S5/CHEMNOTE/ALKANE
P.1
The Alkane - a homologous series
I). Introduction to Organic Chemistry
 Organic chemistry is a study of carbon chemistry.
 Organic compound is applied to all carbon-containing compounds and usually comes from petroleum.
However, the oxides of carbon and carbonates are described as inorganic compounds.
 Carbon - a Unique element
Carbon combines with many other elements, especially hydrogen and oxygen to form millions of
compounds because carbon have the following properties:
1. It tends to share the four electrons with other elements to form four covalent bonds. The four
covalent bonds are actually arranged in a tetrahedral shape.
C
2. Carbon can form stable straight chain, branched chain and ring structure.
Straight chain
branched chain
ring chain
3. Carbon can form strong single, double and triple bonds with another carbon atom
single bond
double bond
triple bond
A). Representing Molecular Structure of Organic Compounds
Molecular formula - formula just showing the number of different atoms in the molecule.
Structural formula - formula showing how the atoms are joined to one another in the molecule.
Molecular
C4H10
C4H10
formula
Structural
formula
Name
Molecular
formula
Structural
formula
Name
butane
2-methylpropane
C3H6Cl2
C3H6Cl2
C3H6Cl2
1,1-dichloropropane
1,2-dichloropropane
2,2-dichloropropane
S5/CHEMNOTE/ALKANE
P.2
Condensed Structural Formula - structural formula written in a condensed form
Structural formula
H
H
H
H
H
C
C
H
H
H
H
H
H
C
C
H
H
H
C
Condensed structural formula
H
H
C
C
H
H
H
H
C
C
H
H
H
C
H
H
H
H
C
C
H
H
C
C
H
H
H
H
H
H
H
H
C
C
H
H
H
H
H
H
O
C
C
H
H
H
C
C
O
H
H
C
H
H
H
H
C
C
C
H
H
H
H
H
H
H
O
C
C
C
H
H
O
H
B). Molecular models
 The structure of organic compounds are actually three-dimensional.
 The structural formulae provide only two-dimensional representations
 The actual molecular shapes can be represented by
1. ball - and - stick model
2. space-filling model
C). Hydrocarbons
 Organic compounds that contain only carbon and hydrogen atoms are called ___________
 Hydrocarbons with carbon atoms forming only carbon-carbon single bond (i.e. CC) are said to
H
H
H
C
C
H
C
H
H
C
C
H
__________
 Hydrocarbons containing one or more carbon-carbon multiple bonds (that is CC or CC) are said to be
H
H
H
H
H
H
C
C
H
_____________.
 Do Classwork in P.22
H
H
H
C
H
C
H
H
S5/CHEMNOTE/ALKANE
P.3
II). The Alkanes
A). Introduction
Petroleum and natural gas are mainly mixtures of a type of hydrocarbon called alkanes.
Name
methane
Molecular formula
Structural formula
ethane
propane
butane
pentane
Can you construct a general molecular formula of the alkane molecules?
B). Homologous Series
Alkanes can be represented by the general formula ___________.
The Alkanes is a group of organic compounds called a homologous series.
Members in homologous series have the following properties.
1. with the same general formula
2. successive members differing by a _________ unit
3. show a graduation in physical properties.
4. show similarity in chemical properties.
C). Physical properties of alkanes
Number of
Name
Molecular
Melting
Boiling
Density
State at
carbon atoms
formula
R.T.P.
point (C)
point (C)
1
methane
CH4
-183
-162
gas
2
ethane
C2H6
-172
-89
gas
3
propane
C3H8
-188
-42
gas
4
butane
C4H10
-135
-0.5
gas
5
pentane
C5H12
-130
36
0.62
liquid
6
hexane
C6H14
-95
69
0.66
liquid
7
heptane
C7H16
-91
98
0.68
liquid
 The C1 to C4 members of alkane are gases, the C5 to C17 are colourless oily liquids while higher
members are solids.
 As the number of carbon atoms in the alkane chain ________, the melting and boiling points _______.
It is because as the molecules become larger and heavier, the forces of attraction (________________
forces ) among the molecules also ________. As a result, _______ energy will be needed to separate the
molecules during melting and boiling.
100
50
0
Boiling
point
-50 1
2
3
4
5
6
-100
-150
-200
number of carbon atoms
7
S5/CHEMNOTE/ALKANE
P.4
D). Chemical Properties of alkanes
Alkanes are chemically unreactive because their carbon atoms form only carbon-carbon single bonds (i.e.
they are saturated.) They do not react with acids, alkalis, oxdizing agents or reducing agents.
1). Combustion
a).Alkane burns completely in excess air or oxygen to give ____________ and _________ as the only
products.
Equation: C3H8 +
O2

C7H16 +
O2

C8H18 +
O2

 The presence of carbon and hydrogen in an alkane can be investigated using the apparatus shown
below:
suction
pump

Dry
alkane
jet of
burning
alkane
Lime water
dry blue cobalt(II)
chloride paper
 ________ gas (from carbon in alkane) turns the lime water ________.
 ________ (from hydrogen in alkane) turns dry blue cobalt(II) chloride paper ________.
 A large amount of heat is produced in this exothermic reaction. So alkanes are mainly used as fuels.
b). If insufficient oxygen is provided for burning, methane and other alkanes will burn with a _____
flame and produce ______. This means the combustion is _________.
2). Reaction with halogens (substitution reaction)
 When methane and bromine are mixed in the dark, no reaction occurs.
 In diffused sunlight (ultraviolet light or heating), alkane react slowly with bromine to give a mixture of
halogenalkanes
equations:
(chain reactions)
 The observation is :The red colour of bromine slowly disappears.
 The hydrogen atoms are successively replaced or substituted by bromine atoms. This is an example of
substitution reaction.
 Substitution reaction is a reaction in which an atom (or a group of atoms) in a molecule is replaced by
another atom (or a group of atoms).
 When chlorine is used instead of bromine, a similar and ______ reaction takes places.
 Iodine has almost no reaction with methane.
 The reaction of methane with halogens indicates the reactivity of the halogens _______ down the group.
III). Naming the alkanes (IUPAC Names)
A). Straight-chain Alkanes
 All the names of alkanes end with the suffix `-ane`
 The number of carbon atoms are indicated by the prefix:
Number of carbon atoms Molecular
Prefix(the
formula
beginning of the
word)
1
meth2
eth3
prop4
but5
pent6
hex7
hept8
oct9
non10
dec
Suffix (the end
of the word)
-ane
Name
S5/CHEMNOTE/ALKANE
P.5
B). Branched-chain alkane
The IUPAC name for a branched-chain alkane consists of 2 parts:
side groups main carbon chain
The IUPAC rules for naming branched-chain alkanes are as follows:
1. Locate the longest continous chain of carbon atoms.
examples

C




C
 C C
 C







 


C
 C C C C C
CCCC

 


 C
CCC
C


 

______ chain
______ chain
_______ chain
2. Number the longest continous chain, starting with the carbon atom at the end nearest the point at
which branch chains are attached.
Examples:
 


CCCC

C

 


CCCC

C

3. Specify the position number of the carbon atoms to which each branch chain or side group is
attached. Name each side group using the prefix shown below
branch chain
CH3 CH3CH2 CH3CH2CH2
F
Cl
Br
I
/side chain
methyl
ethyl
propyl
fluoro
chloro
bromo
iodo
prefix
Example:
H H H H


 
HCCCCH




H H CH3 H
______________
4. If two or more identical groups are attached to the carbon chain, specify this by using the following
prefixes:
no. of identical groups
two identical group
three identical group
four identical group
prefix
di
tri
tetra
Example:
H H H H


 
HCCCCH




H CH3 CH3 H
___________________
H H CH3 H




HCCCCH




H H CH3 H
____________________
H Cl


HCCCl


H Cl
___________________
5. If two or more different side groups are attached to the main carbon chain, list them in alphabetical
order.
A comma (,) is written between two numbers A hyphen (-) is written between a number and an
alphabet.
Example:
H H H H H H


 


HCCCCCCH


 


H H H CH3 CH3 H

CH3
______________________
H H H Br H


 

HCCCCCH


 

H H H CH3 H

CH3
________________________
S5/CHEMNOTE/ALKANE
a).
P.6
Classwork: Give the IUPAC names to the following compounds.
b).
H CH3 H H H H H





 
HCCCCCCCH






H H CH3CH3 H H

CH3
c).
CH3CHCHCH2CH2CH3


CH3 CH3
__________________________
d).
CH3CHCH2CH2


CH3
CH3
_____________________
___________________________
e).
CH3
|
CH3

CH3CH2CCH3
|
CH3
|
CH3
Br

CH3CH2CCH2Cl

Br
______________________
________________________
IV). Writing Structural Formulae from IUPAC Names
If the IUPAC name of a compound, its structural formula can be written, following the steps given
below.
1. Write down the main carbon chain.
2. Number the main chain.
3. Add the side groups to correct places in the main chain.
4. Complete the structural formula by adding the correct number of hydrogen atoms. Check that each
carbon atom has 4 bonds attached to it.
Examples:
a). 3-ethyl-2-methylhexane
b). bromotrichloromethane
c). 2-bromo-1-chlorobutane
Classwork
a). 2-methylpentane
b). 2,2-dimethylpropane
c).2,2,3-trimethylbutane
d). 1,1,1-trichloroethane
V). The Organic Compounds with other Functional Groups
H H


H  C C H


H H
H H


H C C O H


H H
 Consider the compounds ethane
and ethanol
. They have similar
structures but they have very different properties.
Ethane
Ethanol
State at R.T.P.
Gas
Liquid
Reaction with sodium
No reaction
Reacts to give hydrogen gas
CH3CH2OH + Na  CH3CH2O Na + H2
Ethane is saturated compound which usually does not react with metal. But the (-O-H) hydroxyl group in
ethanol make it react with sodium. The (-O-H) is called a functional group
 A family of compounds containing the same functional group form a homologous series.
Some homologous series with the functional group each contains are shown below
Name of
Functional suffix in Naming Examples
homologous series
group


Alkenes
C = C 
Akynes


C  C 
Alkanols
Alkanals
-OH
Alkanoic acids
O

COH
Amines
H

NH
O

CH
S5/CHEMNOTE/ALKANE
Examples:
a).
CH3CH2CH2CH = CHCH3
__________________________
c).
CH3 Cl


CH3CH2CH2CH3

OH
P.7
b). 3-methylpent-1-ene
d). 3-methlybutan-2-ol
__________________________
e).
f). 2-methylpropanoic acid
Cl
CH3 O



CH3CH3CH2 CHCOH
__________________________
VI). Structural Isomerism
 Isomers are compounds with the same molecular formula but different structural formulae.
 Structural Isomers have the same chemical properties because they contain the same functional
group.
 Structural Isomers have different physical properties. (e.g. m.p., b.p. density.) This is because the
structures and shapes of the molecules are different. The intermolecular forces between the molecules
are affected.
Examples:
1. C4H10
2. C4H8
3. C4H9OH
4. C3H7COOH