Carbon–carbon bond formation Module 6

Mark scheme

Examiner's comment

Many candidates scored all 3 marks here showing that the mechanism is well known. Some struggled with the skeletal formula given but were given credit for drawing out structural or displayed formula, provided that the central C showed correct number of bonds. Most common reasons for losing marks: • KCN and KBr shown in the mechanism rather than ions • arrow from lone pair or negative charge on N of CN • dipole on C-Br the wrong way round • missing the negative charge on CN- • curly arrows shown in the wrong direction Remind candidates that curly arrows should only be used to show the movement of a pair of electrons and not the formation/breaking of an ionic bond. A significant number of candidates chose to write more than one version of the mechanism without crossing out the one they did not wish to be marked. This led to marks being lost through contradictions. Teachers should remind candidates of the importance of submitting only one response to each question.

Notes from the mark scheme and report

• arrow from lone pair or – charge on C of CN– to C of C–Br
• dipole AND arrow from C–Br bond to Br
• product AND Br– (KBr does NOT show Br–)
• no double- or half-headed arrows
• ALLOW SN1

Mark scheme

Examiner's comment

The majority of candidates recognised this to be nucleophilic substitution. The most common incorrect response seen was nucleophilic addition.

Notes from the mark scheme and report

• nucleophilic substitution

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

This question was well-answered with around a half of candidates scoring 8 or 9 marks. The full range of marks was seen across the whole cohort. The most common reasons for losing marks were for the addition of extra, contradictory reagents or for missing hydrogens from structures. Bromoethane to amine: The amine structure was mostly correct with a few adding an extra hydrogen to the amine i.e. CH3CH2NH3. Most gave the correct reagent as NH3 but some omitted either ethanol or excess, so did not score here. Bromoethane to alcohol: The alcohol structure was usually correct. However, many lost the reagent mark here for the addition of acid/ H+/H2SO4 alongside the hydroxide. Bromoethane to nitrile: A suitable cyanide was often used, but again many included an acid catalyst or aqueous conditions (aq) so lost the mark here. Ideally candidates would react in ethanol to prevent hydrolysis of the haloalkane. Nitrile to carboxylic acid: Most gave the correct carboxylic acid structure, while occasionally ethanoic acid was given instead of propanoic acid. Most identified the need for acid as a reagent but some omitted water or (aq). Nitrile to amine: H2 was often seen as the reagent but sometimes without a catalyst. Many gave an incorrect structure here either with a missing C (i.e. ethylamine) or missing hydrogens on the first carbon i.e.

Notes from the mark scheme and report

• NH3 in ethanol/excess NH3 gives CH3CH2NH2
• OH–(aq) gives ethanol
• KCN in ethanol (DO NOT ALLOW HCN, CN–/H+ or aqueous)
• H+(aq) gives propanoic acid
• H2 AND Ni gives CH3CH2CH2NH2 (3 C not 2 C)
• DO NOT ALLOW OH– or NH2– as structures
• structure with H missing penalised once
• A useful resource for teaching about organic synthetic routes including functional groups, reagents and two-step processes can be found in the Topic Exploration pack on Teach Cambridge.

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

Answer

D

Mark scheme

Examiner's comment

The most common incorrect response seen here was B. Candidates understood that nitriles can be hydrolysed but did not realise that it would give pentanoic acid not butanoic. Those who drew out the structures of the products for each were more likely to give the correct answer, D.

Mark scheme, with the levels and indicative points

Examiner's comment

This challenging level of response question was generally well attempted. Many candidates recognised the reagents required in this synthesis but fewer candidates were able to deduce three correct reagents in the right order with equations to achieve Level 3. Most candidates achieved Level 2 4 marks. Many correctly identified suitable reagents but carried out the stages in the wrong order. A common error was to carry out electrophilic addition with HBr first then react with cyanide ion, not realising both Br groups would react and the reaction would not be selective. Alternatively, having introduced the nitrile group then carried out the reduction first, not realising that the C=C would also be reduced. The lowest scoring responses were often incomplete and despite identifying some reagents did not give equations. Candidates are encouraged to read the questions carefully so they can make sure their response covers all the detail required. Many candidates used molecular formula in their equations. It is usually preferrable in organic chemistry to give structures. Some gave surplus information such as the mechanisms and reaction conditions for each reaction. The key to answering this question well was knowing reagents for different functional group interconversions as well as planning each step to make sure of a logical synthesis. Some candidates were seemingly confused by the term ‘intermediate’ and gave an intermediate as in a mechanism, e.g. carbocation.

Notes from the mark scheme and report

• order matters: CN– (ethanol) first (it would substitute both Br atoms after HBr), then HBr across C=C (Markownikoff), then H2/Ni (would reduce C=C if before HBr)
• ALLOW LiAlH4 as stage 2 or 3
• IGNORE conditions
• A useful resource for teaching how to identify functional groups and practice at devising synthetic routes is the Topic exploration pack on Organic synthesis.
• This should be used in conjunction with the reaction pathways summaries.

Mark scheme

Examiner's comment

There were many excellent examples of precisely drawn mechanisms for the reaction of acrolein with sodium cyanide in acidic conditions and so most candidates gained at least 3 marks. The importance of accuracy when drawing curly arrows needs to be emphasised when teaching mechanisms - arrows must start at lone pairs or negative charges or come from bonds. Many candidates lost marks due to incorrect arrows. Common errors included the use of NaCN or HCN rather than the cyanide ion, the first curly arrow coming from the N of CN-, omission of partial charge across the C=O double bond and addition of partially charges to hydrogen or oxygen. Lower scoring responses often included an intermediate and/or product containing sodium. Some attempted electrophilic addition using HCN across the double bond. A few gained some credit for the mechanism for a competing reaction with nucleophilic addition on CH2 of C=C. This is not covered in the A Level specification and no candidates scored full marks for this alternative.

Notes from the mark scheme and report

• arrow from lone pair/charge on C of CN– to C of C=O
• dipole on C=O AND arrow from C=O bond to O
• intermediate with O– (DO NOT ALLOW δ– on O) and arrow to H+ (ALLOW H of H2O)
• product hydroxynitrile
• 1,4-conjugate addition credited as alternative

Mark scheme, with the levels and indicative points

Examiner's comment

Overall, candidates performed well when answering this question. They were required to identify that compound A shows optical isomerism and to choose a reaction for each of the three functional groups. Candidates were also expected to use structures for the organic products. To achieve the highest level of response, a description of optical isomerism should be accompanied by 3D diagrams of the optical isomers. Optical isomerism was usually identified, with associated diagrams with almost all candidates identifying the chiral centre. Most attempted 3D diagrams but candidates do need to take care that the groups attached to the chiral C atom are those in compound A and that no parts of chains are omitted. Optical isomers do also require use bold and dashed wedges to be used. Most candidates showed good knowledge and understanding of reactions for the three functional groups. • For the primary alcohol, most chose H+/Cr2O72–, with distil (→ aldehyde) or reflux (→ carboxylic acid); a significant number chose a concentrated acid (→ alkene) or Br–-/H+ (→ haloalkane) • For the ketone, most chose NaBH4 (→ secondary alcohol) • For the nitrile, most chose either H2/Ni (→ amine) or H+(aq) (→ carboxylic acid). Clear diagrams of the products were usually seen although many omitted a CH2 from the amine branch for hydrolysis of the nitrile or an extra CH2 in the aldehyde or carboxylic acid branch from oxidation of the primary alcohol. Some candidates chose 2,4-DNP for a reaction of the ketone and treated the question as one requiring tests, and then proving that the compound was a ketone from no reaction with Tollens’ reagent. The question asked for the organic product and the 2,4- DNP product is beyond the demands of this specification (although this was seen very rarely). Candidates adopting this reaction were limiting the extent of their response and candidate should have considered this requirement before selecting 2.4- DNP. This exemplar shows a good response that lacks 3D diagrams for the optical isomers. The candidate has clearly given reagents and conditions and has shown the organic products. In the response, you can see that the candidate initially showed an extra CH2 in the –COOH branch, and a mistake in the amine branch. The absence of 3D structures limits the response to Level 2 and 4 marks have been awarded for choosing correct and relevant reagents and conditions, and for the clear communication of the structures.

Notes from the mark scheme and report

• optical isomerism with chiral centre/non-superimposable mirror images and 3D diagrams with wedges
• ketone: NaBH4 or HCN
• –OH: H+/Cr2O72– (reflux gives acid, distil gives aldehyde), NaBr/H+, acid catalyst elimination
• –CN: H2/Ni to amine or H+/H2O to COOH
• reagents and products must be linked

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

Answer

C

Mark scheme

Examiner's comment

This question discriminated well, with higher ability candidates correctly identifying C as the volume of hydrogen produced. Many candidates overlooked the hydrogenation of the CN group and incorrectly selected option A.

Mark scheme

Examiner's comment

The majority of candidates were able to identify at least one of the structures. A significant number of candidates did not check the number of bonds of each atom in their structures and frequently had too many or too few hydrogen atoms attached. Most candidates identified that acidic conditions were required but some missed the aqueous condition that was also required for the mark.

Notes from the mark scheme and report

• with C6H5COOH the OH group is esterified
• with (CH3)2CHOH the COOH group is esterified
• nitrile to acid needs H+/H2O (ALLOW HCl/H2O, H2SO4/H2O)
• NaBH4 box is the ketone CH3CH2COCOOH

Mark scheme, with the levels and indicative points

Examiner's comment

This question differentiated well. Candidates who were given Level 3 (5-6 marks) understood the term aliphatic and were able to provide two different mechanisms that produced a C-C bond. The most common responses seen involved the nucleophilic substitution of a halogenoalkane and a cyanide ion and the nucleophilic addition of a carbonyl with a cyanide ion. Some candidates offered radical substitution mechanisms, detailing initiation, propagation and termination steps, with the termination step producing a C-C bond. Candidates who scored Level 2 (3-4 marks) frequently detailed a reaction involving aromatic compounds or polymerisation of alkenes for which they were unable to give a mechanism. If any of your students’ results are not as expected, you may wish to consider one of our post-results services. For full information about the options available visit the OCR website. We send a weekly roundup to tell you about important updates. You can also sign up for your subject specific updates. If you haven’t already, sign up here. Attend one of our popular CPD courses to hear directly from a senior assessor or drop in to a Q&A session. Most of our courses are delivered live via an online platform, so you can attend from any location. Please find details for all our courses on the relevant subject page on our website or visit OCR professional development. ExamBuilder is the question builder platform for a range of our GCSE, A Level, Cambridge Nationals and Cambridge Technicals qualifications. Find out more. ExamBuilder is free for all OCR centres with an Interchange account and gives you unlimited users per centre. We need an Interchange username to validate the identity of your centre's first user account for ExamBuilder. If you do not have an Interchange account please contact your centre administrator (usually the Exams Officer) to request a username, or nominate an existing Interchange user in your department. Review students' exam performance with our free online results analysis tool. It is available for all GCSEs, AS and A Levels and Cambridge Nationals. It allows you to: • review and run analysis reports on exam performance • analyse results at question and/or topic level • compare your centre with OCR national averages • identify trends across the centre • facilitate effective planning and delivery of courses • identify areas of the curriculum where students excel or struggle • help pinpoint strengths and weaknesses of students and teaching departments. Find out more. Post-results services Keep up-to-date OCR Professional Development Signed up for ExamBuilder?

Notes from the mark scheme and report

• haloalkane with NaCN in ethanol by nucleophilic substitution
• aldehyde/ketone with NaCN/H+ (HCN) by nucleophilic addition
• curly arrows from lone pair/– charge on C of CN–, dipoles, intermediate with O– then protonation

Answer

C

Mark scheme

Mark scheme

Examiner's comment

Question 16(d)(ii): Many candidates did not correctly answer this question with products being given that had too few carbon atoms. .

Notes from the mark scheme and report

• cyclohexene ring with the two COOCH3 groups on adjacent carbons
• cyclohexa-1,4-diene with two adjacent methyl groups on a C=C from but-2-yne

Mark scheme, with the levels and indicative points

Examiner's comment

Question 19(b): This question differentiated well.

Notes from the mark scheme and report

• 20.2 g (n = 0.0400 × 100/25 = 0.160)
• partly correct: 1.265 g (yield inverted), 5.06 g (yield omitted)
• stage 1 CN– in ethanol to C6H5CH2CN
• stage 2 H+/H2O acid hydrolysis giving NH4+

Skills: M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities

Answer

A

Mark scheme

Answer

B

Mark scheme

Answer

C

Mark scheme

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

This question required candidates to apply knowledge of organic reactions from across the specification and discriminated well. Candidates with a good knowledge of reagents and conditions frequently scored over five marks. More detailed feedback is discussed with Exemplar 7, below. Phenylethanone is the starting point of this flowchart which shows the synthesis of compounds D and E. Synthesis of D The first step in the synthesis of D is the reduction of the ketone group. This response demonstrates a common error seen by examiners. The candidate has correctly deduced that than alcohol is formed but has omitted a hydrogen atom. Candidates are advised to take care when drawing structures. If a carbon atom is displayed it should be shown to be making four bonds. The final step of the synthesis is the dehydration of the alcohol with an acid. This candidate has used H3PO4. H2SO4 was also frequently seen. Lower ability responses included reference to water or steam and this was not credited. Synthesis of E The first step of the synthesis of E from phenylethanone is the reaction with NaCN(aq)/H+(aq). This candidate identifies that the product of the reaction is a hydroxynitrile but unfortunately this cannot be credited due to the incorrect linking of the OH group via the H atom. Candidates should be aware that, when drawing structures, groups must be bonded via the correct atoms. The next step of the synthesis is the substitution of the OH group. The candidate correctly recognises that NaBr is a suitable reagent but omits an acid, e.g. H2SO4, which is also required for this reaction. This error was seen frequently by examiners. The final two steps involve the substitution of the Br atom and acid hydrolysis of the nitrile group. In this case the candidate chooses the reaction with excess NH3. The structure of the correct product of this reaction is shown and the synthesis is completed by identifying a suitable reagent for the final step. Other candidates opted to perform these steps in the reverse order and the mark scheme allowed full credit for either approach. Common errors in the final steps were omissions. Some candidates simply stated NH3 without mentioning ethanol. Others identified water as important for the hydrolysis but did not include reference to an acid.

Notes from the mark scheme and report

• 1-phenylethanol then acid catalyst (DO NOT ALLOW H2O/steam/(aq)) for D
• hydroxynitrile, NaBr/H2SO4 (ALLOW HBr), acid hydrolysis of CN, excess NH3/ethanol (last two stages in either order)
• DO NOT ALLOW OH– or NH2– for the groups

Answer

C

Mark scheme

Examiner's comment

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

Answer

B

Mark scheme

Examiner's comment

Candidates found this question difficult, presumably as it involved reactions of different functional groups within the same compound. Many candidates identified B as the correct response. The most common incorrect responses were C and D. Section B overview Section B includes a mixture of short answer and extended response questions, including two questions marked using a level of response mark scheme (20d and 22d). This section of the paper is worth 85 marks.

Mark scheme

Examiner's comment

This question proved difficult and although the majority of candidates scored in some parts, only the very best responses secured all five marks. More detailed feedback is discussed with Exemplar 8. Cinnamaldehyde was the starting point for this flowchart of reactions. The most frequently scored mark was correct identification of the reagents required for the formation of the hydroxynitrile. This response uses NaCN/H+. Other candidates used HCN which was also acceptable. The flowchart shows two different reactions of this hydroxynitrile. The first is the reaction with excess hydrogen in the presence of Ni. Most candidates scored one mark for their product. As in this exemplar, the double bond was often reacted to form a saturated chain. Some candidates identified that the CN group would also react but instead of writing CH2NH2 they replaced the CN group with just NH2, effectivity removing a carbon atom from the chain. The second reaction of the hydroxynitrile is acid hydrolysis of the CN group. This response identifies the correct carboxylic acid. However, this reaction seemed unfamiliar to many candidates and a range of incorrect responses were frequently seen. The final reaction is the reduction of cinnamaldehyde with NaBH4. Many candidates recognised this reaction, but as can be seen in this response the alcohol group is shown on the incorrect carbon atom. This was a common error. Candidates are advised to number carbon atoms present if provided with a complex structure, such as cinnamaldehyde. Numbering will ensure that each carbon is considered when drawing reaction products and would minimise errors, such as those demonstrated in the reduction product.

Notes from the mark scheme and report

• NaCN/H+ or HCN for reagent
• NaBH4 reduces CHO only (C=C unchanged)
• excess H2/Ni reduces both C=C and C≡N (1 mark each
• IGNORE hydrogenation of benzene ring)
• H+(aq) hydrolyses nitrile to COOH

Mark scheme

Examiner's comment

This part was answered well with most choosing nucleophilic addition. Credit was given just for ‘addition’.

Notes from the mark scheme and report

• addition
• IGNORE nucleophilic/electrophilic
• DO NOT ALLOW addition–elimination, condensation or polymerisation

Mark scheme

Examiner's comment

This part was one of the most challenging on the paper. Candidates needed to link the earlier information for combining two ethanal molecules to derive the product for combining two propanone molecules. Despite the challenge, the highest ability candidates were able to come up with the correct structure.

Notes from the mark scheme and report

• (CH3)2C(OH)CH2COCH3
• connectivity not assessed

Mark scheme

Notes from the mark scheme and report

• curly arrow from lone pair or negative charge on C of CN– to C of C–Cl
• dipole on C–Cl AND arrow from bond to Cl
• product AND Cl–
• IGNORE NaCl
• ALLOW SN1

Mark scheme

Notes from the mark scheme and report

• G = methanal
• Reaction 2 H2 AND Ni (ALLOW LiAlH4
• DO NOT ALLOW NaBH4)
• Reaction 3 aqueous acid e.g. HCl(aq)
• IGNORE names (formulae asked for)