The ratio of 4-bromotoluene to 2-bromotoluene in the unpromoted reaction is 66.7

Question

The ratio of 4-bromotoluene to 2-bromotoluene in the unpromoted reaction is 66.7:33.1. How does this compare to your result for the zeolite promoted reaction? In the mass spectra, why do peaks for the molecular ions of the bromotoluene isomers appear at 170 and 172 instead of 171? The answer can be found in the mass spectrometry technique section of your lab manual. Given the shapes of the products and the Na-Y cavities, provide an explanation for the product ratio observed upon Na-Y promotion of the reaction versus the unpromoted reaction. Both 2-bromotoluene and 4-bromotoluene can enter and leave the zeolite with approximately equal ability. How is this result with zeolite catalysis valuable for synthesis? Why was it necessary to protect the reaction mixture from light? The answer can be found in your lecture text.

Explanation / Answer

1. Depending on what zeolite to choose, we can get maximum 99% of selectivity. This is almost 100% of the product in this promoted reaction. However, in the unpromoted reaction, when there is no zeolite, one of the product will be 66% formed and another will be 23%. So, unpromoted reaction will be unselective while promoted reactio will be highly selective.

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