Seneca
Sign up

Alkenes

Applied Science: BTEC Level 3 National Extended Diploma, topic 4.3.7

Alkenes

Alkenes are hydrocarbons with the functional group C=C. This means that alkenes have a carbon-carbon double bond.

Unsaturated

Unsaturated

  • Alkenes are unsaturated hydrocarbons because they contain a double bond between two of their carbon atoms (C=C).
  • This means that an alkene contains two fewer hydrogen atoms than the alkane with the same number of carbon atoms.
Formula

Formula

  • A homologous series is a series of compounds with the same general formula.
  • CnH2n is the general formula for alkenes.
Reactive

Reactive

  • Alkenes are more reactive than alkanes.
  • This is because the double bond is of high electron density.

The Structure of Alkenes

Alkenes are unsaturated hydrocarbons that contain at least one C=C double bond.

The CC bond

The C=C bond

  • The C=C bond is made up of one sigma (σ) bond and one pi (π) bond.
  • The π bond involves electrons in p-orbitals existing above and below the plane of the bond. This means that there is no overlapping (like with the σ bond).
  • The π bond causes the C=C bond to be unable to rotate.
  • This gives alkenes a fixed shaped around the C=C bond.
Bond angles around the CC bond

Bond angles around the C=C bond

  • The carbon atoms at either end of the double bond each have three regions of negative charge around them.
  • The electrons in these bonding regions repel each other to be equally far apart.
  • The resulting arrangement is the bonds existing 120o apart from each other but all in the same plane.
  • This gives the effect of an equilateral triangle about each carbon atom.

The Four Shortest Alkenes

The four shortest alkenes, in ascending (increasing) size order, are:

Ethene

Ethene

  • Ethene has the formula C2H4.
Propene

Propene

  • Propene has the formula C3H6.
Butene

Butene

  • Butene has the formula C4H8.
Pentene

Pentene

  • Pentene has the formula C5H10.

Test yourself on Alkenes

  • Which of the following are properties of alkenes?

  • Why are alkenes more reactive than alkanes?

  • Why are alkenes more reactive than alkanes?

  • Which bonds make up a C=C bond?

  • Which of the following is true of the structure of alkenes?

Practise this topic for free