CfE
Chemistry
Nature’s
Chemistry
Alkanes, Alkenes
and Cycloalkanes
Covalent Bonding
The hydrogen and carbon in hydrocarbons are bonded covalently.
A covalent bond is formed between non metal atoms. These atoms
share electrons to achieve a full outer shell of electrons like their
nearest noble gas and become stable.
Lets use Hydrogen as an example.
Hydrogen molecule H2
A molecule is two or more non-metal atoms joined together by
covalent bonds.
Hydrogen can be described as a diatomic molecule as it is made
from 2 atoms. (Di means 2)
The atoms are held together by the electrostatic attraction
between the positive nuclei of each atom and the negatively
charged electrons.
Covalent Structure
Covalent substances that are bonded covalently can exist in
two different ways.
Covalent Molecules
Covalent molecules are compounds that are made up of a
discrete number of atoms and can exist as solids, liquids or
gases.
Examples include water (H2O), carbon dioxide (CO2) and
hydrocarbons.
Water
Carbon Dioxide
Propane
Covalent molecules tend to have low melting and boiling
points. This is because during melting and boiling the strong
covalent bonds within the molecules are not broken. It is the
weak forces of attraction between each molecule that is broken
allowing the molecules to move further away from each other.
This is the property which allows crude oil to be separated into
fractions.
Covalent Networks
Covalent networks are large 3-dimensional structures where all the
atoms are covalently bonded together and are less common than
covalent molecules.
Examples include carbon in the form of diamond and graphite and
sand (silicon oxide, SiO).
Diamond (C)
Silicon Oxide (SiO)
Graphite (C)
Covalent networks all have high melting and boiling points and are
very strong structures because all covalent bonds have to be broken to turn them from solids into liquids or gases.
Covalent substance do not conducts electricity as the electrons
are localised in the bonds and are not free to move and conduct
the charge.
Graphite is the exception to the rule as each carbon only makes 3
bonds and delocalises the fourth electron allowing it to conduct a
charge.
Hydrocarbons
Crude oil is made up of compounds called hydrocarbons. They are
molecules made from carbon and hydrogen atoms covalently
bonded together.
Alkanes
The alkanes are a family of hydrocarbons, they have the same
general formula and similar chemical properties.
The smallest member of the family is methane, CH4.
Actual Shape
Carbon atoms always have 4 unpaired electrons in their
outer shell and so can form 4 bonds.
‘Tetrahedral’
The bonds keep as far apart as possible; the hydrogen atoms
are at the corners of a pyramid. The normal bonds show two
hydrogens are in the plane of the paper, the wedged bond
shows the hydrogen is coming out of the paper and the
dashed line shows one hydrogen is going back.
Structural Formula
It is much easier to draw flat pictures to show how each
of the atoms are joined to each other.
Having as many as 4 bonds allows carbon atoms to form chains,
the alkane family is made up of chains where all the carbon atoms
join together with a single bond. Due to all the bonds being single
bonds, alkanes are described as saturated (nothing can be
added).
Naming hydrocarbons
The surname is –ane to tell you it is a member of the alkane
family, the first name tells you the number of carbon atoms in
each molecule.
Prefix
Number of carbon atoms in the
molecule
meth-
1
eth-
2
prop-
3
but-
4
pent-
5
hex-
6
hept-
7
oct-
8
There are different mnemonics to help you remember the prefixes:
1
Monkeys
2
Eat
3
Peanut
4
5
6
7
8
Butter Perched High High Overhead
No. of
carbon
atoms
Name
Full structural Formula
Shortened Structural
Formula
Formula
CH4
CH4
CH3CH3
C2H6
CH3CH2CH3
C3H8
CH3CH2CH2CH3
C4H10
CH3CH2CH2CH2CH3
C5H12
CH3CH2CH2CH2CH2CH3
C6H14
H
1
Methane
H
C
H
H
2
3
4
5
6
Ethane
Propane
Butane
H
H
H
Pentane H
Hexane
H
H
H
C
C
H
H
H
H
H
H
C
C
C
H
H
H
H
H
H
H
H
C
C
C
C
H
H
H
H
H
H
H
H
H
H
C
C
C
C
C
H
H
H
H
H
H
H
H
H
H
H
C
C
C
C
C
C
H
H
H
H
H
H
H
H
Branched Chain Alkanes
In a straight-chain hydrocarbon, all the carbon atoms in the
molecule are linked one after the other in a single continuous
chain. In a branched-chain hydrocarbon, the molecule has one
or more side chains of carbon atoms coming from the main chain.
As a result of branching, the same molecular formula can have a
different structural arrangement, when this happens the molecules
are called isomers. Clearly both of them cannot have exactly the
same name.
When writing shortened structural formulae for branched alkanes,
the branches are often put in brackets, e.g.
CH3CH(CH3)CH2CH3
or
CH3C(CH3)2CH3
Isomers are molecules with the same molecular formula but
a different structural arrangement
Examples
Butane one has two isomers:
H
H
H
H
H
C
C
C
H
H
H
C
C
C
H
C
H
H
H
H
H
H
H
H
H
C
H
H
H
Pentane has three isomers:
H
H
H
H
H
H
H
H
H
H
C
H
H
H
C
C
C
C
C
H
H
H
H
H
H
H
C
C
H
H
C
H
C
C
H
H
H
H
H
H
C
C
C
H
H
H
C
H
H
H
Naming Rules for Alkanes
1.
The longest chain defines the main chain and the last part of
the name
2.
Numbering of the main chain starts from the end that gives
the lower overall number positions for side branches
3.
Side branch names end in ‘-yl’ and depend on the number of
carbon atoms in them: methyl for 1 carbon, ethyl for 2 carbon
atoms, propyl for 3 carbon atoms, etc.
4.
Alphabetical order is used if different side branches appear in
the same structure (ethyl before methyl).
5.
Hyphens are used before or after numbers that come next to
letters within a name (2-ethyl-3-methyl..)
6.
Commas are used between numbers showing more than one of
the same side branch (2,2,3-trimethyl..)
Homologous Series
Homologous Series is a group of molecules with the same
general formula, similar chemical properties and graded
physical properties.
The alkanes are an examples of a homologous series of molecules.
Each molecule differs from the previous molecule by the same
amount, —CH2 —, ( homo- = same, logous = difference). This
makes it possible to write a General Formula for the alkane family.
General Formula: CnH2n + 2
Where n is the number of carbon atoms in the chain. This means
are always twice plus two hydrogens for any number of carbon
atoms.
As each molecule in the family varies by the same mass each
molecule shows graded physical properties. This means they show
a gradual increase in melting and boiling point and similar
solubility. As they also have very similar structures they have
similar chemical properties, meaning they all react in the same
way.
Alkenes
The alkenes are a family of hydrocarbons, they have the same
general formula and similar chemical properties.
The first and smallest member of the family is Ethene, C2H4.
The Alkene family is made up of chains of carbon atoms where
most of the carbon atoms join together with a single bond, but two
of the carbon atoms will have joined with a double bond. Due to
the double bond, alkenes are described as unsaturated.
Actual Shape
Carbon atoms always have 4
unpaired electrons in their outer
shell and so can form 4 bonds in
total.
Double Bond
In the alkene family, two of the
carbon atoms will share two pairs of
electrons, as shown in the diagram.
Structural Formula
It is much easier to draw flat
pictures to show how each of the
atoms are joined to each other.
Naming Alkenes
Alkenes are named using the same prefixes as the alkanes but the
end section of the name is now –ene.
No. of
carbon
atoms
2
3
Name
Full structural Formula
Ethene
Propene
H
H
C
C
H
H
H
H
H
C
C
C
H
4
Butene
Pentene
H
H
H
H
C
C
C
C
H
H
Hexene
H
H
H
H
H
H
C
C
C
C
C
H
H
H
H
6
H
H
H
H
H
H
C
C
C
C
C
C
H
H
H
H
H
Formula
CH2=CH2
C2H4
CH2=CHCH3
C3H6
CH2=CHCH2CH3
C4H8
CH2=CHCH2CH2CH3
C5H10
CH2=CHCH2CH2CH2CH3
C6H12
H
H
5
H
Shortened Structural
Formula
H
H
Naming Alkenes
1. Select the longest continuous carbon chain containing the
double bond and name it after the appropriate alkene.
2. Number the chain from the end nearest the double bond and
indicate the position of the double bond between the prefix and
–ene, eg but-2-ene.
3. Name any branch(es) and indicate the position(s) of them.
Examples
Isomers
There are even more isomers possible in the alkene family. Again
there are straight-chain alkenes and branched-chain alkenes. In
addition, it is possible to change the position of the double bond to
introduce even more different structural formulae.
Examples
But-1-ene has three isomers. The two straight chain molecules
with the double bond moved and one branched chain molecule
with the double bond.
H
H
H
H
C
C
C
C
H
H
H
But-1-ene
H
H
H
H
H
H
C
C
C
C
H
H
H
H
C
H
C
C
C
H
H
H
H
But-2-ene
H
H
2-methylprop-1-ene
The alkenes have the following general formula:
CnH2n
There are always twice as many hydrogens atoms for any number
of carbon atoms.
This is different from the alkane family. Since the alkenes have a
double bond, there is no need for the two hydrogens on the ends
of the molecule.
As the alkenes are a homologous series, each molecule differs
from the next one in the series by the same amount, they show
similar chemical properties and graded physical properties.
Cycloalkanes
The cycloalkanes are another series of hydrocarbons made up of
only carbon and hydrogen. They are made up of rings where all of
the carbon atoms join together with a single bond. Molecules with
single bonds only are described as saturated since they have the
maximum number of hydrogens attached.
The first and smallest member of the cycloalkanes is cyclopropane.
Actual Shape
Carbon atoms are arranged in a ring with the
hydrogens pointing out of the ring.
H
H
The hydrogens are above and below the ring,
C
H
C
C
H
H
and dashed bonds.
H
H
Structural Formula
H
It is much easier to draw flat pictures to show
C
H
C
H
which is why they can be drawn with wedged
C
H
H
how each of the atoms are joined to each
other.
No. of
carbon
atoms
Name
Full structural Formula
H
3
C
Cyclopropane
H
C
H 2C
H
H
C
H
CH 2
H2
C
H
H 2C
H
C
H
H
H
H 2C
H
Cyclopentane
H
C
C
H
C
H
H
6
Cyclohexane
C
H
H
H
H
H2 C
H
C
C
H
H
C
CH 2
H
H2 C
C
C
C5H10
C
H2
H
H
CH 2
H 2C
C
H
CH 2
H
C
C4H8
C
H2
H
H
5
CH2
C
C
Formula
C3H6
H
C
H
Cyclobutane
H2
C
H
H
4
Shortened Structural
Formula
C6H12
CH 2
H
H
H2 C
CH 2
H
Naming Cycloalkanes
The same system is used to name the cycloalkanes as was used to
name the alkanes.
The surname is -ane to tell you it is like a member of the alkane
family (only single bonds), the first name tells you the number of
carbon atoms in each molecule. Cyclo tells you the carbons are in
a ring.
Just like alkanes and alkenes, the cycloalkanes can also have
branches and the same system for naming these branches is used.
H
H
This molecule is 1-methylcyclopropane.
H
C
C
H
C
H
C
H
H
H
H
H
H H
C
H
H
1C
H
H
C
H
C
3
C
C
This molecule is 1,3-dimethylcyclohexane.
H
H
2
C
H
C
NOT 1,4-dimethylcyclohexane.
H
H
H
H
As the cycloalkanes are a homologous series they all have similar
chemical properties, graded physical properties and the same
general formula. The general formula is:
CnH2n
The cylcoalkanes are similar to both the alkanes and the alkenes.
They are saturated molecules (all single bonds) like the alkanes
but due to the ring structure they have the same general formula
as the alkenes.
Properties of the Alkanes, Alkenes and Cycloalkanes
Melting and Boiling Points
As the number of carbons in the chain increases so does the
boiling point.
Flammability
The flammability increases as the hydrocarbon chains get longer.
Solubility
Alkane molecules of all sizes, are insoluble (immiscible) in water
but will dissolve (are miscible) in similar non-polar liquids. As most
alkanes are liquids they form a layer on top of water as they are
(usually) less dense than water.
Properties Explained
The properties of the alkanes can all be explained by the weak
forces of attraction that exist between molecules. As the molecules
get bigger (more carbon and hydrogen atoms) the forces become
stronger requiring more energy to overcome the interactions. As a
result they have increasing boiling points and become less
flammable and soluble as they get bigger.
Increasing carbon chain, increasing intermolecular
Reactions of Alkanes, Alkenes and Cylcoalkanes
Bromine Test
The alkanes, alkenes and cycloalkanes react differently with bromine and so this test can be used to identify each family. Bromine
is an orange liquid and decolourises (becomes clear) when it reacts
with an alkene but not with an alkane or cycloalkane. This test is
really a test of saturation, as the unsaturated alkene will react but
the saturated alkane and cycloalkane will not.
Family
General Formula Type of C to C
bond
Result with
bromine
Alkanes
CnH2n+2
Single
(saturated)
Bromine stays
brown
(decolourises
slowly)
Cycloalkanes CnH2n+2
Single
(saturated)
Bromine stays
brown
(decolourises
slowly)
Alkenes
One Double
(unsaturated)
Bromine
decolourises
immediately
CnH2n
Addition Reactions
The bromine test is an example of an addition reaction.
An addition reaction is when two molecules react together
to make one molecule.
Eg ethene reacting with bromine
These are important reactions that only molecules with C=C bonds
(unsaturated) can do. They can be used to make many useful
products.
Halogenation
This is an addition reaction but with halogen molcules (F2, Cl2, Br2
and I2), or with hydrogen halide molecules (H-F,H-Cl,H-Br,H-I). In
each case the molecule will break into two parts and they will add
on to the C atoms making up the C=C double bond.
The alkenes are described as
unsaturated because of the
presence of the carbon to
carbon double bond, whereas
their products have only
single bonds so are therefore
saturated and have the –ane
ending to reflect this.
Hydrogenation
The alkenes can be made into alkanes by adding hydrogen and this
reaction is called hydrogenation. The reaction is an addition reaction, the hydrogen molecule breaks and adds across the Carbon to
Carbon double bond.
This reaction is used by food scientists to produce margarine.
Animal fats, like butter, contain mainly saturated fats (C-C single
bonds) while vegetable oils have more unsaturated fats (C=C
bonds) and are healthier for us.
Hydrogenation is used to convert some of the unsaturated fats in a
liquid oil into saturated fats to produce a more solid form, margarine, which is still healthier than butter and has the advantage that
it is spreadable.
Butter has more saturated
fats (C-C single bonds)
Vegetable Oil has
more unsaturated fats
(C=C double bonds)
Margarine has more
saturated bonds than
vegetable oil but less
than butter.
Cracking
The crude oil fractions with small hydrocarbon molecules, such as
naptha, are more useful than the long chained molecules. These
long chained molecules can be broken down by the process of
cracking into smaller, more useful molecules.
It is not possible for all the molecules produced from cracking to
be alkanes as there is not enough hydrogens, as a result there will
always be an alkene produced. There is also no way of controlling
where the long chain molecule breaks meaning the alkanes and
alkenes produced are random.
Cracking in the lab
Cracking can be carried out in the lab to show the products of
cracking.
Aluminium Oxide
In this experiment the vapour of liquid paraffin (a mixture of
saturated hydrocarbons) is cracked by passing it over a heated
catalyst, aluminium oxide. The mixture of gaseous short-chain
hydrocarbons produced is collected over water and can then be
tested for unsaturation with bromine water. There will be a positive
bromine test (bromine water will change from orange to clear) as
there will be an alkene present.
To view a video clip of this experiment, go to:
http://media.rsc.org/videoclips/demos/Crackingahydrocaron.mpg
Uses of Alkanes, Alkenes and Cycloalkanes
The hydrocarbons have many different uses. As alkanes release
large amounts of energy when burn, they are usually used as
fuels.
Hydrocarbons, including alkanes, were used in many of the early
dry-cleaners as they could dissolve many stains. Modern
dry-cleaners use less flammable molecules derived from simple
hydrocarbons.
Though alkenes release large amounts of energy when burnt,
they have much more important uses so are rarely used as fuels.
Alkenes, particularly ethene are important in the manufacture of
other chemicals such as plastics, ethanol, ethanoic acid, esters,
halogen derivatives etc.
Alkanes
plastics
Examples

Plastics bags

Vinyl paints
Alkenes
alcohols
substituted
alkanes
Examples

Propellants

Refrigerants
Examples

Anti-freeze

Hand-gels
acids
esters
Examples

Flavourings

Perfumes
Examples

Vinegar

Aspirin