Experiment_10

docx

School

Northeastern University *

*We aren’t endorsed by this school

Course

2205L

Subject

Chemistry

Date

Feb 20, 2024

Type

docx

Pages

Uploaded by djjo1116

CHM 2205L Exp.10 Wittig Reaction July 28 th , 2017 Nicholas Tyndall Lab Partner: Abhi Ahuja Professor: Dr. Chandrasekhar Abstract The purpose of this experiment was to use the Wittig Reaction method along with using green chemistry to form (E)-1-(4-bromophenyl)-2-phenylethene. This was done by using a mortar and pestle and grinding the materials together to get the reaction to go to completion; vacuum filtration was also used in this experiment to isolate the (E)-1-(4-bromophenyl)-2-phenylethene compound. Also, IR Spectroscopy was used in this experiment to determine the functional groups in this final compound. The results of the experiment were that 0.0011g of (E)-1-(4- bromophenyl)-2-phenylethene was obtained the theoretical yield is 0.133g, so the percent yield of the compound was 0.83%. The melting point of (E)-1-(4- bromophenyl)-2-phenylethene from the experiment was 134.3˚C- 136.5 ˚C, and the literature value was 134-135°C, and because of the values being so close one is able to characterize and identify the crystal compound as (E)-1-(4-bromophenyl)- 2-phenylethene.

Introduction The importance of the Wittig reaction is that it converts a C=O bond into a C=C bond. The importance of this Wittig experiment was that it was done with green chemistry so there was no solvent used in this experiment for the reaction to go to completion. Experimental The equipment, methods, and procedures used can be found in the lab manual. 1 The only change in the procedures were that instead of mixing the sample of the product with ethanol on a watch glass it was mixed in an Eppendorf tube. Results Table 1. Mass and Percent Yield Compound Initial Mass(g) Obtained Mass (g) Theoretical Mass (g) Percent Yield (E)-1-(4- bromophenyl)-2- phenylethene 0.72g 0.0011g 0.133g 0.83% Table 2. Melting Point Compound Melting Point Lit. Melting Point (E)-1-(4-bromophenyl)- 2-phenylethene 134.3˚C- 136.5 ˚C 134-135°C Discussion The appearance of the product was small white crystals, the melting point from the experiment was 134.3˚C- 136.5 ˚C and the literature value was 134-135°C, this being said one is able to identify the compound obtained from the experiment as 1 Chandrasekhar. Experiment 10. Wittig Reaction . (2017)

(E)-1-(4-bromophenyl)-2-phenylethene. When looking at the IR Spectrum (Appendix) from this product an analysis of it first shows that because of the weak peak at 3018 cm -1 , that there is a C=C in the compound. From there going down the two aromatic compounds are between 1582 cm -1 and 2017 cm -1 , as seen by there being many weak peaks in between this range.Going on down to the end of the chart there is a sharp deep peak at 811 cm -1 and at 965 cm -1 , these are the C-H bonds bending from the aromatic compounds.Finally, the bromine bonded to the aromatic bond cannot be seen on the chart because the x-values did not go to a small enough range, but if it were to show it the bromine bond would be somewhere between 650-490 cm -1 . The advantages of doing the experiment without a solvent is that it is green chemistry and that there will be a lot less excess waste from this experiment because no solvent is used, so it is better for the environment. The advantage of a solvent based reaction is that you would not have to manually force the reaction by crushing, that the reaction would happen faster with a solvent then by mixing by hand. It was very important to grind for 20 minutes because for the reaction to occur all the compounds had to be crushed together for the reaction to fully occur and if it was not ground up vigorously then it would take much longer for the reaction to go to completion. The purpose of using TLC was to see if reaction did go to completion, if the reaction had gone to completion then the dot of the product after being mixed up would show up higher on the TLC slide than the dot from the 4-bromobenzaldehyde, if the compound’s dot was much higher and almost at the front of the solvent and the other one was midway that means the reaction went to completion.Mechanism for Wittig reaction starts off with the two reactants, the chlorine anion attacks the aldehyde bond, the consequence of this is it causes the electrons from the double bond to go to the oxygen giving an oxygen anion connected by a single bond now. From there the two reactants are now connected together, once they are connected together the oxygen anion attacks the P(Ph 3 ) cation, this causes a four-membered ring to form but now the anion and cation are stabilized, since the ring connected that gives the phosphorus and the carbon of the ring a partial positive change, meanwhile giving the chlorine and the oxygen a partial negative charge. But the ring is very unstable so it breaks apart while doing this the electrons between the oxygen and the carbon go to the oxygen, and the electrons between the chlorine and the phosphorous go to the chlorine. The negative chlorine leaves the molecule and attacks the H of the aldehyde to form HCl. But now because of the breaking away, there is an oxygen and P(Ph 3 ) molecule connected together by a double bond, and now a double bond has formed to give a carbon-carbon double bond to connect the reactants together.

Your preview ends here

Eager to read complete document? Join bartleby learn and gain access to the full version

Access to all documents
Unlimited textbook solutions
24/7 expert homework help

The product is then in the E isomer form because of it is the most stable isomer, due to the Z isomer having a high degree of the steric hindrance. The purpose of washing out the mortar and pestles with water was to get all of the compound out of the mortar and to get it off the pestle. Ethanol was used in particular because two products were formed from the reaction and the E isomer of 1-(4- bromophenyl)-2-phenylethene is polar and is the most soluble in ethanol, because of this ethanol was used to isolate the final product because the Z isomer is soluble in ethanol. The E isomer was the major product that was obtained, this is due to the E product being the most stable isomer, it is the most stable due to it have much less steric hindrance than the Z isomer does. IR spectrum can be used to differentiate between E and Z isomers, this is shown in the appendix how the trans configuration gives a characteristic band at 965 cm -1 , while the cis configuration would of given a weak band at 1640 cm -1 and a medium band at 3020 cm -1 . 2 The spectroscopic method that should be could be also used to differentiate E and Z isomers from each other would be to use H-NMR instead, because E isomers will be more significant and Z isomers will be less significant. Conclusion The objectives of this experiment were met, green chemistry methods were used in this experiment to form (E)-1-(4-bromophenyl)-2-phenylethene. This was shown by melting point being 134.3˚C- 136.5 ˚C and the literature value being 134- 135°C, so the compound was able to be identified as (E)-1-(4-bromophenyl)-2- phenylethene. The results also were that the yield of (E)-1-(4-bromophenyl)-2- phenylethene compared to the theoretical was 0.83%. References Chandrasekhar. Experiment 10. Wittig Reaction . (2017) Acknowledgements This experiment was done with help from classmates in Organic Chemistry 2 Lab, in particular Danielle Salick, and Matthew Green for sharing their information, and their IR Spectrum of (E)-1-(4-bromophenyl)-2-phenylethene. 2 http://www.spectra-analysis.com/documents/AppNote025IRAppliedToIsomers.pdf

Appendix Calculation for theoretical yield of (E)-1-(4-bromophenyl)-2-phenylethene: 0.2024g of benzyltriphenylphosphonium chloride * (1 mol/388.875g) = 5.204 x 10 - 4 mol 0.0951g of 4-bromobenzaldehyde * (1 mol/185.02g) = 5.140 x 10 -4 mol (5.140 X 10-4 mol) * (259.146 g/mol) = 0.133g Percent yield calculation: (Actual yield/ Theoretical yield) X 100= Percent Yield (0.0011g/0.13g) = 0.0083 0.0083 X 100 = 0.83% Yield