Alkanes and Alkenes Notes Including Mechanisms

Topics: 1.6, 2.8 and 2.9 of syllabus.

I was having trouble understanding the curly arrow diagrams, and the mechanisms revolving around Alkanes and Alkenes so made the following revision notes on the topics. They're rather detailed, but cover the whole of the text book and more!

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Alkanes and Alkenes
Alkanes are saturated hydrocarbons, containing only carbon-carbon, and
carbon-hydrogen single bonds. They are among the least reactive organic
compounds. They are often used as fuels and lubricants so are useful to industry.
Main source of alkanes is crude oil which contains many different types of alkane.
Physical Properties:
Polarity: almost non-polar because electronegativity of carbon and hydrogen are so
similar (Carbon 2.5 and Hydrogen 2.1). Therefore the only intermolecular forced are
weak van der Waals, larger the molecule the stronger the force.
Boiling Point: increased intermolecular forces result in an increased boiling point;
therefore larger length chains have larger boiling points. However, branched chains
result in lower boiling points because they cannot pack as closely together
decreasing van der Waals forces.
Solubility: alkanes are insoluble in water, because the H-bonding in water is stronger
than the van der Waals between alkane molecules. However alkanes do mix with
relatively non-polar liquids.
Reactivity: relatively unreactive as will not react with acids, bases, oxidising/ reducing
agents however, will react with halogens. Will burn in plentiful supply of oxygen to
form Carbon Dioxide and Water (or in restricted supply to form CO and Water).
Fractional Distillation:
The different hydrocarbons in crude oil have different boiling points. This is because
the chain length varies. The greater the number of carbon atoms in the chain, the
longer the chain length. This results in more Van der Waal's forces acting between
the molecules and a greater intermolecular attraction. Thus more energy is needed
to separate the molecules and the boiling point is higher. It is the difference in boiling
points of the different hydrocarbons in crude oil which is used to separate them
from each other.
The crude oil is passed into a tall tower called a fractionating column. This is very
hot near the base but much cooler near the top. When the crude oil is passed into
the tower, near the bottom, most of the mixture boils and starts to rise up the
tower. As they rise up the tower, they start to cool down and will gradually condense
back into liquid form. They are then tapped off. The larger hydrocarbons, with higher
boiling points, will condense first and be tapped off near the base of the column. The
smaller hydrocarbons, with smaller boiling points, will condense later and be tapped
off near the top of the column. Thus the separation is achieved. Note that the
process involves breaking intermolecular forces only; the molecules themselves
are unaffected by this process.
This process does not actually separate the crude oil mixture into pure hydrocarbon
components, but into mixtures called fractions. Fractions are mixtures of

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In many cases these fractions can be
used directly, but sometimes further separation is required into purer components.
As shorter chain molecules are in greater demand but are yielded less from crude oil
to meet demand longer chain molecules are broken down into shorter chains to
meet economic demands by cracking. Cracking results in a longer chain hydrocarbon
being broken down into a smaller chain alkane and an alkene.…read more

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Thermal Cracking:
In thermal cracking, the bonds are broken using a high temperature (400 ­ 900oC)
and a high pressure (70 atmospheres).
The high temperatures mean that the molecule breaks near the end of the chain,
giving a high percentage of small alkenes such as ethene.
Most thermal cracking reactions involve the formation of one of more small alkane
molecules and one alkene molecule. Naphtha (C7 ­ C14) is usually used as the starting
material.…read more

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Pollution problems associated with burning hydrocarbons
Carbon dioxide:
Although carbon dioxide is not poisonous and is naturally removed from the
atmosphere by plants, the enormous quantities of hydrocarbons burned in recent
years have caused carbon dioxide levels to rise significantly.
Carbon dioxide, along with various other compounds, prevents the earth's heat from
escaping into space and is resulting in an increase in the earth's temperature. This is
known as global warming.…read more

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Most crude oil deposits contain sulphur as an impurity. Oil refineries are increasingly
treating the petrol fractions to lower the sulphur content, but some sulphur is still
present in most hydrocarbon fuels. When the fuel is burned, the sulphur also burns,
producing sulphur dioxide:
S(s) + O2(g) SO2(g)
This gas dissolves in rainwater forming a very acidic solution, known as acid rain. This
causes various problems, including erosion of buildings and statues, killing of plants
and trees, and killing of fish through contamination of lakes.…read more

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Ways of reducing pollution levels
A number of ways have been developed to reduce the polluting effects associated
with the burning of fossil fuels. Two examples are given here:
a) Flue gas desulphurisation
Many factory chimneys contain alkaline materials such as lime (calcium oxide). These
absorb the acidic gases such as SO2 and thus prevent them from escaping:
SO2 + CaO CaSO3
Further reactions result in the formation of CaSO4 (gypsum) which is used to make
plaster.…read more

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A haloalkane is a group 7 element joined to an alkane. Nomenclature is obvious
fluoro, bromo, chloro and iodo are used. Numbers are used to indicate how many
and on which carbon the halogen is bonded. The prefixes di, tri and tetra are used to
indicate the number present.
Halogens are listen in alphabetical order if more than one is present, not by carbon
Bond polarity: The bond is polar because the halogen has a greater electronegativity
than carbon.…read more

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The lone pair of electrons of a nucleophile is attracted towards a partially positively
charged carbon atom. A curly arrow starts at a lone pair of electrons and moves
towards carbon delta-plus.
The lower curly arrow shows the electron pair in the C-X bond moving to the halogen
atom, X making it a halide ion. The halide ion is the leaving group.
The rate of substitution depends on the halogen, fluoro- compounds are unreactive
due to the strength of the C-F bond.…read more

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Reactions with Ammonia:
If a haloalkane is heated with ethanolic ammonia in a sealed tube, a primary amine is
R-X + 2NH3 R-NH2 + NH4X
The mechanism is again nucleophilic substitution:
Ammonia is a nucleophile because it has a lone pair of electrons that it can donate
(although it has no negative charge). The nitrogen ion has a delta-minus charge.
Because ammonia is a neutral nucleophile, a proton, H+ must be lost to form the
neutral product called a primary amine.…read more

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This is v good, you are so helpful thank you;)

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