Edexcel Jun 2010 Paper 5 Q24

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PMT 24 Answer ALL the questions. Write your answers in the spaces provided. SECTION C A major function of the chemical industry is the manufacture of perfumes and flavourings. Perfumes were originally made from natural products, such as spermaceti from whales or civetone from the civet cat. The use of synthetic equivalents is generally cheaper and the supply is more reliable, and does not require the animals to be killed which obtaining spermaceti did. Synthetic perfumes and flavourings are usually nature-identical, although naturally-occurring molecules that have stereoisomers are sometimes produced synthetically as mixtures since the stereospecific synthesis required might be difficult to achieve economically. Some chiral molecules have a different taste depending on which enantiomer is present. The enantiomer ()-carvone tastes and smells of spearmint, and its mirror-image, (+)-carvone, of caraway or dill. In some cases only one enantiomer has any taste; this is true for glucose. The mirror image molecule of naturally occurring glucose has no taste and cannot even be absorbed or metabolized. Many drug molecules are chiral, though paracetamol is not. The wrong isomer present in a drug may be positively damaging, which was the case with thalidomide. In order to synthesize optically-pure drug molecules, it is important to understand the mechanism of any reaction used. Using an SN1 reaction which involves the chiral centre would result in the product mixture being racemic. It is advantageous to use stereospecific catalysts wherever possible, and industry on the whole prefers to use heterogeneous rather than homogeneous catalysts. (a) The skeletal formula of carvone is shown below. Draw a circle around the chiral carbon atom. (1) *N36507A01928* 19 Turn over<br /> PMT (b) Explain why the synthesis of paracetamol is more efficient than the synthesis of a single enantiomer such as ()-carvone. (3)(c) Carvone contains two types of functional group. For each of these, give a suitable chemical test to show its presence, and state what you would see in each case. (4)20 *N36507A02028*<br /> PMT (d) Carvone can be reduced in a variety of ways. (i) On reduction with hydrogen, in the presence of a platinum catalyst, 4.5 g of carvone reacted with 1.44 dm3 of hydrogen. Use these data to deduce the skeletal formula of the reduction product. [Molar mass of carvone is 150 g mol1; molar volume of hydrogen at the temperature and pressure of the experiment is 24 dm3 mol1.] (3) Working Hence skeletal formula of reduction product. *N36507A02128* 21 Turn over<br /> PMT (ii) Reduction of carvone, using hydrazine in potassium hydroxide solution, gives limonene. How would you show from a comparison of the infra-red spectra of carvone and of limonene that this reduction had occurred? You should quote appropriate data, from the data booklet, to support your answer. (2)22 *N36507A02228*<br /> PMT (2) (2) (e) (i) Draw the skeletal formula of the molecule that would be obtained if carvone were to be reacted with an excess of hydrogen bromide in an inert solvent. (ii) If the product from (e)(i) were to be heated with ethanolic potassium hydroxide solution, elimination would occur and HBr would be lost. Would the resulting molecule necessarily be carvone? Explain your answer.*N36507A02328* 23 Turn over<br /> PMT (f) *(i) Give the mechanism of one nucleophilic reaction of your choice, which either gives rise to a racemic mixture or results in the inversion of the chirality of the starting material. Explain what your mechanism predicts about the stereochemistry of the product. (5)(ii) Suggest why industrial chemists prefer to use heterogeneous rather than homogeneous catalysts. (1)(Total for Question 24 = 23 marks) TOTAL FOR SECTION C = 23 MARKS TOTAL FOR PAPER = 90 MARKS 24 *N36507A02428*<br />

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