Aldehydes, Ketones and Carboxylic Acids — Comprehensive Study Notes Summary & Study Notes

🎯 Objectives

After studying this unit you should be able to name and write structures of aldehydes, ketones and carboxylic acids, explain major methods of preparation, describe important reactions (including mechanisms for selected reactions), relate physical properties to structure, and understand factors that affect acidity and reactivity.

🏷️ Nomenclature

There are two naming systems: common names and IUPAC names. Aldehydes: replace the ending –e of the parent alkane with -al (e.g., $HCHO$ = methanal; common name formaldehyde). Ketones: replace –e with -one (e.g., $CH_3COCH_3$ = propanone, common name acetone). For aromatic aldehydes use -carbaldehyde (e.g., benzaldehyde = benzenecarbaldehyde). Common names of ketones often name the two alkyl groups (e.g., methyl phenyl ketone = acetophenone).

🔬 Structure of the Carbonyl Group

The carbonyl group ($>C=O$) has an $sp^2$ carbon; the three atoms attached lie in one plane with ~120° angles. The bond is polarized: oxygen is nucleophilic (Lewis base), carbonyl carbon is electrophilic (Lewis acid). Resonance between neutral and dipolar forms explains the polarity and reactivity toward nucleophiles.

🧪 Major Methods of Preparation

• Oxidation of primary alcohols → aldehydes; secondary alcohols → ketones (oxidants: $CrO_3$, $KMnO_4$, PCC for mild oxidation).
• Dehydrogenation of alcohols (industrial, catalysts like Cu, Ag).
• Ozonolysis of alkenes and hydration of alkynes (gives aldehydes/ketones depending on substrate).
• Reduction of nitriles/esters with DIBAL-H to aldehydes; Rosenmund reduction (acyl chloride → aldehyde).
• Friedel–Crafts acylation (aromatic → aromatic ketone).
• For carboxylic acids: oxidation of primary alcohols/aldehydes, hydrolysis of nitriles/esters, Grignard + $CO_2$.

🌡️ Physical Properties

Carbonyl compounds are polar and have higher boiling points than hydrocarbons/ethers of similar mass due to dipole–dipole interactions, but lower than alcohols (no intermolecular O–H hydrogen bonding). Low members are water soluble (hydrogen bonding with $H_2O$); solubility decreases with chain length. Many aldehydes/ketones are fragrant and used in flavours/perfumes.

⚗️ Reactions: General Principles

Aldehydes and ketones commonly undergo nucleophilic addition to the carbonyl carbon. The nucleophile attacks perpendicular to the carbonyl plane, converting $sp^2$ carbon into $sp^3$ and giving a tetrahedral alkoxide intermediate that is protonated to give the addition product.

➕ Nucleophilic Addition: Key Examples

• HCN → cyanohydrins (useful intermediates).
• Alcohols → hemiacetals and acetals (acid-catalysed); acetals protect carbonyls.
• NH3 / derivatives → imines (Schiff bases), oximes, hydrazones, semicarbazones.
• NaHSO3 → bisulphite addition products (useful for purification).
• Grignard reagents add to give alcohols after hydrolysis.

🔁 Oxidation and Reduction

• Aldehydes are easily oxidised to carboxylic acids (e.g., Tollens’ reagent, Fehling’s reagent). Ketones resist mild oxidation and require vigorous conditions (C–C cleavage product mixture).
• Tollens test: aldehyde + Tollens' reagent → silver mirror (oxidation to carboxylate).
• Clemmensen reduction and Wolff–Kishner reduction convert carbonyl to methylene ($>CH_2$).
• NaBH_4 reduces aldehydes and ketones to alcohols; LiAlH_4 reduces esters and carboxylic acids as well.

✳️ Reactions Due to α-Hydrogen

The α-hydrogens are acidic (stabilised by resonance in the enolate). Important reactions include:

• Aldol condensation: enolate formation → attack on another carbonyl → β-hydroxy aldehyde/ketone (aldol), which may dehydrate to α,β-unsaturated carbonyl compounds.
• Cross-aldol: condensation between two different carbonyl compounds (mixtures possible unless one has no α-hydrogen).
• Halogenation at α-position (e.g., Hell–Volhard–Zelinsky for carboxylic acids).

⚖️ Special Aldehyde/Ketone Reactions

• Cannizzaro reaction: non-enolizable aldehydes (no α-H) disproportionate in concentrated base → one molecule reduced to alcohol, another oxidised to carboxylate.
• Iodoform test: methyl ketones ($CH_3CO-$) give a yellow precipitate of iodoform ($CHI_3$) with $I_2$/base.

🧾 Carboxylic Acids: Nomenclature & Structure

Carboxylic acids contain the carboxyl group ($-COOH$). IUPAC names use -oic acid (e.g., acetic acid = $CH_3COOH$ = ethanoic acid). The carboxyl carbon is $sp^2$ and resonance stabilisation of the carboxylate anion explains acidity.

⚖️ Acidity and Substituent Effects

Carboxylic acids are more acidic than alcohols and most phenols due to resonance stabilisation of the conjugate base. Acid strength is expressed by $pK_a$ (smaller $pK_a$ = stronger acid). Electron-withdrawing substituents (e.g., $-NO_2$, $-CF_3$, halogens) increase acidity by stabilising the carboxylate; electron-donating groups decrease acidity. Example $pK_a$ values: HCl ~$pK_a=-7.0$, trifluoroacetic acid ~$pK_a=0.23$, acetic acid ~$pK_a=4.76$.

🏭 Preparation of Carboxylic Acids

• Oxidation of primary alcohols/aldehydes (strong oxidants).
• Oxidation of alkylbenzenes side chains to benzoic acids (e.g., $KMnO_4$).
• Hydrolysis of nitriles/amides.
• Grignard reagent + $CO_2$ → carboxylate salt → acid on acidification.
• Hydrolysis of esters/acid chlorides/anhydrides.

🔧 Reactions of Carboxylic Acids

• Esterification with alcohols (acid-catalysed Fischer esterification).
• Conversion to acid chlorides (e.g., $SOCl_2$, $PCl_5$).
• Reaction with ammonia → ammonium salts → amides on heating.
• Reduction to primary alcohols (e.g., $LiAlH_4$, diborane).
• Decarboxylation: heating salts with lime (soda lime) removes $CO_2$.
• α-Halogenation (Hell–Volhard–Zelinsky) for acids with α-H.

✅ Uses and Industrial Importance

• Formaldehyde: disinfectant, polymer manufacture (Bakelite, urea-formaldehyde resins).
• Acetaldehyde: precursor to acetic acid, esters, polymers.
• Acetone: common solvent.
• Benzoic acid and derivatives: food preservatives and fragrance esters.
• Fatty acids: soaps, detergents, nylon production (dicarboxylic acids).

🧭 Summary

• The carbonyl group is central to reactivity: polar, planar, electrophilic carbon.
• Aldehydes are generally more reactive than ketones in nucleophilic addition due to steric and electronic effects.
• Key reagents: PCC (mild oxidation), $CrO_3$/$KMnO_4$ (strong oxidation), DIBAL-H (partial reduction to aldehydes), NaBH_4 and LiAlH_4 (reductions), Tollens and Fehling (distinguish aldehydes), Grignard + $CO_2$ (prepare acids).
• Carboxylic acids are acidic because their conjugate base is resonance stabilised; substituents modulate acidity by inductive/resonance effects.

Use these notes to build reaction pathways, practise naming, and work through mechanism steps (nucleophilic attack, proton transfers, leaving-group elimination) to master the unit.