Unit 23: Carbonyl Compounds and Carboxylic Acids — Long Questions

Aldehydes and ketones are both classes of organic compounds containing the carbonyl group, C=O, as their characteristic functional group.
• In aldehydes, the carbonyl carbon is bonded to at least one hydrogen atom and one alkyl/aryl group (or two H atoms, as in methanal): general formula R–CHO.
• In ketones, the carbonyl carbon is bonded to two alkyl/aryl groups, with no H directly attached to the carbonyl carbon: general formula R–CO–R′.

Preparation of aldehydes Aldehydes are prepared by the oxidation of primary alcohols using acidified K₂Cr₂O₇ or acidified KMnO₄, added dropwise to the warm alcohol; the aldehyde formed is distilled off immediately (at a temperature above its own boiling point but below that of the starting alcohol) to prevent its further oxidation to a carboxylic acid:
CH₃CH₂OH + [O] →(K₂Cr₂O₇(aq)/H₂SO₄, Heat)→ CH₃CHO + H₂O

Preparation of ketones Ketones are prepared similarly, by the oxidation of secondary alcohols with acidified K₂Cr₂O₇ or KMnO₄. Since ketones are not further oxidized to carboxylic acids under these conditions, the product need not be distilled off immediately:
CH₃CH(OH)CH₃ + [O] →(K₂Cr₂O₇/H₂SO₄, Heat)→ CH₃COCH₃ + H₂O

1) Oxidation of primary alcohols and aldehydes: refluxing with excess acidified KMnO₄ or K₂Cr₂O₇.
CH₃CH₂OH + 2[O] →(K₂Cr₂O₇/dil. H₂SO₄, Heat)→ CH₃COOH + H₂O
CH₃CHO + [O] →(K₂Cr₂O₇/dil. H₂SO₄, Heat)→ CH₃COOH

2) Hydrolysis of alkane nitriles: acid or alkaline hydrolysis (alkaline hydrolysis gives the salt first, which is acidified to the acid).
CH₃CN + 2H₂O + HCl → CH₃COOH + NH₄Cl

3) Hydrolysis of esters: refluxing with dilute acid or dilute alkali (alkaline hydrolysis gives the carboxylate salt, then acidified).
CH₃COOC₂H₅ + H₂O →(H₃O⁺)→ CH₃COOH + C₂H₅OH

4) Oxidation of alkylbenzenes (preparation of benzoic acid): refluxing methylbenzene with acidified (or alkaline) KMnO₄ or K₂Cr₂O₇.
C₆H₅CH₃ + 3[O] →(KMnO₄, dil. H₂SO₄, Heat)→ C₆H₅COOH + H₂O

i) With HCN (nucleophilic addition, forming a cyanohydrin):
HCN itself is a poor nucleophile/weak acid, so it is generated in situ and a slight excess of KCN is used to keep free CN⁻ available:
Step 1: KCN + HCl → HCN + KCl (Excess KCN + limiting HCl → HCN, the actual reagent)
Step 2: The nucleophilic CN⁻ ion attacks the electrophilic (δ+) carbonyl carbon; the π electrons of C=O shift fully onto oxygen, giving a tetrahedral alkoxide intermediate bearing a C–CN bond.
Step 3: The alkoxide oxygen is protonated (by HCN or H⁺ in the medium), giving the hydroxynitrile (cyanohydrin) product and regenerating CN⁻ to continue the cycle.
Both steps are reversible, but the equilibrium lies well to the product side, giving a good yield of cyanohydrin.

b) With NaBH₄ (nucleophilic addition of hydride):
Step 1: NaBH₄ supplies a hydride ion, H⁻, which as a nucleophile attacks the electrophilic (δ+) carbonyl carbon; the π electrons of C=O shift onto oxygen, forming a tetrahedral alkoxide ion.
Step 2: The alkoxide ion is protonated by water (H–OH) in the alkaline aqueous work-up, giving the alcohol (primary alcohol from an aldehyde, secondary alcohol from a ketone) and OH⁻.

The acidic strength of a carboxylic acid depends on the stability of its conjugate base (the carboxylate ion): the more stable the carboxylate ion, the more readily the acid ionizes, and the stronger the acid.
Chlorine is a highly electronegative atom and exerts an electron-withdrawing inductive (–I) effect. When one or more H atoms on the alkyl chain of ethanoic acid are replaced by Cl, the –I effect of chlorine pulls electron density towards itself, dispersing and stabilizing the negative charge on the carboxylate ion. A more stable (delocalized) carboxylate ion is less likely to re-accept H⁺, so the acid ionizes more and is therefore a stronger acid, with a lower pKa.
The effect is cumulative — each additional chlorine atom further withdraws electron density and further stabilizes the carboxylate ion, so acidity increases steadily with the number of chlorine atoms:
CH₃COOH (pKa = 4.74) < ClCH₂COOH (pKa = 2.86) < Cl₂CHCOOH (pKa = 1.26) < Cl₃CCOOH (pKa = 0.64)
(ethanoic acid) (chloroacetic acid) (dichloroacetic acid) (trichloroacetic acid, comparable in strength to some mineral acids)
The effect also weakens with distance: a chlorine atom closer to the –COOH group has a stronger inductive effect than one further away, since the inductive effect falls off rapidly through the chain.

The three types of C–Cl bond differ greatly in how readily they hydrolyse with water, because of differences in the electrophilicity of the carbon bonded to chlorine and in the nature of the C–Cl bond itself.

1) Acyl chlorides (e.g. ethanoyl chloride, CH₃COCl) hydrolyse fastest, reacting rapidly with water even at room temperature:
CH₃COCl + H₂O →(Room temperature, rapid)→ CH₃COOH + HCl
The carbonyl carbon carries a large partial positive charge because it is attached to two strongly electronegative/electron-withdrawing atoms — the carbonyl oxygen and the chlorine — both pulling electron density away from it. This makes the carbon strongly electrophilic, so the nucleophilic attack of H₂O (via an addition–elimination mechanism) is very fast.

2) Alkyl chlorides (e.g. chloroethane, CH₃CH₂Cl) hydrolyse much more slowly, needing heating under reflux (and still taking a long time, e.g. about 14 days even near 100 °C for simple cases):
CH₃CH₂Cl + H₂O →(Heat under reflux)→ CH₃CH₂OH + HCl
Here the carbon is attached to only one electronegative atom (Cl) and ordinary alkyl/H groups, so it carries a smaller partial positive charge than the acyl carbon, making it a weaker electrophile and the nucleophilic substitution by water correspondingly slower.

3) Halogenoarenes (aryl chlorides, e.g. chlorobenzene) do not hydrolyse at all under these conditions ("No Reaction" with water):
In chlorobenzene, the carbon–chlorine bond is part of the aromatic ring, and one of chlorine's lone pairs is delocalized into the π system of the benzene ring. This conjugation imparts partial double-bond character to the C–Cl bond, making it shorter and much stronger, and also reduces the partial positive charge on the ring carbon. As a result, the C–Cl bond resists heterolytic cleavage and does not undergo nucleophilic substitution by water.

Overall order of ease of hydrolysis: acyl chlorides ≫ alkyl chlorides ≫ halogenoarenes (no reaction).