OCR A Jun 2016 Paper 4 Q6 (with Explained Solutions) | - LearnAh!

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OCR A Jun 2016 Paper 4 Q6

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There are several isomeric alcohols with the formula C5H11OH. 16 (a) Pentan-1-ol, CH3(CH2)3CH2OH, can be prepared in the laboratory by the reduction of an aldehyde. State a suitable reducing agent for this reaction and write an equation to show the preparation of pentan-1-ol. Use [H] to represent the reducing agent in the equation. Reducing agentEquation[2] (b) Compound F is a structural isomer of C5H11OH. Compound F is converted to compound G when heated under reflux with acidified potassium dichromate(VI) solution. Compound G reacts with 2,4-dinitrophenylhydrazine to form an orange solid but compound G does not react with Tollens reagent. The 13C NMR spectrum of compound G is shown below. 200 180 160 140 120 100 80 60 40 20 ppm Compound H is a carboxylic acid. In a titration, 0.211 g of carboxylic acid H requires 22.8 cm3 of 0.125 mol dm3 NaOH for neutralisation. Compound F reacts with compound H in the presence of concentrated sulfuric acid to form organic compound I.OCR 2016br / Identify compounds F, G, H and I and draw their structures in the boxes below. Show your working only for the identification of compound H. 17[7]OCR 2016 Turn overbr / 18 (c) Compound J is another structural isomer of C5H11OH. The 1H NMR spectrum of J is shown below. The numbers next to each peak are the relative peak areas. 10 6 3 Chemical shift, (cid:98)(cid:3)/ ppm Identify compound J and draw its structure in the box below. END OF QUESTION PAPER [1] [Total: 10]OCR 2016br / Show answer F324 Question 6 (a) (b) Answer Reducing agent NaBH4 / sodium tetrahydridoborate(III) / sodium borohydride Equation CH3(CH2)3CHO + 2[H] (cid:5) CH3(CH2)3CH2OH M1 Compound F structure is a secondary alcohol with the formula C5H11OH M2 Compound F = CH3CH(OH)CH(CH3)CH3 M3 Compound G = CH3COCH(CH3)CH3 Mark (cid:51) (cid:51) (cid:51) (cid:51) (cid:51) 17 Mark Scheme June 2016 Guidance ALLOW LiAlH4 / lithium tetrahydridoaluminate(III)/lithium aluminium hydride ALLOW correct structural OR displayed OR skeletal formulae OR a combination of above ALLOW C4H9CHO + 2[H] (cid:5) C5H11OH ALLOW molecular formulae: C5H10O + 2[H] (cid:5) C5H12O DO NOT ALLOW COH for aldehyde ANNOTATE WITH TICKS AND CROSSES ETC. ALLOW correct structural OR displayed OR skeletal formulae OR a combination of above as long as unambiguous IGNORE names if structures are given ALLOW 3-methylbutan-2-ol if structure not given ALLOW ECF from an incorrect secondary alcohol for M3 e.g. pentan-2-ol (cid:5) pentan-2-one e.g. pentan-3-ol (cid:5) pentan-3-one ALLOW (3-)methylbutanone if structure not given IGNORE any discussion of the reactions of compound G with 2,4-dinitrophenylhydrazine and/or Tollens reagent. ALLOW 3 SF up to calculator value correctly roundedbr / F324 Question Mark Scheme June 2016 Guidance IF M(compound H) = 74 award 2 marks (M4 + M5) ALLOW ECF from incorrect calculation of amount of NaOH ALLOW propanoic acid if structure not given ALLOW ECF from incorrect compound F (alcohol) and/or incorrect compound H (carboxylic acid) to form compound I (ester). Compounds F, G, H and I must be placed in the correct box or correctly labelled for M2. M3, M6 and M7 ALLOW correct structural OR displayed OR skeletal formulae OR a combination of above as long as unambiguous ALLOW 2,2-dimethylpropan-1-ol Answer M4 n(NaOH) = (0.125 x 22.8/1000) = 0.00285 (mol) M5 M(compound H) = (0.211/0.00285 =) 74(.0) (g mol-1) M6 Compound H = / CH3CH2COOH M7 Compound I = Mark (cid:51) (cid:51) (cid:51) (c) The structural isomer is: (cid:51) (cid:51) Total 10 18br / Question 1 2 3 4 5 6

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