The document summarizes the Wittig reaction used to synthesize β-carotene. The aldehyde used is retinal and the symmetrical diylide containing two equivalents of phosphorus that produces 6 stereoisomers is (E,E,E)-1,3,5-trimethyl-1,3,5-hexatriene ylide.

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.
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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]
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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.
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4. Step 2:
Examine each possible carbonyl-diylide combination which meets the criteria given in
the question.
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5. carbonyl
compound
aldehyde?
symmetrical diylide
no. of
geometric
isomers
(a)
no
2
(b)
yes
6
√
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√
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6. carbonyl
compound
aldehyde?
symmetrical diylide
no. of
geometric
isomers
(c)
no
>6
(d)
yes
>6
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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
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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
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