OCR A Jun 2015 Paper 4 Q2

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2 Chemists often use two different structures to represent a molecule of benzene, as shown below. structure A structure B (a) (i) Describe, with the aid of suitable diagrams showing orbital overlap, the difference in bonding between structure A and structure B. In your answer, you should use appropriate technical terms, spelled correctly.[4]OCR 2015 Turn over<br />
 (ii) The table below shows the enthalpy changes for the reactions of cyclohexene, C6H10, and benzene, C6H6, with hydrogen. reaction enthalpy change / kJ mol1 C6H10 + H2 C6H6 + 3H2 C6H12 C6H12 119 208 Using this information, suggest and explain whether structure A or structure B is a better representation of benzene.[2] (b) Benzene compounds can undergo nucleophilic substitution reactions. Add curly arrows to the diagram below to show the two-step mechanism of C6H5N2 + with F. + N step 1 step 2 F[2] (c) Benzene can react with halogenoalkanes in the same way as with bromine, as shown in reaction 1 below. + (CH3)2CHBr FeBr3 reaction 1 CH(CH3)2 + HBr Write an equation to show the formation of the electrophile that reacts with benzene in reaction 1.OCR 2015 [1]<br />
 H2N NH2 1,3-diaminobenzene (d) The types of reaction in (b) and (c) can be used to synthesise compound D, as shown in the flowchart below. (i) Complete the boxes below to suggest formulae for the reactants involved in the synthesis of compound D. Give structures for organic compounds. +N2 F N2 FeBr3 NH2 N H NH2 compound D OH [3] Turn overOCR 2015<br />
 (ii) In a synthesis of compound D from 1,3-diaminobenzene shown in the flowchart, 1.73 g of compound D was prepared. These structures have been repeated below: N H NH2 H2N NH2 1,3-diaminobenzene compound D The overall percentage yield of compound D was 40.0%. Mr of compound D = 346.0 Calculate the mass of 1,3-diaminobenzene needed for this synthesis. OH mass =g [3]OCR 2015<br />
 (iii) Compound D has been developed for possible use as a drug to treat heart conditions. When compound D, prepared in this synthesis, was given to patients, only 25% of the dose was effective in treating their heart conditions. Explain why only 25% of the dose was effective. Suggest how the synthesis of compound D might be changed to make the dose more effective.[3] [Total: 18]OCR 2015 Turn over<br />

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