Lesson 17: Carbonyl Compounds

The carbonyl group, C=O, is the most versatile functional group in A-level chemistry — the gateway to aldehydes, ketones, carboxylic acids, esters and acyl chlorides, and to the condensation polymers that make modern materials. Its polar double bond invites nucleophilic attack, while its tests (Tollens', Fehling's, 2,4-DNPH) are laboratory staples. This lesson covers nucleophilic addition to aldehydes and ketones, the distinguishing tests, the chemistry of carboxylic acids and esters, the vigorous reactions of acyl chlorides, and condensation polymerisation with its degradability.

Aldehydes and ketones — nucleophilic addition

Aldehydes (RCHO) have the carbonyl carbon bonded to at least one H; ketones (RCOR′) have it bonded to two carbons. The C=O bond is polar (Oδ⁻, Cδ⁺), so nucleophiles attack the δ+ carbon. With HCN (KCN, acidified — the HCN is generated in situ because it is lethally toxic as a gas): CN⁻ attacks the carbonyl carbon, forming an alkoxide intermediate, which is protonated to a hydroxynitrile — e.g. ethanal → 2-hydroxypropanenitrile. The planar carbonyl is attacked from either side with equal probability, so a racemic mixture forms when the product is chiral — the same stereochemical argument as S_N1 (Lesson 13).

Reduction with NaBH₄ (in aqueous or methanolic solution) adds hydride, H⁻: aldehydes → primary alcohols, ketones → secondary alcohols. NaBH₄ is selective — it reduces C=O but not C=C, unlike H₂/Ni, which reduces both. Exploit this selectivity in synthesis: reduce the carbonyl of an unsaturated aldehyde without touching its double bond. (LiAlH₄, in dry ether, is stronger still, reducing esters and carboxylic acids to primary alcohols.)

Nucleophilic addition is the carbonyl's signature mechanism. It parallels electrophilic addition to alkenes but with reversed polarity: there, the π electrons attack an electrophile; here, a nucleophile attacks the δ+ carbon while the π electrons move onto oxygen. Comparing the two mechanisms explicitly is excellent synoptic revision.

Distinguishing tests — Tollens', Fehling's and 2,4-DNPH

2,4-dinitrophenylhydrazine (2,4-DNPH, Brady's reagent) gives an orange (or yellow) precipitate with any aldehyde or ketone — the test for the carbonyl group. The precipitate's melting point can identify the specific carbonyl compound by database comparison. Tollens' reagent ([Ag(NH₃)₂]⁺, warmed) distinguishes aldehydes: the aldehyde is oxidised to a carboxylic acid while Ag⁺ is reduced to a silver mirror on the glass; ketones give no reaction. Fehling's (or Benedict's) solution — a blue Cu²⁺ complex, warmed — likewise gives a brick-red Cu₂O precipitate with aldehydes only. Acidified dichromate also distinguishes: aldehydes turn it green (oxidised to acids); ketones show no change.

Why the difference? An aldehyde has a hydrogen on the carbonyl carbon that can be removed by oxidation; a ketone's carbonyl carbon has no H, and further oxidation would require breaking C–C bonds — far more vigorous conditions. This structural explanation is the mark-earning part of "explain why" questions; do not just state the observations.

In qualitative-analysis flowcharts the sequence runs: 2,4-DNPH (carbonyl present?) → Tollens' (aldehyde or ketone?) → IR/NMR for identity. Practise writing these as decision trees.

Carboxylic acids — formation and reactions

Carboxylic acids (–COOH) form by oxidising primary alcohols or aldehydes (reflux with acidified dichromate) or by hydrolysing nitriles (HCl(aq), heat — this extends the chain by one carbon). They are weak acids, partially dissociating: CH₃COOH ⇌ CH₃COO⁻ + H⁺. Typical reactions: with metals → salt + H₂; with carbonates → salt + CO₂ + H₂O (effervescence — this distinguishes acids from phenols, which do not react); with alkalis → salt + water; with alcohols → esters (concentrated H₂SO₄ catalyst, warm).

Acid strength runs carboxylic acids > phenols > alcohols. The explanation is anion stability: the carboxylate ion's negative charge is delocalised over two oxygens; phenoxide delocalises into the ring (less effective); an alkoxide has no delocalisation at all. Electron-withdrawing substituents increase acid strength (chloroethanoic acid > ethanoic acid) by stabilising the anion — the inductive effect applied to acidity. K_a and pK_a quantify all of this (Lesson 18).

Naming: the -oic acid suffix (ethanoic acid, benzoic acid); salts are alkanoates (sodium ethanoate). Dicarboxylic acids such as ethanedioic acid form two series of salts.

Esters and acyl chlorides

Esters (RCOOR′) form from carboxylic acids + alcohols (concentrated H₂SO₄, warm — an equilibrium, Lesson 6) or, much more vigorously, from acyl chlorides + alcohols (room temperature, HCl fumes). They are named alkyl alkanoate (ethyl ethanoate) and are used as solvents, plasticisers, flavourings and perfumes. Esters hydrolyse: with dilute acid → acid + alcohol (reversible); with NaOH → carboxylate salt + alcohol (saponification — soap manufacture, effectively irreversible).

Acyl chlorides (RCOCl) are made from carboxylic acids + SOCl₂ (the products SO₂ and HCl are both gases, so separation is easy) or PCl₅. They react vigorously with water (white fumes of HCl; the carboxylic acid forms), alcohols (esters, at room temperature), concentrated ammonia (amides) and amines (substituted amides). The mechanism is nucleophilic addition–elimination: the nucleophile attacks the δ+ carbonyl carbon, the tetrahedral intermediate collapses, and Cl⁻ — an excellent leaving group — is expelled.

Why use acyl chlorides? Their vigour drives ester and amide formation to completion at room temperature, avoiding the equilibrium limitations of the acid + alcohol route — a yield advantage worth the extra step. Safety: work in a fume cupboard with dry apparatus, because acyl chlorides hydrolyse in moist air. In synthesis questions, converting an acid to its acyl chloride before esterification is the "high-yield route".

Condensation polymers

Condensation polymers form from monomers with two functional groups, eliminating a small molecule (H₂O or HCl) at each link. Polyesters: a dicarboxylic acid + a diol — benzene-1,4-dicarboxylic acid + ethane-1,2-diol gives Terylene (PET), used in clothing and bottles. Polyamides: a dicarboxylic acid + a diamine — nylon-6,6 from hexanedioic acid + 1,6-diaminohexane; Kevlar is an aromatic polyamide whose rigid rings give extreme strength for body armour. Proteins are natural polyamides (amino acids joined by peptide — amide — bonds).

Draw condensation polymers by linking the monomers and removing H₂O at each junction; show the repeating unit in brackets. To find the monomers from a polymer section, locate the ester or amide links and add H₂O back across each one (H to the N or O side, OH to the carbonyl side) — this reverse-condensation technique solves every "draw the monomers" question.

Environmental evaluation: condensation polymers hydrolyse (acids and enzymes break ester and amide links), so they degrade faster than addition polymers — but "biodegradable" still needs the right conditions (industrial composting, not a cold ocean). PET recycling (bottles → fibres) is well established. Compare with addition polymers (Lesson 12) in 6-mark questions: structure (hydrolysable links vs inert C–C chains) → property (degradability) → environmental consequence.

Worked examples — deducing structure and planning with acyl chlorides

Worked example 1: deduce the carbonyl compound. A compound C₃H₆O gives an orange precipitate with 2,4-DNPH and a silver mirror with Tollens' reagent, but does not react with Na₂CO₃. The DNPH result proves a carbonyl; Tollens' proves an aldehyde (ketones give no mirror); the carbonate result excludes a carboxylic acid (and C₃H₆O could not be one anyway). The only C₃ aldehyde is propanal, CH₃CH₂CHO. Follow-up check: it is oxidised by refluxing acidified dichromate to propanoic acid, and its IR spectrum shows C=O near 1725 cm⁻¹ with aldehyde C–H absorptions near 2720 and 2820 cm⁻¹.

Worked example 2: why the acyl chloride route wins. Making ethyl ethanoate from ethanoic acid + ethanol (concentrated H₂SO₄, warm) reaches equilibrium with K_c ≈ 4, so roughly a third of the acid remains unreacted. Converting the acid to ethanoyl chloride first (SOCl₂) and then adding ethanol gives a fast, essentially complete reaction at room temperature — the addition–elimination has no equilibrium to fight because Cl⁻ is an excellent leaving group and HCl escapes as gas. The price is one extra step and strict dryness; in synthesis-marking, stating this trade-off is what lifts an answer into the top band.

Going deeper — fats, soaps and the chemistry of ester hydrolysis

Alkaline hydrolysis of esters is not just a laboratory reaction: saponification of fats and oils (triesters of glycerol with long-chain fatty acids) with NaOH produces glycerol and the sodium salts of the fatty acids — soap. The soap anion is amphiphilic: a charged carboxylate head (hydrophilic) and a long hydrocarbon tail (hydrophobic), so it forms micelles around grease droplets and lifts them into water. In hard water, Ca²⁺ and Mg²⁺ precipitate the carboxylate as scum — the reason synthetic detergents use sulphonate heads whose calcium salts stay soluble. This is applied equilibrium and solubility (Lessons 6 and 8) in a bathroom context, and an excellent source of synoptic essay material.

Biodiesel is the same functional group chemistry run in reverse direction: transesterification of vegetable-oil triesters with methanol (alkali catalyst) gives methyl esters (biodiesel) plus glycerol, avoiding the waste and fossil feedstock of conventional diesel. Acid hydrolysis of esters, by contrast, is a true equilibrium — dilute acid gives the acid + alcohol mixture part-way both ways — which is why ester hydrolysis experiments with acid need excess water to drive conversion, while NaOH drives it to completion by removing the acid as its salt (Le Chatelier, Lesson 6).

Common mistakes

• Claiming Tollens' gives a "silver precipitate". The silver deposits as a mirror on the glass — and ketones give no reaction, which must be stated, not implied.
• Writing esterification as irreversible. Acid + alcohol ⇌ ester + water is an equilibrium; the acyl chloride route is the one that goes to completion.
• Hydrolysing esters with NaOH and naming the acid as the product. Alkaline hydrolysis gives the carboxylate salt (plus the alcohol); the free acid is obtained only on subsequent acidification.
• Cutting the amide/ester link incorrectly when deducing polymer monomers. Break –CO–O– (or –CO–NH–) and add H₂O across: OH back onto the carbonyl carbon, H onto the O or N.

Exam Experience

Q1. Compound X, C₂H₄O, gives an orange precipitate with 2,4-DNPH and a silver mirror when warmed with Tollens' reagent. (a) Deduce the structure of X and the name of the functional group it contains. [2] (b) Draw the mechanism for the reaction of X with HCN, and name the product. [4] (c) Explain why the product of this reaction is formed as a racemic mixture. [2]

Mark scheme. (a) The DNPH precipitate shows a carbonyl is present [1]; the silver mirror shows X is an aldehyde, so X is ethanal, CH₃CHO (the only C₂H₄O aldehyde) [1]. (b) Curly arrow from the CN⁻ carbon lone pair to the δ+ carbonyl carbon [1]; simultaneous arrow from the C=O π bond onto oxygen, giving the alkoxide intermediate (CH₃)(CN)C(H)–O⁻ [1]; arrow from the O⁻ lone pair to H⁺ (of HCN/H₂O), giving the product [1]; product named: 2-hydroxypropanenitrile [1]. (c) The carbonyl carbon is planar (trigonal) [1]; CN⁻ attacks from either face with equal probability, generating both enantiomers in equal amounts — a racemic mixture [1].