warm-up 11/19
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Warm-up 11/19. Write the abbreviated structural formulae and name all the structural isomers of C 6 H 14 . . 10.2 Alkanes. Reactivity and Reactions. 10.2.1 Alkanes. General Formula C n H 2n+2 Low Reactivity Be able to explain why. Saturated hydrocarbons Strong C-C and C -H bonds - PowerPoint PPT PresentationTRANSCRIPT
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Write the abbreviated structural formulae and name all the structural isomers of C6H14.
Warm-up 11/19
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10.2 AlkanesReactivity and Reactions
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General Formula CnH2n+2
Low Reactivity◦ Be able to explain why.◦ Saturated hydrocarbons◦ Strong C-C and C-H bonds
Relatively high bond enthalpies◦ Low polarity
Why?
Combustion reactions with oxygen
Substitution reactions with halogens
10.2.1 Alkanes
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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”
10.2.2 Combustion Reactions
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1. All carbon atoms are converted into Carbon Dioxide
-fixes the carbon dioxide coefficient2. All hydrogen atoms are converted to water -fixes the water coefficient3. Lastly balance the oxygen molecules
Balancing Combustion Reactions
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C8H18 + O2 CO2 + H2O
First balance the carbons:
C8H18 + O2 8 CO2 + H2O
Example: Complete Combustion of Octane
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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
10.2.2 Incomplete Combustion
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CH4 + O2 CO + H2O
Balanced:
CH4 + 3/2 O2 CO + 2 H2O
2CH4 + 3 O2 2 CO + 4 H2O
Incomplete Combustion
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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)
Reactions of Alkanes: with Halogens
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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
Free Radical Substitutions
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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.
Free Radical Mechanism-The Initiation Step
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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.
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 .
Free Radical Propagation
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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.
Free Radical Propagation II
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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
Termination Steps
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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.
Exercise
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Initiation Cl2 2Cl. Propagation
Solution
Termination 2Cl. Cl2
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Substitution of an alkane with a halogen has 3 steps:
1. Initiation2. Propagation3. Termination
Rate of reaction: Cl2 > Br2 > I2 … Why???
Reactions of Alkanes: with Halogens
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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 free-radical mechanism. Reference should be made to homolytic fission and the reaction steps of initiation, propagation and termination.
Homework
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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?
Warm-up 11/20