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Reactions of alkenes

Reactions of alkenes. Electrophilic addition. Addition reactions of alkenes. +. →. X - Y. C = C. C - C. X Y. Addition reaction. ...a chemical reaction involving two or more reactants resulting in a single product containing all atoms of all components. Breaking the double bond.

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Reactions of alkenes

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  1. Reactions of alkenes Electrophilic addition

  2. Addition reactions of alkenes + → X - Y C = C C - C X Y Addition reaction... ...a chemical reaction involving two or more reactants resulting in a single product containing all atoms of all components

  3. Breaking the double bond high electron density -ve charge δ+ δ- + X - Y C = C C - C X has a greater electronegativity than Y X Y electron-deficient species an electrophile; a species with an electron-deficient centre in its structure

  4. What is an addition reaction? + → X - Y C = C C - C X Y electrophilic addition reaction... ...an addition reaction brought about by the attack of an electrophile on a region of unsaturation in a molecule – a double bond

  5. Mechanism Reaction takes place in two steps 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 → Ä :Y- δ+ δ- X X - Y intermediate – a carbocation

  6. Mechanism 2. The lone pair of electrons on the Y- ion forms a covalent bond with the carbocation C = C C - C X Y C - C → → Ä :Y- δ+ δ- X X - Y intermediate – a carbocation

  7. Mechanism The full mechanism is: C = C C - C X Y C - C → → Ä :Y- δ+ δ- X X - Y

  8. Specific reactions understand the mechanism of electrophilic addition of alkenes with: Br2 bromine HBr hydrogen bromide H2SO4 conc. sulphuric acid

  9. Reaction with bromine The bromine molecule is non-polar... ...until it approaches the C=C double bond... ...the high electron density induces a temporary polarity n the molecule. C = C ...a pair of electrons from the C=C bond forms a bond with the δ+ bromine atom of the bromine molecule... Br δ+ → Br δ- ...and the Br-Br bond breaks to form a Br- ion...

  10. Reaction with bromine The lone pair of electrons on the Br- ion forms a covalent bond with the carbocation... ... and 1,2-dibromoethane is formed C = C C - C 1,2-dibromoethane Br Br C - C Br → δ+ → Ä :Br- Br Br δ- intermediate – a carbocation

  11. Reaction with hydrogen bromide The hydrogen bromide molecule is polar since there is a difference in electronegativity between the hydrogen and bromine C = C ...a pair of electrons from the C=C bond forms a bond with the δ+ hydrogen atom of the hydrogen bromide molecule... H δ+ → Br δ- ...and the H-Br bond breaks to form a Br- ion...

  12. Reaction with bromine The lone pair of electrons on the Br- ion forms a covalent bond with the carbocation... ... and bromoethane is formed C = C C - C bromoethane H Br C - C H → δ+ → Ä :Br- H Br δ- intermediate – a carbocation

  13. 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...

  14. Reaction with bromine The lone pair of electrons on the Br- 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

  15. 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

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