alkenes and bromine n.
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Alkenes and bromine. Starter. Attempt the reaction mechanism sheet. Learning outcomes. • Describe how heterolytic fission leads to electrophilic addition in alkenes, using bromine as an example. • Recall the reactions of alkenes. Addition of the hydrogen halides.

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  • Attempt the reaction mechanism sheet
learning outcomes
Learning outcomes

• Describe how heterolytic fission leads to electrophilic addition in alkenes, using bromine as an example.

• Recall the reactions of alkenes.

addition of the hydrogen halides
Addition of the hydrogen halides
  • Write a brief sep-by-step method for the addition reaction of the hydrogen halides.
  • Remember to use key terms like...
    • Electronegativity
    • Dipole
    • πbond
    • Double bond
    • Heterolytic fission
addition reaction of bromine
Addition reaction of bromine
  • Bromine is a non-polar molecule and yet it is used as a test for unsaturation because it reacts with alkenes.
  • Using what you know about alkenes, bromine and electrons, see if you can come up with a reaction mechanism.
  • Bromine is non-polar. Is there a way that a dipole could be induced in the bromine molecule?
  • Why will the double bond break?
  • What is the first bond to form? What are the two unstable products?

Bromine is non-polar but when it approaches a double bond, the electrons in the π-bond repel the electrons in the Br-Br bond. This inducesa dipole in the Br2 molecule.

  • The Br2 molecule is now slightly polar.
  • The electron pair in the π-bond is now attracted to the slightly positive bromine atom, causing the double bond to break.
  • A new bond forms between a carbon atom and the bromine atom
  • The Br-Br bond breaks by heterolytic fission, with the electron pair going to the bromine
  • A bromide ion and a carbocation are formed

or you could draw this...


Finally, because the positively charged carbocation is unstable, it quickly reacts with the bromide ion to form the organic product 1,2-dibromoethane

  • In this mechanism, the bonds are broken by heterolytic fission and ions are formed as the intermediates
curly arrows
Curly arrows
  • You may be asked to define what the curly arrows signify...
  • Make sure you can draw the reaction of HBr and Br2 with ethene...
  • So try it! Complete the exam questions on your sheet!
peer marking
Peer marking
  • Swap your answers with someone else!

1. (a)

curly 1

dipoles shown correctly on the Br–Br and curly arrow from the Br–Br 1

bond towards the Brδ

correct intermediate shown 1

curly arrow from the lone pair or the negative charge on the Br - to the C+


(b) (i) Hs are diagonal to each other in the trans/ 1

difference clearly shown in a diagram

(ii) (the product is saturated hence) there is no restricted rotation/single 1

bonds allow rotation/because C=C prevents rotation


but i ve got more than one double bond
But... I’ve got more than one double bond!
  • Myrcene (C10H16) is a member of the terpene family of chemicals. These are alkenes built from whole numbers of isoprene (C5H6) molecules.
  • These isoprene molecules can be linked together head to tail forming chains or can be formed into rings.

Tarpenes contain carbon-carbon double bonds.

  • If reacted with excess hydrogen in the presence of a nickel catalyst, all the double bonds will react and form a saturated molecule
flow chart
Flow chart
  • Complete the revision flow chart for the reactions of the alkenes