Identifying Organic Compounds from Formula, Tests and Boiling Point
A school laboratory has four unlabelled bottles, A–D, containing colourless organic liquids. Elemental analysis shows: A has molecular formula C4H10O; B has C4H8O; C and D both have C4H8O2. A is known to be a straight-chain alcohol. The following tests are carried out. A reacts with sodium metal, releasing a gas; it does not decolorize bromine water. B gives a silver mirror with Tollens’ reagent but does not dissolve in sodium hydrogencarbonate solution. C dissolves in sodium hydrogencarbonate with effervescence and has a boiling point of 164 °C. D does not react with sodium hydrogencarbonate; when heated with aqueous NaOH it gives ethanol and sodium butanoate, and its boiling point is 77 °C. The teacher asks the students to identify each compound, explain the boiling-point difference between C and D, and suggest one chiral isomer of A and one ester isomer of D.
1. Identify A, B, C and D, giving a test or reaction that supports each identification.
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A is butan-1-ol. It is a straight-chain C4H10O alcohol; the reaction with sodium releases hydrogen gas: 2 CH3CH2CH2CH2OH + 2 Na → 2 CH3CH2CH2CH2ONa + H2.
B is butanal. It has formula C4H8O, gives a silver mirror with Tollens’ reagent (aldehydes are oxidised to carboxylates while Ag^+ is reduced to Ag), and does not react with NaHCO3 because it is not a carboxylic acid.
C is butanoic acid. It has formula C4H8O2, reacts with NaHCO3 with effervescence because it is an acid: CH3CH2CH2COOH + NaHCO3 → CH3CH2CH2COONa + CO2 + H2O.
D is ethyl ethanoate. It is an ester with formula C4H8O2, does not react with NaHCO3, and alkaline hydrolysis gives ethanol and sodium butanoate: CH3COOCH2CH3 + NaOH → CH3COONa + CH3CH2OH.
2. Explain why butanoic acid and ethyl ethanoate are isomers but have very different boiling points. Which is more soluble in water?
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Both have molecular formula C4H8O2, so they are constitutional isomers, but their functional groups differ. Butanoic acid has a carboxyl group with a strongly polarized O–H bond. Its molecules form strong hydrogen-bonded dimers, so a large amount of energy is needed to separate them; hence its boiling point is 164 °C.
Ethyl ethanoate is an ester. It has a polar C=O group and dipole–dipole attractions, but it has no O–H bond, so it cannot donate hydrogen bonds. Its intermolecular forces are weaker and its boiling point is only 77 °C.
Butanoic acid is more soluble in water because it can both donate and accept hydrogen bonds with water. Ethyl ethanoate can accept hydrogen bonds at its carbonyl oxygen but cannot donate, so it is less soluble.
3. Name one chiral isomer of A and one ester isomer of D. Explain why the isomer of A is chiral.
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A chiral isomer of A (C4H10O) is butan-2-ol, CH3CH(OH)CH2CH3. Its carbon-2 atom is bonded to four different groups: H, OH, CH3 and CH2CH3. It therefore has no plane of symmetry and exists as two enantiomers.
An ester isomer of D is methyl propanoate, CH3CH2COOCH3 (propyl methanoate, HCOOCH2CH2CH3, is also acceptable). These have the same molecular formula C4H8O2 as ethyl ethanoate but a different arrangement of the ester group.