What types of reactions do halogenoalkanes undergo?
[2]:
- Nucleophilic substitution
- Elimination
Nucleophile [definition]:
electron pair donator
e.g. :OH-, :NH3, CN⁻
What does the rate of the substitution reaction depend on? [2]:
- the strength of the C-X bond
- The weaker the bond, the easier it is to break and the faster the reaction
Halogenoalkane + :OH- =
Nucelophilic substitution
Halogenoalkane + :NH3 =
Nucelophilic substitution
Halogenoalkane + CN⁻ =
Nucelophilic substitution
Nucleophilic substitution with aqueous hydroxide ions
[4]::
- halogenoalkane → alcohol
- Reagent: potassium (or sodium) hydroxide
- Conditions: In aqueous solution; Heat under reflux
- Mechanism: Nucleophilic Substitution
Water is a poor nucleophile but…
it can react slowly with halogenoalkanes in a substitution reaction
Nucleophilic substitution with cyanide ions [4]:
- halogenoalkane → nitrile
- Reagent: KCN dissolved in ethanol/water mixture
- Conditions: Heating under reflux
- Mechanism: Nucleophilic Substitution
Nucleophilic substitution with ammonia [4]:
- halogenoalkane → amine
- Reagent: NH3 dissolved in ethanol
- Conditions: Heating under pressure (in a sealed
tube) - Mechanism: Nucleophilic substitution
Elimination with alcoholic hydroxide ions [4]:
- halogenoalkane → alkene
- Reagents: Potassium (or sodium) hydroxide
- Conditions: In ethanol ; heat
- Mechanism: Elimination
Aqueous:
substitution
Alcoholic:
elimination
What reaction does a primary halogenoalkane tend to?
substitution
What reaction does a secondary halogenoalkane tend to?
elimination
Uses of halogenoalkanes [4]:
- Chloroalkanes and chlorofluoroalkanes can be used as solvents
- refrigerants
- pesticides
- aerosol propellants
Why have people mostly stopped using halogenoalkanes?
stopped due to the toxicity of halogenoalkanes and also their detrimental effect on the atmosphere
What is the ozone layer?
- O3
- layer in the upper atmosphere that filters out much of the sun’s harmful UV radiation
What happens when ozone is in the lower layer of the atmosphere? [2]:
- becomes a pollutant
- contributes to the formation of smog
What did man made chlorofluorocarbons (CFCs) cause?
A hole to form in the ozone layer
What happens to CFCs in the upper atmosphere? [2]:
- UV light causes s C–Cl bonds in CFCs to break
- This forms chlorine radicals in upper atmosphere
What do chlorine radicals do to ozone?
catalyse the decomposition of ozone
What do we use instead of CFCs and why? [3]:
- CH2FCF3 are now used for refrigerators
and air-conditioners - safer as they do not contain the C-Cl bond
- C-F bond is stronger than the C-Cl bond and is not affected by UV
