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OCR A Jun 2010 Paper 4 Q1
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![Answer all the questions. 1 Benzene is an important industrial chemical and is used in a wide range of manufacturing processes. Over time our understanding of the structure and bonding of benzene has changed and various models have been proposed. (a) In 1865, Kekul proposed a model for the structure and bonding of benzene, but there is considerable evidence to suggest that Kekuls model may not be correct. Scientists have proposed alternative models for the structure and bonding of benzene. Explain the evidence that led scientists to doubt the model proposed by Kekul.[3]OCR 2010 Turn over<br />
(b) Sudan II is an azo dye which was used as a colourant in chilli powder. However, scientists advised the Food Standards Agency that Sudan II was linked to an increased risk of cancer and it is now no longer used as a food colourant. The flowchart below shows how Sudan II could be prepared in the laboratory from 1,3-dimethylbenzene. (i) Draw the structures of the organic compounds A, B, C and D in the boxes below. Display the functional group in compound C. HNO3, H2SO4 1,3-dimethylbenzene compound A Sn, conc. HCl, heat NaNO2, HCl(aq) compound C compound B alkaline solution of compound D HO Sudan IIOCR 2010 compound D [4]<br />
(ii) Compound A is formed by reacting 1,3-dimethylbenzene with HNO3 and H2SO4. Explain, with the aid of curly arrows, the mechanism for the formation of compound A. Your answer should clearly show the role of H2SO4 as a catalyst. (iii) Deduce how many other structural isomers of compound A could have been formed from the mononitration of 1,3-dimethylbenzene.[1] [Total: 13] [5]OCR 2010 Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/OCR_4_QP_JUN10_question_1-0.jpg)
![F324 Allow Kekul structures throughout Mark Scheme June 2010 Expected Answers Marks Additional Guidance Question 1 Bond length intermediate between/different from (short) C=C and (long) CC (cid:57) (cid:39)H hydrogenation less exothermic than expected (when compared to (cid:39)H hydrogenation for cyclohexene) (cid:57) Only reacts with Br2 at high temp or in presence of a halogen carrier / resistant to electrophilic attack (cid:57) Please annotate, use ticks to show where marks are awarded compound A NO2 if NO2 in wrong position penalise here and ECF for rest of b(i) and b(ii) compound B NH2 compound C + N N ALLOW all carboncarbon bonds the same length ALLOW (cid:39)H hydrogenation less (negative) than expected ALLOW (cid:39)H hydrogenation different from that expected DO NOT ALLOW (cid:39)H halogenation/hydration ALLOW doesnt decolourise/react with/polarise Br2 ALLOW doesnt undergo addition reactions (with Br2) ALLOW any 4-nitro-1,3-dimethylbenzene drawn in any orientation ALLOW NO2 CH3 drawn in any orientation H3C ALLOW any 4-amino-1,3-dimethylbenzene drawn in any orientation ECF amine of incorrect compound A (e.g. position of NO2 or lack of methyl sticks/groups) ALLOW diazonium chloride salt of 1,3-dimethylbenzene ECF diazonium salt/compound of incorrect compound B IGNORE Cl ion + N N not allow + N2 (cid:57) (cid:57) (cid:57) allow<br />
F324 Question Mark Scheme June 2010 Expected Answers Marks Additional Guidance compound D HO ALLOW if + charge is floating between the two Ns only if it is closer to the correct N allow not allow OH O- ALLOW any of OH (cid:57) OH ALLOW O in place of OH<br />
F324 Question ii If NO2 is in correct position do not penalise even if compound A in b(i) is not in correct position Expected Answers mark 1 HNO3 + 2H2SO4 (cid:111) H3O+ + 2HSO4 mark 4curly arrow from CH bond back to reform (cid:83) ring AND correct products (cid:57)+ NO2 + (cid:57) +NO2 NO2 NO2 + H+ mark 3intermediate with (cid:83) ring broken in the correct place (cid:57) mark 2curly arrow from (cid:83) ring to +NO2 (cid:57) mark 5 - H+ + HSO4(cid:111) H2SO4 (cid:57) Link to compound A in part (i)cannot score full marks [in b(i) & b(ii)] if NO2 is not adjacent to a methyl iii 2 (cid:57) Mark Scheme June 2010 Marks ++ NO2 + (cid:111) H2O + NO2 + + and ECF for marks 3 and 4 Additional Guidance + ion (cid:57) Equation to show formation of NO2 ALLOW HNO3 + H2SO4 (cid:111) H2O + HSO4 HNO3 + H2SO4 (cid:111) HSO4 ALLOW mark 2 curly arrow must be from 1,3- dimethylbenzene to NO2 DO NOT ALLOW intermediate+ H2NO3 NO2 (cid:83)-ring must be more than 1/2 way up ALLOW CH3s shown ALLOW H3O+ + HSO4 No other correct response(cid:111) H2O + H2SO4 Total 13<br />](https://learnah.org/wp-content/uploads/2018/08/OCR_4_QP_JUN10_answer_1-0.jpg)
