Edexcel Jan 2012 Paper 5 Q22 (with explained solutions) |

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Edexcel Jan 2012 Paper 5 Q22

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22 (a) Tiglic acid is a compound that is used as a defensive agent by some beetles. (i) Tiglic acid contains, by mass, 60% carbon, 8% hydrogen, with the remainder being oxygen. Show that these data are consistent with the formula C5H8O2. (ii) Tiglic acid contains a carbon-carbon double bond and a carboxylic acid group. Suggest one test for each of these groups in tiglic acid. State what you would do and what you would see as a positive result for the tests. PMT (1) (4) Test for C CTest for COOH*P35302A01328* 13 Turn over<br />
PMT (b) It is suggested that the structure of tiglic acid is either that of A or B. CH3CH2 C C COOH CH3 C C CH3 COOH B (i) State, with a reason, whether B is the E- or Z- isomer. (2)(ii) The mass spectrum of tiglic acid shows two prominent peaks at mass/charge ratios 45 and 55. Write the formulae of the fragments giving rise to each of these peaks. (2) 4555(iii) Does this data from the mass spectrum alone enable you to decide which of A or B is the structure of tiglic acid? Explain your answer. (1)14 *P35302A01428*<br />
PMT (c) The position of a C C double bond in a molecule can be determined by ozonolysis. The compound is reacted with ozone and then dilute acid, two carbonyl compounds being produced as shown below. C C O3 C O O C Ozonolysis of tiglic acid gives two carbonyl compounds, C and D. Compound C gives a silver mirror with Tollens reagent and gives iodoform with iodine in alkali. Compound D does not give a silver mirror with Tollens reagent, but does give iodoform with iodine in alkali. *(i) From the results of the experiments, deduce the functional groups present in C and D. By considering the two possible structures for tiglic acid, give the structural formulae of C and D. From the structures you have drawn, state which of the structures A or B could represent tiglic acid. (6)(ii) Explain whether or not these tests show definitely that your answer to (c)(i) represents tiglic acid. (1)*P35302A01528* 15 Turn over<br />
PMT (d) Lactic acid is a chiral molecule that is found in sweat as the () isomer only. Its structural formula is OH CH3 HOOC H () - lactic acid (i) Lactic acid can be made from ethanol in three steps. C2H5OH ethanol step 1 compound X step 2 OH CH3 C H CN step 3 OH CH3 C H COOH lactic acid Give the structural formula of the intermediate X and the reagents and conditions required for steps 1 and 2. (4) Step 1Step 216 *P35302A01628*<br />
PMT (ii) Classify the type and mechanism of the reaction that occurs in step 2. (1)*(iii) By considering the stereochemistry of the mechanism in step 2, explain why this synthesis would not give a single optical isomer of lactic acid. (2)(iv) Suggest why synthetic pathways for the manufacture of pharmaceuticals may require reactions that are highly stereospecific. (1)(Total for Question 22 = 25 marks) *P35302A01728* 17 Turn over<br />

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Acceptable Answers Question Number 22(a)(i) C 60/12 = 5 Reject Mark H 8/1 = 8 O 32/16 = 2 ALLOW 1 mol = 100 g So 60 %C = C5, etc Acceptable Answers Question Number 22(a)(ii) C=C Reject Mark Test : add bromine water/Br2(aq) (1) Result: From yellow/brown/red- brown/orange to colourless/decolorises (1) OR Test : add (acidified) potassium manganate((VII)) (solution) (1) Result: goes from pink/purple to colourless/brown (1) Test : add alkaline potassium manganate((VII)) (solution) (1) Result: goes green (1) COOH: Test : add NaHCO3 /Na2CO3/sodium carbpnate (solution) (1) Result: Fizzes/bubbles/large volume neutralized (1) Bromine/Br2/Br2(l) clear for colourless clear for colourless PCl5 /LiAlH4 as test NaOH/NaOH(aq) colourless gas evolved 6CH0501 1201<br />
ALLOW gas given off that turns limewater cloudy OR Test : with blue litmus (1) Result: turns red (1) The test can be with any other indicator, including universal Add sodium indicator, with the correct initial and final colour colourless gas evolved ALLOW pH meter (1) pH 4-6 (1) OR Test : add ethanol with conc H2SO4 (and warm) (1) Result: gives pleasant/fruity smell of ester (1) OR Test: add magnesium (1) Result: fizzing/bubbles etc (of hydrogen) (1) ALLOW gas given off that burns with a squeaky pop 6CH0501 1201<br />
Mark Reject Question Number 22(b)(i) Explanation of precedence/priority in terms of atomic numbers/masses of the attached groups OR Highest-precedent/priority groups on each carbon are on opposite sides of the molecule (1) E-/entgegen (1) Mark independently Both CH3/methyl groups on the same side so Z (0/2) Acceptable Answers Question Number 22(b)(ii) 45 Reject Mark COOH+ /CO2H+ (1) 55 C4H7 + (1) OR C3OH3 ALLOW Structural/displayed formulae of ions Absence of + charge (1 max) Acceptable Answers Question Number 22(b)(iii) If they say yes (0) Mark Reject No on its own (No) (Cleavage of the CCOOH bond in) both compounds gives fragment(s) of the same mass OR Both give the same peak(s)/fragment(s) Both give CO2H+/ C4H7 The mark can be scored by referring to just one of the fragments/peaks/masses. + fragments 6CH0501 1201<br />
Question Number *22(c)(i) QWC Acceptable Answers Reject Mark CH3COH 1 max 6 C is CH3CHO (alone) (2) D is CH3COCOOH (alone) (2) so tiglic acid must be B (1) tiglic acid mark can only be awarded if correct structures of either C or D are given. Any one of the following C must be an aldehyde (1) D is a ketone (1) Mention that CH3CO present in either/both compounds (because of formation of iodoform) (1) If one or both of the structures are incorrect any of the last 3 marks can be awarded max 5 If C and D are fully correct, but the wrong way round max 5 Acceptable Answers Question Number 22(c)(ii) Doesnt distinguish E- isomer from Z- isomer/geometric isomers (so no) OR Doesnt distinguish which sides of C=C functional groups are on Reject Mark Just isomers/ stereoisomers/ enatiomers 6CH0501 1201<br />
Acceptable Answers Question Number 22(d)(i) CH3CHO (1) Reject Mark CH3COH Manganate VII/KMnO4 Reflux HCN alone ACCEPT displayed or skeletal Step 1 (heat)using acidified potassium dichromate/or H+/Cr2O7 2- (1) distil (product as formed) conditional on dichromate (1) Step 2 HCN with KCN OR KCN with H+/acid OR KCN with (cold) NaOH(aq)/alkali (1) ALLOW HCN with NaOH/alkali For step 2 Ignore conditions e.g. any references to heat Question Number 22(d)(ii) Acceptable Answers Reject Nucleophilic addition Any recognisable spelling of philic and addition, either order Both words needed Nutrophilic addition Any other or additional words Mark 6CH0501 1201<br />
Question Number *22(d)(iii) QWC Mark Acceptable Answers Reject Ethanal is planar (at the reaction site) OR Ethanal is a planar molecule (1) Attack (from CN- to give the cyanohydrin) is (equally likely) from either side/above or below/from both sides (of the molecule) (so a racemic mixture is formed) (1) Mark independently Intermediate is planar Square planar Can attack carbocation from either side/any reference to SN1/SN2 Acceptable Answers Question Number 22(d)(iv) Receptors for the compound in the body Reject Mark are often stereospecific so only one stereoisomer is pharmacologically active OR Body recognises one (stereo)isomer ALLOW Only one (stereo)isomer is active OR One/the other isomer may be toxic/dangerous/harmful OR One isomer destroys body cells OR (Different) isomers have different biological/pharmacological/biochemical properties 6CH0501 1201<br />

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