Unit 13: Aldehydes and Ketones
Introduction, Nomenclature and Isomerism
Aldehydes
Aldehydes contain the formyl group (-CHO) attached to a hydrogen atom and an alkyl or aryl group. The general formula is R‑CHO, where R may be hydrogen (giving methanal) or any alkyl/aryl substituent. The carbonyl carbon is sp²‑hybridised and planar.
Ketones
In ketones the carbonyl group is bonded to two carbon atoms. The general formula is R‑CO‑R' (or R₂CO when R = R'). The carbonyl carbon is also sp²‑hybridised, but there is no hydrogen directly attached to it.
Isomerism
Aldehydes and ketones exhibit several types of isomerism:
- Chain isomerism – different carbon skeletons, e.g., butanal (CH₃CH₂CH₂CHO) vs. 2‑methylpropanal ((CH₃)₂CHCHO).
- Position isomerism – different location of the carbonyl group along the chain, e.g., pentan‑2‑one (CH₃COCH₂CH₂CH₃) vs. pentan‑3‑one (CH₃CH₂COCH₂CH₃).
- Functional isomerism – aldehydes vs. ketones with the same molecular formula, e.g., C₃H₆O can be propanal (CH₃CH₂CHO) or propanone (acetone, CH₃COCH₃). Additionally, aldehydes that possess an α‑hydrogen can exist in equilibrium with their enol form (functional isomerism via keto‑enol tautomerism).
Preparation
From Alcohols
Primary alcohols can be dehydrogenated or oxidised to aldehydes; secondary alcohols give ketones.
- Dehydrogenation (Cu catalyst, 300 °C):
R‑CH₂OH → R‑CHO + H₂(e.g., ethanol → acetaldehyde). - Oxidation: Using PCC (pyridinium chlorochromate) or PDC stops at the aldehyde stage for primary alcohols:
R‑CH₂OH + [O] → R‑CHO + H₂O. Strong oxidants (KMnO₄, K₂Cr₂O₇) over‑oxidise to carboxylic acids.
Ozonolysis of Alkenes
Cleavage of the C=C double bond with ozone followed by reductive work‑up (Zn/H₂O) yields carbonyl compounds.
R‑CH=CH‑R' + O₃ → R‑CHO + R'‑CHO (if each carbon bears at least one hydrogen). Example: 2‑butene → two molecules of acetaldehyde.
From Acid Chlorides – Rosenmund Reduction
Acid chlorides are reduced to aldehydes using H₂/Pd‑BaSO₄ (poisoned with sulfur) to prevent over‑reduction.
R‑COCl + H₂ →[Pd/BaSO₄] R‑CHO + HCl Example: benzoyl chloride → benzaldehyde.
From Gem‑Dihaloalkanes
Geminal dihalides undergo hydrolysis to give carbonyl compounds.
R‑CHX₂ + 2 H₂O → R‑CHO + 2 HX (X = Cl, Br). Example: CH₃CHCl₂ → acetaldehyde.
Catalytic Hydration of Alkynes
Markovnikov addition of water to terminal alkynes, catalysed by Hg²⁺/H₂SO₄, yields methyl ketones.
RC≡CH + H₂O →[Hg²⁺/H₂SO₄] RC(O)CH₃ Example: propyne → acetone.
Physical Properties
Polar Carbonyl Group
The C=O bond is highly polar (δ⁺ on carbon, δ⁻ on oxygen) giving rise to strong dipole‑dipole interactions. Consequently, aldehydes and ketones have higher boiling points than comparable alkanes but lower than alcohols of similar mass.
Solubility
Lower members (up to C₄) are miscible with water due to hydrogen bonding between the carbonyl oxygen and water molecules. Higher aldehydes and ketones (> C₅) are largely insoluble because the hydrophobic hydrocarbon chain dominates.
Chemical Properties
Structure of the Carbonyl Group
The carbonyl carbon is sp²‑hybridised, trigonal planar, with a bond angle of ≈120°. The C=O π‑bond is susceptible to nucleophilic attack, while the carbonyl oxygen can act as a hydrogen‑bond acceptor.
Distinction Tests
- 2,4‑Dinitrophenylhydrazine (2,4‑DNP) test: Forms an orange‑red precipitate of the corresponding 2,4‑dinitrophenylhydrazone with both aldehydes and ketones.
- Tollens’ reagent (Ag(NH₃)₂⁺): Aldehydes are oxidised to carboxylate ions, reducing Ag⁺ to metallic silver, which deposits as a mirror. Ketones give no reaction.
- Fehling’s solution** (Cu²⁺ in alkaline citrate‑tartrate complex): Aldehydes reduce Cu²⁺ to Cu₂O (red precipitate); ketones do not react (except α‑hydroxy ketones).
Addition Reactions
The electrophilic carbonyl carbon undergoes nucleophilic addition.
- Hydrogenation (H₂, Ni/Pt/Pd):
R‑CHO + H₂ → R‑CH₂OH(primary alcohol);R₂CO + H₂ → R₂CHOH(secondary alcohol). - Hydrogen cyanide (HCN): Forms cyanohydrins.
R‑CHO + HCN → R‑CH(OH)CN. Example: acetaldehyde + HCN → lactonitrile. - Sodium bisulphite (NaHSO₃): Gives crystalline bisulphite addition products.
R‑CHO + NaHSO₃ → R‑CH(OH)SO₃Na. Useful for purification of aldehydes and some methyl ketones.
Reactions with Ammonia Derivatives
Condensation with NH₂‑Z yields imines (or related compounds) with elimination of water.
- Hydroxylamine (NH₂OH): Oxime formation.
R‑CHO + NH₂OH → R‑CH=NOH + H₂O. - Hydrazine (NH₂NH₂): Hydrazone.
R‑CHO + NH₂NH₂ → R‑CH=NNH₂ + H₂O. - Phenylhydrazine (C₆H₅NHNH₂): Phenylhydrazone (often crystalline, used for identification).
- Semicarbazide (NH₂NHCONH₂): Semicarbazone.
R‑CHO + NH₂NHCONH₂ → R‑CH=NNHCONH₂ + H₂O.
Aldol Condensation
In the presence of dilute base (e.g., NaOH), aldehydes (or ketones) possessing at least one α‑hydrogen undergo self‑condensation to give β‑hydroxy carbonyl compounds (aldols), which may dehydrate to α,β‑unsaturated carbonyls.
2 CH₃CHO ⟶[NaOH] CH₃CH(OH)CH₂CHO (acetaldehyde aldol). Dehydration yields crotonaldehyde: CH₃CH(OH)CH₂CHO → CH₃CH=CHCHO + H₂O.
Cannizzaro’s Reaction
Non‑enolizable aldehydes (no α‑hydrogen) undergo disproportionation in strong alkali: one molecule is reduced to alcohol, another is oxidised to carboxylate.
2 HCHO + NaOH → CH₃OH + HCOONa (formaldehyde). Similarly, benzaldehyde yields benzyl alcohol and sodium benzoate.
Clemmensen Reduction
Reduces carbonyl to methylene using zinc amalgam and concentrated hydrochloric acid.
R₂CO + 4[H] →[Zn‑Hg/HCl] R₂CH₂ + H₂O Example: acetophenone → ethylbenzene.
Wolf‑Kishner Reduction
Converts carbonyl to methylene via hydrazone formation followed by strong base heating.
R₂CO + NH₂NH₂ → R₂C=NNH₂ →[KOH/Δ] R₂CH₂ + N₂ Example: cyclohexanone → cyclohexane.
Action with PCl₅ and LiAlH₄
- Phosphorus pentachloride (PCl₅): Replaces the carbonyl oxygen with two chlorines to give gem‑dichlorides.
R₂CO + PCl₅ → R₂CCl₂ + POCl₃. - Lithium aluminium hydride (LiAlH₄): Strong reducing agent; reduces aldehydes to primary alcohols and ketones to secondary alcohols.
R‑CHO + 4[H] →[LiAlH₄] R‑CH₂OH;R₂CO + 4[H] →[LiAlH₄] R₂CHOH.
Methanal (Formaldehyde)
Reaction with Ammonia – Hexamethylenetetramine (Urotropine)
Six molecules of formaldehyde react with four molecules of ammonia to give a cage‑like heterocycle.
6 HCHO + 4 NH₃ → (CH₂)₆N₄ + 6 H₂O Used as a urinary antiseptic and in rubber vulcanisation.
Reaction with Phenol – Bakelite Resin
Under acidic or basic conditions, formaldehyde undergoes electrophilic aromatic substitution with phenol, followed by condensation to form a cross‑linked phenol‑formaldehyde polymer (Bakelite).
n C₆H₅OH + n HCHO → [–C₆H₄‑CH₂–]ₙ + n H₂O (simplified). Bakelite is an early thermosetting plastic used for electrical insulators.
Formalin
Aqueous solution containing 37‑40 % formaldehyde (by weight) with a small amount of methanol to prevent polymerisation. Widely used as a preservative (biological specimens), disinfectant, and in the production of resins.
Aromatic Aldehydes and Ketones
Benzaldehyde from Toluene
Side‑chain oxidation of toluene with strong oxidising agents (KMnO₄, K₂Cr₂O₇) yields benzaldehyde.
C₆H₅CH₃ + [O] → C₆H₅CHO + H₂O (controlled conditions prevent over‑oxidation to benzoic acid).
Acetophenone from Benzene – Friedel‑Crafts Acylation
Benzene reacts with acetyl chloride in the presence of AlCl₃ to give acetophenone.
C₆H₆ + CH₃COCl →[AlCl₃] C₆H₅COCH₃ + HCl
Perkin Condensation
Aromatic aldehydes (especially benzaldehyde) condense with acetic anhydride in the presence of a base (sodium acetate) to give α,β‑unsaturated aromatic acids (cinnamic acids).
C₆H₅CHO + (CH₃CO)₂O →[NaOAc] C₆H₅CH=CHCOOH + CH₃COOH
Benzoin Condensation
Two molecules of benzaldehyde undergo a cyanide‑catalysed condensation to give benzoin (a hydroxy ketone).
2 C₆H₅CHO ⟶[CN⁻] C₆H₅CH(OH)COC₆H₅
Cannizzaro’s Reaction (Aromatic)
Non‑enolizable aromatic aldehydes (e.g., benzaldehyde, formaldehyde) undergo disproportionation in concentrated alkali.
2 C₆H₅CHO + NaOH → C₆H₅CH₂OH + C₆H₅COONa
Electrophilic Substitution on the Aromatic Ring
The carbonyl group is meta‑directing and deactivating. Nitration and sulphonation proceed mainly at the meta position.
- Nitration:
C₆H₅CHO + HNO₃/H₂SO₄ → m‑nitrobenzaldehyde + H₂O - Sulphonation:
C₆H₅CHO + H₂SO₄ → m‑sulfonylbenzaldehyde + H₂O
Summary
This chapter has provided a comprehensive overview of aldehydes and ketones, covering their nomenclature, isomerism, diverse synthetic routes, characteristic physical behaviours, and a wide array of chemical reactions. The distinction tests, addition reactions, condensations, and reductions discussed are fundamental for both academic understanding and practical applications in organic synthesis, industry, and biochemistry.