Alkenes are hydrocarbons containing at least one carbon-carbon double bond. They have lower melting and boiling points than alkanes due to weaker intermolecular forces. The number of carbons determines an alkene's name and formula. Alkenes undergo addition reactions, combustion reactions, polymerization reactions, and can be used to test for double bonds. They differ from alkanes in bonding, reactivity and ability to cause soot during combustion. Isomers are compounds with the same molecular formula but different structural formulas, resulting in different physical but same chemical properties.

1.  is a hydrocarbons containing at least one C-C
double bond
General Formula :
Functional Group:
Naming of Alkenes:
CnH2n
Number of
Carbon
2 3 4 5 6 7
Formula C2H4 C3H6 C4H8 C5H10 C6H12 C7H14
Name Ethene Propene Butene Pentene Hexene Heptene
Double Bond
, n=2,3,4…
Alkenes

2. Structural Formula of Alkenes
Ethene: C2H4
Propene : C3H6

3. Physical properties of alkenes are
similar to alkanes
Physical
Properties
of Alkenes
Solubility
- dissolve in organic
solvents
- insoluble in water
Melting and
boiling point
- low melting and
boiling point
Density
- less dense than water
Electrical conductivity
- cannot conduct electricity
- because there are no free
moving ions

4. Explain the effect of the increase in
number of carbon atoms in alkene
molecules
Size of molecule increase
Melting point & boiling point
increase
• The higher the number of carbon
atoms, the higher the melting &
boiling point

5. a. Combustion reaction
1. Complete combustion:
produce CO2 + H2O
2. Incomplete combustion:
produce CO/C gas + H2O
C2H4 + O2 → CO2 + 2H2O
C2H4 + 2O2 → 2CO + 2H2O
C2H4 + O2 → 2C + 2H2O
Chemical Properties of Alkenes

6. b. Addition reaction
1. Hydrogenation
 Alkenes react with hydrogen at 180 °C at presence
of nickel/platinum (catalyst) to produce alkanes

7. 2. Hydration
 Alkenes reacts with steam, H2O at 300 °C and 60 atm
in the presence of concentrated H3PO4 (as catalyst)
to produce alcohol

8. 3. Addition of hydrogen halides – HX
 Hydrogen halides: Hydrogen chloride, HCl
hydrogen bromide, HBr or hydrogen iodide, HI
 Alkenes reacts with hydrogen halide, HX at room
temperature to produce haloalkane.
C2H4 + HCl → C2H5Cl
Ethene react with hydrogen chloride
produces chloroethane.
Ethene react with hydrogen bromide
produces bromoethane.
Ethene react with hydrogen
iodide produces iodoethane.

9. 4. Addition of hydroxyl group
 Alkenes react with acidified potassium manganate(VII),
KMnO4 to produce diol compound
C2H4 + [O] + H2O → C2H4 (OH)2
or
 Used to test for the presence of a C-C double bond

10. 5. Halogenation
 No catalyst or ultraviolet is needed
 Alkenes react with halogen at room temperature
in the presence of tetrachloromethane, CCl4
C2H4 + Cl2 → C2H4Cl2
C4H8 + Br2 → C4H8Br2
 Used to test for the presence of a C-C double bond

11. c. Polymerization reaction
 Small alkene molecules undergo an
addition reaction with one another at
high pressure of 1000 atm and
temperature 200 °C

12. Alkanes Differences Alkenes
CnH2n+2 General formula CnH2n
Saturated Types of hydrocarbon Unsaturated
Single bond Type of bonding Double bond
Low Soot (unburnt carbon) High
Substitution Type of reaction
1. Addition
2. Polymerization
No reaction Bromine water
The brown colour
decolourized
No reaction
Acidified potassium
manganite (VII) solution
The purple colour
decolourized
Comparing Alkanes with Alkenes

13.  Isomers are compounds with the same molecular
formula but different structural formula.
 Isomerism is the existence of two or more compounds
that have the same molecular formula but different
structural formulae.
 Isomers have different physical properties because they
have different molecular structures.
 However, isomers have the same chemical properties
because they belong to the same homologous series.
Isomerism

14. Examples of two isomers of butane
The 2 molecules have same molecular formula
but different structural formulae