Analysis of the reaction chain
The examined reaction chain of task 32 includes four consecutive transformations: propanoic acid → calcium propionate → pentan-3-one → pentan-3-ol → pent-2-ene → products of vigorous oxidation. The complexity of the problem is determined by the need to know the specific pyrolysis reaction of calcium salts of carboxylic acids and to correctly set the conditions for dehydration and oxidation.
Key observation in the analysis: the number of carbon atoms changes non-linearly. Propanoic acid has 3 carbon atoms, after the first stage in the salt it becomes 6 (two acid residues per one cation), and in pentan-3-one it is 5. This indicates a decarboxylation reaction with the loss of one carbon atom as carbonate.
Stage 1: Reaction with calcium hydrogen carbonate
Propionic (propanoic) acid CH₃CH₂COOH exhibits all typical acidic properties: it reacts with metals, basic oxides, alkalis, and salts of weak acids. Calcium hydrogen carbonate Ca(HCO₃)₂ is a salt with a weak anion, so the ion exchange reaction goes to completion with gas evolution.
Reaction equation:
- 2 CH₃CH₂COOH + Ca(HCO₃)₂ → (CH₃CH₂COO)₂Ca + 2 H₂O + 2 CO₂↑
The resulting carbonic acid H₂CO₃ is unstable and decomposes into water and carbon dioxide. The product is calcium propionate.
Stage 2: Pyrolysis of the calcium salt — synthesis of a ketone
This is a specific reaction for the preparation of ketones from calcium (or barium) salts of carboxylic acids upon heating. Aldehydes are obtained in a similar way from formates, and symmetrical ketones from salts of longer acids.
Equation:
- (CH₃CH₂COO)₂Ca →(t°) CH₃CH₂–CO–CH₂CH₃ + CaCO₃
Covalent bonds between oxygen atoms and calcium in the starting salt must not be drawn — the bond is ionic. The product is pentan-3-one, a symmetrical ketone. CaCO₃ 'leaves' the salt, which explains the loss of one carbon atom.
Stage 3: Reduction of the ketone to an alcohol
Unlike aldehydes, ketones in organic chemistry are not oxidized by ordinary oxidizing agents, but are reduced by hydrogen to secondary alcohols (aldehydes to primary ones). The reaction proceeds with hydrogen in the presence of a catalyst (Ni, Pt, Pd).
- CH₃CH₂–CO–CH₂CH₃ + H₂ →(Ni, t°) CH₃CH₂–CH(OH)–CH₂CH₃
The product is pentan-3-ol, a secondary alcohol, which is a convenient substrate for subsequent dehydration to an alkene.
Stage 4: Intramolecular dehydration of the alcohol
The conversion of an alcohol into an alkene is carried out under conditions of intramolecular dehydration: concentrated H₂SO₄ and a temperature above 140 °C, usually indicated as 180 °C. Indicating the temperature is mandatory: at lower temperatures (around 140 °C), intermolecular dehydration occurs with the formation of an ether, and points are deducted for omitting this condition.
- CH₃CH₂–CH(OH)–CH₂CH₃ →(H₂SO₄ conc., 180 °C) CH₃CH=CH–CH₂CH₃ + H₂O
The product is pent-2-ene. The position of the double bond is determined by Zaitsev's rule: hydrogen is eliminated from the least hydrogenated neighboring carbon atom.
Stage 5: Vigorous oxidation of the alkene with permanganate
In an acidic medium, KMnO₄ performs hard oxidation of the alkene, cleaving both the π- and σ-bonds between the carbon atoms of the double bond. Since both carbon atoms at the double bond in pentene-2 are secondary (each bearing one hydrogen), both are converted into carboxyl groups.
- 5 CH₃CH=CH–CH₂CH₃ + 8 KMnO₄ + 12 H₂SO₄ → 5 CH₃COOH + 5 CH₃CH₂COOH + 8 MnSO₄ + 4 K₂SO₄ + 12 H₂O
The products are acetic and propanoic acids. Electron balance: each of the two carbons changes its oxidation state from −1 to +3 (giving away 4 electrons each), totaling 8 electrons per alkene molecule; manganese accepts 5 electrons (Mn⁺⁷ → Mn⁺²). Hence the coefficients 5 and 8. Further verification by hydrogen and oxygen confirms the equation is balanced.
Key points
- Carboxylic acid reacts with calcium hydrogen carbonate as a salt of a weak acid, releasing CO₂.
- Pyrolysis of the calcium salt of a carboxylic acid yields a symmetrical ketone and CaCO₃ — a loss of one carbon atom.
- The Ca–O bond in carboxylic acid salts is ionic: covalent dashes must not be drawn.
- Ketones are not oxidized by typical reagents, but are reduced by hydrogen to secondary alcohols.
- Intramolecular dehydration of an alcohol requires concentrated H₂SO₄ and a temperature of 180 °C; specifying the conditions is mandatory.
- Vigorous oxidation of an alkene by KMnO₄/H₂SO₄ cleaves the double bond; secondary carbons yield carboxylic acids.
- Electron balance: alkene gives 8 ē, manganese accepts 5 ē — hence coefficients 5 and 8.
- Symmetrical pent-2-ene upon vigorous oxidation yields a pair of acids: acetic and propanoic.