OCR A Jun 2017 Paper 2 Q16 (with explained solutions) |

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OCR A Jun 2017 Paper 2 Q16

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SECTION B Answer all the questions. 16 This question is about unsaturated hydrocarbons. (a) Compound A and compound B are isomers. compound A compound B Compound A has a lower melting point than compound B. Suggest why.[2] (b) Compound C, CH3CH2CH=CHCH2CH2OH, exists as cis and trans stereoisomers. (i) Name compound C.[1] (ii) Define the term stereoisomers.[1]<br />
(iii) Draw the structures of the cis and trans stereoisomers of compound C. cis trans [2] (c) The C=C group in an alkene contains a -bond. Complete the diagram below to show how p-orbitals are involved in the formation of a -bond. [1]<br />
(d) Compound D, shown below, reacts with hydrogen bromide by electrophilic addition. A mixture of two organic compounds, E and F, is formed. 10 CH2CH3 CH3 C H CH3 compound D HBr Mixture of organic compounds E and F (i) Suggest how an HBr molecule can act as an electrophile.[1] (ii) Draw the structures of the two organic compounds E and F. [2]<br />
(iii) Outline the mechanism of the reaction between compound D and hydrogen bromide to form either compound E or compound F. 11 Include curly arrows and relevant dipoles. (iv) Which of E or F is the major organic product? Explain your answer. [3] Major organic productExplanation[1]<br />
(e) Myrcene, C10H16, is a naturally occurring hydrocarbon containing more than one carbon-carbon double bond. 12 myrcene (i) Reaction of 204 mg of myrcene with hydrogen gas produces a saturated alkane. Calculate the volume of hydrogen gas, in cm3 and measured at RTP, needed for this reaction. Show your working. volume =cm3 [2] (ii) -Carotene is a naturally occurring unsaturated hydrocarbon found in carrots. A -carotene molecule contains 40 carbon atoms, has two rings, and a branched chain. 0.0200 mol of -carotene reacts with 5.28 dm3 of hydrogen gas to form a saturated hydrocarbon. Using molecular formulae, construct a balanced equation for this reaction. Include relevant calculations and reasoning. Equation[4]<br />
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H432/02 Question Answer 16 (a) Compound A (is branched so) has less points of contact / less surface interaction between moleculesInduced dipoledipole interactions / London (dispersion) forces are weaker. AND Require less energy to break (these interactions / forces)(b) (i) Hex-3-en-1-olMark Scheme June 2017 Marks Guidance Both answers need to be comparisons ALLOW ORA throughout DO NOT ALLOW more contact between atoms IGNORE van der Waals forces/VDW for induced dipoledipole interactions (ambiguous as this term refers to both permanent dipoledipole and induced dipoledipole forces) ALLOW fewer induced dipole-dipole interactions. IGNORE it is easier to break the induced dipole- dipole / London forces. (reference to energy required) IGNORE less energy required to separate molecules IGNORE less energy is needed to break the bonds. ALLOW Hex-3-ene-1-ol ALLOW 1-hydroxyhex-3-ene as this is unambiguous Hex-3-enol is not sufficient IGNORE lack of hyphens, or addition of commas<br />
H432/02 Question (ii) Same structural formula Answer Mark Scheme Marks AND Different arrangement (of atoms) in space OR different spatial arrangement (of atoms)(iii) cis trans June 2017 Guidance ALLOW have the same structure/displayed formula/skeletal formula DO NOT ALLOW same empirical formula OR same general formula IGNORE same molecular formula Reference to E/Z isomerism or optical isomerism is not sufficient ALLOW any combination of skeletal OR structural OR displayed formula as long as unambiguous ALLOW one mark if both stereoisomers of compound C are shown but in the incorrect columns ALLOW one mark for correct stereoisomers of compound C in correct columns whereCH2CH2OH is represented as -C2H5O orC2H4OH DO NOT ALLOW incorrect connectivity e.g.CH3CH2 on first occasion but allow ECF in second structure.<br />
H432/02 Question (c) Answer Mark Scheme Marks Two p-orbitals shown as a dumb-bell added to structure on left. AND -bond on structure on right(d) (i) (The H atom of HBr) accepts a pair of electrons(ii) June 2017 Guidance DO NOT ALLOW C=C in diagram DO NOT ALLOW overlapping p orbitals on left hand side in the diagram. DO NOT ALLOW a diagram that contains four lobes on the right hand side. e.g. IGNORE any atoms joined to the bonds Note: labels are not required ALLOW the following diagram to show the - bond ALLOW correct structural OR displayed OR skeletal formulae OR a combination of above as long as unambiguous ALLOW in either order<br />
Answer Mark Scheme Marks H432/02 Question (iii) Curly arrow from C=C bond to H of HBrCorrect dipole shown on HBr AND curly arrow showing the breaking of HBr bond---------------------------------------------------------------------- Correct carbocation AND curly arrow from Br to C+ of carbocationOR 10 June 2017 Guidance ANNOTATE ANSWER WITH TICKS AND CROSSES ALLOW any combination of skeletal OR structural OR displayed formula as long as unambiguous DO NOT ALLOW partial charges shown on C=C double bond ( the second marking point) DO NOT ALLOW + on C of carbocation Curly arrow must come from a lone pair on Br OR from the negative sign of Br ion (then lone pair on Br ion does not need to be shown)<br />
H432/02 Question (iv) Answer Mark Scheme Marks 2-bromo-2-methylpentane AND (the) carbocation intermediate (in the formation of 2- bromo-2-methylpentane) is more stable (than the carbocation in the formation of the other product)(e) (i) n(myrcene) = 204103 136.0 = 1.5(0)103 (mol)Volume of H2 = 31.5(0)10324000 = 108 (cm3)11 June 2017 Guidance Note: the correct product and explanation are both required for the mark The major product may be identified by itscorresponding letter (E or F) from the table in (d)(ii)correct structurecorrect name DO NOT ALLOW product comes from the more stable secondary or primary carbocation IGNORE explanations based on Markownikoffs rule. Correct working required for the first marking point. ALLOW ECF from incorrect moles of myrcene i.e. n(myrcene)324000 Common incorrect answers 108000 cm3 = 1 mark (not converted to g) 12cm3 = 1 mark ( divided by 3) 36 cm3 = 1 mark ( not multiplied by 3) IGNORE Calculations based on pV = nRT<br />
H432/02 Mark Scheme June 2017 Question (ii) Amount of hydrogen Answer Marks Guidance n(H2) = = 0.22(0) (mol)5.28 24.0 ALLOW Evidence of n(H2) = if 0.22 is not 5.28 24.0 Number of double bonds 0.220 0.0200 = 11Formula of saturated product C40H78Equation C40H56 + 11H2C40H78seen Evidence for 11 double bonds could come from 11H2 in equation Formula could be shown as the product of an equation ALLOW ECF from C40H82 and C40H80 only i.e. C40H60 + 11H2C40H82 C40H58 + 11H2C40H80 Total 20 12<br />

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