Aldehydes and Ketones
Aldehydes and ketones are two closely related classes of carbonyl compounds that are central to both organic chemistry and biochemistry. The carbonyl group — a carbon atom double-bonded to oxygen (C=O) — is one of the most important functional groups in chemistry. It is the structural feature that defines aldehydes, ketones, carboxylic acids, esters, amides, and many other biologically critical molecules. For the HAAD examination candidate, understanding aldehydes and ketones is essential for grasping the chemistry of carbohydrates (which are polyhydroxy aldehydes and ketones), the metabolism of fats (ketone bodies), the mechanism of general anesthesia (the original anesthetic ether was chosen because of its similarity to the carbonyl group), and the toxicology of formaldehyde (a preservative and disinfectant) and acetaldehyde (a metabolite of alcohol). This chapter covers the structure, nomenclature, preparation, reactions, and tests for aldehydes and ketones.
The Carbonyl Group: Structure and Bonding
The carbonyl group consists of a carbon atom doubly bonded to an oxygen atom. This arrangement gives the carbonyl carbon three regions of electron density — a trigonal planar geometry with bond angles of approximately 120°. The C=O bond is highly polar (oxygen is significantly more electronegative than carbon), with a bond dissociation energy of approximately 745 kJ/mol (stronger than a C–C single bond but weaker than a C≡C).
The carbonyl carbon is sp² hybridized, with three sigma bonds arranged in a trigonal planar geometry, and one pi bond formed by the sideways overlap of an sp² orbital of carbon with an sp² orbital of oxygen. The oxygen also has two non-bonding (lone) pairs of electrons.
This polarization has two important consequences:
- The carbonyl carbon is electrophilic (electron-deficient) and susceptible to nucleophilic attack at the carbonyl carbon
- The carbonyl oxygen is nucleophilic (electron-rich) and can accept a hydrogen bond
Distinguishing Aldehydes and Ketones
Aldehydes have the carbonyl group at the end of a carbon chain, with the structure R–CHO (where R can be H or an alkyl/aryl group). The carbonyl carbon is bonded to at least one hydrogen atom.
Ketones have the carbonyl group within the carbon chain, with the structure R–CO–R’ (where R and R’ are alkyl/aryl groups). The carbonyl carbon is bonded to two carbon atoms.
The key structural difference: Aldehydes have at least one hydrogen attached to the carbonyl carbon; ketones have two carbon groups attached to the carbonyl carbon.
Nomenclature
Aldehydes
- IUPAC suffix: -al (from Latin: aldehydum)
- The aldehyde carbon is always C1 (no need to specify position)
- For aldehydes with 1–4 carbons, the common names formaldehyde, acetaldehyde, propionaldehyde, and butyraldehyde are frequently used
| Formula | Common Name | IUPAC Name |
|---|---|---|
| HCHO | Formaldehyde | Methanal |
| CH₃CHO | Acetaldehyde | Ethanal |
| CH₃CH₂CHO | Propionaldehyde | Propanal |
| CH₃(CH₂)₂CHO | Butyraldehyde | Butanal |
For branched aldehydes: CH₃–CH(CH₃)–CHO = 2-Methylpropanal (isobutanal)
Ketones
- IUPAC suffix: -one
- Number the chain to give the carbonyl group the lowest possible number
- For ketones with 3–5 carbons, common names are frequently used (acetone, methyl ethyl ketone)
| Formula | Common Name | IUPAC Name |
|---|---|---|
| CH₃–CO–CH₃ | Acetone | Propan-2-one |
| CH₃–CO–C₂H₅ | MEK | Butan-2-one |
| C₂H₅–CO–C₂H₅ | DEK | Pentan-2-one |
For cyclic ketones: cyclohexanone is the most common
Preparation of Aldehydes and Ketones
From Alcohols (Oxidation)
- Primary alcohols → Aldehydes: Use PCC (pyridinium chlorochromate) or pyridinium dichromate (PDC) in anhydrous conditions. KMnO₄ or K₂Cr₂O₇ would over-oxidize to the carboxylic acid.
- Secondary alcohols → Ketones: Can use any oxidizing agent (KMnO₄, K₂Cr₂O₇, PCC, Jones reagent): CH₃–CH(OH)–CH₃ →(oxidation) CH₃–CO–CH₃ (acetone)
From Alkynes (Hydration)
Alkynes undergo hydration (Markovnikov addition of H₂O) catalyzed by Hg²⁺ and H₂SO₄ to give ketones (except acetylene which gives acetaldehyde): CH₃–C≡CH + H₂O →(Hg²⁺) CH₃–CO–CH₃ (acetone)
From Gem-dihalides (Hydrolysis)
Aldehydes and ketones can be prepared by hydrolysis of gem-dihalides (two halogens on the same carbon): R–CHCl₂ + 2H₂O → R–CHO + 2HCl R–CO–CHCl₂ + 2H₂O → R–CO–R’ + 2HCl
Ozonolysis of Alkenes (for ketones)
Ozonolysis of alkenes followed by reductive workup (Zn/CH₃COOH or DMS) yields aldehydes or ketones depending on substitution:
- If the double-bonded carbon has one H → aldehyde
- If the double-bonded carbon has two alkyl groups → ketone
Reactions of Aldehydes and Ketones
Aldehydes and ketones undergo three main types of reactions: nucleophilic addition, alpha-substitution, and oxidation. Ketones are generally less reactive than aldehydes because the carbonyl carbon in ketones is bonded to two electron-donating alkyl groups, which partially offset the polarization of the carbonyl group.
Nucleophilic Addition Reactions
The carbonyl carbon is electrophilic. Nucleophiles attack the carbonyl carbon, forming an alkoxide intermediate which is then protonated to give the addition product.
Addition of Hydrogen Cyanide (HCN)
Aldehydes and ketones react with HCN (generated in situ from NaCN + H₂SO₄) to form cyanohydrins: R–CHO + HCN → R–CH(OH)–CN (cyanohydrin)
This reaction is important because cyanohydrins can be hydrolyzed to alpha-hydroxy acids (used in pharmaceutical synthesis).
Addition of Alcohols (Acetal and Ketal Formation)
With aldehydes: Aldehydes react with two equivalents of alcohol under acid catalysis to form acetals (R–CH(OR’)₂): CH₃CHO + 2C₂H₅OH →(H⁺) CH₃CH(OC₂H₅)₂ + H₂O
With ketones: Ketones react with two equivalents of alcohol to form ketals (R₂C(OR’)₂), but this reaction is slower and requires more forcing conditions.
Acetal/ketal formation is reversible — acetals and ketals are hydrolyzed back to the parent aldehyde/ketone in the presence of aqueous acid. This is why alcohols can be used as solvents for aldehydes/ketones without reaction under neutral conditions.
Addition of Grignard Reagents
Grignard reagents (R–MgX) add to the carbonyl carbon to form alcohols after acidic workup:
- Aldehydes + Grignard → secondary alcohols
- Ketones + Grignard → tertiary alcohols
R–CHO + R’MgX →(1) ether →(2) H₃O⁺ → R–CH(R’)–OH
This is a key carbon-carbon bond-forming reaction in organic synthesis.
Addition of Ammonia and Ammonia Derivatives
Aldehydes and ketones react with ammonia and ammonia derivatives to form products useful in identification:
| Reagent | Product | Use |
|---|---|---|
| NH₂OH (hydroxylamine) | Oxime (R–CH=NOH) | Identification |
| NH₂–NH₂ (hydrazine) | Hydrazone (R–CH=N–NH₂) | Identification |
| Phenylhydrazine | Phenylhydrazone | Identification |
| 2,4-DNP (Brady’s reagent) | 2,4-DNP derivative (orange/red crystals) | Crystalline derivative for identification |
| Semicarbazide | Semicarbazone | Identification |
The 2,4-DNP test is particularly important: aldehydes and ketones give an immediate orange/red crystalline precipitate with Brady’s reagent (2,4-dinitrophenylhydrazine). This is a qualitative test for the presence of a carbonyl group.
Nucleophilic Addition of Water and Alcohols
Aldehydes (but not ketones under normal conditions) react with water to form hydrates (gem-diols) — compounds with two –OH groups on the same carbon: CH₃CHO + H₂O ⇌ CH₃CH(OH)₂H (acetaldehyde hydrate)
The equilibrium for ketone hydration lies far to the left (ketones are less reactive). However, chloral hydrate (2,2,2-trichloroethane-1,1-diol) — formed by adding water to chloral (trichloroacetaldehyde) — is a stable hydrate and is used medicinally as a sedative/hypnotic.
Oxidation Reactions
Aldehydes are easily oxidized to carboxylic acids:
- By Tollens’ reagent (AgNO₃ in NH₄OH) — silver mirror test R–CHO + Ag(NH₃)₂⁺ → R–COO⁻ + Ag⁰ (silver mirror on test tube)
- By Fehling’s solution (Cu²⁺ tartrate complex) — brick-red precipitate R–CHO + Cu²⁺ → R–COO⁻ + Cu₂O↓ (brick-red)
- By Benedict’s solution (similar to Fehling’s but with citrate buffer)
- By chromic acid (H₂CrO₄) — aldehydes give green Cr³⁺ color; ketones do not react
Ketones are resistant to oxidation by mild oxidants (Tollens, Fehling’s, Benedict) because they have no hydrogen on the carbonyl carbon. They require strong oxidizing agents (hot KMnO₄ or hot HNO₃) which cleave the carbon skeleton.
Tollens’ test (silver mirror test) is the classic qualitative test to distinguish aldehydes from ketones:
- Aldehyde → silver mirror
- Ketone → no reaction
Fehling’s test distinguishes between:
- Aldehydes → brick-red Cu₂O precipitate
- Ketones → no reaction (except reducing sugars like glucose which give a positive test)
Alpha-Substitution Reactions
The alpha carbon (the carbon adjacent to the carbonyl carbon) has acidic hydrogens (pKa ≈ 20) because the resulting carbanion (enolate) is resonance-stabilized by the carbonyl group.
Halogenation at the alpha position: Aldehydes and ketones react with Br₂ or Cl₂ in acetic acid to form alpha-halo ketones: CH₃–CO–CH₃ + Br₂ → CH₃–CO–CH₂Br (bromoacetone)
The iodoform test (methyl ketone test): Methyl ketones (ketones with the structure CH₃–CO–R) react with I₂/NaOH to give a yellow precipitate of iodoform (CHI₃) and a carboxylate salt: CH₃–CO–CH₃ + 3I₂ + 4NaOH → CHI₃↓ (yellow) + CH₃COONa + 3NaI + 3H₂O
The iodoform test is positive for:
- Methyl ketones (CH₃–CO–R)
- Ethanol (which is oxidized to acetaldehyde then to acetic acid)
- Secondary alcohols containing the CH₃–CH(OH)– group (which are oxidized to methyl ketones)
Acetaldehyde and methyl ketones are the most clinically relevant compounds giving a positive iodoform test.
Biochemical Significance
Acetaldehyde is the toxic intermediate in the metabolism of ethanol. Alcohol dehydrogenase converts ethanol to acetaldehyde, which is then rapidly metabolized by aldehyde dehydrogenase (ALDH) to acetic acid. In many East Asian populations, the ALDH enzyme is deficient, leading to accumulation of acetaldehyde and the characteristic facial flushing reaction when alcohol is consumed.
Formaldehyde (HCHO) is a preservative and disinfectant. It cross-links proteins (by reacting with amino groups) and nucleic acids — this property underlies both its antimicrobial action and its toxicity. Formalin is a 37% w/w solution of formaldehyde in water.
Ketone bodies (acetoacetate, beta-hydroxybutyrate, and acetone) are produced in the liver during fasting and in uncontrolled diabetes mellitus. They are derived from acetyl-CoA and can be used as an alternative fuel source by the brain and heart when glucose is scarce. However, in diabetic ketoacidosis, excessive ketone body production leads to metabolic acidosis.
⚡ Exam tip: Aldehydes give positive Tollens’ test (silver mirror) and Fehling’s test (brick-red Cu₂O). Ketones generally do not. The iodoform test is positive for methyl ketones (CH₃–CO–R) and ethanol. Remember: aldehydes are more reactive than ketones (ketones have two electron-donating alkyl groups that reduce electrophilicity). Acetaldehyde and acetone are the two most clinically significant carbonyl compounds.
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