucleophile, which adds to give a tetrahedral alkoxide intermediate that is then protonated.
• Addition of HCN gives cyanohydrins; addition of NaHSO₃ gives crystalline bisulphite addition products useful for purification.
• Alcohols add to give hemiacetals and acetals (with ketones, ketals) in presence of dry HCl.
• Ammonia derivatives (hydroxylamine, hydrazine, phenylhydrazine, 2,4-DNP) add and then lose water to give oximes, hydrazones and phenylhydrazones.
• Reactivity toward nucleophilic addition is aldehydes > ketones due to less steric hindrance and smaller electron-donating effect (+I) in aldehydes.
• Among aldehydes, reactivity decreases as the size and number of alkyl groups increase (HCHO > CH₃CHO).

Cannizzaro Reaction

• Aldehydes that have no alpha-hydrogen undergo self oxidation-reduction (disproportionation) when treated with concentrated alkali.
• One molecule is oxidised to the carboxylate salt and the other is reduced to the alcohol.
• Example: two molecules of HCHO with concentrated NaOH give sodium formate and methanol.
• Benzaldehyde with concentrated NaOH gives sodium benzoate and benzyl alcohol.
• The reaction is used to distinguish aldehydes without alpha-hydrogen from those that undergo aldol condensation.
• A crossed Cannizzaro reaction uses two different aldehydes without alpha-hydrogen, one commonly HCHO acting as the reducing partner.

Acidity of Carboxylic Acids

• Carboxylic acids are acidic because the carboxylate ion is resonance stabilised, with the negative charge shared equally over two oxygen atoms.
• They are stronger acids than phenols and alcohols but weaker than mineral acids like HCl.
• Electron-withdrawing groups (like -Cl, -NO₂) increase acidity by stabilising the carboxylate ion; chloroacetic acid is stronger than acetic acid.
• The effect is greater when the electron-withdrawing group is closer to the -COOH group and when more such groups are present.
• Electron-donating groups (like -CH₃) decrease acidity, so acetic acid is weaker than formic acid.
• Aromatic carboxylic acids are generally slightly stronger than the corresponding aliphatic acids; ring substituents shift acidity accordingly.

Physical Properties of Aldehydes, Ketones and Carboxylic Acids

• Aldehydes and ketones are polar and have higher boiling points than comparable hydrocarbons and ethers due to dipole-dipole attractions, but lower than alcohols since they lack O-H hydrogen bonding among themselves.
• Lower aldehydes and ketones are miscible with water because their carbonyl oxygen forms hydrogen bonds with water; solubility falls as the carbon chain lengthens.
• Carboxylic acids have higher boiling points than alcohols of comparable mass due to stronger and more extensive hydrogen bonding.
• Carboxylic acids exist as hydrogen-bonded dimers in the vapour and liquid states.
• Lower carboxylic acids (up to 4 carbons) are freely miscible with water; solubility decreases with increasing chain length.
• Lower carboxylic acids have sharp irritating odours, while many higher members are waxy solids.

Alpha-Hydrogen Reactions of Carbonyl Compounds

• The alpha-hydrogen (on the carbon next to C=O) is acidic because the resulting carbanion is stabilised by resonance with the carbonyl group.
• Aldehydes and ketones with alpha-hydrogens undergo halogenation at the alpha carbon in presence of acids or bases.
• Methyl ketones and ethanal give the iodoform (haloform) reaction with I₂/NaOH, forming yellow CHI₃.
• The Hell-Volhard-Zelinsky (HVZ) reaction introduces chlorine or bromine at the alpha carbon of carboxylic acids using X₂ in presence of small amount of red phosphorus.
• Alpha-halo acids are useful intermediates for making alpha-hydroxy and alpha-amino acids.
• Acids and esters that lack alpha-hydrogen do not undergo these alpha substitution reactions.