13.3- ELIMINATION REACTIONS IN HALOGENOALKANES Flashcards
How do halogenoalkanes typically react by?
nucleophilic substitution
Under different conditions what can halogenoalkanes react by?
elimination
What happens in an elimination reaction of a halogenoalkane?
hydrogen halide is eliminated from the molecule, leaving a double bond in its place so alkene formed
Under different conditions what can the OH- ion act as?
base
When the OH- ion is acting as a base what will it do to the halogenoalkane?
remove a H+ ion from the halogenoalkane
What sort of reaction is it when a H+ ion is removed from the halogenoalkane?
elimination
What are the conditions for the elimination reaction for halogenoalkanes? (3)
sodium (or potassium) hydroxide dissolved in ethanol
no water present
mixture heated
What is the elimination reaction of halogenoalkanes useful for?
making molecules with carbon-carbon double bonds
As the hydroxide ion can act as a nucleophile or base what is there between elimination and substitution?
competition
As there is competition between elimination and substitution with hydroxide ions, what is generally produced?
a mixture of an alcohol and an alkene is produced
What factors determine which reaction predominates between elimination and substitution with hydroxide ions? (2)
reaction conditions (aqueous or alcoholic)
type of halogenoalkane (primary, secondary, tertiary)
What conditions would favour substitution ?
hydroxide ion at room temperature, dissolved in water
What conditions would favour elimination?
hydroxide ions at high temperature, dissolved in ethanol
What types of halogenoalkanes tend to react by substitution?
primary and secondary
What type of halogenoalkanes tend to react by elimination?
secondary and tertiary
Is a mixture of isomeric elimination products possible?
yes
What are chlorofluorocarbons?
halogenoalkanes containing both chlorine and fluorine atoms but no hydrogen
Name CCl3F
trichlorofluoromethane
What are chlorofluorocarbons also called?
CFCs
How reactive are CFCs under normal conditions?
very reactive
What state are short chain CFCs in?
gas
Examples of uses of short chain CFCs? (3)
aerosol propellants
refrigerants
blowing agents for foams like expanded polystyrene
What are longer chain CFCs used in?
dry cleaning and de-greasing solvents
Where do CFCs eventually end up?
in the atmosphere
What do CFCs do in the atmosphere?
decompose to give chlorine atoms
What do chlorine atoms decompose in the stratosphere?
decompose ozone, O3
What has been caused as the chlorine atoms have decomposed ozone?
a hole in the Earth’s ozone layer
An example of what CFCs replaced by?
hydrochlorofluorocarbons
