Question

Question 5. One of the major issues with the Wittig reaction is that triphenylphosphine oxide is a by-product, and is difficult to separate from products. This means that tracking the following reaction's success by H NMR cannot be done by simply counting the aromatic integration. What signal is the best choice to monitor? That is what proton in the starting material changes in the product significantly enough that you could easily see its change? Circle the proton in the starting material and product. Describe where you would expect (ppm range) to see it in the starting material, and where you would expect to see it in the product. (5 marks) Pho Ph PhPh PPh3

Answer 1

PhPh. Pho Ph PPh

The aldehyde proton peak of the starting material is the best choice for monitoring the progress of the reaction via ¹H NMR spectroscopy.   The aldehyde proton peak distinctly appears between 9 - 10 ppm as a singlet in the ¹H NMR spectrum while all other peaks appear below 8 ppm.   In the product, the aldehyde proton transforms to olefinic proton which appears between 6 - 7 ppm as a singlet in ¹H NMR. With the progress of reaction, the intensity of the aldehyde singlet will decrease and the intensity of the olefinic singlet between 6 - 7 ppm will increase. The completion of the reaction will be indicated by the absence of the aldehyde singlet in ¹H NMR spectrum.