Indigo is the dye used in blue jeans. It was first obtained from plants of the indigofēra type, but today it is produced almost entirely by synthetic methods.
Indigo is not soluble in water, but this drawback can be removed by converting it into the colourless, water-soluble leuco-indigo. If cloth soaked in a solution of leuco-indigo is allowed to dry in air, the leuco-indigo changes back into insoluble blue indigo, which then precipitates onto the fibres of the cloth.
(a(i))[1]
State the molecular formula of indigo.
(a(ii))[2]
Identify three functional groups present in indigo.
(b(i))[1]
What type of reaction is involved when indigo is converted into leuco-indigo?
(b(ii))[1]
Suggest a laboratory reagent for this reaction.
(c)[5]
Suggest two chemical tests that could be used to tell indigo and leuco-indigo apart. Present your answers in the table below.
(d(i))[2]
When indigo is heated with hydrogen and a nickel catalyst, compound $A$, $\text{C}_{16}\text{H}_{28}\text{N}_2\text{O}_2$, is produced. Suggest a structure for $A$.
(d(ii))[1]
Calculate the volume of hydrogen, measured at room temperature and pressure, that would be absorbed if $2.50\ \text{g}$ of indigo underwent this reaction.
volume $= \ldots\ldots\ldots\ldots\ldots\ldots\ \text{dm}^3$
(e)[3]
Suggest the structure of the product formed when indigo reacts with an excess of $\text{Br}_2(aq)$.
Worked solution & mark scheme
This 16-mark question has a full step-by-step worked solution and mark scheme. One marking point: “The molecular formula is $\text{C}_{16}\text{H}_{10}\text{N}_2\text{O}_2$.” …