Hydration of Alkenes

Reaction

The alkene hydration reaction converts an alkene into an alcohol. This reaction is acid-catalysed, often using aqueous sulfuric acid (H₂SO₄):

The word hydration in the name of this reaction describes the addition of a water molecule across an alkene double bond – the alkene π bond is broken, one alkene carbon gains a hydrogen atom (H), and the other alkene carbon gains a hydroxyl group (OH).

The alkene hydration reaction is an example of an electrophilic addition reaction, in which the π electrons in the alkene double bond act as a nucleophile (or, if you like, an electrophile is added to the double bond). In the alkene hydration reaction, the electrophile is an acidic proton.

Once the alkene has been protonated (that is, a hydrogen atom is now bonded to one of the alkene carbons), a carbocation is formed on the other alkene carbon. Then, a water molecule nucleophilically attacks the carbocation, leading to the alcohol product.

How do we know which alkene carbon will get the proton, and which one becomes a carbocation? This can be predicted based on Markovnikov’s rule, which states that, in an electrophilic addition reaction, the nucleophile (in this case, water) will end up on the more highly substituted alkene carbon. The rationale behind this has to do with carbocation stability. The carbocation will preferentially form on the alkene carbon that has more substituents (that is, the more stable carbocation will preferentially be formed). This is followed by attack of the nucleophile on the carbocation, generating the expected product.

Mechanism

As an example, let’s go through the mechanism of the following alkene hydration reaction:

Like other electrophilic addition reactions, the first step in the mechanism is the nucleophilic attack of the alkene π electrons onto the electrophile. In this case, the electrophile is an acidic proton. So, we draw an arrow from the alkene double bond to the acidic proton (in this example, a proton on sulfuric acid), and break the H-O bond, with the bond electrons going onto the oxygen (giving it a negative charge). However, we must decide which alkene carbon accepts the proton, and which becomes the carbocation. In the following scheme, if we protonate the carbon on the right, we get a tertiary carbocation (top arrow), while if we protonate the carbon on the left, we get a secondary carbocation (bottom arrow):

As Markovnikov’s rule predicts that an electrophilic addition reaction will form the more stable carbocation, the top arrow (which leads to the more stable carbocation) will be favoured. Note that sulfuric acid will also protonate water molecules in this mixture (forming hydronium ions – H₃O⁺), and the alkene could react with a hydronium ion instead of sulfuric acid.

With the carbocation formed, the next step is the nucleophilic attack of a water molecule onto the electrophilic positively-charged carbon. We draw an arrow from one of the lone pairs on the water molecule oxygen to the carbocation carbon:

The resulting positively charged oxygen is not very happy (oxygen is electronegative), so there is a strong drive for it to lose a proton (in other words, it is rather acidic). This acidic proton can be removed by several different bases, such as a water molecule, or another alkene. If we use a water molecule as a base, the lone pair on the water molecule oxygen can attack the proton on the positively charged oxygen, breaking the H-O bond, and giving the bond electrons to the oxygen (neutralizing the positive charge):

This final step forms the alcohol product, and regenerates the acid catalyst. This alcohol is the Markovnikov product, in which the nucleophile (water) has attacked the more highly substituted carbon.

Notes and Related Topics

The mechanism of the alkene hydration reaction is an electrophilic addition reaction, in which the alkene π bond is broken and two new σ bonds are formed with the alkene carbons. As such, the mechanism of alkene hydration is very similar to a number of other electrophilic addition reactions, including alkene halogenation, alkene hydrohalogenation, and alkene oxymercuration. It’s worth focusing on the similarities of these reactions – it will save you time studying, and will allow you to predict the products of new reactions.

The preferential formation of the Markovnikov product from the alkene hydration reaction is an example of regioselectivity, which means that, if there is more than one position where a bond can be formed (or where a bond can be broken), one position is favoured over the other(s).

For the hydration of alkenes, two different products are theoretically possible, depending on which carbocation is formed, and where the hydroxyl group ends up. These two products are known as regioisomers, which are structural isomers that differ based on the position of a substituent (in this case, the hydroxyl group).

For many alkenes, protonation of one alkene carbon is more favourable than the other, based on the different stabilities of the carbocations that are formed. Thus, the formation of one regioisomer is favoured. In other words, this reaction is regioselective. Note that there are other alkene reactions which form the anti-Markovnikov product, such as the hydroboration-oxidation reaction of alkenes. 

Summary

The alkene hydration reaction converts an alkene into an alcohol. This reaction is acid-catalysed, often using sulfuric acid. Alkene hydration is an example of an electrophilic addition reaction, where an alkene nucleophilically attacks an electrophile, a carbocation is formed, and a second nucleophile attacks the carbocation. The major product of the alkene hydration reaction is the Markovnikov product, resulting from the formation of a carbocation on the more highly substituted alkene carbon. This is an example of regioselectivity, where one regioisomer is preferentially formed from a reaction.