Aldehydes, Ketones and Carboxylic Acids
Aldehydes, Ketones and Carboxylic Acids
The Carbonyl Group
Aldehydes (RCHO) and ketones (RCOR') both contain the carbonyl group (C=O). The key difference is position:
- In an aldehyde, the carbonyl is at the end of the carbon chain (at least one H attached)
- In a ketone, the carbonyl is between two carbon groups (no H on the C=O carbon)
The C=O bond is polar — oxygen is more electronegative, making the carbon δ⁺ and the oxygen δ⁻. This makes the carbonyl carbon susceptible to attack by nucleophiles.
Naming
- Aldehydes: suffix -al (e.g. methanal HCHO, ethanal CH₃CHO, propanal CH₃CH₂CHO)
- Ketones: suffix -one (e.g. propanone CH₃COCH₃, butanone CH₃COCH₂CH₃)
- Carboxylic acids: suffix -oic acid (e.g. ethanoic acid CH₃COOH)
Oxidation Reactions
Aldehydes can be oxidised to carboxylic acids because they have a hydrogen on the carbonyl carbon that can be replaced. Ketones cannot be further oxidised (without breaking C–C bonds) because they lack this hydrogen.
Oxidising agent: Acidified potassium dichromate(VI), K₂Cr₂O₇ / H₂SO₄, heated under reflux.
- The orange Cr₂O₇²⁻ is reduced to green Cr³⁺
- RCHO + [O] → RCOOH
Distinguishing Aldehydes from Ketones
Two tests exploit the fact that aldehydes are easily oxidised but ketones are not:
Tollens' reagent (ammoniacal silver nitrate, [Ag(NH₃)₂]⁺):
- Aldehyde → silver mirror forms on the test tube wall (Ag⁺ reduced to Ag)
- Ketone → no reaction
Fehling's (or Benedict's) solution (Cu²⁺ complex, blue):
- Aldehyde → brick-red precipitate of Cu₂O forms (Cu²⁺ reduced to Cu⁺)
- Ketone → no reaction
Nucleophilic Addition
The key mechanism for aldehydes and ketones is nucleophilic addition — a nucleophile attacks the δ⁺ carbonyl carbon.
Reaction with HCN (hydrogen cyanide):
RCHO + HCN → RCH(OH)CN (a hydroxynitrile / cyanohydrin)
Conditions: HCN with a trace of NaCN catalyst (CN⁻ is the actual nucleophile; HCN alone is too weak a nucleophile). Carried out at room temperature.
Mechanism:
1. Curly arrow from the lone pair on CN⁻ to the δ⁺ carbon of C=O
2. Curly arrow from the C=O π bond to the oxygen (O gains the electron pair and becomes O⁻)
3. The alkoxide intermediate is protonated: curly arrow from the lone pair on O⁻ to an H of HCN, forming the OH group and regenerating CN⁻
Importance: This reaction extends the carbon chain by one carbon — useful in synthesis. The –CN group can be hydrolysed to –COOH or reduced to –CH₂NH₂.
Chirality: When the nucleophile attacks a planar carbonyl, it can approach from above or below with equal probability. If the product has a chiral centre, a racemic mixture forms.
Reduction of Aldehydes and Ketones
Both can be reduced to alcohols using NaBH₄ (sodium borohydride) in aqueous solution:
- Aldehyde → primary alcohol: RCHO → RCH₂OH
- Ketone → secondary alcohol: RCOR' → RCHR'OH
The mechanism is nucleophilic addition — the H⁻ (hydride) ion from NaBH₄ acts as the nucleophile, attacking the δ⁺ carbon.
Alternative reducing agent: LiAlH₄ in dry ether (more powerful but reacts violently with water).
Carboxylic Acids
Carboxylic acids (RCOOH) contain the carboxyl group (–COOH). They are weak acids — they partially dissociate in water:
RCOOH ⇌ RCOO⁻ + H⁺ (Ka typically 10⁻⁵ to 10⁻⁴ for simple acids)
The conjugate base (carboxylate ion, RCOO⁻) is stabilised by delocalisation — the negative charge is spread equally over both oxygen atoms.
Reactions of Carboxylic Acids
With bases (neutralisation):
CH₃COOH + NaOH → CH₃COONa + H₂O
With carbonates / hydrogencarbonates:
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂
(Effervescence of CO₂ — a standard test for carboxylic acids)
Esterification (with an alcohol):
RCOOH + R'OH ⇌ RCOOR' + H₂O
Conditions: concentrated H₂SO₄ catalyst, reflux. The reaction is reversible.
Reduction (to a primary alcohol):
RCOOH + LiAlH₄ → RCH₂OH (in dry ether; NaBH₄ is too weak to reduce carboxylic acids)
Acyl Chlorides (Extension for Some Specs)
Acyl chlorides (RCOCl) are more reactive derivatives of carboxylic acids. They react vigorously with nucleophiles via nucleophilic addition-elimination:
- With water: RCOCl + H₂O → RCOOH + HCl (fumes of HCl observed)
- With alcohols: RCOCl + R'OH → RCOOR' + HCl (ester formed quickly)
- With ammonia: RCOCl + 2NH₃ → RCONH₂ + NH₄Cl (amide formed)
- With amines: RCOCl + 2R'NH₂ → RCONHR' + R'NH₃⁺Cl⁻
Exam Tips
- Always show the δ⁺ and δ⁻ on the carbonyl when drawing nucleophilic addition mechanisms
- The nucleophile attacks the carbon, not the oxygen
- NaBH₄ reduces aldehydes/ketones but NOT carboxylic acids; LiAlH₄ reduces all three
- When asked to distinguish aldehyde from ketone, describe both Tollens' AND the expected observation for each