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…

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hydrocarbons with similar boiling points. In many cases these fractions can be
used directly, but sometimes further separation is required into purer components.




Cracking

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…

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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…

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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…

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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…

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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…

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Haloalkanes:
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.…

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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…

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Reactions with Ammonia:

If a haloalkane is heated with ethanolic ammonia in a sealed tube, a primary amine is
formed:

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…

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

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