Acid Catalysed Hydration of Alkene for JEE

The net addition of water across the double bond can convert alkenes to alcohol. This change can be accomplished in a number of different ways. Hydration of alkene  is the process of adding water to alkenes in a definite amount to convert alkene to alcohol. This method is often used for the preparation of alcohol from alkene.

The outcome is the production of a C-H bond and a C-OH bond, as well as the breakdown of the $\pi$ bond in the alkene and an OH bond in water. The reaction is typically exothermic by 10 to 15 $\dfrac{kcal}{mol}$, although the entropy change is -35 to -40 $\dfrac{kcal}{mol}$. As a result, the process's net free energy change is close to zero, and the direct addition's equilibrium constant is close to one. 

Acid catalysed Hydration of Alkenes 

During acid catalysed hydration of alkenes, the pi bond is replaced by a water molecule in acidic medium. Hence, first the hydrogen is added to alkene, forming as carbocation followed by rearrangement and addition of the -OH molecule.  This is done by adding an OH to the more substituted carbon atom, and hydrogen to the less substituted carbon atom. This reaction follows Markovnikov’s rule and may undergo a carbocation rearrangement. Hydration reaction example includes: 

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Features of Reaction 

• The acid catalyst dissociates to give a H+ ion in solution - this H+ can be shown alone, but is typically attached to water to form an H3O+ hydronium ion. 

• Reaction undergoes  carbocation rearrangement due to carbocation intermediates

• The carbocation intermediate is a sp2 hybridised molecule. This allows water to attack the top or bottom of the racemic product.

• This is a solvolytic reaction in which the solvent (water) is involved in the reaction.

• Despite the presence of multiple oxonium ions in this reaction (oxygen with +1 formal charge), it is never the positive oxygen atom that gets attacked. When oxygen is combined with hydrogen, the oxygen atom pulls on the electrons to make itself slightly less positive. This leaves the hydrogen proton partially exposed, partially positive, and resulting in hydrogen (rather than oxygen) getting attacked.

Mechanism 

The mechanism of acid catalysed hydration of alkene involves following steps:-

Activating  the Acid Catalyst 

When a powerful acid is used, the acid dissociates in solution, forming hydronium.

An acid, according to the Arrhenius and Bronsted-Lowry definitions, is a substance that dissociates in solution to produce an H+.

When water is used as the solvent, the result is hydronium. A solvent molecule, in this case water, picks up H+ from the acid to generate a protonated solvent molecule.

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Carbocation formation

The proton adds to the more substituted carbon atom, leaving the less substituted (previous pi bound) carbon deficient, according to Markovnikov's Rule.

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This carbocation rearranges to form the most stable carbocation.

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Attack of water on carbocation 

Water molecules attack the carbocation from above or below the plane to form alcohol.

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The excess H atom is removed in the final phase of this reaction, yielding a neutral alcohol.

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During the acid catalysed hydration of alkenes, the pi bond is replaced by a water molecule in acidic medium hence first the hydrogen adds to alkene forming as carbocation followed by rearrangement and addition of the -OH molecule.

Carbocation Rearrangement

"The migration of the carbocation from an unstable state to a more stable one by utilising various structural reorganizational shifts within the molecule," according to the definition of carbocation rearrangements. When a carbocation lacks a hydrogen atom that is present on the next carbon atom and is readily available for rearrangement, an alkyl shift occurs.

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Regiochemistry of  Hydration of Alkene 

The alkene can be protonated at either carbon in the first stage of the reaction. The more stable carbocation, on the other hand, is preferred. The order of alkyl cation stability is 

$3^{\circ} > 2^{\circ} > 1^{\circ}$

As a result, protonation will take place at the less substituted carbon, resulting in the more substituted carbocation, to which water will be added. Markovnikov's Rule is the addition of a proton to the less substituted carbon and the -OH to the more substituted carbon.

Stereochemistry of Product 

Stereochemistry is concerned with the directionality of the substituent bonds on the product. Stereochemistry is denoted by dashes and wedges, which indicate whether a molecule or atom is moving into or out of the plane of the board. The molecule that is bonded is as likely to be found going into the plane of the board as it is out of the plane of the board whenever the bond is a simple single straight line. This denotes the presence of a racemic mixing in the product.

The stereochemistry of electrophilic hydration is that the substituent is equally likely to bond, pointing into the plane of the board as it is pointing out of the plane of the board. The water molecule can attack both from above or below the plane to form a racemic product.

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Summary

Hydration of alkenes  is the process of adding water to alkenes in a certain amount. During the acid catalysed hydration of alkenes  pi bond is replaced by a water molecule in acidic medium hence first the hydrogen adds to alkene forming as carbocation followed by rearrangement and addition of -OH molecule to form alcohol. During the acid catalysed hydration of alkenes  pi bond is replaced by a water molecule in acidic medium hence first the hydrogen adds to alkene forming as carbocation followed by rearrangement and addition of -OH molecule. The water molecule can attack both from above or below the plane to form a racemic product. The reverse of this reaction is known as acid-catalysed dehydration of alcohol.