Three Vials of C4H6: Deciding Structures from Tests and Hydrogen Uptake
Three unlabelled vials, P, Q and R, each contain a pure hydrocarbon of molecular formula C4H6; all three decolourise bromine in carbon tetrachloride within seconds. Only P gives a cream-white precipitate when shaken with ammoniacal silver nitrate, and that precipitate redissolves on adding dilute nitric acid. Catalytic hydrogenation with excess hydrogen over nickel gives butane from P and from Q, each taking up 2 mol H2 per mol of hydrocarbon. Under the same conditions R takes up only 1 mol H2 per mol, and the product, C4H8, no longer decolourises bromine water. Ozonolysis of R (O3, then Zn/H2O) gives a single product, OHC–CH2–CH2–CHO. The teacher asks for full identification of all three, noting that exactly two acyclic structures remain possible for Q and that a further experiment is needed. The vials are known to contain only straight-chain or simple ring isomers, with no branched skeletons.
1. Identify P, write the reaction responsible for the silver precipitate, and explain why Q and R give no precipitate.
Read the worked answer
P is but-1-yne, CH3CH2C≡CH. The hydrogen on the sp-hybridised terminal carbon is weakly acidic, so the silver diammine complex removes it as a proton and an acetylide precipitates: CH3CH2C≡CH + [Ag(NH3)2]⁺ → CH3CH2C≡CAg↓ (cream-white) + NH4⁺ + NH3. Dilute nitric acid dissolves the precipitate by protonating the acetylide back to the alkyne and releasing Ag⁺, which is the standard confirmation. Q and R give nothing because the test needs a hydrogen attached to an sp carbon that carries a metal-binding lone pair: Q has no terminal alkyne C–H, and R has no sp carbon at all. A negative silver test therefore does not mean 'no unsaturation' - all three decolourise bromine - it only excludes a terminal alkyne. Ozonolysis of P would give propanoic acid plus CO2, which would place the triple bond at C1 and serve as a cross-check.
2. Identify R and explain how the hydrogenation and ozonolysis data together fix its structure.
Read the worked answer
C4H6 has two degrees of unsaturation. R takes up only one mole of H2, so it has only one π bond; the remaining degree of unsaturation must be a ring. The product C4H8 does not decolourise bromine, so the π bond has gone and the ring remains - R is an unsaturated ring, cyclobutene. Ozonolysis with reductive work-up opens the ring at the double bond. Cyclobutene gives a four-carbon chain carrying an aldehyde at each end, OHC–CH2–CH2–CHO, exactly as observed. That product fixes the ring size: a five-membered ring would have given a five-carbon dialdehyde, and a three-membered ring would have given a three-carbon dialdehyde. A side-chain alkene such as but-1-ene would have given two separate fragments, not one molecule. So R = cyclobutene, and its identification rests on two independent pieces of evidence - hydrogen uptake and product skeleton - not on the bromine test alone.
3. Identify the two possible structures for Q, then propose one decisive experiment with predicted observations, and explain why bromine addition alone is not decisive.
Read the worked answer
Q consumes 2 mol H2 to give butane and gives no silver acetylide, so Q has two π bonds and no terminal alkyne. The two candidates are but-2-yne, CH3C≡CCH3, and buta-1,3-diene, CH2=CH–CH=CH2 (but-1-yne is excluded by the negative silver test). Decisive test: ozonolysis. But-2-yne gives two moles of acetic acid per mole, with a vinegar smell and no aldehyde; buta-1,3-diene gives two moles of formaldehyde plus glyoxal, OHC–CHO, so a Tollens test on the products gives a silver mirror and the dialdehyde can be detected separately. A second clean check is reaction with maleic anhydride: the conjugated diene undergoes Diels–Alder addition to give a white crystalline adduct, whereas the alkyne, which is a dienophile rather than a diene, does not. Bromine addition is not decisive because both compounds decolourise bromine. But-2-yne gives a single dibromide, 2,3-dibromobut-2-ene, while buta-1,3-diene gives a mixture of 3,4-dibromobut-1-ene and 1,4-dibromobut-2-ene; the mixture is suggestive but not a clean identification in a first-year laboratory.