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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

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Warm-up 11/19

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warm up 11 19
Warm-up 11/19
  • Write the abbreviated structural formulae and name all the structural isomers of C6H14.
10 2 alkanes

10.2 Alkanes

Reactivity and Reactions

10 2 1 alkanes
10.2.1 Alkanes
  • 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 2 combustion reactions
10.2.2 Combustion Reactions
  • Alkanes are flammable
  • Highly exothermic
  • One of the most common energy sources
  • CnH2n+2 + O2 CO2 + H2O
  • Balancing not difficult if you use the “shortcut”
balancing combustion reactions
Balancing Combustion Reactions
  • 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

example complete combustion of octane
Example: Complete Combustion of Octane

C8H18 + O2 CO2 + H2O

First balance the carbons:

C8H18 + O2 8 CO2 + H2O


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

10 2 2 incomplete combustion
10.2.2 Incomplete Combustion
  • 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
incomplete combustion
Incomplete Combustion

CH4 + O2 CO + H2O


CH4 + 3/2 O2 CO + 2 H2O

2CH4 + 3 O2 2 CO + 4 H2O

reactions of alkanes with halogens
Reactions of Alkanes: with Halogens
  • 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)

free radical substitutions
Free Radical Substitutions
  • 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 mechanism the initiation step
Free Radical Mechanism-The Initiation Step
  • 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.


homolytic fission
Homolytic Fission
  • 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.


free radical propagation
Free Radical Propagation
  • 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 ii
Free Radical Propagation II
  • 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.


termination steps
Termination Steps
  • 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


  • 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.


  • Initiation

Cl2  2Cl.

  • Propagation
  • Termination

2Cl. Cl2


reactions of alkanes with halogens1
Reactions of Alkanes: with Halogens
  • Substitution of an alkane with a halogen has 3 steps:
  • Initiation
  • Propagation
  • Termination

Rate of reaction: Cl2> Br2 > I2 … Why???

  • 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.
warm up 11 20
Warm-up 11/20
  • 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?