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Reactions of alkenes

Reactions of alkenes. Unsymmetrical alkenes. Choice. When HBr is added on to propene ( CH 2 = CHCH 3 ) . ...there are are two products (not equal amounts)... . ...a major one ( 2-bromopropane ). CH 2 = CHCH 3 + HBr → CH 3 CHBrCH 3. ... and a minor one ( 1-bromopropane ).

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Reactions of alkenes

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  1. Reactions of alkenes Unsymmetrical alkenes

  2. Choice When HBr is added on to propene (CH2 = CHCH3 )... ...there are are two products (not equal amounts)... ...a major one (2-bromopropane)... CH2 = CHCH3 + HBr → CH3CHBrCH3 ... and a minor one (1-bromopropane) CH2 = CHCH3 + HBr → CH3CH2CH2Br

  3. Why are there two products? Propene is an ... unsymmetrical alkene

  4. Unsymmetrical alkenes An unsymmetrical alkene is ... ...one which the groups or atoms attached to either end of the carbon-carbon double bond are different, eg propene or but-1-ene In but-1-ene there are a hydrogen and an ethyl group at one end, In propene there are a hydrogen and a methyl group at one end, but two hydrogen atoms at the other end of the double bond but two hydrogen atoms at the other end of the double bond

  5. 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...

  6. Reaction with hydrogen bromide H H H H H H → CC C CC C H H H H + → H H Br H H H H :Br- C = C 2-bromopropane → H CH3 δ+ H H H H H H δ- → H CC C C C C H H H H + Br H Br H :Br- H H bromopropane

  7. What is formed? H H H H H H → CC C CC C H H H H + → H H Br H H H H :Br- C = C 2-bromopropane → H CH3 δ+ H H H H H H δ- → H CC C C C C H H H H + Br H Br H :Br- H H 1-bromopropane

  8. What is formed? H H H H H H → CC C CC C H H H H + → H H Br H H H H :Br- C = C 2-bromopropane → H CH3 δ+ When a compound HX is added to an unsymmetrical alkene... δ- H ...the hydrogen becomes attached to the carbon with the most hydrogens attached to it already. Br

  9. What is formed? H H H H H H → CC C CC C H H H H + → H H Br H H H H :Br- C = C 2-bromopropane → H CH3 δ+ H H H H H H δ- → H CC C C C C H H H H + Br H Br H :Br- H H bromopropane

  10. Primary, secondary and tertiary carbocations H H H H H H → CC C CC C H H H H + → H H Br H H H H :Br- C = C 2-bromopropane → H CH3 δ+ H H H H H H δ- → H CC C C C C H H H H + Br H Br H :Br- H H bromopropane

  11. Why is one favoured over the other? H related to the stability of the intermediate formed... H H CC C H H + ...and so, to the stability of the carbocation formed... H H ...to the stability of the carbocation formed...Carbocations are classed as primary, 1°, secondary, 2° and terrtiary, 3 ° H H H C C C H H + H H

  12. Why is one favoured over the other? related to the stability of the intermediate formed... ...and so, to the stability of the carbocation formed... Carbocations are classed as... primary, 1° secondary, 2° tertiary, 3°

  13. + H H C H 1°, 2° and 3° carbocations - definitions Depends on the number of carbon atoms bonded to the carbon with the + charge H H3C + C H a primary carbocation is more stable than CH3+ Methyl cation Not 1°, 2° or 3°least stable

  14. 1°, 2° and 3° carbocations - definitions Depends on the number of carbon atoms bonded to the carbon with the + charge CH3 H3C CH3 H3C + + C C CH3 H a secondary carbocation is more stable than CH3CH2+ a tertiary carbocation is more stable than (CH3)2CH+

  15. 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...

  16. 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

  17. 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

  18. What is actually formed? H H H H H H hydrolysis  → CC C CC C H H H H + H H → H H H H :-OSO3H C = C OSO3H 2° carbocation → H CH3 H H H H H H hydrolysis →  CC C H H H C C C δ+ H H + H OSO3H H H δ- H :-OSO3H OSO3H 1° carbocation

  19. What is actually formed? MAJOR H H H + H2SO4 CC C H H Propan-2-ol H H → H H Via 2° carbocation C = C OH + H2O → H CH3 H H H MINOR CC C + H2SO4 H H H δ+ Propan-1-ol H OSO3H H δ- Via 1° carbocation OH

  20. The inductive effect Atkinson & Hibbert: Pgs. 202, 203 Questions: 1, 2, 3, 4

  21. The inductive effect The following slides attempt to explain why one carbocation is more stable than another. This is not part of the syllabus but some of you may find it useful.

  22. Stabilization of carbocations via the inductive effect Positively chargedcarbon pulls electrons in  bonds closer to itself +

  23.   Stabilization of carbocations via the inductive effect Positive charge is"dispersed ", i.e., sharedby carbon and thethree atoms attachedto it.  

  24.   Stabilization of carbocations via the inductive effect • Electronic effects transmitted through bonds are called "inductive effects." Electrons in C—Cbonds are more polarizable than thosein C—H bonds; therefore, alkyl groupsstabilize carbocationsbetter than H.  

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