Alkenes are more reactive than alkanes.  Electrons in π bonds are not as strongly attracted to the nuclei as the electrons in the σ bond.  This makes the π bond weaker.

✍️    HL – What is the hybridisation of carbon in this molecule?  If you can’t answer that question, you need to revise this section from bonding.

A model of ethene showing the electron distribution

Distinguishing between alkanes and alkenes

✍️   Review this summary of alkenes and add your own summary to your notes.  Include an example of the reaction that occurs when adding bromine water to an alkene.

✍️  What type of reaction is occurring in the alkene test tube?

✍️   If left overnight, the test tube containing the alkane would also decolourise.  Write a set of equations to explain this and identify the type of reaction occurring.

Addition reactions

Alkenes can react with:

  • halogens eg F2, Cl2, Br2 and I2
  • hydrogen halides eg HF, HCl, HBr and HI and
  • water H2O in the presences of a sulfuric acid catalyst
  • hydrogen in the presences of a Ni catalyst

✍️  Write the equation for the reaction of but-2-ene with each of the following reactants above.  Use structural formulae in the equations and name the product each time.

✍️  What classes of compounds can be made from addition reactions of alkenes?

Addition Polymerisation

View the video found on this page.

Review pages 3 and 4 of this summary and write your own notes.  Make sure you view both the animations.

Key terms to remember here are monomer and polymer.  Make sure you can define both and know how one relates to the other!

Try the polymer puzzles found here.

Alkenes are useful compounds.  You can read about them here.

This concludes the material for standard level.  You should now read the section in your text book (10.2) which is relevant to alkenes and addition polymerisation and add any thing else you find important to your notes.

Practice problems   and    solutions

Electrophilic Addition Reactions – HL only

Study the following image of a general mechanism for electrophilic addition reactions.  What do you think it is showing you?  Think about the following:

  • What do you think the curly arrows are representing?
  • What does r.d.s stand for?
  • Why is the second step faster than the first step?

The above diagram is the general mechanism for electrophilic addition of any halogen (X2), halogen halide (HX) or interhalogen (eg I-Cl or iodine monochloride) with any alkene.

✍️  Draw the mechanism for the bromination of ethene.

Remembering that an electrophile is an electron deficient species, how is Br2 considered an electrophile in this mechanism?

Below is the reaction between propene and HBr.

As you can see, there are two possible products.  One is more likely than the other.

✍️  Draw the mechanism to create both the products.

The Markovnikov rule explains why 2-bromopropane is the major product.  Simply put the Markovnikov rule is…

“The hydrogen rich get richer!”

✍️  Use your textbook to read about this rule and write a summary to explain why the major product is 2-bromopropane and not 1-bromopropane in terms of the stability of the carbocation.

Practice problems  and  solutions

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