The Reaction of Alkenes - ppt slide presentation

Organic Chemistry
Part 4 – Chemical Reaction of The Alkenes

Chemical Reaction of the Alkenes

• Alkenes are much more reactive than alkanes. Under suitable condition, olecules such as bromine, hydrogen and water will add across the C=C double bond.

Bromination (addition of bromine)

• Alkenes decolorise bromine in the dark  at room temperarure.

• When ethene is bubbled through aqueous bromine solution becomes colorless.

• H2C=CH2 (g) + Br2 (aq) 🡪 H2C-CH2

        |  |

       Br  Br

Reddish brown to colorless

Hydrogenation (addition of hydrogen)

• Addition of hydrogen to an alkene changes it into an alkane

• Hydrogenation converts an unsaturated compound into a saturated compound.

• Ethene reacts with hydrogen in the presence of a nickel catalyst to form ethane.

• H2C=CH2(g) + H2(g) ----> H3C-CH3

        Catalyst: Ni

• Unsaturated vegetable oils contain a number of carbon-carbon double bonds. It is an example of what is classified as polyunsaturated food.

• Unsaturated vegetable oils can be partially saturated by hydrogenation to produce a soft solid known as margarine.

• Hydrogenation increases the molecular mass of the vegetable oils. This increases the strength of the intermolecular attractions and hence increases its melting point. Liquid vegetable oil changes into a solid.

Addition of water

• Alkenes do not react with cold water

• Alkenes react with steam under high pressure and temperature and with the help of an acid catalyst to form alcohol.

• Ethene reacts with steam to form ethanol.

• H2C=CH2(g) + H2O(g) 🡪 H3C-CH2

          |

           OH

Combustion reaction of alkenes

• All hydrocarbons burn in oxygen to form carbon dioxide and water

• Complete combustion of alkenes produce carbon dioxide and water

C2H4(g) + 3O2(g) 🡪 2CO2(g) + 2H2O(l)

• Incomplete combustion produces carbon monoxide and soot

C2H4(g) + 2O2(g) 🡪 2CO(g) + 2H2O(l)

C2H4(g) + O2(g) 🡪 2C(s) + 2H2O(l)