Standard Grade Chemistry
Summary Notes
Topic 6: Structures & Reactions Of Hydrocarbons
Learning Outcomes
General
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Alkanes are a subset of the set of hydrocarbons.
Alkanes have an ‘-ane’ ending in their name.
The first eight alkanes are : methane, ethane, propane, butane, pentane,
hexane, heptane and octane.
You should be able to construct for straight chain alkanes (C1 to C8) full and
shortened structural formulae and molecular formulae given the name
You should be able to name an alkane (C1 to C8) given its molecular,
shortened or full structural formulae.
The general formula for alkanes is CnH2n+2 and you should be able to use this
to work out the structural formula of a named alkane.
Alkenes are a subset of the set of hydrocarbons.
Alkenes have an ‘-ene’ ending in their name.
The first five alkenes are : ethene, propene, butene, pentene, hexene
You should be able to name an alkene ( to C6) given its molecular, shortened
or full structural formulae.
You should be able to construct for straight chain alkenes (C1 to C6) full and
shortened structural formulae and molecular formulae given the name
The general formula for alkenes is C nH2n and you should be able to use this to
work out the structural formula of a named alkene.
You should be able to identify from structural formula whether a
hydrocarbon is an alkane or an alkene.
A saturated hydrocarbon contains only carbon to carbon single bonds.
Alkanes are saturated hydrocarbons.
Unsaturated hydrocarbons contain carbon to carbon double bonds.
Alkenes are a subset of the set of unsaturated hydrocarbons.
It is possible to distinguish between an unsaturated and a saturated
hydrocarbon using bromine (solution). Alkenes decolourise bromine solution
rapidly, alkanes do not.
Alkanes are formed from the reaction of an alkene with hydrogen.
An addition reaction occurs when a diatomic molecule adds on across a C=C
double bond in an alkene.
The reactions between alkenes and hydrogen and bromine are addition
reactions.
Fractional distillation of crude oil yields more long chain hydrocarbons than
are useful for present-day industrial purposes.
Cracking is an industrial method for producing smaller, more useful
molecules.
State that cracking produces smaller hydrocarbons, some of which are
unsaturated.
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A homologous series is a series of compounds that
- fit a general formula
- have the same chemical properties
- show a gradual change in physical properties, like boiling points and
viscosity
The alkane family is an example of an homologous series
As the number of carbon atoms increases in the formula then the molecules
become bigger in size. The bigger the molecules, the more energy is required
to get the molecules to move about rapidly. This explains why the alkanes
with the smallest molecules have the lowest boiling points and those with the
largest molecules have the higher boiling points.
The first 4 cycloalkanes are ; cyclopropane, cyclobutane, cyclopentane and
cyclohexane.
The cycloalkane family is an example of a homologous series.
Isomers have the same molecular formula but have different structural
formulae.
The alkene family is an example of a homologous series.
A catalyst allows an addition reaction to take place at a lower temperature.
Cracking produces a mixture of saturated and unsaturated products as the
number of carbon and hydrogen atoms in the cracked alkane must equal the
number of carbon and hydrogen atoms in the products.
Alkanes
Hydrocarbons are compounds of hydrogen and carbon only. Methane is the
simplest hydrocarbon. There are many different hydrocarbons due to carbon
being a very special element. Carbon is special because :
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Carbon can form bonds with itself to form chains of carbon atoms
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Carbon can form bonds with itself to form chains of carbon atoms
with branches off the main chain
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Carbon can form bonds with itself to form rings of carbon atoms
No other element can do this as well as carbon.
The hydrocarbons are divided into families. The simplest family of hydrocarbons
are the alkanes. Methane is the simplest member of the alkane family.
All of the fractions obtained by the fractional distillation of crude oil contain
alkanes. Alkanes are found in many every day products such as petrol, diesel,
aviation fuel, paraffin, camping gases, lighter fuel, lubricating oils, candle wax,
tar etc.
Properties Of Alkanes
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highly flammable – burn in a plentiful supply of oxygen to form carbon
dioxide and water
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pH = 7
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insoluble in water
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very slowly decolourise bromine solution
Alkane Series
Methane, and the mixture of alkanes contained in paraffin, are members of a
large family of alkanes. The name of the alkane compounds always end in –ane.
The first part of the alkane name indicates the number of carbon atoms there
are in the molecule.
For example
BUT
ANE
means there are
four carbons in the
alkane
means the compound
is an alkane
It is important that you know the prefixes (first part of the name) for
molecules with up to eight carbon atoms.
Number of carbon
atoms
1
2
3
4
Prefix
methethpropbut-
Number of carbon
atoms
5
6
7
8
Prefix
penthexheptoct-
Here is a table of data on some members of the alkanes.
Alkane
methane
ethane
propane
butane
pentane
hexane
heptane
octane
Chemical
Formula
CH4
C2H6
C3H8
C4H10
C5H12
C6H14
C7H16
C8H18
Number of
carbon atoms
1
2
3
4
5
6
7
8
Boiling point
(oC)
-161
-89
-42
0
36
69
98
126
State at
25oC
gas
gas
gas
gas
liquid
liquid
liquid
liquid
Boiling point is an example of a physical property of a compound. Physical
properties are those which can be measured in some way. Chemical properties
are those concerning the way the substance behaves e.g. burns to form carbon
dioxide and water.
The boiling point of an alkane can be measured by distillation.
You should see from the above table of data on the alkanes that the boiling
point increases as the number of carbon atoms increases.
As the size of the molecule increases, more energy is needed to separate them.
Since boiling involves separating molecules, the boiling point must increase as
the alkane molecule gets longer?
There are also trends in other properties of the alkanes. Viscosity,
flammability and sootiness of the flame, all vary according to the number of
carbon atoms contained in the molecules.
Although there are trends in some properties of the alkanes they all undergo
the same type of reactions. All the alkanes burn to form carbon dioxide and
water, although the shorter change molecules burn more easily. All the alkanes
only slowly decolourise bromine solution. The alkanes are said to be chemically
similar.
The chemical formula of the alkanes varies by –CH2 as you go up the series.
This common difference between members of the alkanes means that the family
can be represented by a general formula :
CnH2n+2 where n is the number of carbon atoms
A family of compounds, like the alkanes, which are chemically similar and have
the same general formula are called a homologous series.
Alkane Structures
The general formula of the alkanes, CnH2n+2 allows us to work out the molecular
(chemical) formula of any alkane.
e.g. pentane
pent- indicates that the alkane has 5 carbons.
Therefor, molecular formula of pentane is C5H (2X5+2) i.e. C5H12
The molecular formula does not tell us anything about the structure of the
molecule. Each carbon forms four covalent bonds (valency of carbon is 4), and
each hydrogen forms one covalent bond (valency of hydrogen is 1). We also know
from earlier in this topic that carbon can form bonds with itself to form chains
of carbons.
A picture of a methane molecule would show it to have a tetrahedral structure
because the hydrogen atoms sit at the corners of a shape called a tetrahedron.
For ease of drawing the molecule can be represented in this ‘flattened-out’ way.
Drawings can be made of larger straight chain molecules.
These drawings are called full (or extended) structural formulae.
The easiest way to draw the full structural formula of an alkane is to begin by
drawing the correct number of carbons in a line, then join on the hydrogens,
remembering that each carbon must have four bonds, but each hydrogen only
one bond.
e.g. pentane is C5H12
Begin by drawing the 5 carbons atoms in a line.
C–C–C–C–C
Now add the hydrogens, remembering each carbon must have 4 bonds and each
hydrogen 1 bond.
H H H H H
H–C–C–C–C–C–H
H H H H H
Shortened Structural Formulae
The full structural formula of pentane (C5H12) is
H H H H H
H–C–C–C–C–C–H
H H H H H
but its shortened structural formula is CH3CH2CH2CH2CH3
Using the full structural formula, the number of hydrogens on each carbon is
counted. In the example of pentane, the first carbon has three hydrogens
attached, so you write down CH3, the next carbon has two hydrogens attached
(CH2) and so on.
Often, when there a lot of consecutive CH2 groupings to write down, brackets
are used e.g. for pentane, instead of writing CH3CH2CH2CH2CH3, the shortened
structural formula can be written as CH3(CH2)3CH3
Isomers
Full structural formulae can be drawn in different ways for the same molecules
with the same molecular formula.
Consider the following molecules. Both have the molecular formula C5H12.
H H H H H
H C–C–C–C–C–H
H H H H H
H H H H
and
H–C–C–C–C–H
H H
H
H–C–H
H
Compounds with the same molecular formula but different structural formula
are called isomers.
Cycloalkanes
Carbon atoms can join with other carbon atoms to form rings. The hydrocarbons
which have carbons joined in rings are called cycloalkanes. ‘Cyclo’ means joined in
a ring.
The smallest number of carbon atoms which can join to form a ring is three. The
first member of the homologous series is therefore cyclopropane.
Since each carbon in the ring is joined to neighbouring carbons, and since the
valency of carbon is four, each carbon in the ring is also joined to two hydrogen
atoms.
The following table shows the names and chemical formulae of the first four
members of the cycloalkanes and the structural formula of cyclobutane.
Cycloalkane
cyclopropane
cyclobutane
cyclopentane
cyclohexane
Molecular formula
C3H6
C4H8
Structural formula
H H
H–C–C–H
H–C–C–H
H H
C5H10
C6H12
The general formula of the cycloalkanes is CnH2n.
The cycloalkanes have very similar properties to the alkanes. i.e. they burn in
the plentiful supply of oxygen to form carbon dioxide and water, they only
very slowly decolourise bromine solution, they are insoluble in water, and they
have a pH = 7.
Cracking
Crude oil undergoes fractional distillation to separate it into different
fractions. These fractions are mixtures of hydrocarbons and have many
different uses in the modern world. Unfortunately the fractional distillation
process does not produce every fraction in the correct quantity to match the
demand for its various uses.
In general terms, fractional distillation produces more of the long chain
hydrocarbons than are needed and not enough of the short-chain
hydrocarbon fractions.
Because of this greater demand for the shorter-chain hydrocarbons chemists
‘chop up’ some of the surplus longer molecules from the heavier fractions to
get smaller molecules. This process is called cracking. In cracking heat is used
to break up the long chains. However, the use of a catalyst can allow the
process to take place at a lower temperature, as well as speeding up the
process.
Cracking can be demonstrated in the laboratory. The steel wool catalyst is
heated strongly. The heat passes along the boiling tube and causes the liquid
paraffin to evaporate and pass over the heated catalyst. This causes the long
chain hydrocarbons in the paraffin to break up into shorter chain molecules. The
shorter chain gaseous molecules are collected as shown. The gas collected can
be shown to burn and to immediately decolourise bromine solution.
The fact that the gas collected by cracking the liquid paraffin immediately
decolourises bromine solution indicates that a different type of molecule has
been formed. This type of molecule belongs to another hydrocarbon family of
compounds called the alkenes.
The alkenes are special because they have a carbon-to-carbon double bond
between two of the carbons in the molecule. Hydrocarbons with a carbon-tocarbon double bond, like the alkenes are described as being unsaturated.
Alkanes and cycloalkanes are both described as saturated because the
molecules in these families contain only carbon-to-carbon single bonds.
The cracking of long-chain alkane molecules to produce two shorter chain
molecules must result in one of the smaller molecules being unsaturated because
there are not enough hydrogen atoms available to form two saturated
molecules.
C7H16

C5H12
+
saturated
alkane
C2H4
unsaturated
alkene
Alkenes
The alkenes are a homologous series of unsaturated hydrocarbons. They result
from the catalytic cracking of long chain hydrocarbon fractions.
They are said to be unsaturated because they contain one carbon-to-carbon
double bond between two of the carbons in the chain. Since the presence of the
double bond requires two carbons, the first member of the homologous series of
alkenes is the two carbon molecule ethene.
This is the structure of ethene
H H
C=C
H H
The following table gives information on the first few members of the alkenes.
Alkene
ethene
propene
butene
pentene
hexene
Number of
carbons
2
3
4
5
6
Molecular
formula
C2H4
C3H6
C4H8
C5H10
C6H12
Boiling Point
(oC)
-104
-47
-6
+30
+63
State at
25oC
gas
gas
gas
liquid
liquid
The general formula of the alkene, CnH2n.
Note that the general formula of the alkenes is the same as the general formula
of the cycloalkanes. However the alkenes and the cycloalkanes are different in
structure and also in some properties. It should also be noted that since alkenes
and cycloalkanes with the same number of carbons e.g. butene and cyclobutane
have the same molecular formula but different structures, that they are
isomers.
Alkene Reactions
After testing hexane with bromine, we can see that it is easy to tell alkenes
from alkanes.
Alkene
Quick decolourisation of bromine solution.
Alkane
Slow decolourisation of bromine solution.
The reason for the difference in reaction towards bromine must lie with their
difference in structure.
The double bond in the alkenes is responsible for their quick reaction with
bromine. The double bond breaks and the bromine atoms simply add on.
Example
H H
C=C +
H H
Br – Br 
H H
Br – C – C – Br
H H
ethene
bromine
dibromoethane
This type of reaction is typical of unsaturated hydrocarbons such as the alkenes
and is called an addition reaction.
Alkenes can also react with hydrogen in an addition reaction to form the
corresponding alkane. The hydrogen atoms add across the carbon-to-carbon
double bond converting the unsaturated alkene into the saturated alkane.
Addition reactions do not occur with alkanes. Since each carbon in an alkane is
already joined to four other atoms, some atoms have to be removed before
others can join on. This is a much slower type of reaction and is called a
substitution reaction.