Study sheet: Aldehydes and Ketones

📋 Course Outline

1. Structures and Nomenclature
2. Preparation Methods
3. Properties and Nucleophilic Addition
4. Grignard and Derivative Formation
5. Redox Reactions and Tests
6. Condensation and Aromatic Reactions
7. Named Reactions and Conversions

📖 1. Structures and Nomenclature

🔑 Key Concepts & Definitions

• Aldehyde : has the general structure R–CHO, with the carbonyl carbon attached to at least one hydrogen atom
• Ketone : has the general structure R–CO–R′, with the carbonyl carbon attached to two carbon groups
• Carbonyl group : is C=O, and its polarity makes the carbonyl carbon electrophilic because oxygen is more electronegative

📝 Essential Points

📌 The reactivity order is HCHO > RCHO > RCOR′ because aldehydes have less steric hindrance and fewer electron-donating alkyl groups than ketones.

💡 Memory Hook

Aldehyde: carbonyl carbon bears H; ketone: carbonyl carbon bears two carbon groups.

📖 2. Preparation Methods

📝 Essential Points

• Primary alcohols are oxidised to aldehydes, and aldehydes can be further oxidised to carboxylic acids.

• Important aldehyde preparations include: dehydrogenation of primary alcohols with copper at 573 K, Rosenmund reduction of acid chlorides, Stephen reduction of nitriles followed by hydrolysis, ozonolysis of alkenes followed by zinc and water

• Important ketone preparations include: oxidation of secondary alcohols, dehydrogenation of secondary alcohols, reaction of acid chlorides with dialkyl cadmium, Friedel–Crafts acylation of benzene with an acid chloride and AlCl₃

💡 Memory Hook

Alcohol → carbonyl compound → acid or hydrocarbon.

📖 3. Properties and Nucleophilic Addition

★ Must-know

• In nucleophilic addition, a nucleophile attacks the electrophilic carbonyl carbon, producing an alkoxide intermediate that is protonated to give the addition product.

Further detail

• Aldehydes and ketones have higher boiling points than hydrocarbons of similar molar mass because of dipole–dipole forces, but lower boiling points than corresponding alcohols because they do not self hydrogen-bond.

• Lower aldehydes and ketones are water-soluble, and solubility decreases as the hydrocarbon chain length increases.

💡 Memory Hook

Polar C=O → electrophilic carbon → nucleophilic addition.

📖 4. Grignard and Derivative Formation

★ Must-know

• The Grignard products are:

  • methanal → primary alcohol
  • aldehyde → secondary alcohol
  • ketone → tertiary alcohol

• Hydrogen cyanide gives cyanohydrins, sodium bisulfite gives bisulfite addition compounds, and alcohols form acetals from aldehydes or ketals from ketones under acidic conditions.

Further detail

• Ammonia derivatives form:
  • oximes
  • hydrazones
  • phenylhydrazones
  • semicarbazones

💡 Memory Hook

HCHO → primary; aldehyde → secondary; ketone → tertiary alcohol.

📖 5. Redox Reactions and Tests

📝 Essential Points

• Sodium borohydride, lithium aluminium hydride, or hydrogen with nickel reduces aldehydes to primary alcohols and ketones to secondary alcohols.

📌 Clemmensen reduction converts aldehydes and ketones to hydrocarbons using Zn(Hg) and concentrated HCl, whereas Wolff–Kishner reduction uses hydrazine and KOH with heat.

• Tollens’ reagent gives a silver mirror with aldehydes, while Fehling’s solution gives brick-red Cu₂O with aliphatic aldehydes but is generally negative with aromatic aldehydes.

• Iodoform testing gives yellow CHI₃ from methyl ketones and related compounds such as ethanal or ethanol.

💡 Memory Hook

Aldehydes oxidise readily; ketones resist mild oxidation.

📖 6. Condensation and Aromatic Reactions

★ Must-know

• Aldol condensation requires at least one α-hydrogen and produces a β-hydroxy carbonyl compound that can dehydrate on heating.

• Cannizzaro reaction occurs when aldehydes without α-hydrogen undergo disproportionation with concentrated alkali, producing one alcohol and one carboxylate.

• The –CHO and –COR groups are deactivating and meta-directing in electrophilic aromatic substitution.

Further detail

📌 In crossed aldol condensation, two different aldehydes or ketones may react, and selectivity improves when one reactant lacks α-hydrogen.

💡 Memory Hook

α-H present → aldol; α-H absent → Cannizzaro.

📖 7. Named Reactions and Conversions

★ Must-know

• The named reactions transform: acid chloride → aldehyde by Rosenmund reduction, nitrile → aldehyde by Stephen reduction, benzene → aryl ketone by Friedel–Crafts acylation, carbonyl compound → hydrocarbon by Clemmensen reduction, carbonyl compound → hydrocarbon by Wolff–Kishner reduction, α-hydrogen carbonyl compound → β-hydroxy carbonyl compound by aldol condensation, aldehyde without α-hydrogen → alcohol plus carboxylate by Cannizzaro reaction, toluene → benzaldehyde by Etard reaction, carbonyl compound → alcohol by Grignard reaction

Further detail

• The Etard reaction controlledly oxidises the methyl group of toluene with CrO₂Cl₂ to produce benzaldehyde.

• Benzaldehyde is oxidised to benzoic acid, reduced to benzyl alcohol, gives Tollens’ test, generally fails Fehling’s test, and undergoes Cannizzaro reaction because it has no α-hydrogen.

💡 Memory Hook

R-S-F-C-W-A-E: Rosenmund, Stephen, Friedel–Crafts, Clemmensen, Wolff–Kishner, Aldol, Etard.

📊 Synthesis Tables

Aldol and Cannizzaro Comparison

Feature	Aldol condensation	Cannizzaro reaction
α-Hydrogen	Present	Absent
Base	Usually dilute	Concentrated
Products	β-Hydroxy carbonyl or dehydration product	Alcohol and carboxylate
Example	Ethanal	Benzaldehyde

Carbonyl Identification Tests

Test	Positive substances	Observation
Tollens’ reagent	Aldehydes	Silver mirror
Fehling’s solution	Aliphatic aldehydes	Brick-red Cu₂O
2,4-DNP	Aldehydes and ketones	Yellow/orange derivative
I₂/NaOH	Methyl ketones and related compounds	Yellow CHI₃