P, Q and R are structural isomers that all have molecular formula $\text{C}_4\text{H}_8$. Each compound rapidly decolourises bromine when kept in the dark. P and Q do not show stereoisomerism, whereas R is found as a pair of geometrical (cis-trans) isomers. When all three are treated with hot concentrated, acidified potassium manganate(VII), they give the products listed in the table. P gives $\text{CO}_2$ and S ($\text{C}_3\text{H}_6\text{O}$). Q gives $\text{CO}_2$ and $\text{CH}_3\text{CH}_2\text{CO}_2\text{H}$. R gives only $\text{CH}_3\text{CO}_2\text{H}$. S reacts with 2,4-dinitrophenylhydrazine reagent, 2,4-DNPH, to make an orange crystalline product, but it does not react with Fehling’s reagent.
(a)[4]
Write the structural formulae of P, Q, R and S.
(b(i))[2]
Explain what the term stereoisomerism means.
(b(ii))[2]
Draw the displayed formulae for the geometrical isomers of R and name both of them.
(c)[2]
State a reagent that could be used to reduce S and name the organic product formed in this reduction.
Worked solution & mark scheme
This 10-mark question has a full step-by-step worked solution and mark scheme. One marking point: “P = $\mathrm{CH_2=C(CH_3)_2}$” …