C11.5 Alkenes

What is an alkene?

• Unsaturated hydrocarbons

  • They contain at least one double carbon bond C=C

  • This bond can open up and form new bonds, which makes them more reactive than alkanes

How to distinguish alkane from alkene using a test?

• Aqueous bromine test helps check for presence of a C=C bond

• Few drops of aqueous bromine are added to the samples and then shaken

• Results:

  • Alkane + bromine → no colour change, solution remains orange

  • Alkene + bromine → colour does change into a colourless solution, double bond reacts with bromine

What is cracking?

• Thermal decomposition of long-chain alkenes to turn them into:

  • Shorter alkanes

  • One or more alkenes

  • Hydrogen

What are the conditions needed for cracking to occur?

• High temperature (between 600 and 700 celsius typically)

• Catalyst (aluminium/sulfur dioxide typically)

State the process of cracking

• Long-chain alkanes are heated until they vapourise

• Vapours pass over the catalyst which breaks the C-C bonds

• Large molecules randomly split up into mixture of alkanes, alkenes and hydrogen

What happens during an addition reaction?

• Occurs when small molecule adds across double carbon covalent bond

• C=C bonds broken

• New single bonds formed

• Only one product formed

What are examples of addition reactions?

• Bromine: aqueous bromine test

• Hydrogen: reaction of alkene + hydrogen makes an alkane (with use of catalyst)

• Steam: reaction of alkene + steam makes an alcohol (with use of acid catalyst) — also known as hydration reaction