Alkanes and their reactions
1) The C-C bonds and C-H bonds in alkanes are pretty non-polar. But most chemicals are polar - like water, haloalkanes, acids and alkalis.
2) Polar chemicals are attracted to the polar groups on molecules they attack. Alkanes don't have any polar groups, so they don't react with polar chemicals.
3) Alkanes will react with some non-polar things though - such as oxygen or the halogens. But they'll only bother if you give them enough energy.
Alkanes + Halogen ---> Haloalkanes
1. Halogens react with alkanes in photochemical ractions. Photochemical reactions are started by ultraviolet light.
2. A hydrogen atom is substituted (replaced) by chlorine. This is a free-radical substitution reaction.
Free radicals are particles with an unpaired electron, written like this - Cl* or CH3*
Chlorine and methane react with a bit of a bang to form chloromethane: CH4 + Cl2 -> CH3Cl + HCl
The reaction has three stages.
Free radicals are produced
1. Sunlight provides enough energy to break the Cl-Cl bond - this is photodissociation.
Cl2 ---> 2Cl*
2. The bonds splits equally and each atom gets to keep on electron. The atom becomes a highly reactive free radical, Cl*, because of its unpaired electron.
Free radicals are used up and created in a chain reaction.
1. The Cl* attacks a methane molecule: Cl* + CH4 ---> CH3* + HCl
2. The new methyl free radical, CH3*, can attack anoher Cl2 molecule: CH3* + Cl2 -> CH3Cl + Cl*
3. The new Cl* can attack another CH4 molecule, and so on, until all the Cl2 or CH4 molecules are wiped out.
Free radicals are mopped out.
1. If two free radicals join together, they make a stable molecule.
2. There are heaps of possible termination reactions. Here's a couple of them to give you the idea:
Cl* + CH3* ---> CH3Cl
CH3* + CH3 * ---> C2H6
What happens now depends on whether there's too much chlorine or too much methane.
1. If the chlorine's in excess, Cl* free radicals will start attacking chloromethane, producing dichloromethane, CH2Cl2, trichlormethane, CHCl3, and tetrachloromethane, CCl4.
2. But if the methane's in excess, then the product will mostly by chloromethane.
1. Chlorofluorocarbons (CFCs) are haloalkane molecules where all of the hydrogen atoms have been replaced by chlorine and fluorine atoms. E.g. trichloroflurormethane and chlorotrifluoromethane.
2. Both CFCs and chloroalkanes can be used as solvents - they both used to be used in dry cleaning and degreasing.
The Ozone Layer
1. Ozone in the upper atmosphere acts as a chemical sunscreen. It absorbs a lot of the ultraviolet radiation which can cause sunburn or even skin cancer.
2. Ozone's formed naturally when an oxygen molecule is broken into two free radicals by ultraviolet radiation. The free radicals attack other oxygen molecules forming ozone. Just like this: O2 + hv ---> O* + O* ----> O2 + O* ---> O3 You've heard of how the ozone layer's being destroyed by CFCs, right. Well, here's what's happening. a. Chlorine free radicals, Cl*, are formed when the C-Cl bonds in CFCs are broken down by ultraviolet radiation. E.g. CCl3F ---> CCl2F* + Cl* b. These free radicals are catalysts. They react with ozone to form an intermediate (ClO*) and an oxygen molecule. Cl* + O3 ---> O2 + ClO ClO* + O3 ---> 2O2 + Cl* 3. So the overall reaction is... 2O3 ---> 3O2 ... and Cl* is the catalyst.
CFCs and their ban
1. CFCs are pretty unreactive, non-flammable and non-toxic. They used to be used in fire extinguishers, as propellants in aerosols, as the coolant gas in fridges and to foam plastics to make insulation and packaging materials.
2. In the 1970s scientists discovered that CFCs were causing damage to the ozone layer. The advantages of CFCs couldn't outweigh the environmental problems they were causing, so they were banned.
3. Chemists have developed alternatives to CFCs. HCFCs (hydrochlorofluorocarbons) and HFCs (hydrofluorocarbons) are less dangerous than CFCs, so they're being used as temporary alternatives until safer products are developed.
4. Most aerosols now have replaced by pump spray systems or use nitrogen as the propellant. Many industrial fridges use ammonia or hydrocarbons as the coolant gas, and carbon dioxide is used to make foamed polymers.