OCR A Jun 2016 Paper 2 Q2 (with explained solutions) |

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OCR A Jun 2016 Paper 2 Q2

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This question is about the homologous series of alcohols. (a) What is meant by the term homologous series?[2] (b) Ethanol is used more than any other alcohol. One method of preparing ethanol uses yeast. Write the equation for this preparation and state the essential conditions.[2] (c) At room temperature and pressure, the first four members of the alkanes are all gases but the first four alcohols are all liquids. Explain this difference in terms of intermolecular forces.[2] (d) The boiling points of 2-methylpropan-1-ol and butan-1-ol are shown below. Alcohol Boiling point / C 2-methylpropan-1-ol butan-1-ol 108 117 Explain why the boiling points are different.[2]OCR 2016<br />
(e) Alkenes can be prepared from alcohols. Cyclopentene can be prepared from cyclopentanol as shown in the equation below. OH A student plans to prepare 5.00 g of cyclopentene from cyclopentanol. The percentage yield of this reaction is 45.0%. (i) What is the name of this type of reaction?[1] (ii) Calculate the mass of cyclopentanol that the student should use. Show your working. mass of cyclopentanol =g [3]OCR 2016 Turn over<br />
(f) Alcohols can be prepared from halogenoalkanes. 2,2-dimethylpropan-1-ol can be prepared by hydrolysis of a chloroalkane with aqueous sodium hydroxide. (i) Write the equation for this reaction. Use structures for the organic compounds. (ii) Outline the mechanism for this reaction. Show curly arrows and relevant dipoles.OCR 2016 [1] [2] [Total: 15]<br />

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F322 Question 2 (a) Answer (series of compounds with the) same functional group OR same/similar chemical properties/reactions (cid:57) each successive/subsequent member differs by CH2 (cid:57) (b) C6H12O6 (cid:111) 2C2H5OH + 2CO2 (cid:57) warm OR stated temperature between 20 C and 45 C AND anaerobic OR absence of air/oxygen (cid:57) Mark scheme June 2016 Marks Guidance IGNORE reference to physical properties IGNORE same general formula Differs by CH2 is not sufficient (no successive) DO NOT ALLOW same empirical OR molecular formula ALLOW correct molecular OR structural OR displayed OR skeletal formula OR mixture of the above IGNORE state symbols DO NOT ALLOW acidic or alkaline conditions ALLOW conditions shown in the equation A limited supply of oxygen is NOT sufficient IGNORE pressure IGNORE yeast (in question)<br />
F322 Question 2 (c) (d) Answer Alcohols have hydrogen bonds (and van der Waals forces) (cid:57) Hydrogen bonds are stronger than van der Waals forces (in alkanes) (cid:57) Marks 2-methylpropan-1-ol has less surface (area of) contact OR fewer points of contact (cid:57) 2-methylpropan-1-ol has fewer/weaker van der Waals forces OR less energy required to break van der Waals forces in 2-methylpropan-1-ol (cid:57) Mark scheme June 2016 Guidance ANNOTATE ANSWER WITH TICKS AND CROSSES ALLOW reference to specific compounds e.g. comparing methane and methanol Second marking point requires BOTH types of intermolecular forces in response i.e comparison of hydrogen bonds AND van der Waals is essential DO NOT ALLOW the second mark for a comparison of van der Waals and hydrogen bonds between alcohols and water ALLOW more energy required to break hydrogen bonds than van der Waals forces ALLOW it is harder to overcome the hydrogen bonds than van der Waals forces IGNORE more energy is needed to break bonds ANNOTATE ANSWER WITH TICKS AND CROSSES Both answers need to be comparisons ALLOW ORA throughout Reference to just surface area / closeness of molecules is not sufficient IGNORE reference to H bonds IGNORE less energy is needed to break bonds (e) (i) Elimination OR dehydration (cid:57) 10<br />
F322 Question 2 (ii) (e) Mark scheme June 2016 Answer = 0.0735 (mol) (cid:57) IF answer = 14.0 OR 14.1 g award 3 marks --------------------------------------------------------------------------------- actual n(C5H8) produced = 5.00 68.0 theoretical n(C5H9OH) = n(C5H8) = 0.0735Mass of C5H9OH = 0.16386.0 = 14.0 (g) OR 14 g OR 14.1 g (cid:57) (use of unrounded values in calculator throughout) 100 = 0.163 (mol) (cid:57) 45.0 Marks Guidance 100 = 11.111 (g) 45.0 ANNOTATE ANSWER WITH TICKS AND CROSSES ALLOW ECF at each stage ALLOW 3 SF up to calculator value correctly rounded for intermediate values ALLOW expected mass C5H8 = 5.00ALLOW Mass C5H9OH reacted = 0.073586.0 = 6.321 (g) ALLOW Mass of C5H9OH used = 6.321ALLOW 2 SF up to calculator value correctly rounded for mass of C5H9OH Note: 2.84 OR 2.85 g would get 2 marks (use of 45.0/100 instead of 100/45.0) 13.76 OR 13.8 would get 2 marks (use of 0.16 for moles C5H9OH) 100 = 14.0 OR 14 (g) 45.0 11<br />
F322 Question 2 (i) (f) Answer Cl + NaOH Mark scheme Marks OH + NaCl (cid:57) June 2016 Guidance ALLOW correct structural OR displayed OR skeletal formula OR mixture of the above ALLOW equation with OH as reactant and Cl product e.g (CH3)3CCH2Cl + OH (cid:111) (CH3)3CCH2OH + Cl IGNORE equations with KOH/H2O as reactant (question states sodium hydroxide) IGNORE molecular formulae (question requires structures) 12<br />
F322 Question 2 (ii) (f) Answer Mark scheme Marks curly arrow from HO to carbon atom of CCl bond (cid:57) Dipole shown on CCl bond, C+ and Cl AND curly arrow from CCl bond to Cl atom (cid:57) June 2016 Guidance Curly arrow must come from lone pair on O of HO OR OH OR from minus sign on O of HO ion (No need to show lone pair if curly arrow came from negative charge) NOTE: ALLOW mechanism involving ANY halogenoalkane as structures have been assessed in 2(f)(i) ------------------------------------------------------------------------------- ALLOW SN1 mechanism: First mark Dipole shown on CCl bond, C+ and Cl AND curly arrow from CCl bond to Cl atom (cid:57) (cid:71) Cl (cid:71)+ Second mark Correct carbocation AND curly arrow from HO to carbocation OH Note: + is fine for charge (circle used for clarity) Curly arrow must come from lone pair on O of HO OR OH OR from minus sign on O of HO ion (No need to show lone pair if curly arrow came from negative charge) (cid:57) ------------------------------------------------------------------------------ Total 15 13<br />

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