November 2020: Paper 2, Synthesis and analytical techniques

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A

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B

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A

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C

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D

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D

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A

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C

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D

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C

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Examiner's comment

Candidates found Question 10 challenging as they were required to demonstrate knowledge of the term alicyclic and to work out the number of isomers.

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A

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A

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A

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B

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C

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Examiner's comment

Common misconceptions Misconception Question 16(a) Many candidates either reversed the order of the locant numbers for the bromo/dimethyl substituents or did not recognise the need for two locant numbers for the dimethyl group (e.g. 3,3 – often reported as just 3 dimethyl).

Notes from the mark scheme and report

• DO NOT ALLOW 2-bromo-3-dimethylbutane
• DO NOT ALLOW methy/methly/brom
• ALLOW 3,3-dimethyl-2-bromobutane

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Notes from the mark scheme and report

• same structural formula AND different arrangement in space AND optical all needed
• DO NOT ALLOW same empirical/general formula
• IGNORE same molecular formula

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Notes from the mark scheme and report

• four bonds to the chiral C with at least two wedges
• two structures must be mirror images with correct connectivity

Skills: M4.2 Visualise and represent 2-D and 3-D forms including 2-D representations of 3-D objects; M4.3 Understand the symmetry of 2-D and 3-D shapes

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Examiner's comment

Other questions Question 16(c) Often candidates did not use skeletal formula, as required by the question, and did not place the radical dot in the correct position.

Notes from the mark scheme and report

• dot REQUIRED at the correct position on the chain
• 1 mark if both propagation steps correct but dot missing/on wrong C or not skeletal
• IGNORE dots in initiation

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Notes from the mark scheme and report

• further substitution
• substitution at different positions along the chain
• IGNORE separation of products, atom economy and yield

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Notes from the mark scheme and report

• B undergoes addition (dichloro product), C undergoes substitution on the ring

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Examiner's comment

Question 17(a)(ii) Candidates answered this question well.

Notes from the mark scheme and report

• B: π-electrons localised
• C: delocalised π-system
• B has higher electron density so polarises/attracts the electrophile more
• IGNORE charge density
• IGNORE 'B is more reactive' with no reference to electrophile

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Notes from the mark scheme and report

• AlCl3 + Cl2 → AlCl4– + Cl+ and H+ + AlCl4– → AlCl3 + HCl
• first arrow from ring to Cl+
• horseshoe covers 4 of 6 sides with gap towards C–Cl
• arrow from C–H bond back into ring
• DO NOT ALLOW intermediate with a CH3 missing

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Notes from the mark scheme and report

• 3C3H6O → C9H12 + 3H2O
• molecular formulae required
• H2O as by-product

Skills: M0.2 Use ratios, fractions and percentages

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Notes from the mark scheme and report

• C has 3 peaks
• D has 8 peaks

Skills: M4.3 Understand the symmetry of 2-D and 3-D shapes

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Notes from the mark scheme and report

• HNO3 with H2SO4 catalyst (1 of 2 if swapped)
• nitro compound then amine
• CH3COCl or (CH3CO)2O for final stage (IGNORE CH3COOH)
• DO NOT ALLOW AlCl3/FeCl3

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Examiner's comment

Question 18(a)(i) Many candidates did not correctly balance this equation or missed water as a product entirely.

Notes from the mark scheme and report

• K2Cr2O7 AND acid AND reflux
• 4[O] and 2H2O

Skills: M0.2 Use ratios, fractions and percentages

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Examiner's comment

Question 18(a)(ii) Candidates who answered this question well had clear, labelled diagrams.

Notes from the mark scheme and report

• dipoles δ+/δ– on each end of one hydrogen bond
• H bond from lone pair on O to H
• hydrogen bond labelled
• DO NOT ALLOW δ+ on H atoms of CH2

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Notes from the mark scheme and report

• ester link must be displayed
• end bonds MUST be shown
• DO NOT ALLOW more than one repeat unit
• IGNORE brackets and n

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Notes from the mark scheme and report

• ester bond/group can be hydrolysed or broken down
• 'bond breaks' alone insufficient
• IGNORE photodegradable

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Notes from the mark scheme and report

• 2SOCl2 gives ClOC(CH2)2COCl + 2SO2 + 2HCl
• ALLOW PCl5 or PCl3 route

Skills: M0.2 Use ratios, fractions and percentages

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Notes from the mark scheme and report

• each successive member differs by CH2 ('differs by CH2' alone insufficient)
• IGNORE same general formula and physical properties
• DO NOT ALLOW same empirical or molecular formula

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Notes from the mark scheme and report

• C24H48O

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Examiner's comment

Question 19(b) Candidates who found this question difficult often did not give a response that would identify all three of the functional groups (aldehyde, ketone and alkene).

Notes from the mark scheme and report

• each mark needs compound/group, reagent AND observation
• F: Tollens' gives silver mirror
• G: bromine decolourised, 2,4-DNP gives orange precipitate, no change with Tollens'
• IGNORE 2,4-DNP with F

Skills: PAG7 Qualitative analysis of organic functional groups

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Notes from the mark scheme and report

• curly arrow from lone pair on C (or – charge) of –CN to C of C=O
• dipole on C=O AND arrow from bond to O
• arrow from O– of intermediate to H+ (ALLOW to H of H2O)
• nucleophilic addition

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Examiner's comment

Question 19(c)(ii) Candidates often referred to NaCN and HCN in their responses.

Notes from the mark scheme and report

• one bonded atom (O) receives both electrons from the bond
• breaking of a covalent bond ('bond breaking' alone insufficient)
• DO NOT ALLOW both pairs of electrons
• IGNORE formation of ions

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Examiner's comment

Level of response questions Question 20(a) This question differentiated well.

Notes from the mark scheme and report

• mass of cyclopentanol 7.90 g (errors: inverse ratio 3.24 g, ignoring yield 5.06 g)
• neutralise e.g. Na2CO3
• separating funnel with organic layer on top
• dry with anhydrous salt e.g. MgSO4
• redistil at about 44 °C

Skills: M0.2 Use ratios, fractions and percentages; M2.2 Change the subject of an equation; PAG5 Synthesis of an organic liquid

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Notes from the mark scheme and report

• C=C peak at 1620–1680 cm–1 shows alkene
• O–H peak at 3200–3600 cm–1 shows unreacted alcohol
• IGNORE C–O at about 1000 cm–1

Skills: M3.1 Translate information between graphical, numerical and algebraic forms

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Notes from the mark scheme and report

• propan-2-ol
• NaBr/H2SO4 (ALLOW HBr)
• excess NH3 in ethanol gives (CH3)2CHNH2
• HCl for the salt

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Notes from the mark scheme and report

• ester, amide, amine, carboxylic acid (4 correct for 3 marks)
• IGNORE amino acid
• IGNORE formulae
• if more than 4 given incorrect groups counted first

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Notes from the mark scheme and report

• methanol plus two amino acids
• both amino acids must show NH3+ for full marks (acid conditions): 2 of 3 if NH2/NH3 without charge

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Notes from the mark scheme and report

• convert 167 mg to g
• M = 294
• 22.4, 22 or 23 all score 3
• ALLOW ECF and alternative methods

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.1 Recognise and use expressions in decimal and ordinary form; M0.2 Use ratios, fractions and percentages

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Examiner's comment

Misconception Question 22(a) The use of deuterated compounds in NMR analysis was often explained the use in terms of lack of spin but did not give examples (e.g.

Notes from the mark scheme and report

• example AND use needed for each mark: CDCl3 as solvent
• D2O to identify OH or NH protons
• 1 mark for D2O as a solvent

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Examiner's comment

Question 22(b) Candidates achieving Level 2 had most of the key features, e.g.

Notes from the mark scheme and report

• C6H9NO2 from composition and m/z 127
• IR: C=O at 1750 and C≡N at 2280 cm–1
• NMR: quartet 2H at 4.2 (CH3CH2O), quartet 1H at 2.9, doublet 3H at 1.7, triplet 3H at 1.3

Skills: M0.2 Use ratios, fractions and percentages; M3.1 Translate information between graphical, numerical and algebraic forms