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Wittig Reaction- Synthesis of trans-9-(2-phenyl ethenyl) anthracene...

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Victoria Carter No Lab partners Jason Cuce Experiment 10: Wittig Reaction- Synthesis of trans-9-(2-phenyl ethenyl) anthracene

Introduction Background: A Wittig reaction is a reaction type that ultimately utilizes an aldehyde or ketone and a triphenyl phosphonium ylide to yield an alkene. This reaction is a common method for the synthesis of alkenes. In Wittig reactions, the phosphonium salt acts as the strong base. In this experiment, the first step is to produce the ylide. In order to do so, the phosphonium ion must undergo deprotonation and attack the carbonyl group. As a result of deprotonation, the ylide then becomes very nucleophilic. The nucleophilic compound will then attack and in turn, will produce an alkoxide. When the reaction takes place, an intermediate will form, the intermediate state of this reaction forms a 4 membered ring, which is called oxaphosphetane. The oxaphosphetane eventually breaks and produces an alkene. In Wittig reactions, an E or Z alkene could be formed, depending on the stability of the reactant. Some Wittig reactions have modifications, yet still allow for an alkene to be yielded. The Horner Wadsworth Emmons reaction is an example of a modified version of a Wittig reaction. Horner Wadsworth Emmons reactions are significant because they allow for E alkenes to be favored, and yield byproducts that can result in filtration via extraction and eliminates the need for other methods, such as TLC. Horner Wadsworth Emmons reactions synthesize an E alkene due to phosphonate carbanions that are stable and are utilized instead of ylide. The phosphonate is an electron withdrawing group and is able to neutralize a negative charge. This in turn results in an E alkene. Wittig reactions, modified or unmodified, are applicable for synthetic usage. To wit, standard Wittig reactions (unmodified) are commonly used form beta carotene in a synthetic setting to produce commercially. Such is done by the reaction of a ylide with an aldehyde. As well, the modified version, the Horner Wadsworth Emmons reaction (modified) is commonly

utilized asymmetrically. The modified Wittig reaction is commonly a method for the synthesis of natural products such as pyranicin .

Mechanism

Side reaction:

Experimental Setup:

Chemicals used:

Benzyltriphenylphosphonium chloride C₂₅H₂₂ClP

Mm: 388.9 g/mol

Corrosive, acute toxicity, do

mp: 337°C

not inhale, ingest, irritant

Sodium Hydroxide NaOH

Mm: 39.997 g/mol

Highly corrosive, skin

mp: 318°C

irritant, eye irritant, and can

Bp: 1,388°C

cause skin burns

Mm: 206.24 g/mol

Irritant to skin and eyes.

9- anthraldehyde C₁₅H₁₀O

mp: 103-105°C Bp: 305.09°C

Dichloromethane CH₂Cl₂

Mm: 84.93 g/mol

Do not inhale, ingest, may be

mp: -96.7°C

irritant to skin and eyes.

Bp: 39.6°C 1-propanol C₃H₈O

Mm: 60.09 g/mol

Flammable, may be irritant to

Mp: -126°C

skin and respiratory tract

Bp: 97°C

trans-9-(2phenylethenyl)anthracene

Do not inhale, ingest, irritant

Mm: 280.4 g/mol

C₂₂H₁₆

Mp: 125-133°C Bp: 476.4°C

Results:

TLC data Distance traveled by solvent = 4.1 cm Distance traveled by 9-anthraldehyde = 2.8 cm Distance traveled by the alkene (desired product) = 3.7 cm Melting point of the product = 128-131o C Mass of the product = 250 mg

0.3 g benzyl triphenylphosphonium chloride 388.9 g/mol benzyl triphenylphosphonium chloride

= 0.00077 moles benzyl triphenylphosphonium chloride

0.00077 moles benzyl triphenylphosphonium chloride ·

M oles obtained ·

1 mol trans−9−(2 phenylethyl)− anthracene 1 mol benzyl triphenylphosphonium

280.4 g / mol trans−9−(2 phenylethyl )− anthracene 1 0.25 g product 0.216g theoretical yield

= 0.216 g trans − 9 − (2 phenylethyl) − anthracene

· 100 = 115%

Rf =

distance travelled by compound distance travelled by solvent

Rf =

distance travelled by product distance travelled by solvent

=

= 0.00077 moles

2.8 cm 4.1 cm

= 0.68

cm = 0.90 = 3.7 4.1 cm

Discussion: The melting point of the product was 128-131o C, and the desired product of trans-9-(2phenylethenyl)anthracene melting point is 125-133°C. This indicates that the product obtained was trans-9-(2-phenylethenyl)anthracene, however, the percent yield obtained was 115%, this overage indicates contamination or that a side reaction took place. This may have occurred due to reactants not remaining separated long enough. The Rf of the compound was 0.68, and the Rf of the product was 0.90, the product being significantly higher than the compound and solvent indicates that the product is likely still majorly trans-9-(2-phenylethenyl)anthracene. The Rf is supposed to be higher as trans-9-(2-phenylethenyl)anthracene has a lower polarity than the compound, 9-anthraldehyde. This is due to the fact that the dipole dipole bonding is present in the product, however the aldehyde has dipole dipole bonding and hydrogen bonding present.

The 1HNMR spectra obtained in the experiment was a little fuzzy in terms of determining defined peaks, which indicates water may have been a contaminant and cause the results to appear this way. However, the peaks of the 1HNMR were still clear enough to read and compare to the theoretical spectra provided. The peaks on the experimental 1HNMR are slightly lower than the theoretical, furthermore likely due to water contamination. However, the peaks show about 6 different signals. If the NMR was more clear and calibrated, the spectra would have shown two peaks in between 7-8 ppm identifying the presence of the aromatic ring hydrogens seen in the product, as well as peaks near 6ppm to indicate alkenes. The IR spectra indicates a small peak at around 1640, indicating an alkene, however the other peaks found were small peaks near 3000, indicating a C-H alkenyl and C-H alkyl stretch. This IR result however may also be compromised, due to contamination. Conclusion: The objective of this experiment is to react a phosphonium chloride with a base to form a Wittig reagent. Furthermore, an aldehyde in the same solution reacts with the Wittig reagent to give the alkene product. The experiment ultimately provided an alkene product that was contaminated, likely with water. This made it difficult to identify if the desired product was achieved. However, the similarities in the 1HNMR in comparison to the theoretical, indicates that there was an alkene that was achieved, along with a melting point that fits within the range of the theoretical melting point. Wittig reactions are commonly utilized to produce beta carotene commercially, this real life application is useful as it provides health benefits for people, promoting a strong immune system and promoting eye health.

References: Asymmetric Total Synthesis of Pyranicin. https://pubs.acs.org/doi/10.1021/ol900814w (accessed Oct 26, 2020). O-Chem in Real Life: A Wittig Reaction. http://www.asu.edu/courses/chm233/notes/ak/akRL1/wittig.html (accessed Oct 26, 2020)....