Topic 7 – National 4 & 5 Chemistry Summary Notes
Hydrocarbons
A vast number of different hydrocarbons exist and so chemists have grouped
them into “sub-sets” to make them easier to study. In this topic we will study
three different hydrocarbon sub-sets.
The Alkanes
LI 1
The alkanes make up the simplest sub-set of all the hydrocarbons.
Most of the FUELS we use are made from alkanes. They are also useful starting
materials for a huge range of other substances such as PLASTICS and DRUGS.
Name of Alkane
Methane
propane
butane
octane
Uses
(natural gas) for cooking, heating
Used in gas cylinders in homes
Sold in blue cylinders as camping gas
A component of petrol
All the alkanes end in –ANE. A prefix tells you how many carbon atoms are
present in the molecule.
Prefix
Meth
Eth
Prop
But
Pent
Hex
Hept
Oct
Number of Carbon
Atoms
1
2
3
4
5
6
7
8
The alkanes all contain COVALENT bonds. They are different from other
compounds since they are able to form long, chain like molecules containing many
carbon atoms joined together.
1
Physical State of Alkanes
Name of
Alkane
Boiling Point
(⁰C)
Physical State at Room
Temperature (25⁰C)
Methane
Ethane
Propane
Butane
Pentane
Hexane
Heptane
Octane
*Complete the table using the data book pg ______
LI 2
General Formula for the Alkanes
If you look at the full structural formula for the alkanes you should see that
there is a pattern.
Each carbon atom is joined to two hydrogen atoms with two extra hydrogens at
the end of the molecule. This gives the general formula in mathematical terms
as:
Where n = the number of carbon atoms in the molecule
2
LI 3
Drawing and Naming Alkanes
We can represent the alkanes in a variety of different ways:
Molecular Formula
This is the simplest type of formula used for alkanes and other hydrocarbons.
This type of formula shows only the number of carbon and hydrogen atoms
present.
e.g. ethane
C2H6
Full Structural Formula
The full structural formula gives a simplified picture of the structure of the
molecule. All the bonds present between the carbon atoms and the carbon and
hydrogen atoms are shown.
e.g. ethane
Shortened Structural Formula
This type of formula shows how many CH2 and CH3 groups are present in the
molecule.
e.g. ethane
propane
CH3CH3
CH3CH2CH3
3
ALKANE
MOLECULAR
FORMULA
Methane
CH4
Ethane
C2H6
Propane
C3H8
FULL
STRUCTURAL FORMULA
4
SHORTENED
STRUCTURAL FORMULA
Butane
C4H10
Pentane
C5H12
Hexane
C6H14
*The names and molecular formulae of the first eight alkanes are
5 found in the data book pg _______
LI 4
Saturation and Unsaturation
All the carbon to carbon bonds in the alkanes are single C-C bonds. For this
reason they are described as being SATURATED molecules.
In other hydrocarbon families not all the carbon to carbon bonds are single C-C
bonds and are therefore described as being UNSATURATED molecules.
ALKANES ARE SATURATED MOLECULES
LI 5
Homologous Series
A HOMOLOGOUS SERIES is a family of compounds with similar chemical
properties and can be represented by a general formula.
The alkanes form a homologous series because they share the following
characteristics:

they all fit the same general formula

they all have similar chemical properties i.e. they all burn

they show a gradual change in physical properties as the molecules get
bigger in size e.g. boiling point increases as molecular size increases.
Reactions of Alkanes
LI 6
Apart from COMBUSTION the alkanes undergo few other reactions.
Alkanes burn in a plentiful supply of oxygen to produce carbon dioxide and
water.
e.g.
alkane
+
oxygen
CH4
+
2O2
carbon dioxide
+
water
+
2H2O
CO2
It is for this reason that the alkanes are mainly used as FUELS.
6
LI 7
The Alkenes
The alkenes are another homologous series of hydrocarbons
containing covalent bonds. Alkenes are similar to alkanes
in many ways but the main difference is the presence of the carbon-to-carbon
double bond in alkenes, represented as C=C.
All alkenes end in –ENE and the first member of the family is ethene.
Can you think why there is no methene?
Each alkene contains only one C=C
*Each carbon atom only forms 4 bonds so be careful not to add too many
hydrogen atoms when drawing full structural formula!
Since the alkenes are not full of carbon to carbon single bonds they are
described as being UNSATURATED molecules.
ALKENES ARE UNSATURATED MOLECULES
General Formula for the Alkenes
All alkenes fit the same general formula. It should be easy to see a pattern as
the number of carbon atoms increase.
Where n = the number of carbon atoms in the molecule
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ALKENE
MOLECULAR
FORMULA
Ethene
C2H4
Propene
C3H6
Butene
C4H8
FULL
STRUCTURAL FORMULA
8
SHORTENED
STRUCTURAL FORMULA
Pentene
C5H10
Hexene
C6H12
Heptene
C7H14
9
Uses of Alkenes
The C=C present in alkenes is very reactive compared to the C-C in alkanes.
The C=C allows alkenes to be changed readily into new products e.g. ethene and
propene are used in the plastics industry to make polyethene and polypropene.
Alkenes are very important feedstocks for
the PLASTICS INDUSTRY.
LI 8
Reactions of Alkenes
1.Combustion
Like all other hydrocarbons the alkenes burn in a plentiful supply of oxygen to
produce carbon dioxide and water.
alkene +
e.g.
C2H4
oxygen
+
carbon dioxide
3O2
+
water
+
2H2O
2CO2
2. Addition Reactions
The C=C is a reactive bond which breaks easily. This means that small molecules
can add across the double bond, usually with the help of a catalyst e.g.
a) Addition of Hydrogen (H2)
Two hydrogen atoms “add across” the C=C to form the corresponding ALKANE.
e.g.
ethene
+
hydrogen
ethane
10
b) Addition of Bromine (Br2)
ethene
+
bromine
1,2-dibromoethane
Using Bromine Water to Test for Unsaturation
A solution of bromine water Br2(aq) can be used to distinguish an alkene from an
alkane.
Bromine water is an ORANGE colour but when bromine reacts with an ALKENE,
the product of the reaction is COLOURLESS. We say that the bromine has been
DECOLOURISED.
Br2
Br
ethene
bromine
(orange)
Br
1,2-dibromoethane
(colourless)
ALL ALKENES QUICKLY DECOLOURISE BROMINE WATER
ALKANES DO NOT REACT WITH BROMINE WATER
11
Colour the boxes below to show the effects of bromine water reacting with:
a) an alkane
+
alkane
bromine water
(colourless)
stays orange
(orange)
b) an alkene
+
alkene
bromine water
(colourless)
(orange)
LI 9
N5
decolourises
The Cycloalkanes
The CYCLOLKANES are another homologous series of hydrocarbons in which
the carbon atoms are joined to form RINGS.
Cycloalkanes are full of C-C single bonds and are therefore SATURATED
compounds. They will not decolourise bromine water.
The first member of the cycloalkanes is CYCLOPROPANE.
The cycloalkanes all fit the same general formula:
12in the molecule
Where n = the number of carbon atoms
CYCLOALKANE
MOLECULAR
FORMULA
Cyclopropane
C3H6
Cyclobutane
C4H8
Cyclopentane
C5H10
FULL
STRUCTURAL FORMULA
13
SHORTENED
STRUCTURAL FORMULA
LI 10
Branched Chain Alkanes
Alkanes like propane and butane are described as STRAIGHT CHAIN
ALKANES because each carbon atom is joined to no more than 2 other carbon
atoms. However, crude oil and natural gas both contain another type of alkane
called BRANCHED CHAIN ALKANES. In these alkanes some carbon atoms are
joined directly to three or four carbon atoms.
e.g.
Straight chain
bbkfsdokfosdf
Branched chain
Branched chain
Rules for Naming Branched Chain Alkanes
1. Look for the longest continuous chain of carbon atoms and name after the
parent alkane.
2. Number the carbon atoms in the longest chain from the end which gives
the lowest numbers to the carbon atoms that have branches attached.
3. Identify the groups making up the branches ( known as alkyl groups).
Alkyl Group
-CH3
-C2H5 or –CH2CH3
-C3H7 or –CH2CH2CH3
Name of branch
methyl
ethyl
propyl
4. Use the prefixes di-, tri-, tetra etc. to indicate how many of a
particular branch are present.
5. Indicate the position of each branch with a number placed in front of its
name.
6. If more than one type of branch is present, the names are put in
alphabetical order e.g. ethyl would come before methyl.
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Examples:
LI 11
Isomers
Isomers are compounds with the same MOLECULAR FORMULA but
a different STRUCTURAL FORMULA.
Butene and cyclobutane are ISOMERS. These two compounds have the same
number of carbon and hydrogen atoms but they are arranged differently.
Butene contains a C=C whereas cyclobutane does not.
15
*Isomers of straight chain alkanes can be branched alkanes.
*Isomers of alkenes can also be produced by moving the position of the C=C in
molecules with 4 or more carbon atoms.
But-1-ene
16
But-2-ene
*Note the different positions of the Cl atoms in the two isomers.
17
N4
Cracking
In oil refineries, the fractional distillation of crude oil generally produces more
long chain alkanes than are needed by industry. A process called CRACKING
breaks down the long chain alkanes into smaller, more useful molecules which are
in greater demand.
When a long chain alkane is cracked it produces a mixture of ALKANES and
ALKENES. Alkenes are very useful in the plastics industry.
e.g. cracking of decane
C10H22
C6H14
decane
hexane
(saturated)
+
C4H8
butene
(unsaturated)
When an alkane is cracked there are not enough hydrogen atoms to produce
two alkanes and so one product is an unsaturated alkene.
When an alkane is cracked the carbon chain may break at different points on
different molecules which means a mixture of products is usually obtained.
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Cracking in the Lab
aluminium oxide catalyst
delivery tube
water

Cracking can be carried out in the lab using an aluminium oxide catalyst.

To prevent “suck back” the delivery tube should be removed from the
water before heating stops.

To prove the gas is an alkene, bromine water can be added to the test
tube of product gas. The gas decolourises the bromine water.
CRACKING is a process used to meet the demand for
shorter chain alkanes and alkenes.
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National 4 & 5 Topic 7 Hydrocarbons
Number
1
Learning Intention
I will find out about the simplest subset of
hydrocarbons known as the Alkanes
Success Criteria
I can:






2
I will work out the general formula for the
Alkanes after completion of LI3
3
I will find out how to represent alkanes in a
variety of ways
4
I will find out the difference between saturated
and unsaturated molecules
5
I will find out about the meaning of a
homologous series.
State uses for some alkanes
State the physical states of alkanes at room temperature
State that alkanes contain covalent bonds
State that all alkanes end in –ANE.
State that a prefix indicates the number of carbon atoms in the
molecule.
Name the first eight members of the alkanes
I can:
 State the general formula for the alkanes as CnH2n+2
 State that n = a whole number
I can:
 State what is meant by molecular formula, shortened structural
formula and full structural formula.
 Represent the first eight members of the alkane family using
molecular formula, shortened structural formula and full structural
formula
I can:
 State that saturated molecules are full of C-C single bonds
 State that unsaturated molecules contain a C=C
 State that the alkanes are saturated molecules
I can :
 State that all members of a homologous series can be represented by
20
a general formula
State that all members have similar chemical properties
State that there is a gradual change in physical properties as the
molecules get bigger in size e.g. boiling point increases


6
7
I will find out about the different types of
reactions alkanes undergo
I will find out about another homologous series
of hydrocarbons called the alkenes
8
I will find out about the reactions the alkenes
can undergo
9
I will find out about the homologous series of
cycloalkanes
I can:


I can


State that all alkanes burn and are used mainly as fuels
Write and balance combustion formulae equations for alkanes
State that alkenes contain one C=C per molecule
Represent the first 6 members of the alkenes using molecular formula,
shortened structural formula and full structural formula.
Work out the general formula for the alkenes and state this as CnH2n
State that alkenes are important feedstocks for the plastics industry.


I can
 State that all alkenes burn to produce CO 2 and H2O
 State that alkenes undergo addition reactions due to the presence of
the C=C
 State that small molecules can add across the C=C
 Draw the products of addition reactions
 State that alkenes decolourise bromine water whereas alkanes do not.
 State that alkenes are unsaturated molecules
 State that bromine water is used as the standard test for
unsaturation
I can:
 State that the cycloalkanes are hydrocarbons made up of rings of
carbon atoms
 State that cycloalkanes do not react with bromine water and are
therefore saturated compounds
 State the general formula as CnH2n
21
Represent the first 5 members of the cycloalkanes using molecular
formula, shortened structural formula and full structural formula.

10
11
I will learn how to recognise and name
branched chain alkanes
I will find out the definition of an isomer and
how to draw isomers
I can:


I can:




12
I will carry out a cracking experiment
Differentiate a straight chain alkane from a branched chain alkane
Follow a set of rules to name a branched chain alkane
State the definition of an isomer
Draw isomers of straight chain alkanes
State that alkenes are isomers of cycloalkanes i.e. cyclopropane is an
isomer of propene
Draw isomers of alkenes with 4 or more carbon atoms by moving the
position of the C=C
I can:
 State that cracking is a process used to meet the demand for shorter
chain alkane and alkenes.
22