Alkohol till alken
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Acid-Catalyzed Dehydration of Alcohols
In the previous post, we discussed the methods for converting alkenes to alcohols via Markovnikov and anti-Markovnikov addition of water, the strategies to avoid possible rearrangements when the reactions go through carbocation rearrangements, as well as the indirect addition of the OH group via hydrohalogenations and substitution reactions.
Today, we’ll summarize the strategies for converting alcohols to alkenes. There are three main approaches we will discuss in this article: 1)Acid-catalyzed dehydration of alcohols, 2) Using POCl3 as a dehydration agent, and 3) Conversion via alkyl halides followed Zaitsev and Hofmann elimination.
Let’s start with the dehydration of alcohols using strong acids such as H2SO4 and TsOH (p-Toluenesulfonic).
This method works for converting primary, secondary, and tertiary alcohols to alkenes.
Although for all the alcohols, the reaction starts with the protonation of the OH group converting it into a good leaving group, secondary and tertiary alcohols are dehydrated by E1, while primary alcohols by E2 mechanism.
You can check this article for more details about the mec
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The Regiochemistry of Alcohol Addition
In the previous post, we talked about the acid-catalyzed hydration of alkenes where an alcohol is formed as the major organic product:
If an alcohol, rather than water, is used as the solvent a very similar reaction takes place and an ether is produced.
Just like the hydration, it goes through a stepwise mechanism starting with the protonation of the double bond:
Notice that instead of the hydronium ion that we saw in the acid-catalyzed hydration, the immediate protonating species here is the oxonium ion of the alcohol. This ion is strong enough of an acid to protonate the double bond and form a carbocation. Once the carbocation is formed, it can now be attacked by the alcohol:
After the deprotonation of the oxonium ion, the corresponding ether is formed as the final product.
And just like in the acid-catalyzed hydration, all the steps of this transformation are reversible. We only show them with one arrow as we are interested in the forward reaction and trying to shift the equilibrium to that side.
The rate-determining step of the process sis the protonation of the double bond and therefore, the reaction
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Acid-Catalyzed Hydration of Alkenes
There are two main ways of converting alcohols to alkenes, and both are called hydration of alkenes. One is the acid-catalyzed hydration of alkenes, and the other fryst vatten the hydroboration-oxidation of alkenes.
Notice the key difference in the products: the acid-catalyzed hydration of alkenes follows the Markovnikov’s rule, whereas the hydroboration-oxidation places the OH group in the anti Markovnikov position. A quick reminder, Markovnikov’s rule is followed when the X component of the HX reactant (for example HCl, HBr, HI, HOH) adds to the more substituted carbon atom. We have dedicated posts for each of these reactions that you can read, but here, we’ll put a quick summary mechanism for them to understand their regiochemistry. Let’s uppstart with the acid-catalyzed hydration of butene:
Notice that the reaction goes through a carbocation intermediate, and this should be a hint to you to watch out for possible rearrangements. Check out the brev about rearrangements in addition reactions of alkenes. As we have seen in SN1 and E1 reactions, whenever possible carbocation intermediates will undergo a rearrangement to