Aromatic compounds: benzene and phenol Module 6

Mark scheme

Examiner's comment

Most candidates scored at least one mark for recognising that the C-C bond lengths in the delocalised model of benzene are the all the same. Many found it difficult to articulate what they meant, e.g. ‘all the C-C and C=C bond lengths are the same’. Candidates were given the mark for stating that there were ‘equal bond lengths’. Although this is technically not true, as the C-H bonds are not the same length as the C-C bonds, we gave the mark as only skeletal structures are shown here without C-H bonds. Many also scored the second mark but slips were often made by reference to ‘hydration’ or ‘bond’ enthalpy. Some just said ‘enthalpy will be less exothermic’. Some described the hydrogenation value as being lower or higher as we are considering negative values it is much clearer to do this in terms of ‘less exothermic’ or ‘less negative’. A few described the enthalpy of hydrogenation as being ‘more exothermic than expected’. Many candidates were able to quote the actual figures in their response, and while it is not necessary for candidates to learn the values, their effort is commendable. A small number of candidates discussed the lack of reactivity in terms of electrophilic addition – not realising this had already been given in the question in terms of reaction with bromine. STEM Contributors We have produced a STEM Contributors spreadsheet linked to the H432 specification. This includes the work by Kathleen Lonsdale and Linus Pauling on our understanding of the structure of benzene. This is a great resource for broadening understanding of the contribution made by individuals to the topics being taught in the specification.

Notes from the mark scheme and report

• all C–C bond lengths the same/between single and double
• enthalpy of hydrogenation less exothermic than expected
• DO NOT ALLOW enthalpy of hydration
• IGNORE resistance to reaction (given)

Mark scheme

Examiner's comment

Most candidates found this familiar question relatively straightforward, achieving all 3 marks. Some did not gain marks as the electron pair came from -OH rather than from the O or it was suggested that multiple lone pairs on the O were donated to the ring. Some suggested that the presence of the -OH group on the benzene ring disrupted the delocalised electron ring thereby making it less stable and so more reactive. Some also attributed the reactivity solely to the -OH group, suggesting that bromine would react to replace the OH. Some did not gain marks for not giving comparative answers. It was necessary to compare the reactivity of both benzene and phenol here. Some lost the mark for ‘increasing electron density’ as they referred to charge density or electronegativity instead. Lower attaining candidates often just stated that ‘phenol is more reactive than benzene’ without considering why this is the case. However, it was clear that many candidates had practised this type of question and knew what to include.

Notes from the mark scheme and report

• lone pair on O (one pair only) delocalised into ring
• electron density higher than in benzene
• ring more susceptible to electrophilic attack/polarises Br2
• responses must be comparative
• IGNORE activating, charge density, electronegativity

Mark scheme

Examiner's comment

The majority were able to get the mark for the 2,4,6-tribromophenol structure, with some candidates using the relative molecular mass given to confirm their answer. Many were also able to obtain the mark for the balanced equation. Common errors seen in balancing included omitting HBr as a product or only having 1.5 Br2. Some gave H2 as a product. Lower attaining candidates struggled, often replacing the -OH with a -Br group but not accounting for the molar mass.

Notes from the mark scheme and report

• 2,4,6-tribromophenol only (DO NOT ALLOW other positions)
• 3Br2 and 3HBr
• second mark dependent on first

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

This mark was achieved by most candidates. Common errors included: • CH3 or CH4 as reagent • Missing the reagent and only giving the halogen carrier • CH3COCl as halogen carrier or reactant • AgCl3 as halogen carrier

Notes from the mark scheme and report

• CH3Cl AND AlCl3 (ALLOW CH3Br, FeCl3, FeBr3, AlBr3)

Mark scheme

Examiner's comment

Many were confident with this mechanism with approximately half of all candidates achieving all 5 marks. Some lost a mark due to the addition of a lone pair of electrons to the NO2+ and showing the curly arrow moving towards the NO2. A missing or incorrect charge on NO2+ cost 2 marks both for the equation for the formation and for the arrow from the benzene ring to the electrophile. Some also had missing or incorrect charges on HSO4- which lost both equation marks. Some formed H3SO4 (with or without charges) which was then used to form H2 as well as H2SO4 when regenerating the catalyst. Marks were often lost for the intermediate – it was essential that the ring covered 4 out of the 6 sides with the opening orientated towards the -NO2 group. This was sometimes hard to judge if the ring was very small or the hexagon of benzene was uneven. Very few lost a mark for the omission of the methyl group on the intermediate. Some lost a mark for missing H+ with their final product. Lower attaining candidates often omitted the equations to form NO2+ and/or reform H2SO4. Some attempted to show a reaction with H2SO4 despite being given the formula of G in the question.

Notes from the mark scheme and report

• HNO3 + H2SO4 → NO2+ + HSO4– + H2O
• H+ + HSO4– → H2SO4
• arrow from ring to NO2+
• intermediate must keep methyl group with correct position and π-ring covering about 4 sides with gap towards the substituted C
• arrow from C–H bond into ring AND H+

Mark scheme

Examiner's comment

Many were able to identify that the -NO2 group is 3,5-directing as shown in both their explanations and the structure given. Only the highest attaining scored the mark for the explanation as it was necessary to suggest why synthesis 1 was successful as well as why synthesis 2 wasn’t. To do this, candidates had to deduce from synthesis 1 that -CH3 is 2,4-directing. Some suggested -NO2 or -CH3 was 1-directing. This, alongside structures showing the group next to the -NO2 group, suggests some confusion with how counting is achieved around the ring. It was also evident that some, although suggesting the -NO2 group is 3,5-directing, then drew a structure with the -CH3 in position 4. A few candidates suggested that CH3 ‘does not have a directing effect’ or ‘can direct anywhere on the ring’, presumably because it is not on the list of groups they need to know. Some candidates gave explanations based on reactivity, e.g. ‘the -NO2 group is electron withdrawing so makes the ring less reactive’, or that the -NO2 group itself would react with reagents used to add the methyl group, e.g. adding Cl to the ring instead.

Notes from the mark scheme and report

• CH3 is 2,(4)-directing AND NO2 is 3-directing
• IGNORE 'different directing effects' unless one is specified
• product is 3-nitromethylbenzene (mark scheme also shows a 3,5-disubstituted alternative)

Mark scheme

Examiner's comment

This question was answered well, with most candidates identifying that ibuprofen contains a phenol group or that it does not contain a carboxylic acid. A named functional group was required in the answer. Some described phenol incorrectly as not being an acid and this was not credited. It was rare for incorrect functional groups to be seen in candidate responses.

Notes from the mark scheme and report

• it is a phenol / has no COOH: a correct functional group must be named
• IGNORE 'too weak an acid'
• DO NOT ALLOW 'not acidic', ketone or amine

Mark scheme

Examiner's comment

This name caused problems for most candidates. Phenyl methanol was expected, and unambiguous representations were allowed including 1- phenylmethanol, phenylmethan-1-ol and 1-phenylmethan-1-ol Many candidates incorrectly based the name on methylphenol or hydroxymethylbenzene.

Notes from the mark scheme and report

• phenylmethanol
• DO NOT ALLOW hydroxymethylbenzene or methylphenol

Mark scheme

Examiner's comment

This question appeared to be equally challenging for candidates. Common errors included names without the lowest possible numbering. The methyl group was given on the top which could have influenced candidates to start counting from there – more practice is needed to help with naming simple aromatics. Other common errors included:1-ethyl-2,4-methylbenzene (missing di-) and 1-ethyl-2,3- dimethylbenzene (miss counting groups).

Notes from the mark scheme and report

• 1-ethyl-2,4-dimethylbenzene
• smallest numbering needed – DO NOT ALLOW 1,5-dimethyl-2-ethylbenzene
• DO NOT ALLOW misspellings such as dimethy, ethly
• IGNORE alphabetical order

Mark scheme

Examiner's comment

Most candidates were well prepared for this question, with over half of candidates gaining all 5 marks. A significant number of candidates showed halogenation, generating Cl+ as an electrophile, instead of alkylation. Others attempted to substitute the ethylbenzene ring rather than benzene. Further common errors included incorrect connectivity in the ethyl group on the intermediate, curly arrows coming from hydrogen atoms rather than C–H bond to reform the -ring and omission of an H+ ion at the end of mechanism.

Notes from the mark scheme and report

• AlCl3 + CH3CH2Cl → CH3CH2+ + AlCl4–
• arrow from π-ring to the carbocation
• intermediate with horseshoe covering about 4 sides and gap towards the substituted C
• arrow from C–H bond back to ring with H+ lost
• H+ + AlCl4– → AlCl3 + HCl
• no half-headed arrows

Mark scheme

Examiner's comment

Most candidates were able to correctly recognise the correct direction for substitution, with over half gaining all 3 marks. Marks were most often lost for giving multiple substitution products despite being asked for the monosubstituted products. Many unnecessarily drew the same structures but with different orientations i.e. substituting on carbon-3 of a ring is the same as substituting on carbon-5.

Notes from the mark scheme and report

• CN is 3-directing: 3-chlorobenzonitrile only
• NHCOCH3 is 2- and 4-directing: two products
• IGNORE multisubstituted products and HCl
• Ensure students understand the term ‘monosubstituted’ and practise naming compounds to give the lowest possible numbering. This will also help them to recognise the equivalent structures.

Mark scheme

Examiner's comment

The majority of candidates were able to give a suitable tri-substituted product, with many showing the structure although not asked for in the question. Many were also able to give a correct balanced equation too. Some were unsure how phenylamine would react showing the reaction with the amine group or only giving a monosubstituted product. Some didn’t form HCl as another product, reacting phenyl amine with 1.5 Cl2 instead. Others gave hydrogen as the product.

Notes from the mark scheme and report

• 2,4,6-trichlorophenylamine (ALLOW any trichloro structure) + 3HCl with 3Cl2
• 1 mark for a balanced equation using Br2

Mark scheme

Examiner's comment

Similar questions have been seen previously and many candidates were able to give clear and concise responses. The first marking point was the most frequently lost as although many described -NH2 as electron donating, they were not able to fully explain its role. Some understood that a lone pair was donated into the -ring but did not specify that the lone pair was on the nitrogen. Other marks were lost by not making comparison to benzene, e.g. high electron density, polarises Cl2. Some repeated the information from the question regarding phenylamine being more reactive with electrophiles but not explaining why. Lower attaining candidates often described the structure of the benzene ring or referred to phenylamine being more electronegative.

Notes from the mark scheme and report

• lone pair on N (not just NH2) partially delocalised into the π-system
• electron density of ring higher than benzene
• more susceptible to electrophilic attack/polarises Cl2 more – comparisons required
• DO NOT ALLOW two lone pairs
• IGNORE activating, charge density, electronegativity

Mark scheme

Examiner's comment

Another fairly challenging question, however most secured at least one mark for giving an equation for the reaction of sulfuric acid with sodium carbonate. Less confident candidates struggled to gain any marks as they were unable to give correct formula for sodium sulfate, giving NaSO4 for example. Although many attempted the equation showing the reaction of compound G with sodium carbonate, only some correctly identified that only the carboxyl group would react, not the phenol. A small minority of students were able to balance the second equation gaining all 3 marks.

Notes from the mark scheme and report

• Na2CO3 + H2SO4 → Na2SO4 + CO2 + H2O
• only the COOH of G reacts (phenol OH unchanged) giving –COONa
• 2 G per Na2CO3 (ALLOW 1:1 forming NaHCO3)
• ALLOW ionic equations

Mark scheme

Examiner's comment

The best responses correctly identified that using sodium hydroxide was not an improvement and explained this either by stating that it would react with the phenol group or hydrolyse the ester group in compound H. However, most candidates appeared not to consider a reaction with H in their answer. Many focused on the neutralisation of sulfuric acid in a similar way to sodium carbonate and gave responses such as: • stronger base • no effervescence so harder to see when completely reacted • no CO2 produced so easier/safer/higher atom economy/less waste • requires double the moles compared to Na2SO4 to react

Notes from the mark scheme and report

• NaOH would react with the phenol –OH OR hydrolyse the ester (compound H)
• DO NOT ALLOW 'reacts with alcohol'

Mark scheme

Examiner's comment

About half the candidates suggested a range of creditworthy responses with ‘different bond lengths’ and ‘decolorises bromine’ being the most common.

Notes from the mark scheme and report

• (P–N) bond lengths different, or more exothermic hydrogenation, or reacts with bromine by addition
• IGNORE C–C bond lengths
• IGNORE 'alternating single and double bonds'
• IGNORE 'more reactive' without example

Mark scheme

Examiner's comment

This was another question where valuable information: ’all N and Cl atoms are bonded to P atoms’ had been provided. Many of the structures seen ignored this information with chlorine often been shown bonded to a nitrogen atom. Nitrogen atoms were often shown with 1 bond only and chlorine atoms in the ring structure with 2 or more bonds. Most structures contained 6 or 3-membered rings. This was a difficult question, requiring candidates to use the supplied information to come up with realistic structures that met chemical bonding rules. Only about a quarter of candidates could be given any mark. The Kekulé theme in Questions 4 (c) (i) - (iv) should have prompted candidates that a Kekulé structure was likely here. Several other structures were allowed providing that they met normal chemistry bonding rules.

Notes from the mark scheme and report

• six-membered alternating P–N ring
• each P bonded to 2 Cl
• each N with 3 bonds and each P with 5 (or 3) bonds for second mark
• zero marks if N bonded to Cl or N with only 1 bond

Skills: M4.2 Visualise and represent 2-D and 3-D forms including 2-D representations of 3-D objects

Answer

B

Mark scheme

Examiner's comment

The answer to this question was generally well known. The most common incorrect response was A.

Mark scheme

Examiner's comment

Many candidates knew this mechanism well with the majority gaining 4 or 5 marks here. The mark most frequently lost was for the incorrect drawing of the intermediate. Many candidates did not appreciate that two electrons are removed from the  ring in the first stage of the mechanism resulting in the partial delocalisation of electron density across four sides of the benzene ring in the intermediate. In addition, it must have the correct orientation with the opening at the three position where the nitro group is added. Some candidates nitrated the 2 or 4 position instead. Some also showed nitration of benzene then the second nitration. Other common errors included missing or extra charges, missing NO2 group on intermediate, use of NO3+ or other incorrect electrophiles. Lower scoring responses were characterised by unclear diagrams and inaccurate use of curly arrows. Many were often able to score 1 mark for showing C-H bond on benzene ring breaking and electrons going back into the ring. Such responses often included errors in the equations showing the role of the sulfuric acid catalyst, e.g. formation of H3SO4-, or omitted these steps entirely. Others evidently didn’t know what a nitro group was with -NO3 giving just -N and even -NH3 instead.

Notes from the mark scheme and report

• HNO3 + H2SO4 → NO2+ + HSO4– + H2O
• H+ + HSO4– → H2SO4
• arrow from ring to NO2+
• intermediate must keep the original NO2 at the 3-position with horseshoe over about 4 sides, gap towards C with NO2 and H
• arrow from C–H bond into ring
• Encourage candidates to look at more recent mark schemes to check for tips on how to correctly draw out mechanisms. An easy tip for ensuring the correct intermediate for electrophilic substitution is to place a dot inside the ring for each C without the new substituent attached and then connect the dots. Only four out of the six sides should be covered by the broken delocalised  ring (as two electrons have formed the new bond).
• Alternatively, some find it easier to redraw, so horseshoe ring opens upwards (like a smiley face), rather than at an angle.

Mark scheme

Examiner's comment

Overall, this question discriminated well with most candidates gaining some credit. Most candidates were able to suggest a correct product for the reaction of salicylic acid with bromine, with just a few candidates replacing the OH group with a Br atom. Most candidates recognised that the carboxylate salt would be produced from the reaction with Na2CO3 but not all were able to recall that the phenolic OH group is too weak acid to react. Some missed an O on carboxylate i.e. -CO-Na+. Some added Na or CO3 directly to the ring. The reaction with the anhydride proved much more challenging. All sorts of weird and wonderful structures of the main ‘ester’ product were given including joining the CH3 group of propanoic anhydride to the O of the phenol group. Some used the bottom two boxes to form esters with the COOH and OH groups in turn. Many did not identify propanoic acid as a by-product even if they had correctly identified the ester product. Water was a common incorrect product. Marks were also occasionally lost for missing the benzene ring on structures or missing the -OH or - COOH groups that were left unreacted.

Notes from the mark scheme and report

• Br2 substitutes the ring (ALLOW any position, up to 4 Br)
• Na2CO3 reacts with COOH only (COO– or COONa), phenol OH unchanged
• anhydride esterifies the phenol OH giving ester + CH3CH2COOH
• IGNORE COOH forming an anhydride

Mark scheme

Examiner's comment

This question was the hardest part of Question 5 and about half the candidates were not given any marks. Some drew the sodium carboxylate salt of aspirin structure, leaving the ester link intact. A large number of candidates realised that the ester would be hydrolysed. Sometimes the sodium salts were often not shown and, even they were shown, the phenol group was often shown intact. The hardest mark was the formation of 2H2O and a large number of candidates showed the more intuitive but incorrect ‘3H2O’ instead.

Notes from the mark scheme and report

• 3NaOH AND 2H2O
• products sodium salt of phenoxide-carboxylate (ONa and COONa) and CH3COONa
• ALLOW ionic forms
• 1 of the 2 product marks if both shown as COOH and OH

Skills: M0.2 Use ratios, fractions and percentages

Answer

D

Mark scheme

Examiner's comment

This question discriminated well, with higher ability candidates correctly identifying D. Often students overlooked the sigma bonds in the aromatic ring and selected B.

Mark scheme

Examiner's comment

Few candidates knew the test for a phenol group. Frequent incorrect responses involved the production of a gas with a carbonate or stating that bromine water is decolourised but failing to state that a white precipitate is also formed. A significant number of candidates also stated that the reaction with sodium hydroxide confirms the presence of the phenol group.

Notes from the mark scheme and report

• indicator shows acidity AND no reaction with carbonate
• ALLOW bromine giving white precipitate
• ALLOW FeCl3 giving violet/purple colour

Skills: PAG7 Qualitative analysis of organic functional groups

Mark scheme

Examiner's comment

This question required candidates to apply their knowledge of the mechanism of electrophilic substitution. Examiners were encouraged by the number of excellent responses to this question, with the majority of candidates securing 3 out of 4 marks. Common errors included the omission of HCl as product from the regeneration of the catalyst or candidates attempting to substitute at the 2 position.

Notes from the mark scheme and report

• Cl2 + AlCl3 → Cl+ + AlCl4– AND H+ + AlCl4– → AlCl3 + HCl (both for 1 mark)
• arrow from ring to Cl+
• horseshoe over more than half the ring with gap towards C–Cl
• DO NOT ALLOW intermediates substituted at positions 3 or 5
• arrow from C–H bond into ring

Mark scheme

Examiner's comment

The most able candidates scored well on what proved a difficult question for many. Although the vast majority of candidates knew about the reasons behind compounds K and L’s increased reactivity many were unable to express themselves clearly to gain credit. Often answers lacked the specific detail about the lone pair on the oxygen atom in the –OH group being delocalised into the ring. Less successful answers discussed electrons being supplied to the ring, the -OH group providing the electrons to the ring or just that –OH is an activator. More successful answers expressed the increase in electron density and the subsequent increase in compound K and L’s susceptibility to electrophilic attack. This response correctly states that the electron density is higher which result in K and L being more susceptible to electrophilic attack and receives 2 marks. To score the third mark, the candidate needed to identify that that the lone pair is on the oxygen of the OH group.

Notes from the mark scheme and report

• a lone pair on O delocalised into the ring (DO NOT ALLOW two lone pairs)
• electron density higher
• ring more susceptible to electrophilic attack/polarises Cl2 more
• IGNORE activating, charge density
• IGNORE 'react more readily' (given in question)

Mark scheme

Examiner's comment

Candidates were familiar with the reagents required in these two reactions.

Notes from the mark scheme and report

• HNO3 with H2SO4 for nitration
• Sn and HCl for reduction
• IGNORE concentrations
• IGNORE NaOH after Sn/HCl
• IGNORE H2

Answer

C

Mark scheme

Answer

A

Mark scheme

Mark scheme

Examiner's comment

Question 20(a)(i): Candidates who answered this question well had clear mechanisms.

Notes from the mark scheme and report

• stage 1: arrow from lone pair/charge of OH– to H of O–H and arrow from O–H bond to O, H2O formed
• stage 2: arrow from ring to C of CO2 AND arrow from C=O to O
• intermediate horseshoe with gap at C bearing CO2– and phenolic O– present
• arrow from C–H bond into ring AND H+

Mark scheme

Notes from the mark scheme and report

• OH–: base (IGNORE nucleophile / donates electron pair)
• CO2: electrophile / electron pair acceptor (IGNORE lone pair acceptor)

Mark scheme

Notes from the mark scheme and report

• 2.62 : 1.31 : 1.31 : 3.06 gives C6H3N3O7, linked to M = 229.0
• trinitrophenol
• 4 peaks = 4 carbon environments fits 2,4,6 or 3,4,5
• –OH 2,4-directing so 2,4,6-trinitrophenol
• no ECF from wrong molar ratio

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

Answer

A

Mark scheme

Answer

C

Mark scheme

Mark scheme

Notes from the mark scheme and report

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

Mark scheme

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

Mark scheme

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

Mark scheme

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

Answer

C

Mark scheme

Answer

A

Mark scheme

Examiner's comment

This question proved difficult. Many candidates correctly deduced that all three chemicals would react with phenol and selected A. Some candidates did not recognise that phenol would react with HNO3 without the need for a catalyst and selected B. Other candidates did not consider the weak acidity of phenol and selected C.

Mark scheme

Examiner's comment

Candidates found this question difficult. The majority of the cohort referred to the overlap of p orbitals in either the Kekulé or delocalised model. The best responses developed this idea further and made reference to π bonds above and below the bonding carbon atoms. Although some candidates were able describe the difference between the two models, the responses of others lacked detail. Describing conceptual ideas, such as orbital overlap and π bonds, is understandably tricky. Candidates are advised to draw diagrams. These serve as a good basis from which to construct a written response. Exemplar 5 shows how effective the use of diagrams can be. This response starts with a diagram for both the Kekulé and delocalised models of benzene. Each diagram is clear and shows the p orbitals and π bonds separately. This is an excellent response for future candidates to follow. The candidate’s written response is effectively a description of the diagrams they have drawn and describes the similarities and differences between these two models clearly. This response scored full marks and demonstrates the best style of response seen in this part.

Notes from the mark scheme and report

• sideways overlap of p-orbitals (p-orbital label required)
• π bond above and below the plane
• Kekulé alternating/localised π bonds vs delocalised π ring over all six carbons
• IGNORE σ bonds

Mark scheme

Examiner's comment

Candidates were well prepared for this question and the majority of the cohort scored two marks. The most common piece of evidence given was the lack of reactivity of benzene with bromine, with candidates citing the need for a catalyst for the reaction to occur. Responses also included reference to carbon-carbon bond lengths as well as the enthalpy of hydrogenation. A small but significant proportion of the cohort referred to hydration rather than hydrogenation.

Notes from the mark scheme and report

• all C–C bond lengths equal/between single and double
• ∆H hydrogenation less exothermic than expected (IGNORE hydration)
• resistance to reaction needs comparison with alkenes ('benzene is unreactive' not sufficient)

Mark scheme

Examiner's comment

This question required candidates to apply their knowledge of the mechanism of electrophilic substitution to the formation of phenylethanone from benzene. Examiners were encouraged by the number of excellent responses to this question, with the majority of candidates securing four out of five marks. Common errors included accuracy of curly arrows (Exemplar 7 below) and omission of HCl as product from the regeneration of the catalyst. The responses of lower ability candidates also contained errors in the equation for the formation of the electrophile. Such responses used chlorine rather than ethanoyl chloride. This response demonstrates a near perfect attempt at this question. The equations for the formation of the electrophile and regeneration of the catalyst are correct. The first curly arrow is drawn accurately from the circle of the benzene ring to the correct carbon atom of the electrophile. The intermediate is correct, with the π ring over the five carbon atoms and in the correct orientation. Unfortunately the curly arrow drawn to reform the π bond starts at the H atom rather than from the bond. This small error has prevented this candidate from being given full marks.

Notes from the mark scheme and report

• CH3COCl + AlCl3 → CH3C+=O + AlCl4–
• arrow from ring to C of C=O
• intermediate horseshoe over about 4 of 6 sides, gap towards C with COCH3
• arrow from C–H bond back into ring
• H+ + AlCl4– → AlCl3 + HCl

Mark scheme

Examiner's comment

Most candidates correctly named the organic compound as 4-chloro-3,5-dimethylphenol. The numbers caused candidates less issues than the need for the ‘di’ prefix to signify two methyl groups in the name.

Notes from the mark scheme and report

• 4-chloro-3,5-dimethylphenol
• look for dimethyl
• DO NOT ALLOW meth or methy

Mark scheme

Examiner's comment

Most candidates identified the phenol functional group but a satisfactory test proved to be far more difficult. Many candidates identified that there would be no effervescence with sodium carbonate but it was common that the pH being less than 7 was often not seen. Neutralisation with sodium hydroxide was often seen but with no observation. Although not in the specification, the reaction with bromine to produce a white precipitate was often seen as an acceptable alternative test for the phenol group.

Notes from the mark scheme and report

• phenol (DO NOT ALLOW alcohol or hydroxide
• IGNORE hydroxyl/OH)
• acidic to indicator/pH < 7 AND no reaction with carbonate
• ALLOW bromine giving white precipitate

Skills: PAG7 Qualitative analysis of organic functional groups

Answer

C

Mark scheme

Examiner's comment

Almost all candidates identified C (CH3COCl) as the reagent required for this reaction.

Answer

A

Mark scheme

Examiner's comment

The bonding in benzene is well known by candidates at this level and most correctly selected A as their response.

Mark scheme

Examiner's comment

The relative reactivity of phenol is well known by candidates at this level and the majority scored two or three marks. Candidates who did not score full marks gave imprecise or vague explanations. This included reference to ‘higher charge density’ or ‘higher electronegativity’ of the phenol ring, rather than ‘higher electron density’.

Notes from the mark scheme and report

• lone pair on O (p-orbital) delocalised into the ring/π-system
• electron density higher
• more susceptible to electrophilic attack
• IGNORE activating, charge density, electronegativity
• IGNORE 'reacts more readily' with no reference to electrophile

Mark scheme

Examiner's comment

This question required candidates to apply their understanding of electrophilic substitution to the reaction of methylbenzene with sulfur trioxide. Examiners were encouraged by the quality of responses to this question. Most candidates secured full marks in this part. Some candidates did not show the curly arrow for the breaking of the S=O bond, while others omitted the methyl group from the intermediate. Exemplar 4 shows an excellent response. This response demonstrates an excellent example for candidates to follow. Curly arrows are drawn accurately, with each arrow touching the bond it starts from. The intermediate has been drawn clearly, using all the space provided. The ‘horseshoe’ has been drawn accurately over five of the carbon atoms with the positive charge shown neatly in the centre.

Notes from the mark scheme and report

• curly arrow from π-ring to S AND from S=O bond to O
• intermediate horseshoe must cover more than half the ring with gap towards C bearing SO3–
• SO3– fully displayed
• curly arrow from C–H bond back into ring
• DO NOT ALLOW intermediate with CH3 missing

Mark scheme

Examiner's comment

This question assessed different reactions of compound H, 4-hydroxybenzoic acid, and discriminated well. Two of the reactions focused on acid-base chemistry, using the reagents Na2CO3 and NaOH. Many candidates recognised that the carboxylic acid group would react in both cases but only some managed to identify when the phenol group was involved correctly. A number of responses suggested that a phenoxide ion was formed with sodium carbonate but not with sodium hydroxide. The third reaction was substitution with bromine. This reaction appeared more familiar to all candidates with the majority scoring this mark. A small proportion of candidates substituted the phenol OH group or carboxylic acid group.

Notes from the mark scheme and report

• Na2CO3 reacts with COOH only
• NaOH reacts with both COOH and phenol OH
• Br2 substitutes on the ring (ALLOW any ring H, multiple substitution)
• DO NOT ALLOW –COO–Na covalent bond
• 1 mark if top two structures in the wrong boxes

Mark scheme, with the levels and indicative points

Examiner's comment

Overall, this part was answered well. Good answers were well organised, showing clear mechanisms with precisely positioned curly arrows. Most candidates included a clear definition of an electrophile and were able to select appropriate reactions. Most candidates were familiar with the mechanisms for electrophilic addition and electrophilic substitution. Mechanisms of an alkene with HBr or Br2 and benzene with NO2 + or Br+ were the most commonly seen. Most candidates showed the role of a catalyst in electrophilic substitution. Common errors/omissions were the direction of the curly arrow from the aromatic ring to the electrophile, the position of the curly arrow when H+ is lost from an aromatic intermediate, and not showing the lone pair when Br- attacks a carbocation. Some answers lacked detail and gave only two mechanisms with minimal supporting words. Lower ability candidates described nucleophilic addition or substitution as one of their mechanisms or had curly arrows going in the wrong direction. A few candidates answered in prose without including equations or diagrams. Candidates are advised that mechanisms must always be communicated in the usual diagrammatic way. and electrophilic substitution, including the role of the FeCl3 catalyst. Curly arrows are precisely positioned, with correct use of lone pairs and charges. The candidate has demonstrated excellent knowledge and understanding. The response in Exemplar 17 is clearly at a different level. The candidate has chosen an alkane rather than an alkene and has used curly arrows and charges incorrectly. This candidate appears to have been poorly prepared.

Notes from the mark scheme and report

• electrophile as electron pair acceptor
• aliphatic e.g. ethene + Br2 electrophilic addition with dipole, curly arrows and carbocation
• aromatic e.g. nitration/halogenation of benzene electrophilic substitution with generation of electrophile and regeneration of catalyst
• accurately positioned curly arrows and charges

Mark scheme

Notes from the mark scheme and report

• HNO3 + H2SO4 → NO2+ + HSO4– + H2O
• first arrow from the ring to NO2+
• intermediate horseshoe covering about 4 of 6 sides with gap towards C bearing NO2
• H+ + HSO4– → H2SO4 regenerates catalyst

Mark scheme

Notes from the mark scheme and report

• must give rank order: phenol most easily nitrated AND benzoic acid least

Mark scheme

Notes from the mark scheme and report

• lone pair on O (partially) delocalised into the π-system
• –COOH is electron withdrawing
• link electron density of ring to susceptibility to attack by electrophiles
• IGNORE activating/deactivating

Mark scheme

Notes from the mark scheme and report

• Br2 AND AlBr3/FeBr3/Fe
• intermediate 3-bromonitrobenzene
• Sn AND (conc) HCl
• IGNORE names (formulae asked for), conditions and H2

Mark scheme

Notes from the mark scheme and report

• NH2 is 2,4-directing (ALLOW ortho/para)
• 2-bromo- and 4-bromophenylamine
• IGNORE electron donating/withdrawing