Aldehydes, Ketones and Carboxylic Acids

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Aldehydes — R-CHO:

Aldehydes contain a carbonyl group (C=O) attached to at least one hydrogen atom and one carbon or hydrogen: R-CHO. Formaldehyde H₂C=O has two hydrogens on the carbonyl carbon. In IUPAC nomenclature, the suffix -al is used. Propanal (CH₃CH₂CHO), Butanal (CH₃CH₂CH₂CHO). The carbonyl carbon is always carbon 1 in naming.

Ketones — R-CO-R’:

Ketones contain a carbonyl group attached to two carbon atoms (neither can be part of the -CHO or -COOH group): R-CO-R’. Acetone (propanone) CH₃COCH₃ is the simplest and most common ketone. In IUPAC, the suffix -one is used: Butanone (methyl ethyl ketone, MEK), Pentanone. The carbonyl carbon gets the lowest possible number.

Carboxylic Acids — R-COOH:

Carboxylic acids contain a -COOH functional group (carbonyl + hydroxyl attached to the same carbon). The suffix -oic acid is used: Methanoic acid (formic acid, HCOOH, found in ant stings), Ethanoic acid (acetic acid, CH₃COOH, found in vinegar). Propanoic acid, Butanoic acid (rancid butter smell).

⚡ ECAT Tip: Aldehydes can be oxidised to carboxylic acids; ketones cannot be further oxidised without breaking the carbon skeleton (strong oxidising agents cleave C-C bonds adjacent to the carbonyl). This is a key distinction: Tollens’ reagent (AgNO₃ in ammonia) gives a silver mirror with aldehydes but not with ketones. Fehling’s solution (Cu²⁺ in alkaline tartrate) gives a red precipitate of Cu₂O with aldehydes but not ketones.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Nucleophilic Addition Reactions — The Key Mechanism:

The carbonyl carbon is electrophilic (δ+ due to oxygen’s electronegativity) and is attacked by nucleophiles. The C=O bond is shorter and stronger than C-C because of the double bond with oxygen.

Addition of HCN: KCN + dilute acid provides HCN in situ. CN⁻ attacks the carbonyl carbon, forming a cyanohydrin: R-CHO + HCN → R-CH(OH)-CN. Acetone + HCN → acetone cyanohydrin (used in synthesis of methacrylic acid for plastics). This reaction is reversible.

Addition of Alcohols (Acetal/Ketal formation): Aldehydes react with alcohols under acid catalysis to form hemiacetals (R-CH(OH)-OR’), then acetals (R-CH(OR’)₂). Ketones form ketals similarly. These are important in protecting carbonyl groups during organic synthesis.

Reduction: NaBH₄ (mild) reduces aldehydes to primary alcohols and ketones to secondary alcohols. LiAlH₄ (strong) reduces all carbonyls including carboxylic acids. Catalytic hydrogenation (H₂/Pt, Ni, or Pd) also reduces carbonyls but is less selective.

Iodoform Reaction — A Test for Methyl Ketones:

Methyl ketones (RC-CO-CH₃) and acetaldehyde (CH₃CHO) give a positive iodoform test: when treated with I₂ in NaOH, they produce a pale yellow precipitate of CHI₃ (iodoform) with a characteristic antiseptic smell. The reaction proceeds via multiple halogenations (tri-iodination) of the methyl group followed by hydroxide-induced cleavage. Acetone (propanone) gives a positive test. Butanone (methyl ethyl ketone) also gives a positive test. 3-pentanone does not (no methyl group adjacent to carbonyl).

⚡ ECAT Tip: The carbonyl group is planar with sp² hybridised carbon (120° bond angles). The oxygen is also sp² hybridised. In nucleophilic addition, the nucleophile attacks from above or below the plane, and the C=O bond breaks, creating a tetrahedral intermediate. For unsymmetrical ketones (like methyl ethyl ketone), the addition creates a chiral centre, producing a racemic mixture if the ketone is unsymmetrical.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Oxidation Reactions — Distinguishing Tests:

Acidified potassium dichromate (K₂Cr₂O₇/H₂SO₄):

• Aldehydes: orange Cr(VI) reduces to green Cr(III)
• Primary alcohols → aldehydes → carboxylic acids (colour change orange → green)
• Ketones: no reaction (unless under forcing conditions that break the C-C bond)

Tollens’ Reagent (Ag(NH₃)₂⁺):

• Aldehydes: Ag⁺ reduces to metallic Ag, forming a silver mirror on the test tube
• Ketones: no reaction
• This is the classic “silver mirror test” for aldehydes

Benedict’s Solution (Cu²⁺ with citrate/carbonate):

• Aldehydes: blue Cu²⁺ reduces to red Cu₂O precipitate
• Ketones: no reaction
• This is used to test for glucose in urine (a reducing sugar with aldehyde character in its open-chain form)

Carboxylic Acid Derivatives — Reactivity Order:

Carboxylic acids can be converted to:

1. Acid chlorides (RCOCl) — most reactive, using SOCl₂ (thionyl chloride), PCl₃, or PCl₅
2. Acid anhydrides (R-CO)₂O — using acid chloride + carboxylate salt
3. Esters (RCOOR’) — Fischer esterification: RCOOH + R’OH ↔ RCOOR’ + H₂O (acid-catalysed, reversible)
4. Amides (RCONH₂) — using SOCl₂ then NH₃, or directly from acid + NH₄Cl at high temperature

The reactivity order follows the ability of the leaving group to leave: Cl⁻ > -OOC-R > RO⁻ > NH₂⁻. More electron-withdrawing leaving groups increase reactivity.

Biological Significance:

• Formaldehyde (HCHO): preservative, crosslinking of biological tissues
• Acetaldehyde (CH₃CHO): intermediate in alcohol metabolism; causes flushing reaction in people deficient in aldehyde dehydrogenase
• Acetone (CH₃COCH₃): ketone body produced in diabetes and prolonged fasting; exhaled breath indicator
• Acetic acid (CH₃COOH): central metabolite (acetyl-CoA), component of vinegar
• Fatty acids: long-chain carboxylic acids (palmitic C₁₆, stearic C₁₈) are the building blocks of triglycerides and phospholipids in biological membranes

⚡ ECAT Pattern: ECAT chemistry frequently tests: (1) IUPAC naming of aldehydes, ketones, and carboxylic acids (identifying the functional group priority — COOH > CHO > C=O > OH); (2) distinguishing aldehydes from ketones using Tollens’ or Fehling’s test; (3) nucleophilic addition mechanism (HCN, NaHSO₃); (4) reduction products of aldehydes vs ketones; and (5) the iodoform test for methyl ketones. A common ECAT question: “Which of the following will give a positive Tollens’ test: formaldehyde, acetone, acetaldehyde, benzaldehyde?” Answer: All aldehydes (formaldehyde and acetaldehyde) give positive Tollens’. Benzaldehyde (an aromatic aldehyde) also gives positive Tollens’ despite being aromatic — it reduces Ag⁺ to Ag. Acetone (a ketone) does not.