Part 2.6 Chemistry of ALKENES - unsaturated hydrocarbons - addition of water to synthesise alcohols via an acid catalyst

 Index of GCSE level Oil - Useful Products Chemistry Revision Notes

2.6 The reaction of alkenes with steam - addition of water - synthesis of alcohols and the reaction of alkenes with concentrated sulfuric acid

Sub-index for this page

(a) The manufacture of ethanol

(b) The reaction of alkenes with sulfuric acid and the acid catalysed hydration reaction of alkenes

(c) The formation of alkyl hydrogensulfates

(d) The hydrolysis of alkyl hydrogensulfates to give alcohols

(e) The industrial acid catalysed hydration of alkenes to manufacture alcohols

(a) The manufacture of ethanol

The production of ethanol (i) from sugar fermentation or (ii) synthesis from ethene:

(i)  ====> (acid catalysed synthesis in the chemical industry)

(ii)  C₆H₁₂O_6(aq) ====> 2C₂H₅OH_(aq) + 2CO_2(g)  (in the brewing and wine industry)

all the manufacturing processes are described in more detail on the Ethanol manufacture page

(b) The reaction of alkenes with sulfuric acid and the acid catalysed hydration reaction of alkenes

The reactions and mechanism are fully described on ....

Electrophilic addition of sulfuric acid AND electrophilic addition of water [acid catalyst] to form alcohols

So, all I'm repeating are the basic points and some examples for three reactions involving ethene and propene.

However, you should be able to work out the products of these reactions from other alkenes like butenes etc.

(c) The formation of alkyl hydrogensulfates

Concentrated sulfuric acid adds to an alkene to produce an alkyl hydrogensulfate salt.

These are colourless crystals if separated from the reaction mixture.

e.g.

(i) ethene  + sulfuric acid  ===> ethyl hydrogensulfate

H₂C=CH₂  + H₂SO₄  ===> CH₃CH₂OSO₂OH

There is only one product with a symmetrical alkene

Diagram mechanism 68a shows the electrophilic addition of sulfuric acid to ethene gas at room temperature.

In step (1) The sulfuric acid molecule is the electrophile by nature of the highly polar O-H bond (H^δ+-O^δ-) which splits heterolytically to protonate the alkene molecule to form the ethyl carbocation.

Simultaneously the hydrogensulfate ion (HSO₄^-) is formed.

In step (2) the hydrogensulfate ion formed in step (1) combines with the carbocation to give the neutral alkyl hydrogensulfate salt product.

 

(ii) propene  + sulfuric acid  ===> propyl hydrogensulfates

H₂C=CH₂CH₃  + H₂SO₄  ===> CH₃CH₂CH₂OSO₂OH  or  CH₃CH(OSO₂OH)CH₃

There are two isomeric products with an unsymmetrical alkene

Diagram mechanism 68b shows the electrophilic addition of sulfuric acid to propene gas at room temperature.

The steps are identical to those of ethene, but two different carbocation structures are possible.

Step 1: After the initial attack by the sulfuric acid electrophile, a propyl primary carbocation is formed.

This carbocation is less stable than the secondary carbocation formed in mechanism pathway 68c below.

So, in step 2, from Markownikoff's Rule, this alkyl hydrogen sulfate formed is the minority product

 

Diagram mechanism 68b shows the electrophilic addition of sulfuric acid to propene gas at room temperature.

The steps are identical to those of ethene/propene described so far, but a different carbocation results.

Step 1: After the initial attack by the sulfuric acid electrophile, a secondary carbocation is formed.

This carbocation is more stable than the primary carbocation formed in mechanism pathway 68b above.

So, in step 2, from Markownikoff's Rule, this alkyl hydrogen sulfate formed here is the majority product

(d) The hydrolysis of alkyl hydrogensulfates to give alcohols

The overall addition of water to an alkene is an example of a hydration reaction

If the alkyl hydrogensulfates are boiled with excess water, these salts are hydrolysed to an alcohol and the sulfuric acid is regenerated e.g.

(i) ethyl hydrogensulfate  +  water  ===>  ethanol  + sulfuric acid

 CH₃CH₂OSO₂OH  +  H₂O  ===> CH₃CH₂OH  +  H₂SO₄

(ii) propyl hydrogensulfates  +  water  ===> propanol alcohols + sulfuric acid

There are two isomeric alcohols from an unsymmetrical alkene, because there are two possible alkylhydrogensulfates derived from propene.

CH₃CH₂CH₂OSO₂OH  +  H₂O  ===> CH₃CH₂CH₂OH  +  H₂SO₄

giving the primary alcohol propan-1-ol

CH₃CH(OSO₂OH)CH₃  +  H₂O  ===> CH₃CH(OH)CH₃  +  H₂SO₄

giving the isomeric secondary alcohol propan-2-ol

For (ii), propan-2-ol is the major product, which you can work out from the Markownikoff (Markownikov) Rule.

(e) The industrial acid catalysed hydration of alkenes to manufacture alcohols

Alkene vapour and steam are passed over an acid catalyst, water (as H-OH) is added across the double bond e.g.

(i) H₂C=CH₂  +  H₂O  ===> CH₃CH₂OH

Giving ethanol - this is a synthetic method of making alcohols from alkenes from cracking oil hydrocarbons.

The catalyst is phosphoric acid and the reaction carried out at ~300oC and 60 atmospheres pressure.

Mechanism diagram 69a shows the acid catalysed electrophilic addition of water to ethene.

In step (1) The oxonium ion is the electrophile (H₃O⁺) by nature of the highly polar O-H bond (H^δ+-O^δ-) which splits heterolytically to protonate the alkene molecule to form the ethyl carbocation.

Simultaneously, a water molecule is formed.

In step (2) the carbocation formed in step (1) combines with a water molecule to form a protonated ethanol molecule.

In step (3) the protonated ethanol molecule loses a proton to a water molecule to give the ethanol product and regenerating the oxonium ion to complete the catalytic cycle (in industry a phosphoric(V) acid (H₃PO₄) catalyst is used.

 

(ii) H₂C=CH₂CH₃  +  H₂O  ===> CH₃CH₂CH₂OH  or  CH₃CH(OH)CH₃

giving the primary alcohol propan-1-ol and the major product propan-2-ol (Markownikov Rule)

Mechanism diagram 69b shows the acid catalysed electrophilic addition of water to propene via a primary carbocation.

The three steps are similar to those for ethene, but the product is now a propanol alcohol.

In step 1 a primary carbocation is formed, which is less stable than the secondary carbocation formed in mechanism pathway 69c described below.

This means on the basis of the Markownikov rule, propan-1-ol will be the minor product.