Edexcel Jun 2015 (IAL) Paper 6 Q4

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4 Bromobenzene can be prepared from benzene by the following steps. Step 1 Step 2 Step 3 Step 4 Reflux 20.0 cm3 of benzene with 6.0 cm3 of bromine and about 10g of iron filings, by heating on a water bath at 50(cid:113)C. After the reaction has finished, remove the water bath and heat to boiling until no bromine vapour can be seen. Cool the mixture and add 25 cm3 of ethoxyethane (diethyl ether) to extract the bromobenzene. Wash the ethoxyethane layer with aqueous sodium hydroxide. Separate the ethoxyethane layer. Step 5 Wash the ethoxyethane layer with water and repeat the separation. Step 6 Dry the ethoxyethane layer with a suitable drying agent. Step 7 Decant the dried solution. Step 8 Distil the separated solution, collecting the fraction boiling around the boiling temperature of bromobenzene, 156(cid:113)C. (a) Calculate the number of moles of bromine, Br2, used in the experiment. [Density of bromine 3.1 g cm3] (b) Bromine reacts with iron to form iron(III) bromide, which reacts with bromine to produce the attacking electrophile in Step 1. Write the chemical equations for these reactions. (2) (2) 10 *P44884A01016*<br /> (c) Why is the ethoxyethane layer washed with sodium hydroxide solution in Step 4? (1)(d) Draw a diagram of the apparatus used to separate the ethoxyethane layer from the aqueous layer in Step 5. Clearly label the ethoxyethane layer. [Densities: water 1.0 g cm3, ethoxyethane 0.7 g cm3] (e) The bromobenzene formed in this reaction can be nitrated to make 2,4-dinitrobromobenzene. Identify, by name or formula, the chemicals needed for this reaction. (2) (1)*P44884A01116* 11 Turn over<br /> (f) 2,4-dinitrobromobenzene reacts with hydrazine hydrate to make 2,4-dinitrophenylhydrazine crystals. The percentage yields for the reactions are: 75% for the formation of bromobenzene from benzene 70% for the formation of 2,4-dinitrobromobenzene from bromobenzene 70% for the formation of 2,4-dinitrophenylhydrazine from 2,4-dinitrobromobenzene Calculate the overall percentage yield of 2,4-dinitrophenylhydrazine from benzene, for this series of reactions. (1) 12 *P44884A01216*<br /> (g) The purity of the 2,4-dinitrophenylhydrazine crystals can be checked by carrying out a melting temperature determination using the Thiele tube apparatus shown below. thermometer stopper with wedge cut Thiele tube capillary tube with sample (i) The capillary tube must be sealed at one end. Describe how this is done. heat here (1)(ii) When crystals are placed in the capillary tube they often stick in the top. Describe how to ensure the crystals reach the bottom of the capillary tube. (1)(iii) Dibutyl phthalate is often used as the liquid in the Thiele tube. Suggest two properties of dibutyl phthalate that make it a suitable liquid for this purpose. (2)*P44884A01316* 13 Turn over<br /> (iv) The melting temperature for crystals of 2,4-dinitrophenylhydrazine is 201(cid:113)C. Suggest the temperature range over which you would expect the crystals to melt before and after purification by recrystallization. (2) Before recrystallizationAfter recrystallization(Total for Question 4 = 15 marks) TOTAL FOR PAPER = 50 MARKS 14 *P44884A01416*<br />

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