Edexcel Jun 2013 Paper 4 Q19

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19 2-hydroxypropanoic acid, lactic acid, is a chiral molecule which is found in muscles and in sour milk. The 2-hydroxypropanoic acid formed in muscles is optically active but that in sour milk is not. OH C OH H C 2-hydroxypropanoic acid (a) (i) Explain the term chiral, stating the feature of 2-hydroxypropanoic acid that makes it chiral. Label this feature on the formula above. (3)(ii) What is the difference between the 2-hydroxypropanoic acid formed in muscles and that found in sour milk which gives rise to the difference in optical activity? (2)*P41572A01524* 15 Turn over<br /> (b) 2-hydroxypropanoic acid may be prepared in the laboratory from propanoic acid in a two-stage sequence in which 2-bromopropanoic acid is formed as an intermediate: C OH H C Br2 / Red P Stage 1 Br C OH H C Stage 2 OH C OH H C (i) Stage 2 of this sequence was carried out in two steps. Identify the reagent required for each step in Stage 2. (2) First stepSecond step(ii) When an optically active isomer of 2-bromopropanoic acid is used in Stage 2, the resulting 2-hydroxypropanoic acid is also optically active. State and explain what this indicates about the mechanism of the first reaction in Stage 2. (3)16 *P41572A01624*<br /> (c) 2-hydroxypropanoic acid may also be prepared from ethanal in the following sequence: H C C H Reaction 1 OH CN H C Reaction 2 OH C OH H C (i) Name the mechanism and type of reaction occurring in Reaction 1. (2)(ii) Identify the attacking species in Reaction 1. (1)(iii) Give the first step of the mechanism of Reaction 1, showing the formation of the intermediate. *(iv) Explain, by referring to the mechanism in (c)(iii), why the 2-hydroxypropanoic acid formed from ethanal shows no optical activity. (2) (3)*P41572A01724* 17 Turn over<br /> (d) The infrared spectrum of 2-hydroxypropanoic acid is shown below. 100 50 / e c n a t t i m s n a r T 0 4000 3000 2000 1500 Wavenumber / cm1 1000 500 (i) Give the wavenumber of the peak that is present in the infrared spectrum of 2-hydroxypropanoic acid but will not be present in the infrared spectrum of ethanal, identifying the group most likely to be responsible for this peak. Use the data on pages 5 and 6 of the data booklet. (1)*(ii) Identify the bond responsible for absorption peak Q in the spectrum. By considering the wavenumber of this peak, and the data on pages 5 and 6 of the data booklet, explain whether this peak alone can be used to distinguish between ethanal and 2-hydroxypropanoic acid. (3)18 *P41572A01824*<br /> (e) Ethanal and 2-hydroxypropanoic acid can be distinguished by the use of chemical tests. Give two suitable tests not involving indicators. For each test, state the observation associated with a positive result. (4) Test which is positive for ethanal but not for 2-hydroxypropanoic acid.Test which is positive for 2-hydroxypropanoic acid but not for ethanal.(Total for Question 19 = 26 marks) TOTAL FOR SECTION B = 51 MARKS *P41572A01924* 19 Turn over<br />

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