Name: Acid-Catalyzed Hydration of Alkynes: Keto-Enol Tautomerism
Uploaded: 2023-04-30T18:28:38
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Acid-catalyzed&#160;hydration of alkenes involves the addition of water across the &#960; bond in the presence of a strong acid, like sulfuric acid, to form alcohols.
Similarly, alkynes also undergo acid-catalyzed hydration; however, they form aldehydes and ketones instead.
For example, acetylene gives acetaldehyde, whereas other terminal alkynes form methyl ketones.
Unsymmetrical internal alkynes undergo an acid-catalyzed hydration to give a mixture of ketones; in contrast, symmetrical internal alkynes yield a single ketone.
The addition of water to terminal alkynes follows Markovnikov&#39;s rule. However, for internal alkynes the addition is non-regioselective.
Since alkynes are less reactive than alkenes towards the addition of water, a mercuric salt, like mercuric sulfate, is used as a catalyst to facilitate the reaction.
Analogous to the oxymercuration of alkenes, the mechanism of acid-catalyzed hydration of alkynes begins with the nucleophilic attack by one of the alkyne &#960; bonds on the mercuric ion to form a bridged-mercurinium ion intermediate.
Next, water, being a nucleophile, attacks the more substituted carbon from the opposite side of the bridge and opens the ring, accounting for the observed Markovnikov&#39;s regioselectivity.
Thereafter, a proton transfer to the solvent leads to the formation of an organomercuric enol, which rapidly converts into a more stable keto tautomer via the keto-enol tautomerism.
Tautomers are constitutional isomers that interconvert and differ in the locations of a hydrogen atom and a double bond. The equilibrium favors the more stable isomer, in this case, the keto form.
Protonation of the carbonyl group of the keto tautomer forms an oxonium ion, which loses a mercuric ion to give the enol form of the final product.
The last step involves tautomerization of the enol into the desired ketone.
Lastly, acid-catalyzed hydration is most useful for terminal and symmetrical internal alkynes since they produce one final product. Unsymmetrical alkynes form a mixture of products that need to be separated, thus lowering the overall yield.

alkynes to aldehydes and ketones: acid-catalyzed hydration

<h4>Introduction</h4>
Analogous to alkenes, alkynes also undergo acid-catalyzed hydration. While the addition of water to an alkene gives an alcohol, hydration of alkynes produces different products such as aldehydes and ketones.&#160;&#160;&#160;&#160;&#160;&#160;&#160;
<img alt="Figure1" src="/files/ftp_upload/11838/11838_PT_Figure 1.svg" style="text-align: center; height: 180px;" />
Since the rate of acid-catalyzed hydration of alkynes is much slower than alkenes, a mercuric salt like mercuric sulfate (HgSO4) is usually added to facilitate the reaction. Hydration of terminal alkynes follows Markovnikov&#39;s rule; however, for internal alkynes, the addition of water is non-regioselective.
<h4>Mechanism</h4>
The mechanism begins with a nucleophilic attack by the alkyne &#960; bond on the Hg2+ ion resulting in the formation of a cyclic mercurinium ion intermediate. A second nucleophilic attack by water on the more substituted carbon forms an organomercuric enol that rapidly converts into a stable keto form via keto-enol tautomerism. Protonation of the keto intermediate followed by the loss of an Hg2+ ion yields the enol form of the product. The final step proceeds with the tautomerization of the enol to the desired ketone.
<img alt="Figure2" src="/files/ftp_upload/11838/11838_PT_Figure 2.svg" style="text-align: center; height: 280px;" />
<h4>Keto-Enol Tautomerism</h4>
Unlike alkenes, acid-catalyzed hydration of alkynes is irreversible. This is because the enol intermediate formed during the hydration of alkynes is unstable and rapidly isomerizes to a more stable keto form. The chemical equilibrium that exists between the two forms is referred to as keto-enol tautomerism. Since the C=O bond is considerably stronger than the C=C bond, the equilibrium favors the keto isomer. Keto-enol tautomerism is characterized by the migration of a proton and the change in the location of a double bond.
Acid-catalyzed tautomerization is a two-step process:
Step 1: Addition of proton&#160; across the enol double bond
<img alt="Figure3" src="/files/ftp_upload/11838/11838_PT_Figure 3.svg" style="text-align: center; height: 100px;" />
Step 2: Loss of a proton to yield the keto form
<img alt="Figure4" src="/files/ftp_upload/11838/11838_PT_Figure 4.svg" style="text-align: center; height: 100px;" />
<h4>Example</h4>
Acid-catalyzed hydration of 1-propyne initially forms the less stable enol isomer, propen-2-ol, which tautomerizes into a more stable keto product, propan-2-one.
<img alt="Figure5" src="/files/ftp_upload/11838/11838_PT_Figure 5.svg" style="text-align: center; height: 70px;" />
<h4>Hydration of Terminal And Internal Alkynes</h4>
Acid-catalyzed hydration is most useful for terminal and symmetrical internal alkynes because they form only one final product. In contrast, unsymmetrical internal alkynes yield a mixture of products that need to be separated. This lowers the overall yield and makes the process less efficient.

Learn by watching this video about Acid-Catalyzed Hydration of Alkynes: Keto-Enol Tautomerism at JoVE.com

Acid-Catalyzed Hydration of Alkynes: Keto-Enol Tautomerism

Alkynes to Aldehydes and Ketones: Acid-Catalyzed Hydration

Introduction
Analogous to alkenes, alkynes also undergo acid-catalyzed hydration. While the addition of water to an alkene gives an alcohol, hydration of ...