RDC 9729 02 H2 Chemistry Practice 1 QP

1 CHEMISTRY 9729/02Paper2StructuredQuestions 2hoursCandidatesansweronQuestionPaper READTHESEINSTRUCTIONSFIRSTAnswer ALLquestions.Themaximummarksawardedperquestionisindicatedinthesquarebracketsattheendofeachquestion. Donot turnover thebooklet until youaretoldtodoso.Thisdocumentconsistsof 26printedpages,includingthisCoverPage,and1blankpage.

2 QUESTION1Squaricacidisadiproticorganicacid.Figure1.1showstheskeletalformulaofsquaricacid. Figure1.1 (a)(i)Suggestwhattheterm‘diproticorganicacid’means. ……………………………………………………………………………………………………………… …………………………………………………………………………………………………………….[1] (ii)Statethebondanglebetweentwoadjacentcarbonatoms.Explainyouranswer. ……………………………………………………………………………………………………………….. .………………………………………………………………………………………………………………. ..……………………………………………………………………………………………………………… …………………………………………………………………………………………………………….[2] (b)(i)Squaricacidisnotaperfectsquarebecauseoftheunequalbondlengthsbetweenanytwoadjacentcarbonatoms.Explainwhythisoccurs. .………………………………………………………………………………………………………………. ..……………………………………………………………………………………………………………… …………………………………………………………………………………………………………….[1]

3 (ii)Figure1.2showstheresonancestabilisationofthedivalentsquarateanion.Draw,onFig1.2,curlyarrowsandlonepairstoshowhowthisresonancestabilisationisachieved,fromlefttoright. Figure1.2 [1] (iii)Hence, consideringboth(i)and(ii), explainwhythedivalentsquarateanionisaperfectsquare. .………………………………………………………………………………………………………………. ..……………………………………………………………………………………………………………… …………………………………………………………………………………………………………….[1] (c)ComparethepHof1molofsquaricacid1molofethanoicacid,ofequalconcentration.Explaintworeasons. ……………………………………………………………………………………………………………….. .………………………………………………………………………………………………………………. ..……………………………………………………………………………………………………………… ……………………………………………………………………………………………………………….. .………………………………………………………………………………………………………………. ..……………………………………………………………………………………………………………… ……………………………………………………………………………………………………………….. .………………………………………………………………………………………………………………. ..……………………………………………………………………………………………………………… …………………………………………………………………………………………………………….[2]

4 Thedivalent squarateanionisanexampleof theoxocarbonanion,whichisanegativeionconsistingsolelyofcarbonandoxygenatoms,andthereforehavingthegeneralformulaCx Oy n− forsomeintegersx,y,andn.(d)(i)Suggeststructuresforthefollowingoxocarbonanions:● dicarbonate(C2 O5 2− )● ethylenetetracarboxylate(C6 O8 4− ),and● benzenehexolate(C6 O6 6− ). [2] Everyoxocarbonanioncanbematchedinprincipletotheelectricallyneutral variant Cx Oy ,knownastheneutralisedspecies. (ii) By using one of the oxocarbon anions in (i), explainwhether ‘neutralising’ anoxocarbonanionisanoxidationorreductionreaction.Hence,explainwhyasarule,theneutralisedspeciesismoreunstablethantheircorrespondingoxocarbonanion. ..……………………………………………………………………………………………………………… ……………………………………………………………………………………………………………….. .………………………………………………………………………………………………………………. ..……………………………………………………………………………………………………………… …………………………………………………………………………………………………………….[2]

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6 (iii)Eachoxocarbonanioncanalsoformacorrespondinghydrogenatedanion,Hk Cx Oy m− .Findtheaverageoxidationnumberofcarboninthision,leavingyouranswerintermsofk,x,y,andm. ..……………………………………………………………………………………………………………… ……………………………………………………………………………………………………………….. .………………………………………………………………………………………………………………. …………………………………………………………………………………………………………….[1] [Total:13]

7 QUESTION2Tautomers are constitutional isomers of a molecule that readily interconvert via a rapidequilibrium.AnexampleisgiveninFigure2.1. Figure2.1 Thisprocessisknownastautomerisation, whichisdefinedastheinterconversionbetweenconstitutional isomersof amolecule, involvingthetheadditionof aH-atom(proton)atonemolecularsiteanditsremovalfromanother.Itresultsintheswitchbetweenasinglebondtoadoublebondinthemain-bondingframework. (a)(i)Definetheterm‘constitutionalisomers’incontextofFigure2.1. ..……………………………………………………………………………………………………………… …………………………………………………………………………………………………………….[1] Figure2.2showsanaturalaminoacid,threonine. Figure2.2 (ii)Drawthepossiblestereoisomersofthreonine.YoumustuseRtorepresenttheRgroupofthisaminoacid. [1]

8 (iii)Figure2.3showsahypotheticalcompoundthatformsionicbondswiththreonine.Drawany2tautomersforthatcompound, notthreonine. Figure2.3 [1]

9 Figure2.4showspartofthereactionmechanismoftautomerism. Figure2.4 (iv)Consideringfigure2.4,suggestwhyalkenesdonotexhibittautomerism. ..……………………………………………………………………………………………………………… ..……………………………………………………………………………………………………………… …………………………………………………………………………………………………………….[1] Thefollowinglistshowssomefunctionalgroups,withdrawnstructuresforaselectedfew.● FunctionalGroupswithoutstructuresdrawn:❖Alcohol❖Aldehyde❖Ketone❖Amine❖Amide Table2.4showingFunctionalGroupswithStructuresDrawnFunctionalGroup Imine Imidicacid Enol Lactam Lactim Structure

10 (v)Usingcompounds onlyfromthatlistandtable,statepossibletautomersbetweendifferentfunctionalgroups.Expressyouranswerintheformof“tautomerA-tautomerB”.Forexample, if youthinkalcoholandaldehydesaretautomers,writeyouransweras“alcohol-aldehyde”. ..……………………………………………………………………………………………………………… ……………………………………………………………………………………………………………….. .………………………………………………………………………………………………………………. …………………………………………………………………………………………………………….[2] (vi) Zwitterions are also considered tautomers of their respective amino acids. Atdifferent pH, different functional groups in amino acids are either protonated ordeprotonated. Stateandbrieflyexplaintheorderinwhichdifferentdeprotonatedandprotonatedformsofthreonineareformed,aspHincreasesfrom1to14. .………………………………………………………………………………………………………………. ..……………………………………………………………………………………………………………… ..……………………………………………………………………………………………………………… ..……………………………………………………………………………………………………………… ..……………………………………………………………………………………………………………… ..……………………………………………………………………………………………………………… .………………………………………………………………………………………………………………. …………………………………………………………………………………………………………….[2] [Total:8]