Petrol is a complex mixture of many different compounds carefully blended to give
the desired properties. These compounds are obtained from crude oil in several
ways.
Crude oil is a mixture of many hundreds of hydrocarbons (compounds of the
elements hydrogen and carbon only). Crude oil is a thick black liquid but dissolved in
it are gases and solids.
At a refinery, crude oil is separated into fractions (mixtures of hydrocarbons with a
specific boiling range). The process is called fractional distillation. The crude oil is
heated and the vapour passes into a distillation column. There is a temperature
gradient in the column (coolest at the top, hottest at the bottom) and trays at various
levels on to which less volatile hydrocarbons condense (as fractions) and through
which the more volatile ones pass. (Refer to Chemical Storyline pages 23 – 24 for
more information about this).
The gasoline fraction is a mixture of liquids, mostly alkanes with between 5 and 7
carbon atoms, boiling in the range 25 °C to 75 °C. The gasoline and gas oil fractions
are sources of petrol components. Another important fraction, naphtha, is also
converted into high-grade petrol as well as being used in the manufacture of many
organic compounds.
Alkanes
Chemical Ideas 12.1 deals with the Alkane homologous series.
Key terms in this chapter are:
Alkane.
Hydrocarbon.
Homologous series.
Saturated
General formula.
Molecular formula.
Structural formula.
Skeletal formula.
Straight-chain isomer.
Branched-chain isomer.
Structural isomer; structural isomerism.
Cycloalkane.
Tetrahedral bond angle.
Functional group.
Aliphatic.
Aromatic.
Arene.
Alkene.
Alcohol.
Ether.
In this section you are going to learn:
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That the members of the alkane family form the fundamental homologous
series.
How to use a 'general formula' to derive the molecular formulae for a
homologous series.
Ways to represent chemical formula for organic compounds.
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The names of the first ten alkanes.
How to draw structural formulae for the first ten alkanes.
The physical states of the alkanes.
How structural isomers arise in the alkanes.
About cycloalkanes.
How to name alkanes systematically.
Molecules have shapes, and how to draw these for the alkanes.
To plot numerical data using Excel.
To interpret plotted data - ∆H°c related to a homologous series.
A little more about organic chemistry including arenes, alkenes, alcohols and
ethers.
The Alkanes are a family (or homologous series) of hydrocarbons (compounds of
carbon and hydrogen only) characterised by the presence of carbon-carbon single
covalent bonds. Because they have only single bonds (and no double or triple bonds)
they are said to be saturated. They have the general formula CnH2n+2, where n = 1, 2,
3, 4, etc., from which their molecular formulae can be worked out. For example, the
fourth member (n = 4) of the alkane family has the molecular formula C4H10 and is
called butane. The formula of this compound can be represented in a number of
ways:
The table below refers to the straight-chain isomers.
It is important to know the names of the first ten members of the alkane homologous series
because these form the basis for naming other organic compounds.
Q1.
n
Use a chemistry textbook to help you complete the table below.
Molecular
formula
Name
C4H10
Butane
Structural formula
Physical
state (r.t.p.)
1
2
3
4
5
6
7
8
9
10
Gas
Alkyl groups are an important part of organic chemistry. An alkyl group exists
as part of an organic molecule. In theory they are obtained by removing a
hydrogen atom from an alkane, e.g. methyl, CH3-, from methane; ethyl, CH3CH2from ethane, etc. The propyl group is shown below:
Structural Isomerism and Naming Alkanes
All of the structures referred to so far are said to be 'straight chain molecules'. Their
carbon chains are in fact zig-zag, but they have no branching. The molecular formula
C4H10 can also represent a branched hydrocarbon called methylpropane as well as
butane.
The above two compounds are structural isomers.
Structural Isomerism
Structural isomers are different compounds with the same molecular formula but
different structural formulae
It is important to learn the names of the first ten straight-chain alkanes since the
systematic names of all the organic compounds are related to these. You learn how to
name organic compounds as you go along. Here are some rules for naming alkanes:
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Identify the longest straight carbon chain.
Identify the alkyl groups.
Number the longest carbon chain so that the alkyl groups are bonded to the lowest
numbered carbon atoms.
Include the alkyl groups in alphabetical order.
Put dashes between numbers and letters, and commas between numbers.
Q2.
Work out the name of the alkane shown below.
Q3.
Draw the full structural formula for each of the structural isomers of
the molecular formula C6H14. Name each one of these.
As well as straight-chain and branched-chain isomers, there are also alkanes with ring
structures. However, these are isomeric with the alkenes, having the general formula
CnH2n.
Q4.
The skeletal formula of cyclohexane is shown below. By the side of
it draw its full formula.
Q5.
Draw the full structural formulae and skeletal formulae of
cyclopropane, cyclobutane and cyclopentane.
Q6.
Make a molecular model for each of the following alkanes and
cycloalkanes: methane, ethane, butane, methylpropane, pentane,
2-methylpentane, 3-methylpentane, cyclopropane, cyclobutane,
cyclopentane, cyclohexane.
Molecules have shapes
From the model building activity you will have noticed that the alkane molecules
have shapes. For example, the simplest alkane methane is described as tetrahedral in
shape. This is because its carbon atom is at the centre of, and its four hydrogen atoms
are each at the corners of, a perfect tetrahedron.
Note that the H-C-H bond angle is approximately 109°. This is known as the
tetrahedral bond angle. In fact, all of the bond angles in the open-chain alkanes are
tetrahedral bond angles. It is very important to appreciate this, and the shapes of these
molecules, if you are to be able to draw structural isomers and to name them
accurately.
Q7.
Use the 'key' above to draw hexane and represent its
3-dimensional shape on paper. Mark on your diagram a few bond
angles.
Reactions of the Alkanes
The alkanes are not particularly reactive. They undergo combustion reactions in
which they become oxidised (after all they are used as fuels!), and they can be made
to react with halogens such as chlorine (see The Atmosphere unit). For example,
CH4(g) +2O2(g) → CO2(g) + 2H2O(l);
∆H° = -890 kJ mol-1
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Finally in this section, learn a little more about organic chemistry…
All organic compounds belong to one of two broad categories: aliphatic or aromatic.
At its simplest, aromatic compounds are those that contain the benzene ring structure.
You can think of benzene (molecular formula C6H6) itself as the fundamental
aromatic compound. Two ways to represent the structure of benzene are:
Arenes are aromatic hydrocarbons. Methylbenzene is an example.
The alkanes are one homologous series (family) of organic compounds. There are
several others, each characterised by their functional group. The alkenes are another
family of hydrocarbons but these are characterised by the functional group carboncarbon double covalent bond. They have the general formula CnH2n (where n = 2, 3, 4,
etc.).
Q9.
Draw the structures of the first two member of this homologous
series, ethene and propene.
In the alcohols (general formula CnH2n+1-OH) the functional group is the alcohol (or
hydroxyl) group, -OH. Ethanol is the second member of this homologous series:
Isomeric with alcohols are the ethers. Their functional group is an oxygen atom
bridging two carbon atoms (C-O-C). The general formula can be represented as
R-O-R' (where R- and R'- are the same or different alkyl groups). An example is
shown below: