THE WITTIG REACTION
SHAUN LYNN
J9106574
Abstract
Carbon to carbon double bonds can be created by the Wittig Reaction. In this reaction, a
nucleophilic ylid attacks a carbonyl group, and the ringed intermediate collapses to form a
double bond with the acyl carbon in place of the oxygen. In this experiment, trans-9-(2phenylethenyl)anthracene was synthesized from 9-anthraldehyde and
benzyltriphenylphosphonium chloride via the Wittig Reaction. The goal of this experiment
was to obtain a high percent yield and purity for the synthesis of trans-9-(2phenylethenyl)anthracene. A percent yield of 48.68% was obtained for the synthesized
product, and the observed melting point range value was 2°C lower than the literature value
range. The results supported that an efficient synthesis had taken place.
Introduction
Discovered in 1954 by Georg Wittig (for which he was awarded the 1979 Nobel Prize in
Chemistry), the Wittig reaction is a nucleophilic substitution reaction, used in the synthesis
of alkenes.
The Wittig reaction is the overall substitution of a C=O bond to a C=C bond. The double
bond forms specifically at the location of the original ketone or aldehyde (McMurry, 2008).
The aldehyde/ketone will react with a species known as a phosphonium ylid, which is a
neutral molecule with positive and negative charges on adjacent atoms; the positive charge
in this case will be situated on the phosphorus. The ylid is prepared via a two step process:
To begin, an SN2 reaction between triphenylphosphine and an alkyl halide followed by
treatment with a strong base.
The nucleophilic carbon of the Wittig reagent adds to the electrophilic carbon in the polar
carbonyl group. Electrons from the C=O π bond will then be used to form a σ bond to the
positive phosphorus atom. This creates a cyclic intermediate called an oxaphosphetane
(Carey, 2007).
Decomposition of the intermediate by breaking the C-P and C-O σ bonds leads to the
formation of the C=C π bond of the alkene and triphenylphosphine oxide. (Carey, 2007)
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Aim
To prepare a carbon – carbon double bond by converting a carbonyl compound into an
alkene via the Wittig reaction, the mechanism of which can be seen below.
The removal of a proton from benzyltriphenylphosphonium chloride will create an ylide,
which will then attack the 9-anthraldehyde carbonyl group. The disintegration of the 4ringed intermediate will then produce the desired end product.
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Chemical reactivity
J9106574
Hazards
Benzyltriphenylphosphonium chloride - Toxic
Dichloromethane – Toxic, suspect carcinogen
Sodium hydroxide – causes severe burns
9-anthraldehyde – avoid direct with skin and eyes, avoid inhalation.
Method
Benzyltriphenylphosphonium chloride (1.90g) [balance N0017425] and 9-anthraldehyde
(1.19g) were weighed and added to a round bottomed flask (25cm³). Dichloromethane
(6.0cm³) was measured using a measuring cylinder and added to the flask. The flask was
then stirred using a magnetic stirrer at high speed whilst adding sodium hydroxide (50%
w/v 2.6cm³) dropwise using a Pasteur pipette.
After complete addition of the sodium hydroxide, the mixture was left to stir for a further 30
minutes before being transferred to a separate flask, using water (20cm³) and
dichloromethane (20cm³) to complete the transfer. The mixture was shaken and left to
settle, the organic layer was removed to another flask and the aqueous layer was extracted
once more with dichloromethane (10cm³) and then combined with the previous organic
extract. The combined organic layers were dried with anhydrous calcium chloride pellets,
transferred o a conical flask (100cm³) and evaporated to dryness on a steam bath. The
product was then recrystallised from 2-propanol, the melting point and IR spectrum of the
recrystallised product was then recorded.
Discussion
Mass of product obtained = 0.925g
Melting point range = 126.1°C – 128.1°C
According to Dictionary of organic compounds, 6th edition, Chapman and Hall, London, 1996
the melting point range for 9-(2 phenylethenyl)anthracene is 130°C – 132°C.
As the melting point range is only 2°C lower than the cited literature value, this suggests
that the synthesised product was indeed 9-(2-phenylethenyl)anthracene with a very high
purity.
From an examination of the molecular structure of 9-(2 phenylethenyl)anthracene, the
formation of the Z-Cis 9-(2-phenylethenyl)anthracene is highly unlikely due to steric
hindrance, the size of the two groups at either side of the double bond are too large.
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Chemical reactivity
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Therefore the stereoisomer formed is E-Trans 9-(2 phenylethenyl)anthracene exclusively.
Yield calculations
Mr of benzyltriphenylphosphonium chloride [BTP] C25H22PCl = 388.869 (1.90g)
9-anthraldehyde C15H10 O = 206.2293 (1.19g)
1
BTP = (1.90𝑔 𝑥 (388.869 ) = 0.0048859 𝑚𝑜𝑙𝑒𝑠
1
9-anthraldehyde = 1.19𝑔 𝑥 (206.2293 ) = 0.00577 𝑚𝑜𝑙𝑒𝑠
The limiting reagent is therefore Benzyl triphenylphosphonium chloride.
0.0048859𝑚𝑜𝑙𝑠 𝑥 (
388.869
) = 1.90𝑔
1
The theoretical yield of E-Trans 9-(2 phenylethenyl)anthracene is 1.90g
Percentage yield =
Shaun Lynn
0.925𝑔
1.90
𝑥 100 = 𝟒𝟖. 𝟔𝟖%
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IR interpretation
The black ring at 3030.52 indicates the presence of sp 2 hybridised C-H bonds, as well as C-H
bonds on the conjugated system.
The Blue circle at highlights non aromatic C=C bonds at around 1674.96 and aromatic C=C
bonds at around 1575.80
The orange circle indicates the presence of a trans alkene, which supports the fact that the
product is in fact E-Trans 9-(2 phenylethenyl)anthracene.
The green circle highlights aromatic C-H bending.
WORKS CITED
Carey, F. S. (2007). Advanced Organic Chemistry: Reactions and Synthesis (5th Edition ed.). London: Springer.
Hunt, I. (2011, 04 10). The Wittig reaction. Retrieved 04 10, 2011, from University of Calgary:
http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch17/ch17-3-2-3.html
Trippet, S. (2011, 04 10). The Wittig Reaction. Retrieved
http://www.iupac.org/publications/pac/1964/pdf/0902x0255.pdf
04
10,
2011,
from
IUPAC.org:
Unknown. (2011, 04 15). Infrared spectroscopy correlation table. Retrieved 04 15, 2011, from Wikipedia, the
free encyclopedia: http://en.wikipedia.org/wiki/Infrared_spectroscopy_correlation_table
Dictionary of organic compounds, 6th edition, Chapman and Hall, London, Volume 3(& Volume 6), 1996
McMurry, J. (2008). Organic Chemistry. J. McMurry, organic Chemistry . London: Thompson Brooks/Cole.
Wittig Reaction: The Synthesis of trans-9-(2-Phenylethenyl)anthracene Revisited, C. Jaworek and S. Iacobucci
Department of Chemistry, Tufts University, Medford, MA 02155 J. Chem. Educ., 2002, 79 (1), p 111, DOI:
10.1021/ed079p111 ,Publication Date (Web): January 1 2002
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