SAJC Prelim P3_Answers

Section A Answer all the questions in this section. 1 Electrosynthesis in organic chemistry is the synthesis of compounds in an electrolytic cell using inert electrodes. One important process is the Kolbe reaction. The Kolbe reaction is the decarboxylative dimerization of two carboxylate ions. Alkanes can be formed at the anode by the following reaction. (a) Explain why alkanes are generally unreactive. [1] Alkanes have C-C and C-H bonds which are strong and non-polar. (b) (i) Calculate the volume of carbon dioxide produced when a steady current of 5A is passed through the electrolyte for 32 minutes at standard temperature and pressure. [2] Q = It = 32 x 60 x 5 = 9600 C Amt of CO2 = Amt of e = Q/F = 9600/(96500) = 0.09948 mol Vol of CO2 collected = 0.09948 x 22.7 = 2.26 dm3 (ii) Methanoate ions undergo the Kolbe reaction to form only 2 gases. Using the axes given below, sketch the variation of pV/RT against p for one mole of each gas at the same temperature. Briefly explain your answer. [2] pV/RT p 0 1.0 ideal gas

2 CO 2 deviates more from ideal behaviour because it experiences stronger instantaneous dipole-induced dipole interactions / intermolecular forces between the molecules compared to H2 which has a smaller electron cloud. (c) The cathodic reaction for the Kolbe reaction can be represented by the following half equation: 2H2O + 2e  H2 + 2OH-. Write an overall equation for the Kolbe reaction involving ethanoate ions. Hence, calculate ΔGƟ for the Kolbe reaction given that the E Ɵcell value for the reaction is +2.28 V. [2] 2CH3COO-  CH3-CH3 + 2CO2 + 2e 2H2O + 2e  H2 + 2OH- Overall: 2CH3COO-+ 2H2O  CH3-CH3 + 2CO2 + H2 + 2OH- ∆GƟ = − nFE = − (2)(96500)(2.28) = − 440 kJ mol-1 (d) The reaction mechanism of the Kolbe reaction involves a 4-step reaction mechanism as described below: Step 1: Dissociation of carboxylic acid to form a carboxylate ion. Step 2: Oxidation of carboxylate ion to form a carboxyl radical, RCOO•. Step 3: Decomposition of the carboxyl radical to form carbon dioxide gas and an alkyl radical, R•. Step 4: Formation of a covalent bond between two alkyl radicals. (i) Using ethanoic acid as a starting reagent, suggest suitable equations for Steps 1, 2 and 4 on how ethane can be produced using the Kolbe reaction. [3] pV/RT p 1.0 H2 ideal gas CO2

3 [Please Turn Over Step 1: CH3COOH → CH3COO- + H+ Also accept “CH3COOH + H2O  CH3COO- + H3O+”. Step 2: CH3COO-→ CH3COO• + e Step 4: CH3• + CH3• → CH3CH3 (accept C2H6 also) (ii) The decomposition of the carboxyl radical to form carbon dioxide gas and an alkyl radical, R •, in Step 3 can be represented by the following equation: RCOO•  CO2 + R• Outline the mechanism of this step by copying the diagram below and include relevant curly half arrows. [1] (e) In another separate experiment, a new alkane A, C6H12, was produ