11.02.2 Reactions of Alkenes

Syllabus

Alkenes are Hydrocarbons with the functional group C=C.
It is the generality of reactions of functional groups that determine the reactions of organic compounds.
Alkenes react with Oxygen in combustion reactions in the same way as other Hydrocarbons, but they tend to burn in air with smoky flames because of incomplete combustion.
Alkenes react with Hydrogen, water and the halogens, by the addition of atoms across the Carbon -Carbon double bond so that the double bond becomes a single Carbon-Carbon bond.
Students should be able to:

describe the reactions and conditions for the addition of Hydrogen, water and Halogens to alkenes
draw fully displayed structural formulae of the first four members of the Alkenes and the products of their addition reactions with Hydrogen, water, Chlorine, Bromine and Iodine.

What does this mean?

Combustion

As with all Hydrocarbons, complete combustion (burning) will make Carbon Dioxide and Oxygen.

You should be able to write a balanced symbol equation for the first four Alkenes.

Ethene: C₂H₄ + 3 O₂ --> 2 CO₂ + 2 H₂O

Propene: C₃H₆ + 4.5 O₂ --> 3 CO₂ + 3 H₂O

Butene: C₄H₈ + 6 O₂ --> 4 CO₂ + 4 H₂O

Pentene: C₅H₁₀ + 7.5 O₂ --> 5 CO₂ + 5 H₂O

However, Alkenes tend not to burn completely - and incomplete combustion is indicated by smoky flames.

It also suggests that toxic Carbon Monoxide is being made.

Addition reaction 1- Alkenes & Halogens

Alkenes have "space where more atoms could be added" to their molecules.

So they undergo addition reactions - ones where they gain atoms.

They do this very quickly with Halogens like Bromine and Iodine.

The syllabus suggests that we need to be able to draw displayed formulas of the first 4 alkenes and the product of their reaction with Bromine, Iodine and Chlorine are.

We should know that the reaction is fast and doesn't require heating.

We should also know that it takes away the orange colour (in the case of Bromine).

Alkanes can't do this so the decolourising of Bromine is a test to show that a molecule is an Alkene (contains a C=C bond).

The same reaction happens with Iodine, where it is the brown colour of Iodine solution or purple colour of Iodine gas that is removed.

And Chlorine, which is a yellow-green gas or pale yellow solution.