Addition Reactions of Alkenes (OCR AS Chemistry A)

Reactivity of Alkenes

• Alkenes are more reactive than alkanes

  • This is due to the presence of the C=C bond or more specifically the π bond

The σ bond between the two carbon atoms is in the centre of the C=C bond with the π bond concentrated above and below the plane of the σ bond, therefore, exposing the π electrons

• A C-C single bond has a bond enthalpy value of 347 kJ mol^-1 

  • This is equivalent to the σ bond portion of a C=C bond 

• A C=C double bond has a bond enthalpy value of 612 kJ mol^-1 

  • This value represents the bond enthalpy of the σ and π bond

• Using these values, we can estimate the value of the π bond alone

  • 612 - 347 = 265 kJ mol^-1 

• This calculation shows that the π bond of the C=C double bond is weaker than a C-C single bond an explains why alkenes are more reactive

  • The π bond of the C=C bond requires less energy to break than a C-C single bond and consequently reacts more readily

Addition Reactions of Alkenes

• Alkenes are very useful compounds as they can undergo many types of reactions

• They can therefore be used as starting molecules when making new compounds

Hydrogenation

• The reaction between an alkene and hydrogen is known as hydrogenation or reduction

• As well as a nickel catalyst, this requires a temperature of 200 °C and a pressure of 1000 kPa

Catalytic hydrogenation converts alkenes into useful alkanes

• One important application of this reaction is in the production of margarine from vegetable oils

• Vegetable oils are unsaturated and may be hydrogenated to make margarine, which has a higher melting point due to stronger London Dispersion Forces

• By controlling the conditions it is possible to restrict how many of the C=C bonds are broken and produce partially hydrogenated vegetable oils which have which have the desired properties and textures for margarine manufacture

Halogenation

• The reaction between alkenes and halogens is known as halogenation

• It is an example of an electrophilic addition where an electrophile ('electron seeker') joins onto to a double bond

• The C=C double bond is broken, and a new single bond is formed from each of the two carbon atoms

• The result of this reaction is a dihaloalkane

• The reaction occurs readily at room temperature and is the basis for the test for unsaturation in molecules

Halogenation in alkenes

• Halogens can be used to test if a molecule is unsaturated (i.e. contain a double bond)

• Br₂ is an orange or yellow solution, called bromine water

• The unknown compound is shaken with the bromine water

• If the compound is unsaturated, an addition reaction will take place and the coloured solution will decolourise

The bromine water test is the standard test for unsaturation in alkenes

Hydrohalogenation

• Alkenes will react readily with hydrogen halides such as HCl and HBr to produce halogenoalkanes

• This reaction is known as hydrohalogenation 

• It is also an electrophilic addition reaction that occurs quickly at room temperature

Hydrohalogenation reactions in alkenes

• All the hydrogen halides react in this way, but the fastest reaction occurs in the order HI > HBr > HCl due to the increasing bond strength of the hydrogen-halogen bond, so the weakest bond reacts most easily

Hydration

• When alkenes are treated with steam at 300 ^oC, a pressure of 60 atmospheres and sulfuric acid (H₂SO₄) or phosphoric acid (H₃PO₄) catalyst, the water is added across the double bond in a reaction known as hydration

• An alkene is converted into an alcohol

• The reaction processes via an intermediate in which H⁺ and HSO₄^-  ions are added across the double bond

• The intermediate is quickly hydrolysed by water, reforming the sulfuric acid

Hydration in Alkenes

• This is a very important industrial reaction for producing large quantities of ethanol, a widely used solvent and fuel

• The process is much faster and higher yielding that producing ethanol by fermentation