Edexcel Jan 2012 Paper 5 Q24

Answers available below

PMT SECTION C Answer ALL the questions. Write your answers in the spaces provided. 24 Read the passage below carefully and answer the questions which follow. Molecular structure and colour chemistry The sight of tubes of paint or of coloured pencils in an artists supply shop is something that most people enjoy; we love colour. The ability to synthesise brightly-coloured compounds coincides with the rapid growth of the organic chemicals industry. Synthetic organic dyes started to appear in the mid-19th century when William Perkin synthesised Mauve in 1856 at the age of 18. He was trying to synthesise quinine even though he did not know the structure of the molecule. In the 19th century many chemists did not believe that molecules existed. The work of Butlerov, Couper, and notably Kekul showed that molecules not only exist but have specific structures. In 1865 Kekul suggested a ring structure for the aromatic compound benzene which he represented as Kekul knew that benzene does not react with bromine water. Later work showed that the enthalpy change of hydrogenation of the compound is 205 kJ mol1, rather than the value of 360 kJ mol1 that would be expected if the structure was exactly as shown above, given that the enthalpy change of hydrogenation for cyclohexene to cyclohexane is 120 kJ mol1. + H2 + H2 When Greiss in 1856 discovered diazotisation and the azo dyes, he used a reaction characteristic of aromatic amines. Witt, in 1876, found the functional groups in the dye molecule that make it water-soluble and enable it to attach to the cloth fibres. Graebe, Liebermann and Perkin in 1869 patented the synthesis of alizarin, found in madder root grown in Holland and Von Baeyer synthesised indigo in 1880, until then grown in India. Synthetic dyes were made available in large quantities and were cheaper than the sources from plants. Now the organic chemical industry produces a vast range of pigments and dyestuffs for use in paints and for fabrics, inks and other materials, making our world the most colourful that it has ever been. 20 *P35302A02028*<br /> PMT (a) (i) Explain why Perkins attempted synthesis of quinine was almost certain to fail. (1)(ii) Suggest the effect that the growth of the organic chemicals industry in the late 19th century had on Holland and India in particular. (2)(b) (i) What observation did Kekul make to show that benzene does not react with bromine water? Explain the significance of this with reference to his representation of the molecule. (2)(ii) Explain, in terms of the bonding in the benzene ring, why the enthalpy of hydrogenation is less exothermic than would be expected from a molecule with three double bonds. (3)*P35302A02128* 21 Turn over<br /> (c) The first steps in the preparation of an azo dye from benzene are shown below. PMT NO2 step 2 NH2 step 1 nitrobenzene phenylamine benzenediazonium cation Give the mechanism for the reaction in step 1, including the equation for the formation of the electrophile. (4) Equation for formation of electrophile Mechanism 22 *P35302A02228*<br /> (d) (i) Phenylamine is converted into the benzenediazonium cation using sodium nitrite and hydrochloric acid at a temperature between 0 C and 10 C. Explain why the temperature must not be lower or higher than these limits if a good yield is to be obtained. PMT (2)(ii) Draw the structural formula of the benzenediazonium cation showing all the bonds and the charge. (1) (iii) Suggest how you could convert a sample of the benzenediazonium cation into an azo dye. Give the name of the other compound you would use and the skeletal formula of the azo dye you would obtain. (3) *P35302A02328* 23 Turn over<br /> PMT (e) The structural formula of methyl orange is given below. Na+ O3S N N N(CH3)2 Suggest the main features of methyl orange which make it water-soluble, giving your reasons. (2)(Total for Question 24 = 20 marks) TOTAL FOR SECTION C = 20 MARKS TOTAL FOR PAPER = 90 MARKS 24 *P35302A02428*<br />

Show answer