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Alkanes
Copyright© 2006, MRSM Kuantan. All rights reserved.
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1: Classification of Hydrocarbons1.
Alkanes are saturated hydrocarbons:
CH HH
HC C CH HH H
H
H
HH
saturated (only single C-C bonds) hydrocarbon (only C and H)
Copyright© 2006, MRSM Kuantan. All rights reserved.
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Carbon and hydrogen atoms can bond to each other to form very extensive and molecular systems.
2: Bondings in hydrocarbon
Carbon atoms has 4 valence electrons need 4 electrons to reach octet electron arrangement. four bonds to each carbon atom
Hydrogen atoms has 1 valence electron need 1 electron to reach duplet electron arrangement.
one bond to each hydrogen atom
Copyright© 2006, MRSM Kuantan. All rights reserved.
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3: Molecular Formulas of Alkanes
Write the molecular formula for each of the alkanes below.
C CH HH
H
H
HC C C CH HH
H
H
H
H
H
H
H
C2H6 C4H10
Copyright© 2006, MRSM Kuantan. All rights reserved.
5
C5H12
C6H14
Copyright© 2006, MRSM Kuantan. All rights reserved.
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What is the general molecular formula for alkanes?
Alkanes have the general formula
CnH2n+2
n = no.of carbon atoms in the molecule
Copyright© 2006, MRSM Kuantan. All rights reserved.
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4: Nomenclature of Alkanes
Copyright© 2006, MRSM Kuantan. All rights reserved.
1. The name of alkanes consist of 2 parts : the root & ending.
2. Root- tells the number of carbon atoms in the longest continuous carbon chain.
3. Ending-tells the family of the compound.No. of
carbon
atoms
1 2 3 4 5 6 7 8 9 10
Root name
Meth
Eth Prop
But Pent
Hex Hept
Oct Non Dec
The root names for the first ten straight-chain alkanes
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Naming Alkanes• Example : Butane
Name of alkane
Root Ending
Butane But (no. of carbon atoms = 4)
-ane (the family is alkane)
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Methane CH4
Ethane C2H6
Propane C3H8
CH H
H
H
C CH H
H
H
H
H
C C CH H
H H
H
H
HH
Nomenclature / Molecular formula
No. of C
atoms
Structural Formula
3 C
2 C
1 C
Copyright© 2006, MRSM Kuantan. All rights reserved.
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Butane C4H10
C C C CH H
H
H
H
H
H
H
H
H
Nomenclature / Molecular formula
No. of C
atoms
Structural Formula
Hexane C6H14
4 C
6 C
Copyright© 2006, MRSM Kuantan. All rights reserved.
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Structural FormulaShows how the atoms in
molecule are bonded together and by what types
of bonds.C C C CH H
H
H
H
H
H
H
H
H
Example :
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* Low melting and boiling points because these molecules are held together by weak intermolecular forces.
Copyright© 2006, MRSM Kuantan. All rights reserved.
a) Melting and Boiling Point
5: Physical Properties5: Physical Properties
Alkanes- covalent compounds
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Copyright© 2006, MRSM Kuantan. All rights reserved.
a) Melting and Boiling Point
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• The boiling point increases between 20-30 °C per -CH2 group.
• Boiling points and melting points increase as number of carbon atoms per molecule increases.
• Then, size of molecules increases (molecular mass increases)
• Forces between molecules (van der Waals forces) are stronger
• More heat energy is needed to overcome the forces
a) Melting and Boiling Point
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b) Physical State at Room Temperature
CH4 - C4H10 : gasC5H12 - C17H38 : liquid C18H38 : solid
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• -Soluble in non polar solvents (organic solvents),
• -Insoluble in water.
* The density of the alkanes increases with increasing number of carbon atoms, BUT is less dense than that of water. Hence, alkanes form the upper layer in an alkane-water mixture.
Copyright© 2006, MRSM Kuantan. All rights reserved.
c) Solubility
d) Density
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• Cannot conduct electricity • because : alkanes are covalent
compounds, do not have ions but are made up of molecules.
e) Electrical Conductivity
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Chemical Properties of alkanes
• Unreactive compounds • Saturated compounds.• The strong C-C and C-H bonds
must be broken for the alkanes to react.
• Although alkanes do not react with most chemicals, they do react with oxygen and halogens.
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Combustion• The reaction with oxygen• Complete combustion of methane / in
the presence of sufficient oxygen :CH4(g) + 2O2(g) CO2(g) + 2H2O(l)
• Incomplete combustion / insufficient oxygen available :2CH4(g) + 3O2(g) CO(g) + 4H2O(l)CH4(g) + O2(g) C(s) + 2H2O(l)
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Halogenation• Reaction with halogen • Takes place readily in sunlight.• C-H bonds-broken, formed C-halogens
bonds• Halogenation - example of a
substitution reaction• Substitution reaction – when one atom
or a group of atoms in a molecule is replaced by another atom or group of atoms.
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Halogenation• Example-Mixture of methane
and chlorine is exposed to ultravolet light-varying amounts of four different products are formed.
• CH4 CH3Cl chloromethane CH2Cl2 dichloromethane CHCl3 trichloromethane
CCl4 tetrachloromethaneCH4(g) + Cl2(g) CH3Cl(g) + HCl(g)
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Copyright© 2006, MRSM Kuantan. All rights reserved.