- Title of experiment, name and date
- The reaction scheme for the specific reaction you are performing. Do this for both the salt formation and the actual Wittig reaction. No mechanism is included in the scheme. Underneath the structure of reactants write how much you are using, g or mg for solids, mL and density for liquids, also how many mmol of each you are using. Get this information from the procedure or online resources. Indicate in your scheme which reactant is limiting. Indicate in your scheme which product is the desired product and the theoretical yield in mmol and either g or mg. Add above or below the arrow any solvents and conditions for the reaction. All the reactants should be to the left of the reaction arrow.
- Prepare a table of reactants and products and their pertinent information. You only need to do this for the actual Wittig reaction. Column headings are name, structure, MF, MW (g/mol), density (if a solid the say NA), amount used for reactants (g or mg for solids, mL for liquids), mmol used, mmol expected for products, theoretical yield for products g or mg and hazards. Each reactant and organic product is a row in the table.
- Prepare a table of reaction solvents and workup solutions. You only need to do this for the actual Wittig reaction. The columns for this table should be name, structure, bp, density, purpose (be specific i.e. “removes H2O”, “removes ions”, “neutralizes” etc.) and hazards. All of the information for 3 and 4 can be found in the procedure or online. Sigma-Aldrich Catalog is a good source. Chemfinder is another option. You will need to know the mmol of zinc, allyl bromide, and benzaldehyde. Allyl bromide and benzaldehyde are the two major reactants. Which one is limiting? What is the theoretical yield?
- Obtain reference IR spectra for starting material and expected product. You need to do this for the aldehyde and the alkene product. SDBS is a good source. You can also search online using the name along with the type of spectrum you are looking for. Organize IR data of the product(s) into table(s). For the IR table, the two column headings are structure and IR Diagnostic Bands (cm-1). The correct format for listing data in the second column is stretch, frequency range, descriptors. For example a broad, medium sp3C-H stretch from 2800 -3000 cm-1would be: sp3C-H, 2800-3000, b, m. You will be doing a full analysis of all non-fingerprint region bands of any acquired spectra.
- Obtain proton NMR spectra for the starting material and expected product. You need to do this for the aldehyde and the alkene product. SDBS is a good source. You can also search online using the name along with the type of spectrum you are looking for. Organize the NMR information for the expected product into a table. You will need the structure of the product with the non equivalent H's labelled alphabetically. For each H set you will need chemical shift, multiplicity, J values if applicable and how many H are generating the signal.
- Draw out a formal curved arrow electron pushing mechanism for the reaction you are performing in the laboratory. You need to do this starting with the formation of the salt and continue all the way to the formation of the alkene product. You will need to show intermediates but not transition states. You can use chemical drawing software or copy and paste from a reference. If you have a reference be sure and cite it.
Lab 11 Wittig Reaction During Lab
1. Combine 208 mg of p-methylbenzyltriphenylphosphonium chloride, 95 mg of p-bromo benzaldehyde, and 425 mg of K3PO4 in a mortar and grind for 20 minutes, making sure to scrape down the sides every few minutes with a scoopula. Every 5 minutes, dissolve a small amount of the mixture in acetone and spot onto a TLC plate. Develop the TLC plate in 1:4 ethyl acetate/hexanes and visualize using UV light.
Figure 1. Sample TLC plate after reaction is complete. Yellow is the bromobenzaldehyde, green is the mixture, purple is one isomer, and blue is the other isomer.
2. After the reaction is complete add 10 mL of water to the mortar and collect the insoluble materials via vacuum filtration with a Hirsch funnel.
3. Dissolve the expected alkene product in 5 mL of hot ethanol, and allow recrystallization of the alkene to occur in the filtrate.
4. Collect the purified product via vacuum filtration and obtain a melting point, an IR spectrum, and prepare a proton NMR sample. The expected melting point of the product is 214-215 °C.
|Due By (Pacific Time)
||04/11/2015 12:00 pm