Chemistry 9701 · AS & A Level · States of matter

States of matter — practice question

Lawsone is the dye obtained from the henna plant, Lawsoneia inermis. Its natural colour is yellow, but lawsone reacts with proteins in hair and skin to form the familiar brown henna colour. Lawsone can be reduced quite readily to 1,2,4-trihydroxynaphthalene, compound A.
(a(i))[3]

Name any three functional groups in lawsone.

(a(ii))[3]

Describe a reaction, with reagent and conditions, that would let you tell lawsone from compound A. Describe the observations you would make with both compounds.

(a(iii))[1]

Suggest a reagent that could be used in the laboratory to turn lawsone into compound A.

(a(iv))[2]

Draw the structural formula for the compound obtained when lawsone reacts with $\text{Br}_2(aq)$.

(b(i))[2]

Compound A can be oxidised to lawsone by acidified $\text{K}_2\text{Cr}_2\text{O}_7$. Using the Data Booklet, Calculate the $E^\circ_{\text{cell}}$ for this reaction.

(b(ii))[2]

Construct an equation for this reaction, using the molecular formulae of lawsone, $\text{C}_{10}\text{H}_6\text{O}_3$, and compound A, $\text{C}_{10}\text{H}_8\text{O}_3$, in your equation.

(b(iii))[5]

A $20.0\ \text{cm}^3$ sample of compound A was acidified and titrated with $0.0500\ \text{mol dm}^{-3}$ $\text{K}_2\text{Cr}_2\text{O}_7$, and $7.50\ \text{cm}^3$ of the $\text{K}_2\text{Cr}_2\text{O}_7$ solution was required to reach the end-point. Calculate $[A]$ in the solution.

(c(i))

Suggest the identity of compound C, then draw its structure in the box above.

(c(ii))

A second compound, D, is also formed in the reaction above. D is an isomer of C and has the same functional groups as C, but positioned differently. Suggest one possible structure for D.

(c(iii))[3]

Suggest a mechanism for forming D from B and ethanoyl chloride by drawing the relevant structures and curly arrows in the scheme below.

Worked solution & mark scheme

This 21-mark question has a full step-by-step worked solution and mark scheme. One marking point: Three valid functional groups: ketone, alcohol, alkene, arene/aryl/benzene/phenyl

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