OCR A Jun 2015 Paper 4 Q2 (with explained solutions) |

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OCR A Jun 2015 Paper 4 Q2

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2 Chemists often use two different structures to represent a molecule of benzene, as shown below. structure A structure B (a) (i) Describe, with the aid of suitable diagrams showing orbital overlap, the difference in bonding between structure A and structure B. In your answer, you should use appropriate technical terms, spelled correctly.[4]OCR 2015 Turn over<br />
(ii) The table below shows the enthalpy changes for the reactions of cyclohexene, C6H10, and benzene, C6H6, with hydrogen. reaction enthalpy change / kJ mol1 C6H10 + H2 C6H6 + 3H2 C6H12 C6H12 119 208 Using this information, suggest and explain whether structure A or structure B is a better representation of benzene.[2] (b) Benzene compounds can undergo nucleophilic substitution reactions. Add curly arrows to the diagram below to show the two-step mechanism of C6H5N2 + with F. + N step 1 step 2 F[2] (c) Benzene can react with halogenoalkanes in the same way as with bromine, as shown in reaction 1 below. + (CH3)2CHBr FeBr3 reaction 1 CH(CH3)2 + HBr Write an equation to show the formation of the electrophile that reacts with benzene in reaction 1.OCR 2015 [1]<br />
H2N NH2 1,3-diaminobenzene (d) The types of reaction in (b) and (c) can be used to synthesise compound D, as shown in the flowchart below. (i) Complete the boxes below to suggest formulae for the reactants involved in the synthesis of compound D. Give structures for organic compounds. +N2 F N2 FeBr3 NH2 N H NH2 compound D OH [3] Turn overOCR 2015<br />
(ii) In a synthesis of compound D from 1,3-diaminobenzene shown in the flowchart, 1.73 g of compound D was prepared. These structures have been repeated below: N H NH2 H2N NH2 1,3-diaminobenzene compound D The overall percentage yield of compound D was 40.0%. Mr of compound D = 346.0 Calculate the mass of 1,3-diaminobenzene needed for this synthesis. OH mass =g [3]OCR 2015<br />
(iii) Compound D has been developed for possible use as a drug to treat heart conditions. When compound D, prepared in this synthesis, was given to patients, only 25% of the dose was effective in treating their heart conditions. Explain why only 25% of the dose was effective. Suggest how the synthesis of compound D might be changed to make the dose more effective.[3] [Total: 18]OCR 2015 Turn over<br />

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Mark Scheme Mark F324 Question (i) 2 (a) Answer M1 p-orbitals overlap (to form pi/-bonds) (cid:51) M2 -bond(s) are delocalised in structure B (cid:51) M3 -bonds are localised/between two carbons in structure A (cid:51) M4 AND Diagrams show correct position of delocalised and localised -bonds/-electrons OR correct position of p-orbital overlap (cid:51) (cid:4) QWC requires delocalised/delocalized spelled correctly and used in correct context June 2015 Guidance ANNOTATE ANSWER WITH TICKS AND CROSSES ETC IGNORE p-orbitals overlap to form sigma bonds ALLOW electrons are delocalised in structure B IGNORE B has delocalised structure or ring (must be electrons or -bonds) ALLOW -electrons/p-orbital overlap localised/between two carbons in structure A ALLOW p-orbitals overlap with one other carbon IGNORE electrons are localised OR structure A has localised structure (must be -bonds/-electrons/p-orbital overlap) ALLOW labelled diagram showing overlap of p-orbitals between two carbon atoms DO NOT ALLOW C=C in this diagram Diagram for structure A must show the full ring for M4 IGNORE C=C in M4 diagram IGNORE charge density DO NOT ALLOW electronegativity Structures do not need to be labelled A and B if the description matches the structure<br />
F324 Question Answer (ii) structure B/delocalised structure is (more) stable (cid:57) structure B is a better because (enthalpy change of hydrogenation for benzene is) less (exothermic) than (-) 357 (kJ mol-1) Mark Scheme June 2015 Mark Guidance ALLOW structure B is low in energy IGNORE structure B is less reactive (b) (cid:57) F curly arrow from CN bond to N+ curly arrow from lone pair on fluoride ion to positive charge on benzene ring (cid:57) (cid:57) 10 ALLOW enthalpy change/hydrogenation for benzene is less (negative) than 3()119 IGNORE more positive than (-)357 kJ mol-1 ALLOW enthalpy change is less than 3x enthalpy change for cyclohexene ALLOW structure B is more stable by 149 kJ mol-1 (2 marks) DO NOT ALLOW more/less energy needed for the reaction Answer must refer to data given in the question and must be a comparison IGNORE 360 kJ mol-1 No marks can be awarded if structure A is selected First curly arrow must come from bond not from C atom ALLOW first curly arrow to nitrogen atom OR to positive charge on nitrogen atom ALLOW second curly arrow from negative charge on fluoride ion ALLOW second curly arrow to carbon atom with positive charge<br />
F324 Mark Scheme June 2015 Mark Guidance ALLOW correct structural OR displayed OR skeletal formulae OR a combination of above as long as unambiguous ALLOW positive charge anywhere on the electrophile IGNORE AlCl3 OR AlBr3 ALLOW NaNO2 + HCl OR HNO2 + HCl IGNORE conditions/concentration ALLOW correct structural OR displayed OR skeletal formulae OR a combination of above as long as unambiguous ALLOW Question (c) Answer (CH3)2CHBr + FeBr3(CH3)2CH+ + FeBr4 (d) (i) First reactant = HNO2 (cid:57) Second reactant = Third reactant = (cid:57) (cid:57) 11<br />
F324 Question Answer Mark Scheme June 2015 (ii) FIRST CHECK THE ANSWER ON THE ANSWER LINE IF answer = 1.35 (g) award 3 marks IF answer = 0.54 (g) award 2 marks (no scale-up) IF answer = 0.216 (g) award 2 marks (incorrect scale-up) n(compound D) = 1.73/346 = 0.00500 mol (cid:57) n(1,3-diaminobenzene) required = 100/40 x 0.005 = 0.0125 mol (cid:57) Molar mass of 1,3-diaminobenzene = 108 (g mol1) AND Mass of 1,3-diaminobenzene = (108)(0.0125) = 1.35 g (cid:57) (iii) (compound D has) two chiral centres (cid:51) Four optical isomers exist (cid:57) (Synthesis could) use enzymes OR bacteria OR use (chemical) chiral synthesis OR chiral catalysts OR use natural chiral molecules OR single isomers (as starting materials) (cid:57) Mark Guidance ANNOTATE ANSWER WITH TICKS AND CROSSES ETC If there is an alternative answer, check to see if there is any ECF credit possible ALLOW ECF from incorrect amount, scale-up or molar mass Alternative 1 n(compound D) = 1.73/346 = 0.00500 mol Molar mass of 1,3-diaminobenzene = 108 (g mol1) AND Mass of 1,3-diaminobenzene = (0.00500)(108) = 0.540 g Mass of 1,3-diaminobenzene required = (0.540)(100/40) = 1.35 g Alternative 2 346 g gives 108 g 1.73 g gives 108/364 x 1.73 = 0.54 g 0.54/40 x100 = 1.35 g ALLOW (Compound D) has two asymmetric carbons OR has two stereocentres ALLOW four enantiomers OR two pairs of enantiomers INDEPENDENT MARK ALLOW biological catalysts ALLOW chiral transition metal complex/catalyst OR stereoselective transition metal complex/catalyst ALLOW 'chiral pool'/chiral auxiliary Total 18 12<br />

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