A - THE ATMOSPHERE
Introduction
Radiation and The Atmosphere
Ozone
Halogenoalkanes 
Problems in the Troposphere
Halogenoalkanes
Physical Properties
Chemical Reactions
Reactions of Halogenoalkanes
Chlorofluorocarbons (CFCs)
Halogenoalkanes
If halogen atoms replace the hydrogen atoms in alkanes, halogenoalkanes are produced. They derive their name from the alkane, for example:
| CH3Cl | chloromethane |
| C2H5Br | bromoethane |
Two halogenoalkanes can be derived from propane which can be distinguished by numbering the carbon atom to which the halogen is attached, for example:
| CH3CH2CH2Cl | 1-chloropropane |
| CH3CHClCH3 | 2-chloropropane |
CH2ClCH2CHBrCH3 is called 3-bromo-1-chlorobutane. Bromo and chloro are listed in alphabetical order.
Physical Properties
The carbon-halogen bond is polar but this does not have a great influence on physical properties. They are immiscible in water. The boiling point depends on the size of the molecule and the halogen. Compounds with Cl or Br have higher boiling points than those with F.
Chemical Reactions
Reactions of halogenoalkanes involve breaking the C-halogen bond. The bond can break homolytically or heterolytically.
Homolytic fission forms radicals; this reaction can occur in the stratosphere.
CH3Cl + hν CH3 + Cl
Heterolytic fission is more common under laboratory conditions when reactions tend to be carried out in a polar solvent such as water or ethanol. The C - Hal bond can break and form a positive carbocation:
The reaction conditions can often determine how a bond breaks: in a polar solvent, the C - Br bond breaks heterolytically, but in a non-polar solvent such as hexane, or at high temperature, the C - Br bond breaks homolytically.
When halogenoalkanes react, it usually involves the breaking of the C - Hal bond. The stronger the bond the more difficult it is to break. The strength of the bond decreases C - F > C - Cl > C - Br > C - I. The effect of this is that CFCs are not easily broken down, and get into the stratosphere to damage the ozone layer.
Reactions of Halogenoalkanes
Halogenoalkanes are polarised due to the electronegativity of the halogen. As a result of the slightly positively charged carbon, they are susceptible to nucleophilic attack, i.e. attacked by electron rich reagents, e.g. H2O:, :OH-, :NH3, :CN-. The nucleophiles all have lone pairs of electrons. In all these reactions another group replaces the halogen. They are known as nucleophilic substitution reactions, for which the general equation is:
Y- + R - X R - Y + X-
- Water. Halogenoalkanes react slowly with water to form alcohols.
- Hydroxide Ions. Substitution is much quicker with aqueous sodium hydroxide.
- Ammonia. Heating with aqueous ammonia solution yields an amine
- Potassium Cyanide. Refluxing with an alcoholic solution of potassium cyanide gives a nitrile.
CH3OH + HBr CH3OH + NaBr| CH3Br + NH3 |
CH3NH2 + HBr |
| methylamine |
| CH3Br + KCN |
CH3CN + KBr |
| ethanenitrile |
Chlorofluorocarbons (CFCs)
Chlorofluorocarbons are compounds, which contain the elements, chlorine, fluorine and carbon. There are several members of the family with different boiling points, hence different CFCs with different boiling points can be found for different applications. CFCs have some very useful properties. They are very unreactive, have low flammability and low toxicity. Their four most important uses are:
- Propellant for ærosols.
- Refrigerant.
- Blowing agent for expanding plastics.
- Cleaning solvent for elctronic circuits and for dry-cleaning.
The problem with CFCs is that they are too unreactive. Their estimated lifetime in the troposphere is about 100 years. This gives them plenty of time to reach the stratosphere, where they are no longer unreactive.
Replacement compounds are hydrochlorofluorocarbons or hydrofluorocarbons. These molecules have C - H bonds and are broken down in the troposphere.
Cl atoms in the stratosphere rapidly destroy ozone. Fortunately methane, which is produced in large quantities on Earth, removes chlorine atoms by reacting with it.
CH4 + Cl CH3 + HCl
The hydrogen chloride is then removed in rain.
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