Alkenes

  • Alkenes represent hydrocarbons with carbon-carbon double bonds. It’s essential to remember the general formula for alkenes (CnH2n).

  • The double bond in alkenes is able to undergo addition reactions. This is because the bond includes a sigma (σ) bond and a pi (π) bond. The pi bond is relatively weak, making it easier to break and leading to the reactivity of alkenes.

  • Alkenes exhibit a property known as geometrical isomerism. This caused by the restricted rotation around the double bond. You’ll find cis and trans isomers in alkenes with two different atoms or groups on each carbon atom of the double bond.

  • Naming alkenes involves identifying the longest carbon chain that contains the double bond and labelling it with the ending ‘-ene’. If the molecule contains more than one double bond, use the terms ‘diene’, ‘triene’, etc.

  • Adding bromine water to an alkene results in the loss of the original brown colour because dibromoalkanes are formed. This is an example of an electrophilic addition reaction. This test can be used to distinguish alkenes from alkanes as it does not change colour when added to alkanes.

  • Alkenes can be polymers in a process known as polymerisation. Through this process, many small molecules (monomers) join together to form a long chain molecule, the polymer.

  • Another common reaction of alkenes is hydrogenation in the presence of a catalyst. This reaction results in the production of alkanes.

  • Markovnikov’s Rule can be applied to the addition of water to unsymmetrical alkenes. According to this rule, the hydrogen atom from the water molecule will join the carbon atom with the most hydrogen atoms already attached to it, forming the major product.

  • Cracking is a process used in the industry to break down larger alkanes into smaller, more useful alkanes and alkenes. This process gives us usable fuels and essential chemicals for making polymers and other products. Remember, the conditions for catalytic cracking are a zeolite catalyst and a temperature of about 450°C.

  • Environmental concerns related to alkenes include the production and disposal of plastic produced from polymerized alkenes and the emissions produced when alkenes are burned as fuels.