Chemical Properties of Alkane and Alkene

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Alkanes are less reactive chemically because they are
saturated hydrocarbons. The strong C―C and C―H bonds can
only be broken by a large amount of energy.

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They undergo two types of reactions:

Reaction

Combustion Substitution

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Alkanes burn completely in excess oxygen to form
carbon dioxide and water.

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1. Balance C
2. Balance H
3. Balance O (can be in fraction)

CH4 + 2 O2  CO2 + 2 H2O

C2H6 + 7 O2  2CO2 + 3 H2O or
2

2C2H6 + 7 O2  4 CO2 + 6 H2O

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• Alkanes undergo incomplete combustion when there is
not enough or limited supply of oxygen, O2.

• Incomplete combustion of alkanes produces carbon
particles, C (in the form of soot), carbon monoxide gas,
CO which is poisonous, and water, H2O.

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• Occurs when an alkane is mixed with a
halogen (Cl2 and Br2) in the presence of
sunlight or ultraviolet rays.

• In this reaction, each hydrogen atom in the
alkane molecule is replaced by the
halogen atoms (one at a time).

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• Alkenes are chemically more reactive than alkanes due to
the existence of a double covalent bond between two
carbon atoms, C.

• Almost all chemical reactions in alkenes occur at the double
bond.

Chemical reactions

Combustion Addition Addition
polymerisation
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Alkenes burn completely in excess oxygen, O2 to form carbon
dioxide, CO2 and water, H2O.
Example :

1. Balance C
2. Balance H
3. Balance O (can be in fraction)

C2H4 + 3 O2  2 CO2 + 2 H2O

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Incomplete combustion of alkenes produces carbon (in the
form of soot), carbon monoxide, CO and water, H2O.
Example :

C2H4 + 2O2  2CO + 2H2O

C2H4 + O2  2C + 2H2O

Presence of the black carbon solids
- Sooty

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• As alkenes are unsaturated hydrocarbons, they undergo
addition reactions.

HH + XY HH
CC CC
XY
(unsaturated)
(saturated)

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1. Addition of Hydrogen (Hydrogenation)

• Alkenes react with hydrogen at a temperature of 180 °C in the presence
of nickel /platinum as a catalyst to produce the corresponding alkanes.

HH HH

HC C Ni H C C H

H + H2 180C

HH

ethene ethane

→C2H4 + H2 C2H6

Hydrogenation is used to prepare an alkane from an alkene in industry

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2. Addition of Halogen (Halogenation)

• Alkenes react with halogens such as chlorine, Cl2 and bromine, Br2 at

room conditions.

HH HH
HC C
H + Br2 H 1 2C H

C

ethene + Br2 → Br Br

C2H4 1,2‒ dibromoethane

C2H4Br2

Product name: 1,2‒dibromoethane
Observation: The brown (or orange) colour of

bromine water decolourises.

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3. Addition of Hydrogen Halide

• Alkenes react with hydrogen halides, such as hydrogen chloride, HCl or

hydrogen bromide, HBr at room temperature to form haloalkane.

HH HH

H C C H + H Cl H CC H

ethene H Cl

C2H4 + HCl  Chloroethane

C2H5Cl

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4. Addition of Water (Hydration)

• Alkenes react with water (in the form of steam) at high temperature and

pressure, in the presence of phosphoric acid, H3PO4 as a catalyst to
produce alcohol.

HH HH

300C, 60 atm,

H C C H + H2O H3PO4 HCC H

H OH

Ethene 300C, 60 atm, Ethanol
C2H4 + H2O H3PO4 C2H5OH

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5. Oxidation with Acidified Potassium Manganate(VII), KMnO4 Solution
• Alkenes react with acidified potassium manganate(VII), KMnO4 solution.
• In this reaction, two hydroxyl groups OH are added to the double bond.

HH HH H
H C C H + H2O + [O]
12

H CC

Ethene Acidified potassium OH OH
manganate (VII)
Ethane-1,2-diol
C2H4 + H2O + [O] 
C2H4(OH)2

[O] and not O2 is written Product name: Ethane-1,2-diol
Observation: The purple colour of acidified potassium

manganate(VII) solution is decolourised.

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• Small alkene molecules undergo addition reaction with one
another to form long chain molecules.

• Alkene molecules link together to form a long chain of
molecules called polymer, while the smaller alkene
molecules are the basic units called monomers.

• The reaction of alkene monomers to form polymers is called
addition polymerisation.

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Ethene, C2H4 undergoes addition polymerisation at
200 °C and pressure of 1200 atm to produce
polythene.

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