Alkenes

This is a recap of our Year 12 work on alkenes.

We need to be able to:

• identify the functional group
• apply the general formula to predict the number of hydrogen atoms
• draw structural isomers
• name structural isomers

GEOMETRIC ISOMERISM

Because the C=C double bond cannot freely rotate in space, alkenes have a special type of isomerism, called geometric isomerism (or cis/trans isomerism).

DRAWING ALKENES

When given the name of an alkene, we need to be able to draw them. For this assessment, it is useful to draw them around the C=C double bond, as this is where polymerisation occurs. This video goes through how you draw an alkene from its name.

1. It has "ene" in the name, so draw a C=C double bond, and four bonds (2 off each carbon atom)
2. Use the number in the alkene part of name to number the far-left carbon atom
3. Use the prefix (but-, pent-, hex- etc.) to build the rest of the carbon chain. Number every carbon.
4. If it says cis, make sure the carbon chain is all on the same side of the C=C bond (above it or below it on the page. If it says trans, make sure the carbon chain goes across the C=C bond, like in the picture and video.
5. Add any side chains to the appropriate carbon(s) - you had already numbered the carbon atoms so this should be easy :)
6. Fill in all of the hydrogen atoms to the vacant bonds.