SRJC H2 Chem 2013 Prelim P3 Soln

2013 H2 Chemistry Prelim Examinations Paper 3 Suggested Solutions 1 Phosphorus is the thirteenth element in the world to be discovered and due to this is sometimes referred to as the Devil’s element. The reference is also made due to its versatility in forming chemical species with explosive of toxic natures. (a) One example of a highly reactive phosphorus-containing ion is phosphonium, PH4 +. (i) Write a balanced chemical equation for the reaction between phosphonium iodide and potassium hydroxide to form phosphine, PH3. PH4I + KOH  PH3 + KI + H2O (ii) Hence, suggest a suitable method of measuring the rate of reaction. Volume of gas produced / Pressure increase (iii) Draw a ‘dot-and-cross’ diagram showing the electrons (outer shells only) in the phosphonium ion, and use the VSEPR (valence shell electron pair repulsion) theory to predict its shape. You may find both a written description and a 3-dimensional sketch useful in your answer. There are 4 bond pairs of electrons around the P atom. To minimise repulsion, the 4 electron pairs are directed to the corners of a tetrahedron. Shape of PH4 +: tetrahedral (iv) The bond angle in ammonia is 107.8o. Predict the H – P – H bond angle in phosphine and justify your answer. [7] H–P–H bond angle: any specific angle between 90o and 107.8o N atom is more electronegative than P atom. The bond pair is pulled closer towards the N atom. Bond pair-bond pair repulsion is greater in NH 3. Hence, the bond angle in PH3 is smaller than that in NH3. FYI: Because PH3 is trigonal pyramidal in shape, the bond angle should also be larger than that in a molecule with a trigonal bipyramidal shape. Hence, bond angle in PH3 is larger than 90o. 1292

(b) Although phosphorus is present in numerous harmful chemicals, it is a useful feature in organophosphorus compounds. Ylides are a class of organophosphorus compounds that are well-known for their role in the Wittig reaction. The synthesis of methylpropene via the Wittig reaction is shown below. P                     PC H 3                      PC H 2 (i) Name the type of reaction that occurs in step I and outline the mechanism for the reaction. SN2 P CH3 Br           C H H H BrP            PC H 3 (ii) Suggest why 2-bromo-2-methylpropane is not favourable to be used as a reactant in step I. Steric hindrance posed by the three phenyl groups on the P atom of P . (iii) State the role of butyl lithium, C4H9Li, in step II. C4H9Li acts as a base. [5] C4H9Li II CH3Br I O III + Br– + – 1293

(c) Two resonance structures exist to stabilise the ylide used in step III above. PC H 2                       PC H 2 In this step, tetrahydrofuran, THF, is used as an aprotic solvent – one that lacks an acidic hydrogen. (i