Edexcel Jun 2014 (IAL) Paper 5 Q24

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SECTION C Answer ALL the questions. Write your answers in the spaces provided. 24 Stereoisomerism Stereoisomerism occurs when species have the same formula and their atoms are bonded in the same sequence, but the spatial orientation of the atoms is different. Stereoisomers that are non-superimposable mirror images are optical isomers, or enantiomers. These have identical chemical and physical properties, apart from their effect on plane-polarized light. Optical isomerism arises most simply when a tetrahedral atom has four different groups bonded to it, but the presence of such a system does not necessarily result in an optical isomer. Stereoisomers that are not enantiomers are called diastereoisomers. Complex optically active species can be diastereoisomers, but a more familiar example is found with geometric isomers. In organic chemistry, geometric isomerism may arise when there is an energetic barrier to free rotation. Restricted rotation occurs with double bonds, because a (cid:652) bond is formed by the lateral overlap of p orbitals, and also with cyclic compounds. Diastereoisomers have different physical properties and can be separated by conventional techniques. Stereoisomerism is not confined to organic chemistry. Inorganic compounds may also show stereoisomerism and it is a particular feature of transition metal complexes. The simplest geometric isomers in transition metal complexes occur in square planar complexes ML2N2 and octahedral complexes ML4N2, where M is the metal ion and L and N are ligands. Geometric isomerism also occurs with octahedral complexes of the type ML3N3. The most important optical isomers of transition metals are octahedral complexes of the type MB3 where B is a bidentate ligand; the structure of an enantiomer of MB3 is shown below. The stability of complexes with polydentate ligands makes them easier to prepare. M B Because many biochemical reactions are stereospecific, stereoisomerism is of great importance in the pharmaceutical industry. Wherever possible, the industry aims to synthesize specific isomers. 26 *P42979A02632*<br /> (a) 2,3-dihydroxybutanedioic acid (tartaric acid) exists in three isomeric structures, X, Y and Z, two of which are shown below. HO HO HO OH OH HO HO OH Isomer X occurs in many plants including grapes, bananas and tamarinds. (i) Circle the chiral centres of isomer X on the diagram. (ii) Draw the structure of isomer Y, which is an enantiomer of X. (1) (1) *P42979A02732* 27 Turn over<br /> (iii) How can plane-polarized light be used to distinguish the enantiomers X and Y? (2)(iv) Suggest, with a reason, whether or not nuclear magnetic resonance could be used to distinguish between the enantiomers X and Y. (1)(v) An important use of X is in the preparation of other chiral molecules. One example is the substitution of the alcohol groups by bromine atoms. State and explain what may be deduced about the mechanism of this reaction if the product is also chiral. (2)28 *P42979A02832*<br /> (vi) Isomer Z is a diastereoisomer. Suggest why it is not an optical isomer, even though it has two asymmetric carbon atoms. (1)(b) (i) Suggest why the lateral overlap that forms a (cid:652) bond results in a barrier to free rotation about a carbon-carbon double bond. (2)(ii) Draw the geometric isomers of 1,2-dichlorocyclohexane. (1) *P42979A02932* 29 Turn over<br /> (c) Cisplatin, [PtCl2(NH3)2], is an example of a stereospecific anti-cancer drug. (i) Draw the structure of cisplatin and its geometric isomer. (ii) Explain the term stereospecific drug. (1) (1)*(iii) Suggest three reasons why the pharmaceutical industry prefers to use stereospecific synthetic routes. (3)30 *P42979A03032*<br /> (d) The ethanedioate ion, C2O4 (i) Explain the term bidentate ligand. 2, is an example of a bidentate ligand. (1)(ii) Explain why the complexes formed with bidentate ligands are generally more stable than those with monodentate ligands. (1)(iii) The complex, [Fe(C2O4)3]3, exists as two enantiomers. Use the information in the passage to draw the two enantiomers. (2) (Total for Question 24 = 20 marks) TOTAL FOR SECTION C = 20 MARKS TOTAL FOR PAPER = 90 MARKS *P42979A03132* 31<br />

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