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2013 SRJC Preliminary Examination, H2 Chemistry Syllabus 9647, Paper 3 Question 1(d)

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2013 SRJC Preliminary Examination, H2 Chemistry Syllabus 9647, Paper 3 Question 1(d)

  1. 1. An IQ Test or a Chemistry Question? (The question stem is highlighted in green.) 2013 Serangoon Junior College / Paper 3 / Question 1(d) 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. www.wewwchemistry.com Page 1 of 8
  2. 2. β-carotene is a food colouring that can be extracted from the pigmentation found in redorange plants and fruits such as carrots. It can be synthesised using excess of an aldehyde and a diylide, a compound with two equivalents of phosphorus in its structure. β-carotene Suggest the structures of the aldehyde and a symmetrical diylide with 6 stereoisomers that can be used to produce β-carotene. [2] www.wewwchemistry.com Page 2 of 8
  3. 3. Proposed Solution Note: The following solution contains a very detailed explanation of how the answer is arrived. Given that this question is allocated only two marks, students are expected to produce the answer by simple observation. Step 1: Examine the structures of the ylide, the carbonyl compound (propanone) and the product alkene in the given Wittig example. www.wewwchemistry.com Page 3 of 8
  4. 4. Step 2: Examine each possible carbonyl-diylide combination which meets the criteria given in the question. www.wewwchemistry.com Page 4 of 8
  5. 5. carbonyl compound aldehyde? symmetrical diylide no. of geometric isomers (a) no 2 (b) yes 6 √ www.wewwchemistry.com √ Page 5 of 8
  6. 6. carbonyl compound aldehyde? symmetrical diylide no. of geometric isomers (c) no >6 (d) yes >6 www.wewwchemistry.com Page 6 of 8
  7. 7. Aside: The ylide that was used to synthesise carotene has three C=C double bonds. Why does it have 6, and not 23 = 8, geometric isomers? where A = a www.wewwchemistry.com Page 7 of 8
  8. 8. Geometric isomers arising from the three C=C double bonds: C1=C2 (wrt –H and –CH3) cis cis trans cis cis trans trans trans C3=C4 cis cis cis trans trans trans trans cis C5=C6 (wrt –H and –CH3) cis trans cis cis trans cis trans trans } same isomer } same isomer ∴ Total no. of geometric isomers = 6 www.wewwchemistry.com Page 8 of 8

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