The Kekulé structure of benzene suggests it should readily undergo addition reactions.

Discuss two pieces of evidence, one physical and one chemical, which suggest this is not the structure of benzene.

Formulate the ionic equation for the oxidation of propan-1-ol to the corresponding aldehyde by acidified dichromate(VI) ions. Use section 24 of the data booklet.

The aldehyde can be further oxidized to a carboxylic acid.

Outline how the experimental procedures differ for the synthesis of the aldehyde and the carboxylic acid.

Deduce the molecular formula of the compound.

Identify the bonds causing peaks A and B in the IR spectrum of the unknown compound using section 26 of the data booklet.

Deduce full structural formulas of two possible isomers of the unknown compound, both of which are esters.

Deduce the formula of the unknown compound based on its ¹H NMR spectrum using section 27 of the data booklet.

Physical evidence:

equal C–C bond «lengths/strengths»

OR

regular hexagon

OR

«all» C–C have bond order of 1.5

OR

«all» C–C intermediate between single and double bonds

Chemical evidence:

undergoes substitution reaction «more readily than addition»

OR

does not discolour/react with bromine water

OR

substitution forms only one isomer for 1,2-disubstitution «presence of alternate double bonds would form two isomers»

OR

more stable than expected «compared to hypothetical molecule cyclohexa-1,3,5-triene»

OR

enthalpy change of hydrogenation/combustion is less exothermic than predicted «for cyclohexa-1,3,5-triene»

M1:

Accept “all C–C–C bond angles are equal”.

[2 marks]

3CH₃CH₂CH₂OH(l) + Cr₂O₇^2–(aq) + 8H⁺(aq) → 3CH₃CH₂CHO(aq) + 2Cr³⁺(aq) + 7H₂O(l)

correct reactants and products

balanced equation

[2 marks]

Aldehyde:

by distillation «removed from reaction mixture as soon as formed»

Carboxylic acid:

«heat mixture under» reflux «to achieve complete oxidation to –COOH»

Accept clear diagrams or descriptions of the processes.

[2 marks]

«\(\frac{{136}}{{48 + 4 + 16}} = 2\)»

C₈H₈O₂

[1 mark]

A: C–H «in alkanes, alkenes, arenes»

AND

B: C=O «in aldehydes, ketones, carboxylic acids and esters»

[1 mark]

Any two of:

 OR C₆H₅COOCH₃

OR CH₃COOC₆H₅

 OR HCOOCH₂C₆H₅

Do not penalize use of Kekule structures for the phenyl group.

Accept the following structures:

Award [1 max] for two correct aliphatic/linear esters with the molecular formula C₈H₈O₂.

[2 marks]

C₆H₅COOCH₃ «signal at 4 ppm (3.7 – 4.8 range in data table) due to alkyl group on ester

[1 mark]