Elimination Reactions in Halogenolalkanes
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What is eliminated in an elimination reaction of halogenoalkanes?
A hydrogen halide (HBr) is eliminated
What type of product is formed when a halogenoalkane undergoes elimination?
An alkene
In elimination, what role does the OH- ion play?
It acts as a base
What two roles do hydroxide ions (OH-) play in substitution/elimination reactions with halogenoalkanes?
OH- ions act as nucleophiles in substitution reactions, and acts as a base in elimination reactions
What are the reaction conditions for elimination reactions of halogenoalkanes?
Sodium or potassium hydroxide dissolved in ethanol, with no water present, and the mixture is heated
What products are usually formed when OH- reacts with halogenoalkanes
A mixture of an alcohol (from substitution) and an alkene (from elimination)
What two factors determine whether substitution or elimination predominates?
• The conditions of the reaction • The type of halogenoalkane (primary, secondary + tertiary)
How do primary, secondary and tertiary halogenoalkanes usually react with OH-?
Primary halogenoalkanes = substitution Secondary halogenoalkanes = both substitution + elimination Tertiary halogenoalkanes = elimination
What are the conditions involving hydroxide ions that favour substitution reactions?
At room temperature, dissolved in water (aqueous)
What are the conditions involving hydroxide ions that favour elimination reactions?
High temperatures, being dissolved in ethanol
What elements do CFCs contain?
Chlorine, fluorine and carbon, but no hydrogen
What happens to CFCs in the upper atmosphere?
They decompose to form chlorine atoms
How do chlorine atoms affect the ozone layer?
They decompose ozone (O3), leading to ozone layer depletion
What environmental problem is linked to CFCs?
The formation of a hole in the ozone layer
What has been done in response to the environmental impact of CFCs?
CFCs are being replaced by safer alternatives like HCFCs