1. 3.3.3 Halogenoalkanes

2. Introduction
Halogenoalkanes are much more reactive than alkanes. They have many uses, including as refrigerants, as solvents and in pharmaceuticals. The use of some halogenoalkanes has been restricted due to the effect of chlorofluorocarbons (CFCs) on the atmosphere.

3. 3.3.3.1 Nucleophilic substitution
Halogenoalkanes contain polar bonds, as a result of the electronegativity difference between the carbon and halogen atoms.
Definition: A nucleophile is a species which can donate a lone pair of electrons
Nucleophiles you need to know are:
Hydroxide ion :OH-
Cyanide ion :CN-
Ammonia :NH3

4. The lone pair of the nucleophile (:OH-, :CN- or :NH3) can attack the δ+ carbon atom of the C-X bond.
When the nucleophile attacks the carbon atom of the C-X bond it replaces (substitutes) the halogen atom
Name of mechanism: Nucleophilic substitution

5. Mechanisms
You need to be able to outline (draw) the reaction mechanism:
A curly arrow indicates the movement of a pair of electrons.
The direction of the arrow is important.
Curly arrows always start at either a pair of electrons or the centre of a covalent bond.

6. CH3CH2CH2Br + NaOH → CH3CH2CH2OH + NaBr
Hydroxide ions
Name of mechanism: Nucleophilic substitution
Reagents/ conditions: Warm with aqueous NaOH or KOH
Product: Alcohol
Example:
CH3CH2CH2Br + NaOH → CH3CH2CH2OH + NaBr

7. CH3CH2CH2Br + KCN → CH3CH2CH2CN + KBr
Cyanide ions
Name of mechanism: Nucleophilic substitution
Reagents/ conditions: Warm with aqueous alcoholic NaCN or KCN
Product: Nitrile
Example:
CH3CH2CH2Br + KCN → CH3CH2CH2CN + KBr

8. CH3CH2CH2Br + 2NH3 → CH3CH2CH2NH2 + NH4Br
Ammonia
Name of mechanism: Nucleophilic substitution
Reagents/ conditions: Warm with excess ammonia (:NH3)
Product: Primary amine
Example:
CH3CH2CH2Br + 2NH3 → CH3CH2CH2NH2 + NH4Br

9. Halogenoalkanes react with excess ammonia to produce primary amines.
The excess of ammonia reduces the chance of further nucleophilic substitution.

10. Rate of nucleophilic substitution reactions
The rate of the nucleophilic substitution reaction. depends on the strength of the carbon-halogen bond (C-X bond).
The C-F bond is very strong making fluoroalkanes unreactive.
The C-I bond is the weakest and so iodoalkanes are the most reactive.
C-X Bond
Bond enthalpy kJ mol-1
Bond strength
Reactivity
C-F
484
Very strong
Unreactive
C-Cl
338
Strong
Very slow
C-Br
276
Moderate
Reasonable
C-I
238
Weakest
Most reactive

11. Elimination
The hydroxide ion, :OH- can act as a nucleophile and a base (accepting an H+ ion).
When the hydroxide ion acts as a base an elimination reaction takes place.
In elimination the halogenoalkane is converted into an alkene.

12. CH3CHBrCH3 + NaOH → CH3CH=CH2 + H2O + NaBr
Name of mechanism: Elimination
Reagents/ conditions: Hot ethanolic KOH or NaOH
Product: Alkene
Example: 2-bromopropane and hot ethanolic KOH
CH3CHBrCH3 + NaOH → CH3CH=CH2 + H2O + NaBr

13. Only one elimination product (alkene) is possible for a halogenoalkane with 3 carbons. However, for halogenoalkanes with 4 carbons, two elimination products (alkenes) are possible.
A mixture of the two alkenes would be produced.

14. Substitution or elimination?
The hydroxide ion, :OH- can act as a nucleophile or base
The reaction conditions can be changed to favour nucleophilic substitution or elimination
Substitution is favoured by warm aqueous NaOH or KOH
Elimination is favoured by hot ethanolic NaOH or KOH.

15. Ozone depletion
Hydrocarbons in which all hydrogen atoms have been substituted by chlorine and fluorine atoms are called chlorofluorocarbons (CFCs)
Examples:

16. Ozone, formed naturally in the upper atmosphere, is beneficial because it absorbs ultraviolet radiation
Ozone, O3 is formed in the atmosphere by free radical reactions in the presence of UV light
O3 ⇌ O2 + •O•
The rate of ozone production = rate of ozone decomposition. Therefore amount of ozone in the atmosphere stays constant.

17. CFCs and the ozone layer
CFCs can get into the atmosphere because they are so unreactive and volatile.
There the UV light causes the C-Cl bond in CFCs to break, forming chlorine radicals (Cl•)
CF2Cl2 → Cl• + •CF2Cl
The chlorine radical reacts with ozone and decomposes it:
Cl• + O3 → ClO• + O2
ClO• + O3 → 2O2 + Cl•
Overall: 2O3 →3O2
The chlorine radical (Cl•) is not used up and it acts as a catalyst.

18. Because of this, small amounts of chlorine radicals (Cl•) can decompose a large amount of ozone causing a ‘hole’ in the ozone layer.
The hole in the ozone layer exists over the Antarctic
The concentration of ozone above the Antarctic is much lower than expected.

19. At a meeting in Montreal in 1987, 24 countries signed the ‘Montreal Protocol on Substances that Deplete the Ozone Layer’.
The Montreal protocol banned the use of CFCs.
The results of research by different groups in the scientific community provided evidence for legislation to ban the use of CFCs as solvents and refrigerants.
Chemists have now developed alternative chlorine-free compounds.

20. Hydrofluorocarbons, HFCs (containing hydrogen, fluorine and carbon) have replaced CFCs.
HFCs are less likely to form free radicals in the presence of UV light.
Example: 1,1,1,2-tetrafluoroethane, CF3CH2F
The C-F bond is stronger than the C-Cl bond so it is less likely to be broken by UV light.