Unit 2 Section 4 Synthesis of Chloroalkanes

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Synthesis of Chloromethane - Initiation

CH4 + Cl2 >>>(U.V.)>>> CH3Cl + HCl - This is free radical substitution

1. Initiation:

Free radicals are produced. Sunlight provides enough energy to break Cl-Cl bonds - this is photodissociation. 

  • Cl2 >>>(U.V.)>>> 2Cl*

Bond splits equally and each atoms gets to keep one electron. The atom becomes a highly reactive free radical because of its unpaired electron.

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Synthesis of Chloromethane - Propagation

2. Propagation:

Free radicals are used up in a chain reaction. First, Cl* attacks a methane molecule.

  • Cl* + CH4 >>> CH3* + HCl

The new methyl free radical, CH3*, can then attack another Cl2 molecule.

  • CH3* + Cl2 >>> CH3Cl + Cl*

The new Cl* can attack another CH4 molecule, and so on, until all the Cl2 or CH4 molecules are used up.

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Synthesis of Chloromethane - Substitution

3. Substitution:

If chlorine is in excess, the hydrogen atoms on methane will eventually be replaced by chlorine atoms. This means that you will get: CH2Cl2, CHCl3 and CCl4.

  • CH4 + Cl2 >>> CH3Cl + HCl
  • CH3Cl + Cl2 >>> CH2Cl2 + HCl
  • CH2Cl2 + Cl2 >>> CHCl3 + HCl
  • CHCl3 + Cl2 >>> CCl4 + HCl

But If methane is in excess, the chlorine will be used up quickly and the product will be mostly chloromethane:

  • CH4 + Cl2 >>> CH3Cl + HCl
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Synthesis of Chloromethane - Termination

4. Termination:

In termination free radicals are gotten rid of. If two free radicals join together, they make a stable molecule - this terminates the chain reaction.

  • CH3* + Cl* >>> CH3Cl
  • CH3 * + CH3* >>> C2H6
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  • Chloroflourocarbons (CFC's) are haloalkane molecules where all of the hydrogen atoms have been replaced by chlorine and flourine atoms.
  • Both CFC's and chloroalkanes can be used as solvents - they both used to be used in dry cleaning and degreasing.
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The Ozone Layer

Ozone in the upper atmosphere acts as a chemical sunscreen. It absorbs a lot of ultra violet radiation which can cause sunburn or even skin cancer.

Ozone is formed naturally when an oxygen molecule is broken down into free radicals by ultra violet radiation:

  • O2 + U.V. >>> O* + O*

The free radicals attack other oxygen molecules forming ozone:

  • O2 + O* >>> O3
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Chlorofluorocarbons and the Ozone Layer

Ozone can be destroyed by CFC's.

Chlorine free radicals are formed when the C-Cl bonds in CFC's are broken down by U.V. radiation:

  • CCl3F(g) >>>(U.V.)>>> CCl2F*(g) + Cl*(g)

These free radicals are homogeneous catalysts - they're in the same phase as the ozone molecules.

They react with ozone to form an intermediate (ClO*), and an oxygen molecule.

  • Cl*(g) + O3(g) >>> O2(g) + ClO*(g)
  • ClO*(g) + O3(g) >>> 2O2(g) + Cl*(g)

The Cl* is regenerated so it only takes one to destroy a lot of ozone molecules and Cl* is the catalyst. overall reaction:

  • 2O3(g) >>> 3O2(g)
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Environmental Problems of CFC's

CFC's are unreactive, non-flammable and non-toxic. They used to be used in fire extinguishers, as aerosol propellants, as coolant gas in fridges and as foam plastics to make insulation and packaging materials.

In the 1970's scientists discovered that CFC's were causing damage to the ozone layer. Their advantages couldn't outweigh the environmental problems they were causing, so they were banned.

Chemists have developed alternatives to CFC's, such as HCFC's(hydrochloroflourocarbons) and HFC's(hydroflourocarbons). These are less dangerous than CFC's so they're being used temporarily until safer products are developed.

Most aerosols now have been 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 plastic.

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