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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.
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β-carotene is a food colouring that can be extracted from the pigmentation found in red-orange 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]
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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.
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Step 2: Examine each possible carbonyl-diylide combination which meets the criteria given in the question.
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carbonyl compound
alde-hyde? symmetrical diylide
no. of geometric isomers
(a)
no
2
(b)
√
yes
√
6
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carbonyl compound
alde-hyde? symmetrical diylide
no. of geometric isomers
(c)
no
> 6
(d)
yes
> 6
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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
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Geometric isomers arising from the three C=C double bonds:
C1=C2 (wrt –H and –CH3)
C3=C4 C5=C6
(wrt –H and –CH3)
cis cis cis cis cis trans
trans cis cis } same isomer
cis trans cis cis trans trans
trans trans cis } same isomer
trans trans trans trans cis trans
∴ Total no. of geometric isomers = 6
