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Reactions of alkenes

Reactions of alkenes. Industrial manufacture of ethanol. Reaction with conc sulphuric acid. The sulphuric acid molecule is polar: δ + H – δ - OSO 3 H. C = C. ...a pair of electrons from the C=C bond forms a bond with the δ + hydrogen atom of the sulphuric acid molecule. H. δ +. →.

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Reactions of alkenes

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  1. Reactions of alkenes Industrial manufacture of ethanol

  2. Reaction with conc sulphuric acid The sulphuric acid molecule is polar: δ+H – δ-OSO3H C = C ...a pair of electrons from the C=C bond forms a bond with the δ+ hydrogen atom of the sulphuric acid molecule... H δ+ → OSO3H δ- ...and the H-OSO3H bond breaks to form a -OSO3H ion...

  3. Reaction with conc sulphuric acid The lone pair of electrons on the :-OSO3H ion forms a covalent bond with the carbocation... ... and ethyl hydrogensulphate is formed... C = C C - C ethyl hydrogensulphate H OSO3H C - C H → δ+ → Ä :-OSO3H H OSO3H δ- intermediate – a carbocation

  4. Hydrolysis of ethyl hydrogensulphate Ethyl hydrogensulphate reacts readily with water (hydrolysis) to form ethanol ... and regenerate sulphuric acid CH3CH2OSO3H + H2O → CH3CH2OH + H2SO4 Overall... CH2=CH2 + H2O → CH3CH2OH

  5. Hydrolysis of ethyl hydrogensulphate Overall.. CH2=CH2 + H2O → CH3CH2OH more efficient if it could be done directly as a continuous process... ...but it has a very high activation energy so needs very high temperatures and highpressures!

  6. Industrial manufacture of ethanol CH2=CH2 + H2O → CH3CH2OH Use a catalyst!

  7. Catalysts and activation energy Mechanism 1 Mechanism 2 C2H4 + H2O C2H5 OH

  8. Industrial manufacture of ethanol CH2=CH2 + H2O → CH3CH2OH • Pressure: 7 Mpa • Temperature: 300°C • Catalyst: conc. phosphoric acid (H3PO4) on a silica support

  9. Industrial manufacture of ethanol CH2=CH2 + H2O Ý CH3CH2OH • process is continuous... • more reactants added as products are removed • pressure must not rise too high – polmerisation reactions occur

  10. Industrial manufacture of ethanol • other alcohols can be made by this process but ...... ... mechanism is still electrophilic addition so still possibility of more than one product!

  11. Mechanism Reaction takes place in two steps O– H ÄO – H H 1. A pair of electrons from the C = C bond forms a covalent bond with the δ+ X atom, the X – Y bond breaks forming a Y- ion. C = C → → C - C C - C C - C :OH2 Ä intermediate – a carbocation X X X H+ From H3PO4

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