Alkenes - Reactivity (DP IB Chemistry: SL)

• Alkenes are hydrocarbons containing a carbon-carbon double bond
• The atoms around the carbon-carbon double bond adopt a planar arrangement and the bond angle is 120^o

The bonding arrangement around a carbon-carbon double bond

• The presence of the C=C bond gives alkenes a number of chemical properties that are not seen in alkanes
• Since the alkene contains π-bonds, it is possible to break the weaker π-bond and form stronger  σ-bonds with other species without forcing any atoms on the molecule to break off
• As a result alkenes (unlike alkanes) are capable of undergoing addition reactions

Addition reactions in alkenes

• Molecules which contain π-bonds and which can hence undergo addition are said to be unsaturated
• Molecules which do not contain π-bonds and which hence cannot undergo addition are said to be saturated.
• Alkenes are unsaturated and can hence undergo addition
• Addition is the combination of two or more molecules to form a single molecule
• Addition reactions are generally faster than substitution reactions since only weak π-bonds are broken, rather than stronger σ-bonds
• The ability of alkenes to undergo addition means that they are much more reactive than alkanes