Aldehydes, Ketones and Carboxylic Acids

The One Thing

Aldehydes and ketones undergo nucleophilic addition because the polar carbonyl group makes the carbonyl carbon electrophilic. Carboxylic acid strength depends primarily on the resonance stabilisation of the carboxylate ion, with inductive effects modifying that stability.

Reactions, Processes and Experiments

Key Terms

• Carbonyl group: A functional group containing a carbon atom double-bonded to oxygen, written as C=O.
• Aldehyde: An organic compound containing the –CHO group; its general formula is R–CHO.
• Ketone: An organic compound in which the carbonyl carbon is bonded to two carbon groups; its general formula is R–CO–R′.
• Nucleophile: An electron-rich species that donates an electron pair to an electron-deficient atom.
• Electrophile: An electron-deficient species that accepts an electron pair; the carbonyl carbon behaves as an electrophile.
• Nucleophilic addition: A reaction in which a nucleophile attacks the carbonyl carbon, followed by protonation of the oxygen-containing intermediate.
• Cyanohydrin formation: Reaction of an aldehyde or ketone with hydrogen cyanide to form a cyanohydrin.
• Oxime formation: Reaction of an aldehyde or ketone with hydroxylamine to form an oxime.
• Hydrazone formation: Reaction of an aldehyde or ketone with hydrazine to form a hydrazone.
• 2,4-DNP derivative: A coloured derivative formed when an aldehyde or ketone reacts with 2,4-dinitrophenylhydrazine; it is used for identification.
• Acetal and ketal formation: Formation of acetals from aldehydes and ketals from ketones with alcohols in the presence of acid catalysts.
• Cannizzaro reaction: Self-oxidation and self-reduction of an aldehyde without an alpha hydrogen in concentrated alkali, producing an alcohol and a carboxylate salt.
• Aldol condensation: Base-catalysed condensation of an aldehyde or ketone having an alpha hydrogen to form a beta-hydroxy carbonyl compound, which may dehydrate.
• Alpha hydrogen: A hydrogen atom attached to the carbon next to a carbonyl carbon.
• Carboxylic acid: An organic compound containing the –COOH group; its general formula is R–COOH.
• Carboxylate ion: The conjugate base formed when a carboxylic acid loses a proton, written as R–COO−.
• Resonance stabilisation: Spreading of negative charge over both oxygen atoms of a carboxylate ion, increasing its stability and the acid strength of the parent carboxylic acid.
• Inductive effect: Displacement of sigma-bond electrons caused by electronegative or electron-releasing groups.
• Decarboxylation: Removal of carbon dioxide from a carboxylic acid or its salt, commonly using soda lime for sodium salts.
• Carbonyl polarity: The oxygen atom in C=O carries partial negative charge (δ−), while the carbon atom carries partial positive charge (δ+).
• Steric hindrance: Restricted approach to the carbonyl carbon caused by groups surrounding it.
• Electron-withdrawing group: A group that draws electron density away; it increases carbonyl reactivity and carboxylic acid strength.
• Electron-releasing group: A group that donates electron density; alkyl groups decrease carbonyl reactivity and carboxylic acid strength.
• Enolate: A species formed when removal of an alpha hydrogen gives a resonance-stabilised anion adjacent to a carbonyl group.
• Rosenmund reduction: Catalytic hydrogenation of an acid chloride to form an aldehyde.
• Ozonolysis: Cleavage of an alkene by ozone followed by work-up to produce aldehydes and/or ketones.
• Friedel–Crafts acylation: Acylation of an aromatic compound using an acyl chloride and a Lewis acid catalyst.
• pKa: A measure of acid strength; a stronger acid has a lower pKa. Typical simple carboxylic acids have pKa values near 4 to 5.

Easily Confused

• Aldehydes and ketones: Aldehydes contain –CHO and are generally more reactive toward nucleophilic addition; ketones contain R–CO–R′ and have greater steric hindrance.
• Aldol condensation and Cannizzaro reaction: Aldol condensation requires an alpha hydrogen, whereas Cannizzaro reaction occurs in aldehydes without an alpha hydrogen.
• Aldehyde oxidation tests and ketone reactions: Aldehydes give a silver mirror with Tollens reagent and red copper(I) oxide with Fehling solution; most ketones do not.
• Acetals and ketals: Aldehydes form acetals, whereas ketones form ketals.
• Carboxylic acids and alcohols: Carboxylic acids are stronger acids because their carboxylate conjugate bases are resonance-stabilised.
• Resonance and inductive effects: Resonance spreads charge over both oxygen atoms of the carboxylate ion; the inductive effect operates through sigma bonds and decreases with distance.
• Controlled oxidation and further oxidation: Primary alcohols can be controlled to form aldehydes, whereas further oxidation may produce carboxylic acids; secondary alcohols form ketones, and further oxidation generally requires carbon–carbon bond cleavage.
• Methyl ketone formation and ethanal formation from alkynes: Terminal alkynes generally form methyl ketones on hydration, but ethyne forms ethanal.

What Gets Asked

• Explain nucleophilic addition to a carbonyl group. The answer must identify the partial positive charge on carbon, the attack by a nucleophile, formation of R2C(O−)Nu, and subsequent protonation.
• Compare aldehyde and ketone reactivity. Marks depend on stating both reasons: aldehydes have less steric hindrance and fewer electron-releasing alkyl groups.
• Predict whether aldol condensation or Cannizzaro reaction occurs. The decisive feature is the presence or absence of an alpha hydrogen.
• Select a preparation method for an aldehyde or ketone. The starting functional group must be matched to the method, while avoiding over-oxidation or carbon–carbon bond cleavage.
• Distinguish aldehydes from ketones experimentally. An aldehyde gives a silver mirror with Tollens reagent and red copper(I) oxide with Fehling solution; most ketones do not.
• Explain and compare carboxylic acid strengths. The answer must refer to resonance stabilisation of the carboxylate ion, the effects of groups such as –NO2, –Cl and –F, the destabilising effect of electron-releasing alkyl groups, and the reduced inductive effect with increasing distance.