General Formula C n H 2n+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
Alkanes are flammable Highly exothermic One of the most common energy sources C n H 2n+2 + O 2 CO 2 + H 2 O 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
C 8 H 18 + O 2 CO 2 + H 2 O First balance the carbons: C 8 H 18 + O 2 8 CO 2 + H 2 O
Next balance water: C 8 H 18 + O 2 8 CO 2 + 9 H 2 O Balance Oxygen: C 8 H 18 + 25/2 O 2 8 CO 2 + 9 H 2 O 2 C 8 H 18 + 25 O 2 16 CO 2 + 18 H 2 O
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
CH 4 + O 2 CO + H 2 O Balanced: CH 4 + 3/2 O 2 CO + 2 H 2 O 2CH 4 + 3 O 2 2 CO + 4 H 2 O
Alkanes do not react with halogens in the dark at room temperature, but will react in the presence of sunlight: C 6 H 14 (g) + Br 2 (g) → C 6 H 13 Br (l) + HBr (g)
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: Cl 2 + CH 4 CH 3 Cl + HCl 11
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. 12
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. 13
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, CH 3. 14
If a methyl radical collides with a chlorine molecule the following occurs: CH 3. + Cl 2 CH 3 Cl + 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. 15
The free radical propagation does not go on for ever. If two free radicals collide the reaction is terminated. 2Cl. Cl 2 CH 3. + Cl. CH 3 Cl CH 3. + CH 3. CH 3 CH 3 16
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. 17
Substitution of an alkane with a halogen has 3 steps: 1. Initiation 2. Propagation 3. Termination Rate of reaction: Cl 2 > Br 2 > I 2 … 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.
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?