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Organic
Chemistry
Lesson 4
Alkanes
Building Organic Compounds
• Use molecular models to make any of the compounds
mentioned in your mind map:
– Draw it (structural and skeletal)
– Name it
– Give it to a friend and challenge them to do the same
• Only go up to 10 carbons
• Only include branched chains for the alkanes
Compounds can often have common or
popular names
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Butter = butryic acid = butanoic acid,
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Acetone = propanone, CH3-CO-CH3
Isopropyl alcohol = propan-2-ol, CH₃CH(OH)CH₃
Formaldehyde = methanal, CH3CHO
Alcohol = ethanol, C2H5OH
Acetic acid = ethanoic acid, CH3COOH
Ether = diethyl ether, C2H5-O-C2H5
Methyl 2-hydroxybenzoate is better known as oil
of wintergreen
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CH3CH2CH2COOH
We Are Here
Lesson 4: Alkanes
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Objectives:
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Explain the stability of the alkanes
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Observe the combustion of alkanes
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Describe the free-radical substitution reactions of alkanes
and its mechanism
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Observe the free-radical substitution of hexane
Trends in physical properties of
alkanes
• Alkanes are non-polar
• Insoluble in water (oil and water don’t mix)
• Nearly all alkanes have densities less than 1.0
g/mL and are therefore less dense than water
(oil and grease float on top of water).
Methane ethane propane butane pentane hexane heptane octane nonane decane
Volatility
Branched chain alkanes
2-methylpropane (-0.11C)
boiling point
straight chain alkanes
butane (-0.5 C)
Combustion of alkanes
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Alkanes really don’t do much
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Complete combustion:
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alkane + oxygen  carbon dioxide + water
Incomplete combustion:
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Combustion is of one of two notable reactions (this is why we use them for fuels)
Alkane + oxygen  carbon + carbon monoxide + carbon dioxide + water
The amounts of C, CO and CO2 will vary depending on conditions
Incomplete combustion depends on the availability of oxygen
Task: Observe the combustion of the gas from the gas taps (propane/butane
mix) and of a small amount hexane (in spirit burners). Hold the end of a clean
boiling tube just over the flame for 15 seconds, this will collect soot (black
carbon) from the flame.
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Record all observations clearly and try to account for them
Include balanced equations to describe the (complete) combustion
Why Good Fuels?
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To do:
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Use Table 12 in the data booklet to help you determine
the trend in energy released per gram by combustion of
the alkanes.
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Use bond enthalpies to help you explain the trend noted
above.
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What do you think should be the characteristics of a good
fuel?
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Use the above to decide and explain which out of
methane and octane is a better fuel.
Why so boring stable?
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There are at least two reasons why alkanes are so
unreactive
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Task: Think back to your knowledge of molecular
structure, and look at the tables of bond enthalpies
in the data booklet to see if you can work out why.
More organic vocabulary
• Alkanes are saturated hydrocarbons. They contain all
single carbon-carbon bonds. [Unsaturated
hydrocarbons contain double or triple carbon-carbon
bonds.]
• Alkanes are also aliphatic, meaning the carbon atoms
form open chains. [In contrast to aromatic compounds
which contain benzene rings.]
Alkanes contain only strong C-C (348 kJ/mol) and C-H
(412 kJ/mol) bonds.
Alkanes are non-polar. They have weak intermolecular
forces and they are not susceptible to attack by
common reactants.
Halogenation
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Alkanes will undergo halogenation if reacted with a
halide in the presence of u.v. light.
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For example:
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C2H6(g) + Cl2(g)
ethane
u.v.
CH3CH2Cl(g) + HCl(g)
chloroethane
This reaction is an example of free radical
substitution
Radicals
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Radicals are species with unpaired electrons
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They are crazy reactive
Halogens form radicals when hit by uv light of the right frequency:
u.v.
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Cl2
2 Cl•
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The dot after the Cl represents the unpaired electron and tells us we have a radical
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This process is called homolytic fission – the bond breaks equally with one
electron going to each chlorine
Task: draw Lewis structures for the Cl2 molecule and each of the Cl• radicals.
Show the process using single-sided curly arrows, sometimes called
fishhooks, to show the movement of a single electron.
Reaction Mechanism: Free Radical Substitution
u.v.
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Cl2
2 Cl•
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Cl• + C2H6  C2H5• + HCl
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C2H5• + Cl2  C2H5Cl + Cl•
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C2H5Cl + Cl•  C2H4Cl• + HCl
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C2H4Cl• + Cl2  C2H3Cl2 + Cl•
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Cl• + Cl•  Cl2
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Cl• + C2H5•  C2H5Cl
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C2H5• + C2H5•  C4H10
Initiation
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Propagation
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Radicals formed by homolytic fission
These steps feed each other the
radicals needed to continue. This type
of reaction is called a chain reaction.
Termination
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Any two radicals can combine to
terminate the reaction
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Concentration of radicals is low so this
is a rare event
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A single radical can cause thousands of cycles of the propagation stage before it reaches
termination
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This same mechanism applies to all of the halogens
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The alkane can be substituted multiple times, until every H has been replaced
Try it Yourself
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Place approx 1 cm3 of hexane into two test tubes.
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Add roughly 1 cm3 of bromine water to each and stopper them, then
give them a good shake.
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Leave one test tube in the classroom but take the other outside and
shake it in direct sunlight.
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Record and explain all observations.
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Write equations showing the mechanism of the reaction.
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Draw full structural and skeletal formulas of at least 6 possible
products, and name each one.
Extension:
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Research the role of free radical reactions in the
depletion of the ozone layer or the combustion of
hydrocarbon alkanes on global warming and climate
change.
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Remember CO2 and H2O are greenhouse gases.
CO is toxic to human.
Unburned carbon is particulate matter.
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Key Points
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Alkanes are fairly unreactive
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They release a lot of energy on combustion, and are
easy to handle, store and transport which makes
them good fuels
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Alkanes undergo free radical substitution to form
halogenoalkanes and a hydrogen halide in the
presence of UV light