Making OH a good leaving group


In organic chemistry, when students learn reactions such as SN1, SN2, E1 and E2, we learn about good vs bad leaving groups. A good leaving group, when it leaves, is very stable on it's own and will not attack the substrate again. Examples of good leaving groups are I (iodine) and Cl (chlorine). As we go down a group, the size of halogens increases making them more capable of stabilizing the negative charge. Thats it why Iodine is one of the best leaving groups.

OH makes a bad leaving group since if it would leave, it would be OH-, a very strong base that is ready to attack. Therefore, we will discuss ways of making OH a good leaving group. What are they? First, we can protonate OH and make it OH2, or water. H2O is a much better leaving group since when it leaves it is very stable and will not go back to attack the molecule. We can protonate OH and make it a good leaving group by using any acid such as HCl, H3O+, H2SO4 etc. Another way to make OH a good leaving group is by reacting it with TsCl, tosylate chloride. Upon this reaction, OH becomes OTs, a very good leaving group. Why? In OTs-, the negative charge on the oxygen is very well resonance stabilized.

What can we do once we turn OH into a good leaving group? Any reactions that involve a leaving group. For example OH plus H2SO4 usually undergoes E1 giving the most substituted and stable double bond. It can also undergo SN1, SN2, and E2 depending on which nucleophile is being used and in which solvent conditions.

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