Chem 344 lab 8 wittig - This is a completed lab report for the second organic chemistry lab. The labs PDF

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This is a completed lab report for the second organic chemistry lab. The labs do not change; they remain the same year to year. ...

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A Stereospecific Wittig Reaction Friday March 15, 2019 Chem 344, Section 10

Objective:

The objective of this experiment was to conduct a Witting reaction in which 9-anthraldehyde reacted with benzyltriphenyl-phosphonium chloride to generate trans-9-(2-phenylethenyl)anthracene and a byproduct of triphenylphosphine oxide. Sodium hydroxide was used as a base and helped with the formation of the nucleophile. Due to the stereospecificity of the reaction, characterization techniques, such as 1H NMR, J-values, and TLC were performed to identify if the trans or cis product was generated. Overall Reaction:

Mechanism:

Stoichiometric Table:

Compound

Molar Mass (g/mol) 206.24 388.87

Density (g/mL) 1.22 1.18

Amount Used 0.1571g 0.2859g

9-anthraldehyde Benzyltriphenyl -phosphonium Chloride 6M NaOH 40.00 1.53 8 drops Methylene 84.93 1.33 2.5mL Chloride Limiting Reagent: Benzyltriphenylphosphonium chloride

Mmol 0.762 0.735

Molar Equivalence 1.1 1.0

--39

excess Solvent

Theoretical Yield: 0.21g of product Procedure & Observations: Procedure 1. Approximately 150mg of 9-anthraldehyde and 285mg of benzyltriphenyl phosphonium chloride were added to a 10mL r.b. flask. About 2.5-3.0mL of methylene chloride was added as well. 2. With vigorous stirring, about 7-8 drops of 50% NaOH was added to the solution. This was stirred for 30 minutes. 3. The stirring was stopped, and about 1mL of dichloromethane and water was added. The stirrer was removed and the flask was capped and shaken vigorously with frequent venting. 4. The organic layer was removed with a pipet and transferred into a 25mL E.flask. The aqueous layer was extracted with another 2mL portion of dichloromethane and combined with the first organic layer.

5. The organic layer was dried with potassium carbonate for 10-15 minutes. This was filtered with rinsing into a clean, tared r.b. flask. 6. The solvent was evaporated using a rotovap. The crude product was weighed and a melting point was obtained. A few crude crystals were reserved for TLC. 7. The crude product was recrystallized from isopropanol. It was re-weighed and the yield

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Observations 0.15 g of 9-anthra Looked like turmeric, very powdery 0.2984 g of benzyl Looked like salt/sugar

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Solution looked like apple juice

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First extraction: -top layer lighter yellow -bottom layer dark yellow -oily consistency Second extraction: -top layer, opaque yellow -bottom layer: clear, but contained yellow hue Rich yellow color; created oily residue 1st tare: 43.3665g

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2nd tare: 43.7185g Crude weight: 0.3520g Mp of crude product: 99.7-99.9ºC Crude product looked like turmeric Still wet, slushy looking Prude product was bright yellow Looked like the sugary coating on the

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was calculated again. The melting point was retaken and a TLC plate was spotted with the crude and purified product. It was developed in 1:1 ethyl acetate: hexanes. 8. A 1H NMR spectrum was obtained. The identity and stereochemistry was identified for the two alkenyl protons by matching coupling constants and calculating the jvalues.

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peeps candy Weight: 0.0429g Mp: 128.6-130.9ºC Confirmed trans product was generated

Results & Calculations: Compound

Crude Product Yield (g)

Crude Percent Yield (%)

Pure Product Yield (g)

Theoretica l Yield (g)

% Yield

Trans-9-(2phenylethenyl) -anthracene

0.3520

167.6

0.0429

0.21

20.4

Experimental Melting Point (ºC)

Actual Melting Point (ºC)

% Recovery from Recrystallization

128.6-130.9

130-132

12.2

TLC Characterization: Eluent used: 1:1 ratio of ethyl acetate to hexanes Solvent moved: 6.1cm

b y U n k n o w n A u t h o r

b y U n k b n y Number on Picture Distance Traveled o (cm) w U n n 1. 4.1 A k ut n 2. 4.0 h o 3. 3.5 or w 1 H NMR Characterization: is n li c A e u n t se h d o u r

Rf Value 0.67 0.66 0.57

Identity Crude product Pure product (Ph)3P=O

li c e n s e d u n d e r

i c e n s e d

Proton Ha Hb Hc

Integration 1.00 1.05 1.10

Shift (ppm) 6.96 un 7.93 d 8.39 e

Splitting Doublet Doublet Singlet

r

J Values (Hz) Jab=16.4 3 Jab=16.8 None 3

Calculations: *Mmol of 9-anthraldehyde: 0.1571g of 9-anth x

1 mol 206.24 g

x

1000 mmol 1 mol

= 0.762mmol of 9-anth

*Molar Equivalents: 0.762g of 9-anth/0.735g of benzyl x 100 = 1.1 *Theoretical Yield: 0.2859g of benzyl x

1 mol 338.87

x

1 mol of product 1mol

*Percent Yield of Crude product: 0.3520g/0.21g x 100 = 167.6% *Percent Yield of Pure Product: 0.0429g/0.21g x 100 = 20.4% *Percent Recovery from Recrystallization: 0.0429g/0.3520g x 100 = 12.2% *Rf Value of Spot 1: 4.1 cm = 0.67 6.1 cm

x

280.36 1 mol of product

= 0.21g of product

Discussion: In this experiment a Wittig reaction was conducted in which 9-anthraldehyde reacted with benzyltriphenylphosphonium chloride to generate trans-9(2-phenylethenyl)-anthracene. Sodium hydroxide was used as a base and helped with the formation of the ylide. Due to the stereospecificity of the reaction, characterization techniques, such as 1H NMR, J-values, and TLC were performed to confirm that the trans product was generated. The first step of the mechanism was to generate a triphenylphosphonium salt. This was achieved via an SN2 mechanism, in which the lone pair in triphenylphosphine attacks the carbon in the alkyl group, displacing the chlorine as a leaving group. Once the salt was formed, sodium hydroxide deprotonated that salt by removing the hydrogen attached to the carbon that was bonded to the phosphorus atom. This lead to the formation of the ylide because a negative charge was left on the carbon due to the lone pairs from the removal of the hydrogen. The ylide was then resonance stabilized and formed another double bond between the phosphorus and carbon atoms. Once the ylide was formed, a Witting reaction was able to take place. The ylide acted as a nucleophile and attacked the carbonyl carbon of 9-anthraldehyde. As a result, a four-centered intermediate, Betaine, was formed. The structure of Betaine included a negatively charged oxygen atom and a positively charged phosphorus atom. To stabilize the molecule, one of the lone pairs on the carbonyl oxygen attacked the positively charged phosphorus atom in an intramolecular closure reaction to form an oxaphosphetane. Even though oxaphosphetane is more stable than Betaine, it is still not stable. So to further stabilize the molecule, the electrons moved around causing the four-membered ring to fragment. Eventually, triphenylphosphine was eliminated, producing two new pi bonds. The end products were an alkene (trans-9(2phenylethenyl)-anthracene) and triphenylphosphine oxide as a by-product. As stated, a four-centered intermediate, Betaine, was formed. The stereochemistry of this intermediate ultimately determines the stereochemistry of the alkene product. The Betaine contained a trans-selective variation that allowed the bulky groups to be far away from each other, resulting in less steric train. Since this type of variation is most favored, the final product was presumed to be trans-9(2-phenylethenyl)-anthracene. To confirm this, characterization techniques such as 1H NMR, melting point, and TLC were performed. When looking at the 1H NMR spectrum, J-values were taken into consideration. If the product were cis, the dihedral would be around 20º, giving a J-value of 6-12Hz. If the product were trans, the dihedral angle would be 180º, giving a J-value of 12-18Hz. According to the obtained spectrum, the calculated Jvalue for proton Ha was 3Jab = (6.962-6.921) x 400 MHz = 16.4Hz. The calculated J-value for proton Hb was 3Jba = (7.931-7.889) x 400 MHz = 16.8Hz. Both fell into the J-value range that corresponded with the trans product, thus confirming that the trans product was generated. Both protons were doublets with the same 3J coupling constant which was expected since they were the only protons splitting each other. The spectrum also revealed that proton Hc was the most downfield due to the surrounding benzene rings.

Melting point was another characterization technique utilized to confirm that the trans product was generated. The experimental melting point range obtained was 128.6-130.9ºC compared to the actual melting point range of the trans product: 130-132ºC. Although the experimental melting point is close in proximity to the actual, it is still a little off. This could be a result of the presence of the starting materials, 9-anthraldehyde and benzyltriphenylphosphonium chloride, in the final product. They probably did not dissolve thoroughly, causing it to disrupt the arrangement of the final product resulting in a lower melting point. When performing recrystallization, the impurities may have not all been removed with the solvent. This also disrupts the arrangement of the final product, resulting in a low melting point as well. Thin Layer Chromatography (TLC) revealed three spots in total. The crude spot on the plate revealed the presence of both trans-9-(2-phenylethenyl)-anthracene and the byproduct triphenylphosphine oxide. Once recrystallization was performed, the TLC plate was spotted with the pure product, and as such, the pure spot only revealed the presence of trans-9-(2-phenylethenyl)-anthracene. This made sense since the recrystallization of the hot isopropanol removed the byproduct. Upon completion of the experiment, the final mass of the product was obtained and a theoretical yield of 0.21g was calculated. A percent yield of 167.6% was calculated for the crude product due to the large presence of triphenylphosphine oxide adding weight. The percent yield of the pure product was calculated to be 20.4% with a percent from recrystallization of 12.2%. These low yields could have been the result of some of the product being lost to excess solvent during the recrystallization process. Some of the product could have also been left in the reaction vessel when transferring it onto the filter paper for vacuum filtration....