Hydrolysis of alkene | ethene, propene hydration with dilute H2SO4 | mechanism
Alkene compounds can be hydrated and alcohols are given as products. Dilute sulfuric acid is used for alkene hydration. A water molecule is added to the carbon atoms which has the double bond.
In this tutorial, we discuss about followings
- Hydrolysis of alkene and products
- Markovnikov rule in alkene hydration with mechanism
- Examples of alkene hydration
Hydrolysis of alkene in the presence of H2SO4 acid
Alkene hydration with dilute sulfuric acid
All alkene react with dilute sulfuric acid and give alcohols. This is a hydration reaction. A water molecule is added through double bond and may give primary, secondary or tertiary alcohols. Alkene hydration is also decided by Markovnikov rule which is explained below. Dilute sulfuric acid is behaved as a catalyst in hydrolysis of alkene.
Markovnikov rule
Negative part of the addendum (OH) joins with carbon atom which carries the smaller number of hydrogen atoms and positive part (H) goes to the carbon atom which has more hydrogen atoms.
Products of hydrolysis of alkenes
The product forming according to the Markovnikov rule is called as the major product because it is the most forming product. Other products are formed in very less amounts. They are called minor products.
Alkene and dilute sulfuric acid reaction examples
When alkene react with dilute sulfuric acid, sometimes it gives single alcohol and sometimes it gives a mixture of alcohol products. But in given alcohol product mixture, one alcohol composition is very large. That alcohol is given according to the Markovnikov rule. You can see some examples of alkene hydration and products of them in following reactions.
Examples of hydrolysis of alkene
Ethene hydration gives ethanol | ethene and H2SO4
Ethene has only two carbon atoms and it is a symmetrical molecule too. Therefore we do not need to apply markovnikov rule to decide, where hydrogen and OH parts are added. Hydrolysis of ethene gives ethanol as the only product and it is a primary alcohol.
Propene acidic hydration
Propene hydration gives 2-propanol as the major product. We know hydrogen atom is attached to the carbon atom which had more hydrogen atoms. Then OH part is attached to the other carbon atom in giving major product. Also 1-propanol forms as the minor product.
2-butene hydration gives 2-butanol
Hydrolysis of 2-butene will give 2-butanol as the only product. 2-butene is a symmetrical alkene. Therefore, there is no need to application of markovnilov rule.
1-butene hydration
1-butene is a unsymmetrical molecule. So markovnikov rule is required to decide the major product. 1-butene hydration gives 2-butanol as the major product and 1-butanol as the minor product.
Alkene hydration mechanism
- Electrons density in the double bond of alkene is high. Therefore this double bond can attack positively charged parts such as H+Sulfuric molecule is a polarized molecule such as Hδ+ and Oδ-. Hδ+ is attacked by double bond of alkene. Meanwhile H-O bond is braked in the H2SO4 molecule.
- Then a carbocation is formed. That carbocation is attacked by O- in the -OSO3H.
- next, H2O is hydrolyzed and H2SO4 is released again. Hence H2SO4 is behaved as an catalyst.
Questions asked by students
hydrolysis of pentene to give ester?
There is not a method to produce an ester directly from pentene. When pentene is hydrolyzed from dilute H2SO4, 2-pentanol is given as the product which is an alcohol.
Catalyst used during hydrolysis of alkene can be used in alkyne hydrolysis?
Alkyne hydrolysis is difficult than alkene hydrolysis. Therefore HgSO4 is added as an extra catalyst dilute with H2SO4 in alkyne hydrolysis.
Can I identify alkenes and alkynes from alkene hydrolysis reaction?
Yes. You can. Alkenes and alkynes react differently with dilute sulfuric acid. Alkenes are hyrolyzed by dilute sulfuric acid. But alkynes are not. After addition of sulfuric acid, do some experiments to identify given products (products are given only by alkenes).
After hydrolysis of alkene, primary alcohol or secondary alcohol is given. So we want to identify an alcohol and alkyne now.
When alkyne does not have an acidic hydrogen
If alkyne does not have an acidic hydrogen, we can do sodium test. When sodium is added, alcohol will react and emit hydrogen gas. But there is no change in alkyne.
When alkyne have an acidic hydrogen atom
Add alkaline potassium permanganate to both alcohol and alkyne. alkaline potassium permanganate does nothing with alcohol. But with alkyne, it reacts. So purple colour of alkaline potassium permanganate is changed to brown colour mangenese dioxide (MnO2).
What will give when alkene reacts with water with H2SO4 catalyst it forms?
It will give a primary alcohol or secondary alcohol or tertiary alcohol according the alkene molecule.
How isomers of C4H8 are hydrated?
We can draw different isomers for C4H8. But according to the arrangement of C4H8 molecule in different isomers, given alcohol type is different as primary, secondary and tertiary.
hydration of alkene 3-bromo-1-propene?
When 3-bromo-1-propene reacts with dilute H2SO4, hydration is occurred and give 3-bromoprop-2-ol.