Edexcel Jan 2015 (IAL) Paper 4 Q22 (with explained solutions) |

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Edexcel Jan 2015 (IAL) Paper 4 Q22

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22 Butanone, CH3CH2COCH3, is used as an industrial solvent. (a) State each stage in the procedure in which 2,4-dinitrophenylhydrazine is used to confirm the identity of a carbonyl compound thought to be butanone. Detailed practical descriptions are not required. (3)(b) Butanone can be converted into 2-hydroxy-2-methylbutanoic acid, CH3CH2C(OH)(CH3)COOH, in two steps: CH3CH2CCH3 Step 1 HCN/KCN OH Step 2 CH3CH2CCH3 CN OH CH3CH2CCH3 COOH (i) Classify the type and mechanism of the reaction taking place in Step 1. (2)(ii) Identify the reagent(s) and conditions for the reaction taking place in Step 2. (2)*P45044A01528* 15 Turn over<br />
(iii) The incomplete mechanism for Step 1 is shown below. CH3 CH2 CH3 CN Step A CH3 CH2 O CN CH3 CN CH3 CH2 O CN CH3 Step B OH CH3 CH2 CH3 + CN CN On the incomplete mechanism above, draw the curly arrows and the relevant lone pairs of electrons to complete the mechanism. (iv) Explain why the 2-hydroxy-2-methylbutanoic acid produced in this reaction is not optically active. (3) (3)16 *P45044A01628*<br />
(c) Draw two repeat units of the polymer that could be formed from 2-hydroxy-2-methylbutanoic acid. (2) (Total for Question 22 = 15 marks) *P45044A01728* 17 Turn over<br />

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Question Number 22(a) Question Number 22(b)(i) Just crystallisation if the precipitate has not been filtered Just characteristic melting temperature First mark mix/add the reagents and filter OR react butanone/ketone with Bradys reagent/2,4-dinitrophenylhydrazine and filter OR filter the (yellow/orange) precipitate formed (1) Second mark recrystallize OR description of recrystallization ALLOW this mark even if the ppt is not filtered (1) Third mark measure the melting temperature (of derivative of butanone) and compare with data book /reference / literature value (1) Stand alone marks Acceptable Answers nucleophilic (1) addition (1) answers can be in any order IGNORE heterolytic Acceptable Answers Reject Mark (3) Reject hydrolysis/ reduction SN1 or SN2 Mark (2) WCH0401 1501<br />
Question Number 22(b)(ii) Method 1 acid hydrolysis Acceptable Answers Name or formula of any strong acid eg (dilute) hydrochloric acid/ (dilute) sulfuric acid (1) IGNORE dilute acid / H+(aq)/ just H+ Boil/heat /reflux Conditional on acid as the only reagent ALLOW high temperature (1) Method 2 alkaline hydrolysis Sodium hydroxide solution/ dilute sodium hydroxide/ NaOH(aq) and boil/heat /reflux (1) then add dilute acid / H+(aq)/dilute hydrochloric acid/ dilute sulfuric acid (1) Mark (2) Reject Just concentrated sulfuric acidPotassium dichromate(VI) and dilute sulfuric acid Just warm WCH0401 1501<br />
Question Number 22(b)(iii) Acceptable Answers Reject Mark First mark both curly arrows on the first diagram arrow from C of CN to C of carbonyl and arrow from double bond to O ALLOW curly arrow from the - sign but not from the N (1) IGNORE correct dipoles Second mark lone pair on C of CN correct (1) IGNORE other lone pairs, even if incorrect Third mark both curly arrows on the third diagram arrow from O to H and from bond to C of CN ALLOW curly arrow to gap between C and N (1) (3) full charges on C / O incorrect dipole on C=O arrow directly to N of CN WCH0401 1501<br />
Acceptable Answers Question Number 22(b)(iv) First mark a racemic mixture/racemate forms OR equal amounts of the two optical isomers /enantiomers / D-L isomers / (+) and (-) isomers /R-S isomers (1) Second mark the molecule is (trigonal) planar around C=O /carbonyl group /reaction site (1) Third mark (equal probability of) the CN- ion/nucleophile attacking (the C of C=O) from above or below/either side/both sides of (the plane) (1) Mark (3) Reject For second mark only: mention of carbocation OR C=O/carbonyl carbon atom is planar OR intermediate is planar OR the molecule /butanone / ketone is planar Question Number 22(c) Acceptable Answers Reject Mark (2) in polymer scores (0) more than 1 H missing from a bond OR ALLOW any combination of displayed structure/ structural formula /skeletal formulae ester group correct ALLOW COOC- (1) rest of polymer correct ALLOW C2H5 ALLOW more than 2 repeat units Conditional on ester group correct (1) IGNORE n and square brackets (Total for Question 22 = 15 marks) WCH0401 1501<br />

Question 21 22 23 24

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