IB Chemistry: Alkenes

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10.3 Alkenes

  • Alkenes contain the general formula CnH2n
  • Alkenes are unsaturated, and contain a double-double carbon bond.
  • In an exam, be VERY careful that you don’t get alkane and alkanes mixed up – See, I was trying to test you here ;). Alkanes are alkanes, alkenes are alkanes, make sure you read very carefully.

10.3.1 Describe, using equations, the reactions of alkenes with hydrogen and halogens.

Alkenes react with hydrogen in a process called hydrogenation. The result is an alkane, as shown below.

Conditions for this reaction:

    • Pressure
    • Temperature of around 200°C
    • Metallic catalyst (Usually platinum, nickel, or palladium)

Alkenes react with halogens to form something called a di-halogeno compound.  This process is called halogenation. 

Ethene + Bromine → 1,2-dibromoethane

10.3.2 Describe, using equations, the reactions of symmetrical alkenes with hydrogen halides and water.

Alkenes react with water in a process called hydration. This process converts an alkane into an alcohol.


    • Concentrated sulfuric acid (H2SO4), which acts as a catalyst.

Lastly, alkenes react with hydrogen halides (HCl, HBr)  to form halogenoalkanes.


  • The reaction takes place fastest at room temperature.

10.3.3 Distinguish between alkanes and alkenes using bromine water.

The most popular test is the bromine-water test. As alkenes go through addition reaction, adding bromine water to the suspected alkene will quickly help determine whether the substance is an alkane or alkene.

 If the solution is decolorized, then an alkene is present

10.3.4 Outline the polymerization of alkenes.

Alkenes can react with itself to form longer chains called polymers. This can be done n times, and we have to learn how to draw this out.

 Consider the example ethane reacting with ethane to form poly- ethane. When alkane reacts with the itself, to name the result, simply tag a poly- before the alkanes name. So if you polymerize butene, you get poly-butene.  This process is called polymerization. 

Although this can potentially repeat forever, we can generalize the chain lengths by repeating unit length of n. n  can be any value, such as 1, 2, 2000, 5 million.

 Additionally, as we see above, the double-bond bond in ethene must be broken to form a polymer.

 The optimized way to write the chain above is:

n basically suggests that the chain is very long, and it saves you a lot of time if you want to give a broad picture of what happens during a polymerization reaction without actually having to draw CH2 thousands of times.


Bromoethene to Poly-bromoethene

10.3.5 Outline the economic importance of the reactions of alkenes.

There are potentially three uses you can talk about:

    • Making margarine
    • Making plastics (polymers)
    • Making ethanol

Making margarine

If you hydrogenate (react with hydrogen) vegetable oil, you get margarine.

Making Plastics

Polymerization of ethene to form poly-ethene, or polymerization of propene to form poly-propene. These form plastics, which have multiple uses, such as making plastic bags, which allows mass distribution of shopping bags – however, this has a severe negative impact on the environment.

Making ethanol

React ethene with water to make ethanol, a type of fuel.


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