PMT GCE. Chemistry A. Advanced GCE. Unit F324: Rings, Polymers and Analysis. Mark Scheme for January Oxford Cambridge and RSA Examinations

Transcription

1 GCE Chemistry A Advanced GCE Unit F324: Rings, Polymers and Analysis Mark Scheme for January 2012 xford Cambridge and RSA Examinations

2 CR (xford Cambridge and RSA) is a leading UK awarding body, providing a wide range of qualifications to meet the needs of candidates of all ages and abilities. CR qualifications include AS/A Levels, Diplomas, GCSEs, CR Nationals, Functional Skills, Key Skills, Entry Level qualifications, NVQs and vocational qualifications in areas such as IT, business, languages, teaching/training, administration and secretarial skills. It is also responsible for developing new specifications to meet national requirements and the needs of students and teachers. CR is a not-for-profit organisation; any surplus made is invested back into the establishment to help towards the development of qualifications and support, which keep pace with the changing needs of today s society. This mark scheme is published as an aid to teachers and students, to indicate the requirements of the examination. It shows the basis on which marks were awarded by examiners. It does not indicate the details of the discussions which took place at an examiners meeting before marking commenced. All examiners are instructed that alternative correct answers and unexpected approaches in candidates scripts must be given marks that fairly reflect the relevant knowledge and skills demonstrated. Mark schemes should be read in conjunction with the published question papers and the report on the examination. CR will not enter into any discussion or correspondence in connection with this mark scheme. CR 2012 Any enquiries about publications should be addressed to: CR Publications P Box 5050 Annesley NTTINGAM NG15 0DL Telephone: Facsimile: publications@ocr.org.uk

3 Annotations available in Scoris. Annotation Meaning Benefit of doubt given Contradiction Incorrect response Error carried forward Ignore Not answered question Benefit of doubt not given Power of 10 error mission mark Rounding error Error in number of significant figures Correct response 1

4 Abbreviations, annotations and conventions used in the detailed Mark Scheme (to include abbreviations and subject-specific conventions). Annotation D NT ALLW IGNRE ALLW Meaning Answers which are not worthy of credit Statements which are irrelevant Answers that can be accepted ( ) Words which are not essential to gain credit ECF AW RA Underlined words must be present in answer to score a mark Error carried forward Alternative wording r reverse argument Annotations should be placed to clearly show where they apply within the body of the text (i.e. not in margins) for: Question 1 c(iii) Question 2 a(i), (b) Question 3 a(i), a(ii), b(i) Question 4 b(ii), (c) 2

5 1 (a) (i) The p R point at which the zwitterion exists 1 ALLW p/point at which there is no overall/net charge IGNRE p/point at which there is no charge/ neutral charge ie overall/net is required ALLW p/point at which contains C AND N 3 + (ii) 3 N C C 3 N C C ALLW C 3 C(N 3 ) + C ALLW C 3 C(N 3 ) + C ALLW C 2 and C 2 C 3 C 3 2 ALLW + charge on N or : ie + N 3 or N 3 + D NT ALLW charge on C: ie C D NT ALLW or C 3 missing ALLW correct structural R displayed R skeletal formula ALLW combination of formulae as long as unambiguous 3

6 1 (a) (iii) p < 3: C ALLW carboxyl group R carboxylic acid D NT ALLW acid R just carboxylic (without acid ) p > 10: N 2 2 ALLW amino group R amine D NT ALLW if give correct formula but wrong name or correct name and wrong formula eg N 2 and amide IF any carbon chain is shown attached to BT functional groups ALLW 1 mark eg C 2 C AND C 2 N 2 for 1 mark C 3 C AND C 3 N 2 for 1 mark RC AND RN 2 for 1 mark (b) IF functional groups are shown the wrong way round, ALLW 1 mark i.e. N 2 C D NT ALLW more repeat units IGNRE brackets and n ALLW end bonds shown as D NT ALLW if end bonds are missing peptide link must be fully displayed, i.e. ALLW terminal N on right (R C= on left), ie TW repeat units shown correctly 2 IF peptide bond is shown not displayed, i.e. CN, 2nd mark can still be awarded 4

7 1 (c) (i) There is no chiral carbon ALLW there is no asymmetric carbon R there is no asymmetry in the molecule 1 R it has no non-superimposable mirror image R there are not four different atoms/groups of atoms (attached to carbon) R there are only three different atoms/groups of atoms (attached to carbon) R because there are two hydrogen atoms on the carbon (ii) ALLW Add the same 3-D structure repeated but with 2 groups swapped as after rotation the 2nd isomer is a mirror image of the first, i.e. 2 Connectivity: Chiral C must be linked to the C of the C, the C of the C 2 S and the N of the N 2 (ie connectivity is being tested) ie, ALLW as in the example but D NT ALLW an attempted N shown as below: 2 The 2nd mark is for the mirror image of CRRECT optical isomer only CARE: may be orientated differently D NT penalise connectivity more than once Each structure must have four central bonds, with at least one wedge in AND one wedge out 5

8 For bond into plane of paper, ALLW: For bond out of plane of paper, a solid wedge is expected, either way around: ALLW a hollow wedge for in bond R an out bond, provided it is different from the other in or out wedge eg: ALLW examples of other 3-D representations provided they are possible: i.e. CARE: This is a 3-D representation so this is possible and the bonds are clearly not 90º to one another 6

9 1 (c) (iii) ANNTATINS MUST BE USED Disadvantages: any two from: (one stereoisomer might have harmful/adverse) side IGNRE harmful/adverse effects only effects reduces the (pharmacological) activity/effectiveness cost of separating stereoisomers R difficulty in separating stereoisomers 2 ALLW a response that implies an increased dose IGNRE it takes time to separate Synthesis of a single optical isomer any two from: using enzymes or bacteria using (chemical) chiral synthesis R using chiral catalysts using (natural) chiral molecules/compounds 2 ALLW biological catalysts ALLW chiral transition metal complex/catalyst R stereoselective transition metal complex/catalyst ALLW 'chiral pool' R L-amino acids / D-sugars Quality of Written Communication For full marks to be awarded for this question chiral R enzyme R bacteria R catalyst must be spelled correctly at least once in the correct context 7

10 1 (d) amino acid isoleucine leucine tyrosine number of peaks mark for each number (e) ALLW correct structural R displayed R skeletal formula ALLW combination of formulae as long as unambiguous N N Common errors: Look for N 2 on first structure and N on second structure N N valine anhydride proline anhydride 2 Total 19 8

11 2 (a) (i) Response requires three stages Acceptable sequence of stages are: chlorination nitration, reduction, chlorination nitration nitration, chlorination, reduction, reduction chlorination, nitration, reduction Reduction must be a later stage than nitration Mark according to which sequence chosen. Stage 1 organic product: Cl R N 2 chemicals: Cl 2 AND AlCl 3 R N 3 AND 2 S 4 Stage 2 organic product: Cl N 2 R N 2 For organic products, ALLW C 6 5 N 2 R C 6 5 Cl R C 6 5 N 2 ALLW N 2 AND N 2 D NT ALLW ClC 6 4 N 2 (formula ambiguous) D NT ALLW molecular formulae IGNRE any additional structures shown eg 2- (ortho) and 3- (meta) substituted isomers In chemicals boxes, IGNRE temperatures IGNRE catalyst For chlorination chemicals, ALLW Cl 2 AND FeCl 3 R Cl 2 AND Fe R Cl 2 AND halogen carrier For nitration chemicals, concentrated not required for N 3 R 2 S 4 BUT... D NT ALLW dilute chemicals: N 3 AND 2 S 4 R Sn AND Cl Stage 3 chemicals: Cl 2 AND AlCl 3 R Sn AND Cl 5 For reduction chemicals, concentrated Cl not required but D NT ALLW dilute For Sn/Cl ALLW addition of Na also IF it is clear that it is a second step BUT. D NT ALLW Sn AND Cl AND Na IGNRE catalyst 9

12 2 (a) (ii) diazonium ion ALLW + sign up to halfway along triple bond from left-hand N Cl N N N N group MUST be displayed IGNRE presence of Cl D NT ALLW Cl substituent on benzene ring D NT ALLW: Cl N 2 azo dye Cl N N N=N group MUST be displayed 2 In azo dye, ALLW as alternative to phenol group: R Na + R Na ALLW phenol part substituted at any carbon (ie 2,3 or 4 position for ) i.e. N N N N IGNRE geometry/shape, i.e. ALLW N=N Mark independently D NT ALLW if Cl is missing from benzene ring in EITER structure 10

13 2 (b) δ mark 2 S δ+ S mark 1 step 1 mark 4 + intermediate = mark 3 4 ANNTATINS MUST BE USED mark 1 curly arrow from π-delocalised ring in benzene to S δ+ in S 3 ALLW curly arrow from the ring R from within the ring mark 2 curly arrow from one S= double bond to the (to produce a S ) ALLW curly arrow to any in S 3 mark 3 intermediate showing delocalisation over 5 carbons Intermediate must have correct S 3 structure FULLY displayed D NT ALLW intermediate with broken ring less than halfway up in correct orientation: + mark 4 curly arrow from C bond reforming π- delocalised ring in benzene Stand alone mark IGNRE responses after STEP 2 11

14 ALLW Kekulé mechanism δ mark 2 S δ+ S mark 1 mark 4 step 1 + intermediate = mark 3 ALLW double bonds shown in other Kekulé arrangement 2 (c) (i) Various possibilities, eg: Br ALLW 1, 2, 3 or 4 Br atoms substituted on phenol ring at carbon atoms 2, 3, 5 or 6 BUT must be in correct position shown D NT ALLW or Na 3 C C N ALLW for side chain: C 3 CN but aromatic part of structure must be shown Br IGNRE any additional inorganic products in boxes (even if incorrect Reaction with Na ALLW Na R as alternative to Na + D NT ALLW Na R Na (i.e. Na without charge) 3 C C N - Na + Na must be in correct position shown 2 ALLW for side chain: C 3 CN but aromatic part of structure must be shown IGNRE any additional inorganic products in boxes (even if incorrect) 12

15 2 (c) (ii) ydrolysis with Na(aq) n BT structures, ALLW Na R as alternative to Na + D NT ALLW Na R Na (i.e. Na without charge) 3 C C - Na + Na must be in correct position shown on 2nd structure ALLW C 3 CNa/ C 3 C 2 Na R C 3 C / C 3 C 2 2 N - Na + ALLW one mark for carboxylic acid AND phenol, rather than sodium salts: Mark independently 2 3 C C 2 N ALLW N 2, C 3 Total 15 IGNRE any additional inorganic products in boxes (even if incorrect) 13

16 3 (a) (i) ne mark is for positive carbonyl test (Add) 2,4-dinitrophenylhydrazine AND orange/yellow/red precipitate ALLW errors in spelling ALLW 2,4(-)DNP R 2,4(-)DNP ALLW Brady s reagent or Brady s Test ALLW solid R crystals R ppt as alternatives for precipitate ne mark is for negative aldehyde test EITER (Add) Tollens reagent/tollens test AND no change R no reaction R no silver (mirror) ALLW AgN 3 /N 3 (Formulae must be correct) R ammoniacal silver nitrate ALLW Fehling s solution R Benedict s solution AND no (brick-red) precipitate ALLW any response that implies that nothing happens ie no change R no reaction R no silver (mirror) ALLW the aldehyde/pentanal gives a silver mirror (ii) R (Add) 2 S 4 AND K 2 Cr 2 7 AND no change R no reaction R no green colour 1st mark Take melting point of orange crystals/derivative/product from 2,4-DNP 2 ALLW + AND Cr (Formulae must be correct) ALLW any response that implies that nothing happens IGNRE responses using NaB 4 (as no observations) NTE: a(ii) is marked completely independently of a(i) 2nd mark Compare melting point with known values R compare melting point with value in database/reference book 2 Mark independently of response for 1st mark D NT ALLW 1st or 2nd marks for taking and comparing boiling points R chromatograms 14

17 3 (b) (i) Synthesis 1 6 NTE: ALL Structures MUST have s shown IGNRE bond angles D NT ALLW more than one repeat unit C C C IGNRE brackets and n Ester linkage must be fully displayed ALLW terminal on right (R C= on left), i.e. C C C ALLW end bonds shown as D NT ALLW if structure has no end bonds Synthesis 2 C C C C C C 2 15

18 Synthesis 3 Mark each structure independently must be connected correctly on BT structures C C C C C C D NT ALLW more repeat units IGNRE brackets and n C C C C C C ALLW terminal on right (R C= on left), i.e. C C C C C C ALLW end bonds shown as D NT ALLW if structure has no end bonds D NT ALLW ECF from wrong structure in previous boxes 3 (b) (ii) Synthesis 1: condensation AND Synthesis 2: addition AND Synthesis 3: condensation Total 11 1 All three correct responses required for the mark 16

19 4 (a) (C 3 C) 2 + C 3 C()C 3 C 3 CC(C 3 ) 2 + C 3 C 1st mark Correct structure of ester: C 3 CC(C 3 ) 2 2nd mark Equation contains correct formulae for (C 3 C) 2, C 3 C()C 3 AND C 3 C 2 ALLW correct structural R displayed R skeletal formula ALLW combination of formulae as long as unambiguous D NT ALLW molecular formulae ALLW (C 3 ) 2 CCC 3 R (C 3 ) 2 CCC 3 (b) (i) (relative) solubility 1 IGNRE partition (ii) The esters would have similar retention times AND similar structures/molecules R same functional groups R similar polarities R similar solubilities Alcohol would have short retention time AND alkane would have long retention time 2 D NT ALLW adsorption R absorption IGNRE similar properties 17

20 4 (c) Elemental analysis and molecular formula 2 marks Use of percentages (to find EF) AND 144 Molecular formula = C marks ANNTATINS MUST BE USED Working C : : = 66.63/12 : 11.18/1 : 22.19/ : : : 8 : 1 Alternative method: carbon: (144 x 66.63/100)/12 = 8 hydrogen: (144 x 11.18/100)/1 = 16 oxygen: (144 x 22.19/100)/16 = 2 ester structure 4 marks C 3 3 C C C 2 C C 2 C 3 C 3 4 marks ALLW correct structural R displayed R skeletal formula ALLW combination of formulae as long as unambiguous N ECF from earlier structures If not fully correct award following marks: If structure an ester of formula C R the organic structure contains C(C 3 ) 3 If structure is an ester of formula C AND ester contains C(C 3 ) 3 If structure is an ester of formula C AND ester contains C 2 C(C 3 ) 3 AND ester contains C 3 C 2 C i.e. If the ester link is reversed C 3 C 3 C 2 C C 2 C C 3 IGNRE any name C 3 18

21 NMR analysis 4 marks NTE: Each peak can be identified from: its δ value: ± 0.2 ppm a range, eg the peak between 2 and 3 its relative peak area (CARE two peaks have an area of 2) its splitting (CARE: two peaks are singlets) labelling on the spectrum Triplet (at δ 1.3) shows an adjacent C 2 R triplet (at δ 1.3) shows (C with) 2 adjacent s/protons (because of splitting: so triplet) QWC: triplet must be spelled correctly ALLW neighbouring s for adjacent to s Peak at (δ) 2.2 shows adjacent to C= AND adjacent to (C with) no hydrogens (because of no splitting: so singlet) For peak at (δ) 2.2 ALLW singlet at (δ) 2.2 ALLW singlet labelled 2 Peak at (δ) 4.2 shows C AND adjacent C 3 R 3 adjacent s/protons (because of splitting: so quartet) Peak at (δ) 0.9 show 3 x C 3 (because of singlet and area 9) 4 marks Total for 4(c) 10 Total 15 For peak at (δ) 4.2 ALLW quartet (labelled 2) Check back for any responses added to spectra ADD ^ MARK T TE SPECTRUM PAGE T SW TAT IT AS BEEN LKED AT 19

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