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)
