Edexcel Jun 2011 Paper 4 Q15 (with explained solutions) |

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Edexcel Jun 2011 Paper 4 Q15

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PMT SECTION B Answer ALL the questions. Write your answers in the spaces provided. 15 A sequence of reactions for the production of lactic acid is shown below. CH2 CH2 step 1 CH3 CHO step 2 CH3 step 3 CH3 HO C CN HO C CO2H lactic acid (a) (i) Name the type and mechanism of the reaction in step 2. (2)(ii) Which two substances need to be added to ethanal to carry out the reaction in step 2? (2)(iii) Give the mechanism for the reaction in step 2, using curly arrows to show movements of electron pairs. (3) *P38482A0924* Turn over<br />
PMT *(iv) The product of step 2 is not optically active even though it has a chiral carbon atom in its formula. Explain, by reference to the mechanism, the reason for the lack of optical activity. (2)(b) What reactant, or combination of reactants, is needed to carry out step 3? (1)(c) (i) What is the systematic name of lactic acid? (1)(ii) Lactic acid molecules can combine to form a biodegradable polymer, poly(lactic acid) or PLA. Draw a section of the polymer with two units of the polymer chain and showing all bonds. (iii) Suggest why PLA is biodegradable. (1) (1)10 *P38482A01024*<br />
PMT (iv) Lactic acid can be prepared from ethene as shown in the scheme. Lactic acid also forms when milk turns sour. Suggest one reason why it would be advantageous to make lactic acid from milk rather than from ethene. (1)(Total for Question 15 = 14 marks) *P38482A01124* 11 Turn over<br />

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Section B Mark Mark Question Number 15 (a)(i) Question Number 15 (a)(ii) Acceptable Answers Addition (1) Nucleophilic (1) Either order Acceptable Answers Reject SN1 SN2 Reject Hydrogen cyanide / HCN (1) Potassium cyanide / KCN/ sodium cyanide/ NaCN (1) OR Potassium cyanide / KCN (1) With hydrochloric acid / sulfuric acid (to generate HCN) (1) Ignore concentration of acids Mark for HCl etc is consequential on KCN OR Hydrogen cyanide / HCN (1) With sodium hydroxide / other base (to make cyanide ions) (1) Mark for NaOH etc is consequential on HCN Just CN- Just CN- Just acid/ H+ any weak acid Just OH- 6CH04/01 1106<br />
Question Number 15 (a)(iii) Acceptable Answers Mark Reject C=O breaking before attack by CN- Arrows from atoms when they should be from bonds and vice versa Both arrows in first step of mechanism above correctly drawn (1) Correct intermediate with charge (1) Both arrows in second step with correct organic product (CN- is not required) (1) Use of HCN for first step max 2 marks Allow omission of lone pair on CN- and O- Allow curly arrow from negative charge or elsewhere on cyanide ion Allow arrow from O- in 2nd step to H+ (no other product or only one product) or H2O ( with OH- formed) 6CH04/01 1106<br />
Question Number *15 (a)(iv) Question Number 15 (b) Question Number 15 (c)(i) Acceptable Answers Reject Attack (by nucleophile on the C) is from both sides (equally)/ above and below (at the planar reaction site in the aldehyde group) (1) So a mixture of two enantiomers/(optical)isomers in equal proportions forms OR racemic mixture forms (1) First and second marks are independent Attack on intermediate in reaction mechanism is from both sides Attack from both ends/two angles Just both enantiomers form Acceptable Answers Reject Any named (aqueous) strong acid or its formula. Allow (aqueous) sodium hydroxide followed by named acid or formula Ignore references to concentration Water H+ Potassium dichromate + sulfuric acid Carboxylic acids Acceptable Answers Reject 2-hydroxypropanoic acid 2- hydroxylpropanoic acid 2- hydroxopropanoic acid 2-hydroxypropan- 1-oic acid Mark Mark Mark 6CH04/01 1106<br />
Question Number 15 (c)(ii) Question Number 15 (c)(iii) Question Number 15 (c)(iv) Mark Mark Mark Acceptable Answers Reject A dimer Missing H atoms Missing bonds at ends OR CH3 C H O C O CH3 C H C O O CH3 C H O C O CH3 C C O H All bonds in ester link must be shown More than 2 units may be shown but structure shown should be a repeat unit Ignore brackets/n Acceptable Answers Ester (link/bond) in PLA can be hydrolysed/broken down (by enzymes) OR Ester (link/bond) in PLA can be broken down Reject Just it can be hydrolysed Acceptable Answers Reject Ethene is (from crude oil so) non-renewable/ milk is from a renewable source/ energy required to make ethene is high/ high temperatures needed to make ethene/ energy requirements for process from sour milk less/ process from milk doesnt use toxic chemicals / process from milk doesnt use cyanide Allow process from ethene requires many steps so expensive/so loss of material occurs at each step /so more reagents needed Ignore references to cost, unless answer gives a reason for lower cost. Milk is more readily available Greater atom economy No other chemicals needed in process from milk Just process from ethene requires many steps Just cheaper 6CH04/01 1106<br />

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