Transcript 10.2 Alkanes

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Write the abbreviated structural formulae and
name all the structural isomers of C6H14.
Reactivity and Reactions
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General Formula CnH2n+2
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Low Reactivity
◦ Be able to explain why.
◦ Saturated hydrocarbons
◦ Strong C-C and C-H bonds
 Relatively high bond enthalpies
◦ Low polarity
 Why?
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Combustion reactions with oxygen
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Substitution reactions with halogens
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Alkanes are flammable
Highly exothermic
One of the most common
energy sources
CnH 2n+2 + O2  CO2 +
H2O
Balancing not difficult if you
use the “shortcut”
1.
All carbon atoms are converted into Carbon
Dioxide
-fixes the carbon dioxide coefficient
2. All hydrogen atoms are converted to water
-fixes the water coefficient
3. Lastly balance the oxygen molecules
C8H18 + O2  CO2 + H2O
First balance the carbons:
C8H18 + O2  8 CO2 + H2O
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Next balance water:
C8H18 + O2  8 CO2 + 9 H2O
Balance Oxygen:
C8H18 + 25/2 O2  8 CO2 + 9 H2O
2 C8H18 + 25 O2  16 CO2 + 18 H2O
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If there is not enough
oxygen available,
incomplete combustion
occurs and CO and C are
also produced as products
CO—colorless, odorless,
toxic gas
C—black smoke
These products along with
lead additives in gasoline
are a major source of
pollution
CH4 + O2  CO + H2O
Balanced:
CH4 + 3/2 O2  CO + 2 H2O
2CH4 + 3 O2  2 CO + 4 H2O
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Alkanes do not react with halogens in the
dark at room temperature, but will react in
the presence of sunlight:
C6H14 (g) + Br2 (g) → C6H13Br (l) + HBr (g)
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Many organic molecules undergo substitution
reactions.
In a substitution reaction one atom or group
of atoms is removed from a molecule and
replaced with a different atom or group.
Example:
Cl2 + CH4  CH3Cl + HCl
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The ultraviolet light is a source of energy that
causes the chlorine molecule to break apart
into 2 chlorine atoms, each of which has an
unpaired electron
The energies in UV are exactly right to break
the bonds in chlorine molecules to produce
chlorine atoms.
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Free radicals are formed if a bond splits
evenly - each atom getting one of the two
electrons. The name given to this is
homolytic fission.
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The productive collision happens if a chlorine
radical hits a methane molecule.
The chlorine radical removes a hydrogen atom
from the methane. That hydrogen atom only needs
to bring one electron with it to form a new bond to
the chlorine, and so one electron is left behind on
the carbon atom. A new free radical is formed this time a methyl radical, CH3 .
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If a methyl radical collides with a chlorine
molecule the following occurs:
CH3. + Cl2  CH3Cl + Cl.
The methyl radical takes one of the
chlorine atoms to form chloromethane
In the process generates another chlorine
free radical.
This new chlorine radical can now go
through the whole sequence again, It will
produce yet another chlorine radical - and
so on and so on.
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The free radical propagation does not go on
for ever.
If two free radicals collide the reaction is
terminated.
2Cl.  Cl2
CH3. + Cl .  CH3Cl
CH3 . + CH3.  CH3CH3
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Write the steps in the free radical mechanism for
the reaction of chlorine with methyl benzene. The
overall reaction is shown below. The methyl group
is the part of methyl benzene that undergoes
attack.
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Initiation
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Propagation
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Cl2  2Cl.
Termination
2Cl.  Cl2
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1.
2.
3.
Substitution of an alkane with a halogen has
3 steps:
Initiation
Propagation
Termination
Rate of reaction: Cl2 > Br2 > I2 … Why???
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1. Explain why alkanes have such low reactivity.
2. What types of reactions do alkanes readily
undergo? Give an example.
3. Use an equation to describe the incomplete
combustion of alkanes.
4. Describe, using equations, the reactions of
methane and ethane with chlorine and bromine.
5. Explain the reactions of methane and ethane
with chlorine and bromine in terms of a freeradical mechanism. Reference should be made to
homolytic fission and the reaction steps of
initiation, propagation and termination.
1.
When propane undergoes complete
combustion in air, for each mole of propane
burnt, how many moles of oxygen are
consumed and how many moles of water
are formed?