A Stereospecific Wittig Reaction PDF

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Summary

Organic Chemistry 2 Lab A Stereospecific Wittig Reaction...

Description

A STEREOSPECIFIC WITTIG REACTION Kushal Nellore 3/05/2020 Section: 344-02 Objective The goal of this experiment was to form cis- or trans-9-(2-phenylethenyl)-anthracene, through a Wittig reaction of benzyltriphenylphosphonium chloride with 9-anthraldehyde in NaOH. Melting point, TLC and 1H NMR spectroscopy procedures were done on the product recovered to determine whether the cis or trans product is favored. Reaction

Mechanism

Stoichiometry Table Compound

Amount Started

MW (g/mol)

Density (g/mL)

mmol

Stoichiometric Ratio

Molar Equivalents

benzyltriphen ylphosphoniu m chloride

285 mg

388.87

-

0.733

1

0.733/0.727=1.01

9anthraldehyde

150 mg

206.24

-

0.727

1

0.727/0.727= 1

NaOH 50%

0.5 mL

40.00

1.515

9.5

1

excess

9-anthraldehyde is the limiting reagent in the reaction. NaOH is in excess. Procedure

Observations

1. 150 mg of 9-anthraldehyde, 285 mg of The stirring took longer than half an hour, and benzyltriphenylphosphonium chloride the NaOH was added very slowly and 2.5-3.0 mL of methylene chloride were added to a 10 mL r.b. flask with a stir bar. 7-8 drops of 50% NaOH solution were added dropwise using a Pasteur pipet, and stirred for 0.5 h. 2. 1 mL each of dichloromethane and water were added after stirring had ceased. The stirrer was removed and the flask was capped and shook with frequent venting.

The solution built up pressure which popped the cap open if the solution was not vented frequently

3. The organic layer was removed via pipet to a 25 mL E. flask. The aqueous layer was extracted with another 2 mL portion of dichloromethane and combined with the first organic layer.

The organic layer was yellow in color, while the aqueous was more clear

4. The organic layer was dried with potassium carbonate for about 10-15 min and filtered.

It took longer than expected to dry the solution

5. The solvent was evaporated under reduced pressure.

A sand bath was used for this procedure

6. The mass of the crude product and mp were acquired. A few crystals were reserved for tlc.

0.198g of crude product mp range of crude product: 122.7-125.2 °C

0.163g of pure product 7. The crude product was recrystallized in isopropanol. The mass of the pure mp range of pure product: 126.3-127.2 °C product and mp were acquired. A tlc of crude vs pure product was run in CH2Cl2 solvent. 8. 1H NMR spectrum for the product was obtained.

Results 0.198g of crude product recovered mp range of crude product: 122.7-125.2 °C 0.163g of pure product recovered mp range of pure product: 126.3-127.2 °C 0.727 mmol of 9-anthraldehyde = 0.727 mmol of 9-(2-phenylethenyl)-anthracene (0.727 mmol of 9-(2-phenylethenyl)-anthracene) x (280.4 g/mol) = 0.204g of product Pure Percent Yield = (0.163g/0.204g) x (100%) = 77.9% Crude Percent Yield = (0.198g/0.204g) x (100%) = 97.1% TLC The solvent was CH2Cl2. The starting point, pure product, crude products and solvent front are marked on the TLC plate. Points on TLC Starting Point Crude Product 1 Crude Product 2 Pure Product Solvent Front

Placement (cm) 0.20 3.90 4.70 4.60 5.05

Rf value 0.00 0.76 0.93 0.91 1.00

1H NMR Table Chemical Shift (ppm) 6.953-6.994 7.915-7.956 J value calculation:

Integration 1.00 1.29

Splitting Pattern Doublet Doublet

Identity HA HB

J Value (Hz) 16.4 16.4

(6.994 ppm – 6.953 ppm) x (400 MHz) = 16.4 Hz (7.956 ppm – 7.915 ppm) x (400 MHz) = 16.4 Hz Conclusion The goal of this experiment was to form cis- or trans-9-(2-phenylethenyl)-anthracene, through a Wittig reaction of benzyltriphenylphosphonium chloride with 9-anthraldehyde in NaOH. Melting point, TLC and 1H NMR spectroscopy procedures were done on the product recovered to determine whether the cis or trans product is favored. The melting point range measured from the acquired pure product was 126.3-127.2 °C. This melting point range of the pure product was within the melting point range of trans-9-(2-phenylethenyl)-anthracene from literature, which is 125-133 °C. The melting point range of cis-9-(2-phenylethenyl)-anthracene is not available in literature in order to compare to the product. The crude melting point range was a bit lower but still very close to the trans-9-(2-phenylethenyl)-anthracene melting point range, as it was 122.7-125.2 °C. The reason why the crude melting point range was slightly different from literature could be due to the presence of impurities in the crude product, which were not present in the pure product. According to the melting point data, the trans product seems to be the favored product. The theoretical yield of the product was 0.204g of 9-(2-phenylethenyl)anthracene, while the crude product was 0.198g and the pure product mass was 0.163g. This resulted in relatively high percent yields, 97.1% for the crude product, and 77.9% for the pure product. The reason for loss of some products could be due to some impurities being present. The TLC of the pure product only shows the pure product, which is expected, as no

impurities should be found in the pure product. On the other hand, the crude product seems to contain the pure product and another product, which can be assumed to be an impurity. The pure product is a single dot on the TLC product with a Rf of 0.91. The crude product contains two products with Rf of 0.76 and 0.93 respectively. The product with a Rf value of 0.93 can be assumed as the product, while the product with 0.76 is assumed to be an impurity. The 1H NMR for the product is also what is expected of the pure product. The two protons which are important in order to determine which product is preferred are HA and HB. The J value of the signals produced by these two protons would help determine if the preferred product would be cis or trans, as a trans product would have a higher J value than the cis product. Both of these NMR signals would have the same J value. The determined J value was 16.4, which is very high, and would indicate an angle close to 180 degrees. This means that the 1H NMR suggests that the trans product is preferred over the cis product. This is probably because the cis product would be more sterically hindered compared to the trans product. The other signals on the 1H NMR are all benzylic hydrogens. The mechanism for this reaction begins with the formation of the ylide by OH- taking a hydrogen from the non-benzylic carbon bonded to phosphorus, which gives the phosphorus a positive charge and the carbon a negative charge. This reactant is the ylide. This is resonance stabilized with a double bond between the two atoms. Then negatively charged carbon attacks the 9-anthraldehyde at the carbonyl carbon, which breaks the double bonded CO group and gives the O a negative charge. This negatively charged oxygen forms a bond with the positively charged phosphorous, which forms a square with the oxygen, phosphorous, previously carbonyl carbon and the carbon from the ylide. In this square, the bond between the phosphorous and the ylide carbon is broken and the electrons are used to from a double bond between the ylide carbon

and the previously carbonyl carbon. The bond between oxygen and the previously carbonyl carbon is broken and the electrons are used to from a double bond between the phosphorous and oxygen. This forms the product, 9-(2-phenylethenyl)-anthracene, as well as the byproduct, triphenylphosphine oxide. The trans product is favored as it is less sterically hindered compared to the cis product....