Alkanes
Alkanes are saturated hydrocarbons
They contain only carbon – carbon single bonds and carbon – hydrogen single
bonds
Very unreactive
Used as fuels, lubricants and often as starting materials for many different
products
General formula for all chain alkanes is CnH2n+2
Unbranched Chains
H3C
H2
C
H2
C
C
CH3
H2
Unbranched chains are often called straight chains but this in fact is not true. The
C-C-C angle is 109.5 degrees so the chains are not actually straight.
Branched Chains
An example of this would be methylbutane which is an isomer of pentane
H3 C
H2
C
CH3
C
H
CH3
Alkanes
Ring Alkanes
These have the general formula CnH2n. This is because the “end” hydrogen's are not
needed.
Example: Cyclohexane C6H12
Physical Properties
H2C
H2C
H2
C
C
H2
CH2
CH2
Alkanes are almost non polar due to the relative similarities between the
electronegativities of hydrogen and carbon. The only intermolecular forces between the
molecules are the weak “Van der Waals forces”, though the larger the molecule, the
stronger these become.
As the chain length (numbers of carbons) increases, the melting and boiling points of the
alkanes gradually increase for these compounds. The shorter chains are gases at room
temperature, pentane with five carbons is a liquid at room temperature though has a
very low boiling point. Once we reach about 18 carbons the alkanes become solids at
room temperature and have a waxy feel.
Reactions of Alkanes
Branched alkanes have a lower boiling point than straight chain alkanes due to
the fact that they cannot pack together so well.
Alkanes in general are quite unreactive. Their strong, non polar bonds make
them unreactive to electrophiles, nucleophiles, acids, bases, oxidising or
reducing agents. But they do burn and they will react with halogens under the
right conditions.
Combustion
Short chain alkanes burn completely in a good supply of oxygen to give carbon
dioxide and water.
CH4(g) + 2O2(g) -> CO2 (g) + 2H2O(l) ΔH = -890 kJ mol-1
These combustion reactions give out large amounts of heat and have a very
high negative enthalpy value, making them incredibly important to us as fuels.
Examples: Methane (natural gas), Propane (camping gas), Butane (calor gas),
petrol (a mixture, averaging 8 carbons in length) and paraffin (a mixture
between C10-C18)
Reactions of Alkanes
Incomplete combustion
In a limited supply of oxygen, the poisonous gas carbon monoxide, CO is formed,
for example:
C3H8 (g)+ 3½O2 (g) --> 3CO (g) + 4H2O (l)
This is called incomplete combustion. With even less oxygen, carbon (soot) is
produced. This often happens with longer chain hydrocarbons, which need more
oxygen to burn compared with shorter chains.
Reaction with halogens
A halogen and an alkane will react if a bright light source is present. It is actually
the UV component of the light which starts the reaction. Alkanes do not react with
halogens in a dark room at room temperature.
C6H14(g)
+
Br2(l)
-->
C6H13Br(l) +
HBr(g)
Hydrocarbons:Fuels
Crude Oil
Currently the worlds main source of organic chemicals. It was produced
over millions of years by the breakdown of plant and animal remains at
high pressure and temperatures deep below the sea. Because it was made
so long ago it is known as a fossil fuel and is not renewable.
Crude oil is a mixture mostly of alkanes, both unbranched and branched.
Crude oils from different sources have different compositions. Below is
the composition of Gippsland Crude Oil.
Hydrocarbons:Fuels
Crude Oil
Crude oil contains small amounts of other compounds, for example some
that contain sulphur. These produce sulphur dioxide, SO2, when they are
burned. This is one of the causes of acid rain, it reacts with oxygen and
water in the atmosphere to form sulphuric acid.
Fractional Distillation of Crude Oil
To convert crude oil into useful products we have to separate this mixture.
We can do this by heating and then collecting the fractions. This can be
done because each of the fractions have a different boiling point.
Hydrocarbons:Fuels
Fractionating Tower
The crude oil is first heated in a furnace so that it vaporises.
The vapours pass into a tower that is cooler at the top than the bottom.
They pass up the tower via a series of trays containing bubble caps until
they arrive at a tray that is sufficiently cool (at a lower temperature than
their boiling point). Then they condense to a liquid.
The mixture of liquids that condenses on each tray is piped off.
The shorter chain hydrocarbons condense in the trays nearer to the top
of the tower, where it is cooler, because they have lower boiling points.
The thick residue that collects at the base of the tower is called bitumen
or tar and is used for road surfacing.
Industrial Cracking
Cracking is needed because when fractional distillation of crude oil occurs, you get a
very high percentage of long chain hydrocarbons. So the two useful results of cracking
are:
Shorter, more useful chains are produced
Some of the products produced are alkenes. These can be converted to other
compounds very easily and are therefore very useful.
There are a number of different types of cracking.
Thermal Cracking
This reaction involves heating alkanes to a high temperature, under high pressure.
Carbon-carbon bonds break homolytically and two shorter chain are produced each
ending in an unpaired electron. As there are not enough hydrogens to produce two
alkanes, one of the new chains must have a carbon-carbon double bond.
In thermal cracking any number of carbon-carbon bonds can break, and anywhere on
the chain.
Industrial Cracking
Catalytic Cracking
This kind of cracking takes place at a much lower temperature (approx 800K)
and uses a zeolite catalyst which consists of silicon dioxide and aluminium
oxide. Zeolites, as well as being acidic, have a honeycomb structure which
has an enormous surface area. This form of cracking is used to produce
hydrocarbons such as motor fuels and aromatic compounds.
The products of catalytic cracking are mainly gases, which means chain
lengths of less than five. The mixture also decolourises bromine showing that
it contains alkenes as well.
Industrial Cracking
A number of other processes are carried out on the products of
fractional distillation and crack, to improve motor fuels.
Isomerisation
Straight chain alkanes make relatively poor moter fuels. Straight chain
alkanes burn too quickly causing what is often called “knocking” in the
engine. Branched chain alkanes burn more slowly and steadily
promoting efficient combustion.
To produce branch chain alkanes you heat the straight chains under
pressure with a platinum catalyst.
Reformation
In reformation we heat straight chain alkanes under pressure with a
catalyst of platinum and alumina. This removes hydrogen in steps
ultimately ending up with cyclic hydrocarbons.
Crude Oil and the Environment
Crude oil started off providing us with fuel and lubricants, but from the 1940s
onwards, it became a starting point for other synthetic materials such as dyes and
plastics as well as drugs etc. This is often known as a chemical feedstock.
Crude oil based fuels
As you know already fuels based on crude oil are produced by distillation and
cracking. They are alkanes which include methane (CH4, butane (C4H10) and octane
(C8H18) which is the main constituent of petrol. They all burn in air to give out large
quantities of heat.
Example
C4H10
+
61/2O2 --> 4CO2
+
5H2O
ΔH= -2877 kj mol-1
The heat given out is the difference between the energy that has to be put in to break
the bonds in the reactants and the energy that is given out when the bonds are
formed into products. These fuels give out a LOT of heat per gram when burnt. They
are therefore very energy dense.
Crude Oil and the Environment
All hydrocarbon fuels produce polluting products when they burn. These
include:
Carbon monoxide – Toxic gas
Unburnt hydrocarbons – These may form photochemical smog
Carbon dioxide – A greenhouse gas
Nitrogen dioxides – Contribute to acid rain
Sulphur dioxides – Contribute to acid rain
As I hope you are well aware, crude oil is a fossil fuel, meaning that it takes
millions of years to be produced from plant and animal decay and hence
cannot be renewed in the short term, so new alternatives source of energy
much be found.
Crude Oil and the Environment
Biofuels
These are fuels that are obtained from fast growing plants so they are renewable.
Some biofuels can be used as they are, for example straw or wood can be burnt
directly. Other crops such as sugar or rape seed oil need chemical treatment to
turn them into useable sources of energy.
Ethanol
Ethanol (C2H5OH), when made from fermentation of sugar and other
carbohydrates obtained from plants, is a biofuel. It burns in oxygen to form carbon
dioxide and water.
C2H5OH +
3O2
-> 2CO2
+
3H2O
ΔH -1367 kJ mol-1
Ethanol has an energy density of about 30 kJ per gram, significantly less than
alkane fuels (around 50 kJ per gram). With some modification, normal car engines
can run on pure ethanol, or ethanol/petrol mixtures. This is currently being done
in Brazil, so it reduces the need to import crude oil.