organic reaction mechanismsDoc Brown's GCE Chemistry  Revising Advanced Level Organic Chemistry

A Level Revision Notes PART 10 Summary of organic reaction mechanisms

A mechanistic introduction to organic chemistry and explanations of different types of organic reactions

10.3 Reaction mechanisms of ALKENES

Synthesis of organic hydrogensulfates and alcohols by reacting alkenes with sulfuric acid or water

Part 10.3 ALKENES - introduction to the reaction mechanisms of alkenes. Electrophilic addition of sulphuric acid and electrophilic addition of water [acid catalyst] to form alcohols. These revision notes include full diagrams and explanation of the ionic electrophilic addition reaction mechanisms of the reactions of alkenes and the 'molecular' equation and reaction conditions and other con-current reaction pathways and products are also explained.


10.3.5 The electrophilic addition of conc. sulphuric acid to alkene

  • Alkenes are reactive molecules, particularly when compared to alkanes.

  • They are reactive towards electron pair accepting electrophiles because of the high density of negative electron charge associated with the pi electrons of the double bond.

  • What is the reaction mechanism for the addition of concentrated sulfuric acid to an alkene?

  • Can isomeric products be formed in the addition of sulphuric acid to an alkene?

  • R2C=CR2 + H2SO4 ==>  R2CH-C(OSO2OH)R2    [see mechanism 28 below]

    • Isomeric products can be formed if the alkene not symmetrical.

organic reaction mechanisms

mechanism 28 - electrophilic addition of sulphuric acid (conc.) to an alkene

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

  • In step (2) the hydrogensulphate ion formed in step (1) combines with the carbocation to give the alkyl hydrogensulphate product.

  • FURTHER COMMENTS

    • The Markownikoff rule predicts for a non-symmetrical alkene, the proton from the sulphuric acid will mainly combine with the carbon atom with the most number of hydrogen atoms.

      • So for propene the majority product is CH3-CH(OSO2OH)CH3 with a little HOSO2OCH2-CH2CH3 

    • On warming the product with water, the alkyl hydrogensulphate is hydrolysed to an alcohol is. (see also mechanism 29 below).

      • R2CH-C(OSO2OH)R2 + H2O ==> R2CH-C(OH)R2 + H3O+ + HSO4- 

      • In terms of organic synthesis, for example, propene eventually forms mainly propan-2-ol.

        • stage 1: CH2=CHCH3 + H2SO4 ==> CH3-CH(OSO2OH)CH3 

          • stage 2: CH3-CH(OSO2OH)CH3 + 2H2O ==> CH3-CH(OH)CH3 + H3O+ + HSO4- 

    • If the product of the addition can exhibit optical isomerism, an optically inactive racemic mixture will be formed (see ethene + HBr for fuller discussion).


10.3.6 The acid catalysed electrophilic addition of water to alkene

  • What is the mechanism for the catalysis reaction on addition of water to an alkene?

  • Can isomeric products be formed in the addition of water to an alkene?

  • R2C=CR2 + H2O ==>  R2CH-C(OH)R2    [see mechanism 29 below]

    • Isomeric products can be formed if the alkene is not symmetrical about the C=C bond.

    • This one stage synthesis is equivalent to the two stage synthesis of alcohols from alkenes using concentrated sulphuric acid and hydrolysing the product (see above), but the catalysis mechanism has three steps!

organic reaction mechanisms

mechanism 29 - the acid catalysed electrophilic addition of water to an alkene

  • In step (1) the oxonium ion (the electrophile from the acid) protonates the alkene to form the carbocation.

  • In step (2) a water molecule, acting as an electron lone pair donor, bonds with the carbocation to form the protonated alcohol.

  • A proton transfer occurs in step (3) as a water molecule accepts a proton from the protonated alcohol to leave the free alcohol product and reforming the catalytic oxonium ion H3O+.

  • FURTHER COMMENTS

    • The Markownikoff rule predicts for a non-symmetrical alkene, the proton from the sulphuric acid will mainly combine with the carbon atom with the most number of hydrogen atoms.

      • So for propene CH3CH=CH2 the majority product is propan-2-ol CH3-CH(OH)CH3 

        • with a little propan-1-ol CH3CH2-CH2OH

    • In the industrial process, catalysed by phosphoric(V) acid, the proton can come from H3PO4, in step (1)

      • and the H2PO4- ion formed in step (1) will remove the H+ from the protonated alcohol in step (3).

    • If the product of the addition can exhibit optical isomerism, an optically inactive racemic mixture will be formed (see ethene + HBr for fuller discussion).


keywords phrases: electrophile mechanism steps reagents reaction conditions formula intermediates organic chemistry reaction mechanisms steps electrophilic addition of water and sulphuric/sulfuric acid to alkenes ethene propene butene R2C=CR2 + H2SO4 ==> R2CH-C(OSO2OH)R2 R2CH-C(OSO2OH)R2 + H2O ==> R2CH-C(OH)R2 + H3O+ + HSO4- CH2=CHCH3 + H2SO4 ==> CH3-CH(OSO2OH)CH3  CH3-CH(OSO2OH)CH3 + 2H2O ==> CH3-CH(OH)CH3 + H3O+ + HSO4- R2C=CR2 + H2O ==>  R2CH-C(OH)R2 CH3CH2-CH2OH


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