In this reaction you used observation of a precipitate as evidence that reaction

Question

In this reaction you used observation of a precipitate as evidence that reaction occurred if there were no precipitate you would have had to isolate, purify and characterize the products of the reactions. This would have been a LOT OF WORK! But let's imagine that you did all of that for the reaction shown below, i.e. you isolated the iodide product of reaction of the bromide. Which analytical technique, mass spectrometry, infrared spectroscopy or proton NMR spectroscopy would you use to show that the iodide product is different from the bromide starting material, and that reaction occurred? Assume further that you don't have an authentic sample of the iodide product on hand to compare your spectra to. You need to show that the product is actually the iodide and not just that it is different from the starting bromide. Argue why the method you choose is best and why the other two methods are not as good.

Explanation / Answer

Out of given tecniques we will use Mass spectroscopy.

Let us discuss why we are not using the other two given spectroscopic techniques

a) IR spectro: It will give us the peaks related to functional group. There will not be a remarkable difference in the IR of alkyl bromide and alkyl iodide, as both have halogen group. There is not great modification in the functional group.

b) NMR: It shows peak due to hydrogen atoms. The peak pattern of hydrogens will not be affected (much) due to change in the halogen group

So we have to use Mass spectroscopy for the difference , which should be used because

a) Bromine (present in the reactant) shows two molecular ion peaks (M+ and M+2) of same intensity due to the two isotopes of bromine element. The two isotopes have same abundance in nature so we obtain two molecular ion peaks of same intensity. Such two peaks will be absent in the product which has iodine.

b) the product will show a peak absence of 127 (due to removal of Iodine) and a significant peak at 127 in the mass spectra of product. Such pattern is absent in the mass spectra of reactant.

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