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 Index of organic chemistry technical terms and mechanism pages

 Index of ALL advanced revision notes on ALKENES

 All my advanced A level organic chemistry notes

10.3 Reaction mechanisms of ALKENES

Synthesis of organic alkyl 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.

• 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  π 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 diagram 28 below]

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

  • R = H or alkyl.

  • Ethene gives ethyl hydrogensulfate

  • H₂C=CH₂ + H₂SO₄ ==>  H₂CH-CH₂OSO₂OH    [see mechanism diagram 28 below]

• In step (1) The sulfuric 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 hydrogensulfate ion formed in step (1) combines with the carbocation to give the alkyl hydrogensulfate 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 CH₃CH(OSO₂OH)CH₃ with a only a little of the isomeric HOSO₂OCH₂-CH₂CH₃ 

  • On warming the product with water, the alkyl hydrogensulfate is hydrolysed to an alcohol is.

    • (see also mechanism diagram 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.

      • step 1: CH₂=CHCH₃ + H₂SO₄ ===> CH₃CH(OSO₂OH)CH₃ 

      • step 2: CH₃CH(OSO₂OH)CH₃ + 2H₂O ==> CH₃CH(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).

• 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 diagram 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!

  • R = H or alkyl.

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 CH₃CH(OH)CH₃ 

      • with a little propan-1-ol CH₃CH₂CH₂OH

  • In the industrial process, the addition of water to an alkene is catalysed by phosphoric(V) acid, the proton will come from H₃O⁺ via 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).