HOME PAGE * KS3 SCIENCES * GCSE BIOLOGY  CHEMISTRY  PHYSICS * ADVANCED LEVEL CHEMISTRY

Organic Chemistry: Alkenes - reaction with sulfuric acid, steam & synthesis of alcohols

Part 2. The chemistry of ALKENES - unsaturated hydrocarbons

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry

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


INDEX of ALKENE revision notes

All Advanced A Level Organic Chemistry Notes

Index of GCSE/IGCSE Oil - Useful Products Chemistry Notes

[SEARCH BOX]



The manufacture of ethanol

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

(i)  doc b oil notes ====> (acid catalysed synthesis in the chemical industry)

(ii)  C6H12O6(aq) ====> 2C2H5OH(aq) + 2CO2(g)  (in the brewing and wine industry)

all the manufacturing processes are described on the Ethanol manufacture page


The reaction of alkenes with sulfuric acid and the acid catalysed hydration reaction of alkenes

The reactions 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.


(2.6.1) 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

alkyl ethyl hydrogensulfate Diagram mechanism electrophilic addition of sulfuric acid to ethene advanced A level organic chemistry doc brown's revision notes

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 (Hδ+-Oδ-) 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 electrophilic addition of sulfuric acid to propene primary carbocation advanced A level organic chemistry doc brown's revision notes

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 electrophilic addition of sulfuric acid to propene secondary carbocation advanced A level organic chemistry doc brown's revision notes

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

 


(2.6.2) The hydrolysis of alkyl hydrogensulfates to give alcohols

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.


(2.6.3) The acid catalysed hydration of alkenes

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.

acid catalysed electrophilic addition mechanism of water to ethene to form ethanol advanced A level organic chemistry doc brown's revision notes

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 (Hδ+-Oδ-) 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)

acid catalysed electrophilic addition mechanism of water to propene to form propan-1-ol 1-propanol advanced A level organic chemistry doc brown's revision notes

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.

 

acid catalysed electrophilic addition mechanism of water to propene to form propan-1-ol 1-propanol advanced A level organic chemistry doc brown's revision notes

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

The three steps are similar to those for propene already described, but the product is now another isomeric propanol alcohol.

In step 1 a secondary carbocation is formed, which is more stable than the primary carbocation formed in mechanism pathway 69b described above.

This means on the basis of the Markownikov rule, propan-2-ol will be the major product.

 


TOP OF PAGE

Doc Brown's Advanced Level Chemistry Revision Notes

What next?

Index of all the ALKENE revision notes

All Advanced Level Organic Chemistry Notes

Index of GCSE/IGCSE Oil - Useful Products Chemistry Revision Notes

[SEARCH BOX]

 

KS3 BIOLOGY QUIZZES ~US grades 6-8 KS3 CHEMISTRY QUIZZES ~US grades 6-8 KS3 PHYSICS QUIZZES ~US grades 6-8 HOMEPAGE of Doc Brown's Science Website EMAIL Doc Brown's Science Website
GCSE 9-1 BIOLOGY NOTES GCSE 9-1 CHEMISTRY NOTES and QUIZZES GCSE 9-1 PHYSICS NOTES GCSE 9-1 SCIENCES syllabus-specification help links for biology chemistry physics courses IGCSE & O Level SCIENCES syllabus-specification help links for biology chemistry physics courses
Advanced A/AS Level ORGANIC Chemistry Revision Notes US K12 ~grades 11-12 Advanced A/AS Level INORGANIC Chemistry Revision Notes US K12 ~grades 11-12 Advanced A/AS Level PHYSICAL-THEORETICAL Chemistry Revision Notes US K12 ~grades 11-12 Advanced A/AS Level CHEMISTRY syllabus-specificatio HELP LINKS of my site Doc Brown's Travel Pictures
Website content Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. Exam revision summaries & references to science course specifications are unofficial.

 Doc Brown's Chemistry 

*

TOP OF PAGE