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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

Organic synthesis of dibromoalkanes and bromoalcohols by reaction of bromine and bromine water with alkenes

Part 10.3 ALKENES - introduction to the reaction mechanisms of alkenes. Electrophilic addition reaction of bromine, electrophilic addition to alkenes with pure bromine or in non-polar solvent (non-aqueous Br2(l/solvent)) to give dibromoalkanes or electrophilic addition using bromine water [aqueous Br2(aq)] to give bromo-alcohols. These revision notes include full diagrams and explanation of the ionic electrophilic addition reaction mechanisms of alkenes and the 'molecular' equation and reaction conditions and other con-current reaction pathways and products are also explained.


10.3.3 The electrophilic addition of bromine to alkene (non-aqueous media)

  • 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.

  • Typical bromine addition reactions of unsaturated alkenes to give saturated dibromoalkanes
    • (i) ethene + bromine ==> 1,2-dibromoethane
    • colour of mixture changes from orange to colourless - the simple test for alkenes
    • doc b oil notes doc b oil notes doc b oil notesarrow doc b oil notes.... or
    •   doc b oil notes doc b oil notes doc b oil notes doc b oil notes doc b oil notes
    • (ii) propene + bromine ==> 1,2-dibromopropane
    • doc b oil notes doc b oil notes doc b oil notes doc b oil notes doc b oil notes.... or
    •   doc b oil notes doc b oil notes doc b oil notes doc b oil notes doc b oil notes
  • What is the reaction mechanism for the addition of bromine to an alkene?

  • Does the mechanism change if the solvent is changed?

  • Do the products of the reaction depend on the solvent used?

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

  • R2C=CR2 + Br2 ==> R2CBr-CBrR2 [see mechanism 4 below]

  • The alkene is mixed with bromine liquid or a solution of bromine in an organic (non-aqueous, non-polar) solvent.

organic reaction mechanisms

mechanism 4 - electrophilic addition of bromine to an alkene in non-aqueous media

  • In step (1) The non-polar bromine molecule is the electrophile, and becomes polarised on collision with the traces of water or ions on the reaction vessel surface*. The collision causes the bromine molecule to split heterolytically so that that the equivalent of a Br+ bonds to one of the double bond carbons to give a carbocation. The electrophilic Br+ accepts the pair of electrons from the C=C double bond to form the 1st new C-Br bond.

    • *Apparently completely dry bromine and alkene, and a paraffin coated reaction vessel, produce zero reaction!

  • In step (2) the bromide ion formed in step (1) rapidly combines with the carbocation to form the dibromoalkane, by donating a pair of electrons to make the new 2nd C-Br bond.

  • FURTHER COMMENTS

    • There is considerably evidence (beyond the academic scope of the page) to show that the 1st stage in the mechanism of bromine addition (non-aqueous or aqueous) actually goes via a triangular bromonium ion shown in mechanism 43 below. However many exam boards and older textbooks seem happy with the carbocation mechanism 4 shown above. Chlorine reacts similarly via a chloronium ion.

    • organic reaction mechanisms

    • The reaction mechanism is similar for non-aqueous chlorine.

    • The Markownikoff rule does NOT apply to this reaction, whatever the mechanistic details, because the reagent itself is symmetrical i.e. Br-Br, so different isomeric products are NOT expected. However the rule does apply when using aqueous bromine (see mechanism 5 below) or using a mixed halogen reagent (see next point)

    • Addition of mixed halogen compounds (inter-halogen compounds), such as iodine(I) chloride ICl, will also add to the alkene double bond. 

      • e.g. CH3CH=CH2 + ICl ==> CH3CHI-CH2Cl or CH3CHCl-CH2I 

      • From the Markownikoff rule 2-chloro-1-iodopropane should be the principal product because chlorine is more electronegative than iodine, so think of it as the addition of Iδ+-Clδ-.

    • There is considerably evidence (beyond the academic scope of the page) to show that the 1st stage in the mechanism of bromine addition (non-aqueous or aqueous) actually goes via a triangular bromonium ion shown in mechanism 43 below. However many exam boards and older textbooks seem happy with the carbocation mechanism 4 shown above. Chlorine reacts similarly via a chloronium ion.

    • organic reaction mechanisms

    • The reaction mechanism is similar for non-aqueous chlorine.

    • The Markownikoff rule does NOT apply to this reaction, whatever the mechanistic details, because the reagent itself is symmetrical i.e. Br-Br, so different isomeric products are NOT expected. However the rule does apply when using aqueous bromine (see mechanism 5 below) or using a mixed halogen reagent (see next point)

    • -


10.3.4 The electrophilic addition of bromine to alkene (aqueous media, bromine water)

  • R2C=CR2 + 2H2O + Br2 ==> R2C(OH)-CBrR2 + H3O+ + Br- [see mech 5 below]

    • If the alkene is not symmetrical about the C=C bond, isomeric products can be formed.

    • with the chance of a little R2CBr-CBrR2 too via the small concentration of bromide ion.

organic reaction mechanisms

mechanism 5 - electrophilic addition of bromine to an alkene in aqueous media

  • In step (1) The bromine molecule is the electrophile, and becomes polarised on collision with water. It splits heterolytically so that that the equivalent of Br+ bonds to one of the double bond carbons to give a carbocation. The electrophilic Br+ accepts the pair of electrons from the C=C double bond to form a new C-Br bond. So step (1) is the same for non-aqueous bromine, however step (2) is different!

  • In step (2), unlike with non-aqueous bromine, the much greater concentration of water, compared to the bromide ion, ensures the most probable addition to the carbocation is a water molecule. Water acts as an electron pair donor and on rapid combination with the carbocation, a protonated alcohol is formed.

  • In step (3) a 2nd water molecule then removes a proton to leave the bromo-alcohol product.

  • Evidence for ionic mechanism, if chloride ions present in the bromine water, bromo-chloro-alkanes are formed.

  • FURTHER COMMENTS

    • Evidence for ionic mechanism, if chloride ions present in the bromine water, bromo-chloro-alkanes are formed.

    • The triangular bromonium ion mechanism described above for non-aqueous bromine also applies here and the reaction is similar with chlorine water.

    • There is of course a small chance that a bromide will combine with the carbocation, so a little of the dibromoalkane is formed to.

    • The Markownikoff rule for a non-symmetrical alkenes does apply here, so the initially added 'Br+' will end up combined with the carbon atom of the double bond with the most hydrogen atoms and the H2O/OH ends up bonded to the C atom with the least number of H atoms e.g.

      • from propene CH3CH=CH2, the majority product is 1-bromopropan-2-ol, CH3CHOHCH2Br and the 

      • minority products are 2-bromopropan-1-ol, CH3CHBrCH2OH and 1,2-dibromopropane, CH3CHBrCH2Br 

    • The reaction is similar with chlorine water.

 


10.3.2 The electrophilic addition of hydrogen bromide to alkene now on separate page

10.3.5 The electrophilic addition of conc. sulphuric acid to alkene now on separate page

10.3.6 The acid catalysed electrophilic addition of water to alkene now on separate page

10.3.7 The free radical addition polymerisation of an alkene now on separate page


keywords phrases: electrophile mechanism steps reagents reaction conditions formula intermediates organic chemistry reaction mechanisms steps electrophilic addition of bromine water to alkenes ethene propene butene  R2C=CR2 + Br2 ==> R2CBr-CBrR2 CH3CH=CH2 + ICl ==> CH3CHI-CH2Cl or CH3CHCl-CH2I R2C=CR2 + 2H2O + Br2 ==> R2C(OH)-CBrR2 + H3O+ + Br- CH3CHOHCH2Br CH3CHBrCH2OH CH3CHBrCH2Br


APPENDIX -  COMPLETE MECHANISM and Organic Synthesis INDEX (so far!)


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