Edexcel Jun 2012 Paper 6 Q4 (with explained solutions) |

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Edexcel Jun 2012 Paper 6 Q4

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4 A student attempted to make a sample of methyl 3-nitrobenzoate using the following reaction. COOCH3 COOCH3 + HNO3 o + H2O NO2 methyl benzoate methyl 3-nitrobenzoate Procedure 1. Transfer 9 cm3 of concentrated sulfuric acid into a 100 cm3 conical flask and cool it to below 10qC in an ice bath. Add 5.0 g of methyl benzoate, swirling the flask. Mix 3 cm3 of concentrated nitric acid with 3 cm3 of concentrated sulfuric acid in another small flask and cool it in ice. 2. Add the mixture of nitric and sulfuric acids, drop by drop, to the methyl benzoate solution, making sure that the temperature stays below 15qC. 3. Take the mixture out of the ice bath and leave it to stand for 10 minutes at room temperature. Pour the mixture over 40 g of crushed ice and collect the solid product by filtering the mixture under suction. Wash the precipitate, first with cold water, then with ice-cold ethanol. Keep the washings obtained with the ethanol for a further experiment. 4. Purify the impure methyl 3-nitrobenzoate by recrystallization, using ethanol as the solvent, cooling the solution in an ice bath to assist recrystallization. 5. Dry the recrystallized product and determine the yield. (a) The student wore goggles and a laboratory coat. For each of the processes below, state the hazard and give one further safety precaution which should be taken. (i) Working with concentrated nitric and sulfuric acids. (1)(ii) Carrying out the recrystallization using ethanol. (1)10 *P39311A01016*<br />
(b) What is the purpose of adding sulfuric acid to the nitric acid in this reaction? (1)(c) (i) Calculate the number of moles in 5.0 g of methyl benzoate. Assume the molar mass of methyl benzoate is 136 g mol1. (1) (ii) Methyl benzoate is a liquid at room temperature. What is the volume of 5.0 g of methyl benzoate? The density of methyl benzoate is 1.09 g cm3. (iii) After recrystallization and drying, 3.4 g of methyl 3-nitrobenzoate was obtained. Calculate the percentage yield of methyl 3-nitrobenzoate, assuming that an excess of nitric and sulfuric acids was present. (1) (3) *P39311A01116* 11 Turn over<br />
(d) One reason for the low yield in this experiment is that methyl 2-nitrobenzoate is also formed. This compound dissolves in ethanol and would be present in the ethanol washings from step 3. Methyl 2-nitrobenzoate and methyl 3-nitrobenzoate are both pale yellow. (i) Describe how to make a chromatogram with the ethanol washings from step 3 in order to separate methyl 2-nitrobenzoate and methyl 3-nitrobenzoate. The chromatogram can be made on a plate covered with a layer of silica, and you may assume that a suitable solvent is available. (4)(ii) How would you improve the chromatogram to confirm that both methyl 2-nitrobenzoate and methyl 3-nitrobenzoate are present in the washings? You may show this on a diagram if you prefer. (1)12 *P39311A01216*<br />
(e) The table below gives data about the solubility of methyl 3-nitrobenzoate in two solvents. This data may be used to select the best solvent for recrystallization. Solubility of methyl 3-nitrobenzoate / g per 100 g solvent Temperature / qC Solvent 1 Solvent 2 10 70 6.0 11.0 2.0 9.5 (i) Explain why using Solvent 1, rather than Solvent 2, would lead to a lower yield of recrystallized methyl 3-nitrobenzoate. (1)(ii) 50 g of Solvent 2 was saturated with methyl 3-nitrobenzoate at 70 qC, and the solution was then cooled to 10 qC. Calculate the mass of methyl 3-nitrobenzoate crystals which would be obtained. (f) What method, other than spectroscopy or chromatography, could be used to assess the purity of the methyl 3-nitrobenzoate? How would the result of the experiment indicate if it was pure? (1) (2)(Total for Question 4 = 17 marks) TOTAL FOR PAPER = 50 MARKS *P39311A01316* 13<br />

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Question Number 4(a)(i) Question Number 4(a)(ii) Question Number 4(b) Correct Answer Reject Corrosive so wear gloves ALLOW burns skin/damages skin ALLOW exothermic reaction so keep cool/add acids drop by drop Toxic Irritant/irritates skin Correct Answer Reject (In)flammable so use water bath/ electric hotplate/keep away from naked flames Keep away from heat Use fume cupboard Correct Answer Reject (Reacts with nitric acid) to make + /to make nitronium ion/to NO2 make an electrophile Mark Mark Mark Mark Mark To make a nitrating agent/ NO2/ nitrate/ nitro group/nitrite ion Just as a catalyst/to speed up reaction Just Drying agent Oxidizing agent Reducing agent Just a correct equation Reject Reject Question Number 4(c)(i) Question Number 4(c)(ii) Correct Answer (5.0/ 136 = 0.0367647) =0.0368/ 0.037 (mol) IGNORE sf except 1 sf Correct Answer (5.0 / 1.09 = 4.587156) = 4.59/ 4.6 (cm3) IGNORE sf except 1 sf<br />
Reject Mark Question Number 4(c)(iii) Mr for product = 181 (1) Correct Answer Max yield = (181 x 0.0367647 = 6.6544118 ) = 6.65/ 6.7g (1) % yield = (3.4/6.6544118 x 100 = 51.09392) = 51.1/ 51 (1) OR Mr for product = 181 (1) Moles product = (3.40/181 = 0.0187845) = 0.0188/ 0.019 (1) % yield = (0.01878/ 0.036747 x 100 = 51.111854) = 51.1/ 51 (1) IGNORE sf except 1 sf ALLOW final answers rounding to 51 which will depend how number of moles is rounded. Working need not be shown, but if incorrect molar mass used max (2) TE from (c)(i) and in intermediate stages<br />
Mark Correct Answer Reject Put the solvent on the plate Put the sample under the solvent level Use of electric current First mark: Use a spot/small drop (of the ethanol washings) (1) Then any 3 points from the following: put spot near the bottom of the plate/on a marked line/on a datum line (1) Put plate in a sealed container (1) with the (suitable) solvent below the spot (1) Leave until the solvent has moved to near the top of the plate/till solvent has risen up/until separated (1) ALLOW any of these points shown on a diagram. ALLOW use of paper instead of plate IGNORE references to spraying with a reagent to make spots visible Question Number 4(d)(i)<br />
Mark Mark Mark Mark Correct Answer Reject Add pure samples (of methyl 2- nitrobenzoate and methyl 3- nitobenzoate) to chromatogram and get two spots at different levels corresponding to pure samples This may be shown on a diagram GIVE THE MARK FOR THIS QUESTION IF THIS PROCEDURE IS DESCRIBED IN 4(d)(i) OR measure Rf values on chromatogram of washings and compare with Rf of pure samples Use of electrophoresis Spectroscopy Correct Answer Reject Higher proportion of product would remain in solution/ more product would stay dissolved/ less product would crystallize out/ product is more soluble in solvent 1 at room temp Correct Answer ( 9.5 -2) 2 =3.75 (g) IGNORE sf except 1 sf Solvent is more soluble Just It dissolves more in solvent 1 Reject Correct Answer Reject Measure the melting temperature ALLOW measure the boiling temperature / measure the melting and boiling temperature (1) Should be sharp ALLOW Should match data book value (1) Recrystallization Just compare with data book TOTAL FOR PAPER = 50 MARKS Question Number 4(d)(ii) Question Number 4(e)(i) Question Number 4(e)(ii) Question Number 4(f)<br />

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