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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 - Electrophilic addition of bromine

Organic synthesis of dibromoalkanes and bromo-alcohols 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.

• 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.

• 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
  • .... or
  •  
  • (ii) propene + bromine ==> 1,2-dibromopropane
  • .... or
  •  

• 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.

Remember: An electrophile is an electron pair acceptor and will attack the pi electron rich bond of an alkene

• In step (1) The non-polar bromine molecule is the electrophile, and becomes polarised on collision with the pi electron cloud - an induced dipole effect, converting the non-polar bromine molecule into an electrophile.

  • 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 π electrons from the C=C double bond to form the 1st new C-Br bond.

  • Apparently with completely dry bromine and alkene, and a paraffin coated reaction vessel, produce zero reaction but with the traces of water or ions on the reaction vessel surface it goes rapidly! Can't explain this!

• 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.

• Chlorine reacts similarly via a chloronium ion i.e. 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 I^δ+-Cl^δ-.

• 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)

• -

• R₂C=CR₂ + 2H₂O + Br₂ ==> R₂C(OH)-CBrR₂ + H₃O⁺ + Br^- [see mechanism5 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.

• 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 π 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.

Mechanism diagram 43b shows the addition of bromine water via the bromonium ion, leading to the formation of a bromoalcohol - this is the more correct mechanism.

• FURTHER COMMENTS

  • Evidence for ionic mechanism:

    • If chloride ions present in the bromine water, bromo-chloro-alkanes are formed.

    • If any carbocation ion is present, then ANY negative ion can stand some chance of adding to it, giving products not necessarily present in the 'usual' reaction equation.

  • The triangular bromonium ion mechanism described above for non-aqueous bromine also applies here and the reaction is similar with chlorine water (see diagram 43b above).

  • 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 - just swap Cl for Br.