Edexcel Jun 2015 Paper 5 Q25 (with explained solutions) |

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Edexcel Jun 2015 Paper 5 Q25

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SECTION C Answer ALL the questions. Write your answers in the spaces provided. 25 Read the passage below carefully and answer the questions which follow. The general formula of most naturally occurring amino acids can be written RCH(NH2)COOH, though in some amino acids, such as proline, the nitrogen atom is part of a five-membered ring. Amino acids are water soluble, though the extent of their solubility varies. All but one of the naturally occurring amino acids show optical activity. The formula of the R group for some naturally occurring amino acids is shown below. Amino acid Formula of R group serine lysine phenylalanine leucine iso-leucine alanine CH2OH (CH2)4NH2 CH2C6H5 CH2CH(CH3)2 CH(CH3)CH2CH3 CH3 Mixtures of amino acids can be separated by electrophoresis. This method depends on amino acids moving different distances through paper or gel when an electric field is applied. They can also be separated by chromatography. Ninhydrin, shown below, is the chemical which is used to locate the position of the amino acids on chromatograms. OH OH Ninhydrin detects ammonia and primary and secondary amines. When it reacts with primary amines, a deep blue or purple colour is produced in a complex series of reactions. With proline, a yellow compound forms. Ninhydrin is widely used to detect fingerprints. Sweat secretions from ridges on the finger contain dipeptides and proteins. These are left on porous surfaces such as paper, and react with ninhydrin. When ninhydrin reacts with amino acids, carbon dioxide is released from the carboxylic acid group. Archaeologists have used this reaction to release the carbon from proteins in ancient bones, and, by comparing the proportions of carbon and nitrogen in the remains, they have obtained evidence for the diets of these animals. 20 *P45073RA02028*<br />
(a) (i) At a pH of 5.68, serine exists as a zwitterion. Draw the formula of serine at this pH. (1) *(ii) At pH 5.68, serine and lysine can be separated by electrophoresis. By considering the structures of the amino acids at this pH, suggest why this separation occurs. (2)*(b) Serine is very soluble in water, whilst phenylalanine is much less soluble. Explain the difference, disregarding any effect of zwitterion formation. (2)(c) The naturally occurring amino acid which does not show optical activity is not shown in the table. Give the formula of the R group for this acid. (1) *P45073RA02128* 21 Turn over<br />
(d) The optical activity of equimolar solutions of naturally occurring samples of leucine and iso-leucine can be measured in an experiment using plane-polarized light. (i) What measurement is made to show the optical activity of amino acids? (1)(ii) By considering the structures of iso-leucine and leucine, explain why iso-leucine has more stereoisomers than leucine. Amino acid leucine iso-leucine Formula of R group CH2CH(CH3)2 CH(CH3)CH2CH3 (1)(e) The amino acid proline, shown below, does not contain a primary amine group, but it can still form peptide bonds. OH NH Draw the structure of the dipeptide formed when the carboxylic acid group of alanine reacts with proline. Circle the peptide group on your drawing. (2) 22 *P45073RA02228*<br />
(f) The first steps of the reaction of ninhydrin with alanine can be summarised in the equation shown below. OH OH + CH3 CH(NH2)COOH (cid:314) O Y NH2 + CO2 + H2O + Z (i) By balancing the equation, suggest the structural formula of the product Z. (ii) In the final stage, Y reacts with another molecule of ninhydrin to form a dye, Q, shown below. What is the molecular formula of Q? (1) (1) *P45073RA02328* 23 Turn over<br />
(iii) On combustion, 1.000 g of Q produces 2.614 g carbon dioxide, 0.2673 g water and 0.04620 g nitrogen. Use these data to calculate the percentage composition by mass of Q, and hence its empirical formula. Show whether your answer is consistent with the molecular formula of Q. (iv) Evidence for the structure of Q is obtained from data in its mass spectrum, and the number of peaks in its low resolution nmr spectrum. Suggest one piece of evidence from each type of spectroscopy which would support the structure shown in (f)(ii). Give data where appropriate. (4) (2)(Total for Question 25 = 18 marks) TOTAL FOR SECTION C = 18 MARKS TOTAL FOR PAPER = 90 MARKS 24 *P45073RA02428*<br />

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Question Number 24b(vi) Question Number 25(a)(i) Acceptable Answers Reject Mark C6H2(CH3)2NH2 ALLOW any substitution positions C6H3(CH3)2NH2 H- C6H2(CH3)2NH2 Kekule structure Acceptable Answers Total for Question 24 = 10 marks Reject Mark H +H3N C COO CH2OH OR HOCH2CH(NH3 ALLOW Charge on NH3 shown on N atom +)CO2 6CH0501 1506<br />
Acceptable Answers Question Number *25(a)(ii) EITHER Reject Mark + / lysine has an At pH 5.68 both NH2 groups on lysine will be NH3 overall positive charge while serine has no overall charge (1) So only lysine will move to negative terminal / in an electric field (1) OR lysine has a greater positive charge (than serine) (1) Lysine will move faster / further in an electric field (1) ALLOW They have different charges so will move different distances/ at different speeds 1 mark max IGNORE Answers based on molecular mass Question Number *25(b) Acceptable Answers COMMENT Ignore references to H bonds from amino and carboxyl groups and assume that phenylalanine cannot form H bonds refers to R group in the compound. Serine has an OH (alcoholic) group which can form H bonds with water (1) Benzene ring in phenylalanine is bulky /is large and non-polar / is hydrophobic / London forces between phenylalanine molecules are significant ALLOW van der Waals etc for London ALLOW If MP1 is not scored, phenylalanine cannot form H bonds with water can score MP2 (1) Reject VdW / id-id Mark 6CH0501 1506<br />
Question Number 25(c) Acceptable Answers H, -H, H- ALLOW NH2CH2CO2H / NH2CH2COOH Reject Mark Acceptable Answers Reject Question Number 25(d)(i) Mark Just polarimetry (Measure) the amount of / angle of / degree of /extent of rotation (of plane of polarization/ plane polarized light) IGNORE direction they rotate the light Question Number 25(d)(ii) (Leu has 1 chiral C but) Ile has 2 chiral Acceptable Answers C / 2 asymmetric C atoms OR The R group in Ile contains a chiral C (Leu has 2 optical isomers but Ile has 4 optical isomers) ALLOW Ile has (1) more chiral C (than leu) Chiral centres shown on formulae Reject Leu has no chiral C Mark 6CH0501 1506<br />
Question Number 25(e) Acceptable Answers Reject Just N-C circled CON circled with N as part of ring (1) Fully correct structure (1) ALLOW -CH(CH3)NH2 shown skeletally OR Dipeptide using NH2 from alanine and COOH from proline, with correct structure and CONH circled This scores max (1) Acceptable Answers Question Number 25(f)(i) CH3CHO / CH2CHOH ALLOW Displayed or semi-displayed formula C2H4O Reject CH3COH (unless with correct structure) Acceptable Answers Question Number 25(f)(ii) C18H9NO4 Reject Mark Mark Mark 6CH0501 1506<br />
Reject Mark Question Number 25(f)(iii) Several methods are possible. Acceptable Answers Amount of O should have been calculated and final answer should be consistent with masses/mols in calculation. There should be evidence of working at each stage for 4 marks MP1 2.614 g CO2 contain 0.7129 g C 0.2673 g H2O contain 0.0297 g H OR 2.614 g CO2 contain 0.059409 mol CO2 0.2673 g H2O contain 0.01485 mol H2O (1) MP2 Mass O in Q = (1.000.7129- 0.0297 -0.04620) = 0.2112 g (1) % mol/100g ratio C 71.29 5.94 18 H 2.97 2.97 9 N 4.62 0.33 1 O 21.12 1.32 4 MP3 Number of moles of each element OR Number of mol in 100g Allow TE for incorrect mol of Hsee calculation below (1) MP4 Ratio stated must follow from numbers in calculation (so empirical formula is consistent with the molecular formula of Q) (1) OR Calculation following error in H scores max (3) If mass H is wrongly calculated as 0.01485 g, then mass O will be 0.22605 (TE for MP2) (1) % mol/100g ratio C 71.29 5.94 18 H 1.48 1.48 4.5 N 4.62 0.33 1 O 22.60 1.41 4.27 6CH0501 1506<br />
Question Number 25(f)(iv) Number of moles of each element (TE for MP3) (1) Ratio stated and empirical formula said not to be consistent (TE for MP4) (1) Acceptable Answers Reject Mark Mass of molecular ion in mass spectrum = 303 / any sensible fragment with mass eg 302 / 158 / 145 or smaller fragment (1) ALLOW Molecular ion based on formula in f(i) Number of peaks in nmr = 2 / 3 ALLOW 5 (1) Total for Question 25 = 18 marks 6CH0501 1506<br />

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