What is meant by acid catalyzed hydration of alkene? [closed]








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My teacher told me that acid catalyzed hydration needs acid because h20 is a weak acid and conjugate base of OH- is unstable.Can anyone tell me exactly how this is sure in terms of equilibrium?I don’t know the pka of carbocation formed to compare with acidity of H20 and see where equilibrium lies.




This is a bit tough to answer because your question indicates confusion with multiple concepts.


First, you don't measure the $\mathrm{pK}\_{a}$ of a carbocation. It is a Lewis acid, but that's not the correct context for this problem. It is an **electrophile**. It will accept a nucleophile, which in this problem is water (the lone pairs on the oxygen make it an OK nucleophile). The resultant product, an alcohol, has the appropriate functional group (OH) because it comes from the water molecule that is added.


Second, the slow part of this reaction is generating the carbocation. In order to do so, you need to add a $\ce{H+}$ to the alkene. Unfortunately, water is too weak of an acid to do this efficiently. Instead, we use a strong acid, which in water, forms hydronium $\ce{H3O+}$. This is a strong acid and able to protonate the alkene to create the carbocation. As discussed above, the cation goes on to react water to generate the alcohol.


One additional point is that after adding water to the cation, we actually don't have the alcohol directly; we have the oxonium ion: $\ce{ROH2+}$. In order to generate the alcohol, we need to remove $\ce{H+}$. The hydrogen ion goes back onto a water molecule to reform the hydronium ion that we used to start the reaction. By regenerating the hydronium, the reaction is catalytic in hydronium, i.e., it's not used up in the reaction (and technically, it makes the reaction go faster). That's why this reaction is **acid-catalyzed**.


I strongly recommend going back to review the mechanism and trying carefully to identify which role each of the components plays in this reaction.


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