1. Introduction to halogenoalkanes

2. Starter
Match the definitions

3. Learning objectives
• Describe the structure, general formula and uses of halogenoalkanes.
• Describe and explain the nature of the carbon– halogen bond and its susceptibility to nucleophilic attack.
• Define the term nucleophile as an electron pair donor.

4. Halogenoalkanes
Compound where a halogen has replaced one of the hydrogen atoms in the alkane chain
Were very widely used as refrigerants, aerosol propellants, degreasers, dry-cleaning solvents, however their use has changed (we will look at this in later lessons)
They are useful in organic synthesis and to prepare many useful materials

5. Halogenoalkanes
What is the general formula for a halogenoalkane? (where X represents the halogen)
CnH2n+1X
Prefixes for naming halogenoalkanes
Halogen
Prefix F
Fluoro- Cl
Chloro- Br
Bromo- I
Iodo-

6. Naming the halogenoalkanes
Only contain single bonds, therefore the names are based on the ALKANE homologous series
Appropriate prefix is added to the name of the longest alkane chain
Position of the halogen is indicated
If there is more than one halogen, they are listed alphabetically

7. Example Five carbons in the longest chain The name is based on pentane
There is a bromo group on the third carbon and a chloro group on the second carbon
The name is 3-bromo-2-chloropentane

8. Questions
Complete question 1 on page 157

9. Reactivity of the halogenoalkanes
Haloagenoalkanes have a polar bond between the carbon and halogen atoms.
Why is this bond polar?
It is polar because of the different electronegativities of the carbon and halogen atoms
Halogen atoms are more electronegative than carbon atoms
The bonded electron pair is attracted more towards the halogen atom than towards the carbon atom
The result is a polar bond

10. Polarity in a molecule of bromomethane

11. Electronegativity of halogens
Decreases down the group
Decrease in polarity of the carbon-halogen bond from fluorine to iodine
The electron deficient carbon atom attracts nucleophiles such as H2O, OH- and NH3
Substitution reactions
Nucleophile replaces the halogen

12. Hydrolysis of halogenoalkanes
Key definition – hydrolysis
A reaction with water or aqueous hydroxide ions that breaks a chemical compound into two compounds
In halogenoalkanes a nucleophilic substitution reaction occurs
Reaction is carried out under reflux
The product is an alcohol
For example:
hydrolysis of 1-choropropane
CH3CH2CH2Cl (aq) + OH-(aq) CH3CH2CH2OH(aq) + Cl-(aq)
1-chloropropane  propan-1-ol

13. Questions
Complete questions 2 and 3 on page 157