3.3 Halogenoalkanes
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What is a halogenoalkane?
Compounds which one or more hydrogen atoms in an alkane have been replaced by halogen atoms
What are the halogen prefixes?
Fluoro Chloro Bromo Iodo
What are some nomenclature rules for halogenoalkanes?
The different halogens are listed alphabetically
What are some uses for halogenoalkanes?
Refrigerator Solvents Anaesthetics Aerosol propellants Insecticides
What is a primary, secondary and tertiary structure?
Primary- only has 1 other carbon attached to the carbon with the halogen attached Secondary- has 2 carbons attached to the carbon with the halogen attached Tertiary - has 3 carbons attached to the carbon with the halogen attached
What is the ozone layer?
O3 forms naturally in the atmosphere It protects us from harmful UV radiation
What affects the ozone layer?
CFC’s - Chlorofluorocarbons They form holes in the layer
What are used instead of CFC’s?
HFC’s - Hydrofluorocarbons
What happens when CFC’s reach the ozone layer?
Chlorine free radicals are formed when CFC’s are exposed to UV radiation
What reaction shows what happens to the ozone layer? Type of reaction?
O3 + Cl• -> ClO• + O2 ClO• + O3 -> 2O2 + Cl• Chain reaction
Name the types of reactions that can take place with halogenoalkanes?
Free radical substitution Nucleophilic substitution Elimination
What is free radical substitution?
A chain reaction Where one thing replaces another with 3 stages
What are the three stages of free radical substitution?
Initiation Propagation Termination
What is the initiation mechanism?
Cl2 ——> 2Cl• With UV light over the arrow
What takes place in initiation?
Homolytic bond fission When the bond breaks each atom takes an electron from the bond now called free radicals
What is meant by propagation step?
Where free radicals are used up and then regenerated
What is the mechanism for propagation in free radical substitution? (Use CH4 and Cl)
CH4 + Cl• -> •CH3 + HCl • CH3 + Cl2 -> CH3Cl + Cl•
What is the mechanism for termination in free radical substitution? (Use CH4 and Cl)
Cl• + Cl• -> Cl2 • CH3 + Cl• -> CH3Cl • CH3 + •CH3 -> C2H6
In free radical substitution how can you make sure you get more of one particular product?
Use an excess of the reactant that will help produce more of the product Therefore collisions are more likely between the one in excess and the other free radical
What are the strongest to weakest bonds of carbon-halogens?
Strongest C-F C-Cl C-Br C-I Weakest
What is a nucleophile? What do they do?
Attracts positive charge Attack a halogenoalkane and form a bond with it As the carbon will have a delta + charge therefore the nucleophile will be attracted to it
Examples of a nucleophile?
:OH- (hydroxide ion) :NH3 (ammonia) :CN- (cyanide ion)
How do you know it’s likely nucleophilic substitution?
A polar molecule - nucleophilic Saturated - substitution
How does carbon-halogen bonds affect reactions?
The weaker the carbon-halogen bond the more reactive it will be