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OCR A Jun 2009 Paper 2 Q3
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![3 Alkenes are unsaturated hydrocarbons used in the industrial production of many organic compounds. (a) Complete the flowchart below to show the organic product formed in each addition reaction of methylpropene. H2 / Ni Br2 CH3 CH3 HBr mixture of isomers [4] (b) Curly arrows are used in reaction mechanisms to show the movement of electron pairs during chemical reactions. Use curly arrows to outline the mechanism for the addition reaction of methylpropene with bromine. The structure of methylpropene has been drawn for you. Include relevant dipoles in your answer. CH3 CH3OCR 2009 [4]<br />
(c) Alkenes can be prepared by the dehydration of alcohols with an acid catalyst. Cyclohexene can be prepared by the dehydration of cyclohexanol, shown below. OH H2O A student reacted 7.65 g of cyclohexanol, C6H12O, and obtained 0.0268 mol of cyclohexene. (i) What is the molecular formula of cyclohexene?[1] (ii) Calculate the percentage yield of cyclohexene. (d) Percentage yield has been used for many years to measure the success of a reaction. Recently, chemists have turned their thoughts also to the atom economy of a reaction. answer =% [3] (i) Explain the term atom economy.[1] (ii) Cyclohexene can also be prepared by the reaction below. Explain why the atom economy of this cyclohexene preparation is higher than that from cyclohexanol in (c).[2]OCR 2009 [Total: 15] Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/OCR_2_QP_JUN09_question_3-0.jpg)
![F322 Question 3 (a) CH3 Expected Answers Mark Scheme Marks CH3 H2 CH3 CH3 HBr Br2 Br CH3 Br CH3 Br CH3 CH3 CH3 Br CH3 one mark for each correct structure (cid:57) (cid:57) (cid:57) (cid:57) (b) curly arrow from double bond to Br+ and curly arrow from Br Br bond pair to Br in 1st step (cid:57) curly arrow in 2nd step from bromide ion (cid:57) correct dipole shown on Br2 (cid:57) correct carbocation shown (cid:57) H H CH3 CH3 CH3 Br (cid:71)(cid:14) (cid:71)(cid:16) Br Br CH3 Br 14 June 2009 Rationale ALLOW skeletal formula OR displayed formulae IGNORE molecular formulae IF two answers given e.g. name and structure then both must be correct to be given a mark ALLOW methylpropane OR (CH3)3CH (cid:57) ALLOW 1,2-dibromo-methylpropane OR CH2BrCBr(CH3)2 (cid:57) ALLOW 1-bromo-methylpropane OR CH2BrCH(CH3)2 (cid:57) ALLOW 2-bromo-methylpropane OR CH3CBr(CH3)2 (cid:57) ALLOW ecf if wrong carbon skeleton is used in all of the structures mark first structure wrong and then apply ecf for the rest Curly arrow must start from the double bond and not a carbon atom, other curly arrow must start from BrBr bond ALLOW curly arrow from any part of bromide ion The bromide ion does not need to show a lone pair Dipole must be partial charge and not full charge Carbocation needs a full charge and not a partial charge (charges do not need to be surrounded by a circle) ALLOW carbocation on carbon 1 where electrophile attacks carbon 2 i.e. +CH2CBr(CH3)2<br />
Mark Scheme Marks June 2009 Rationale ALLOW full marks for correct answer with no or limited working out ALLOW ecf from wrong molar mass i.e. 7.65molar mass ALLOW ecf from wrong amount in moles i.e. [0.0268moles]100 ALLOW 35% ALLOW two marks for 0.35% If Mr of 82 is used then % yield will be 28.7 or 29 and this is worth two marks ALLOW (sum of) the molecular masses of the desired productsum of molecular masses of all reactants100 (cid:57) ALLOW no by products formed ALLOW other substances formed OR cyclohexene is not the only product 15 F322 Question (c) (i) C6H10 (cid:57) (ii) Expected Answers Mr(cyclohexanol) = 100 (cid:57) amount of cyclohexanol = 0.0765 mol (cid:57) percentage yield = 35.0% (cid:57) (d) (i) (sum of) the molecular masses of the desired productsum of molecular masses of all products100 (cid:57) (ii) this preparation is addition OR has 100% atom economy OR there is only one product (cid:57) preparation from cyclohexanol has less than100% atom economy OR H2O is produced as well OR calculated atom economy = 82% (cid:57) Total 15<br />](https://learnah.org/wp-content/uploads/2018/08/OCR_2_QP_JUN09_answer_3-0.jpg)
