13.3 Elimination reactions in halogenoalkanes Flashcards
What two ways do halogenoalkanes react
. Usually react by nucleophilic substitution
. However they react by elimination under different conditions
What happens in an elimination reaction
(with OH- as the base)
A hydrogen halide is eliminated from the molecule, leaving a double bond in its place so an alkene is formed
Elimination reaction with OH- ion acting as a base
. When OH- acts as a base, it can remove a H+ ion from the halogenoalkane
. Eg bromoethane reacting with potassium hydroxide
Where a molecule of hydrogen bromide is eliminated which then reacts with potassium hydroxide to make KBr and H2O
Bromoethane + KOH- —> Ethene + KBr + H2O
What are the conditions of the reaction of OH- acting as a base, and a halogenoalkane
. The sodium or potassium hydroxide is dissolved in ethanol, then mixed with the halogenoalkane
. NO WATER PRESENT
. The mixture is heated
Mineral wool is soaked in the reaction mixture and is then heated in a test tube bung with tube leading to the products water, and gases
Describe the mechanism of elimination of hydrogen bromide
with :OH- base
In bromoethane, there will be 3 curly arrows
-One from the lone pair of :OH- to a H
-Another from that C-H bond to C-C bond
- Another from a different C-Br bond to Br
This produces ethene , H2O and :Br-
. The OH- ion uses its lone pair to bond with a hydrogen atom on the other carbon, not the one in the C-Br bond
. These hydrogen atoms are slightly f+
. The electron pair from C-H bond now becomes part of a C=C double bond
. The bromine takes a pair of electrons from the C-Br bond and leaves as a bromide ion :Br-
Why are elimination reactions good
They are useful for making molecules with C=C double bonds
Describe the competition between substitution or elimination reactions with OH- ions
- Hydroxide ion will react with halogenoalkanes as a nucleophile or as a base
- Usually a mixture of an alcohol (from nucleophile) with an alkene (from elimination) is produced
The reaction that predominates (elimination or substitution)
depends on two factors
. The reaction conditions (aqueous or ethanolic solution)
. Type of halogenoalkanes (primary, secondary, tertiary)
Halogenoalkanes are classified based on where the halogen atom is (eg if its at the end of hydrocarbon chain it is primary, if it is in the body of the chain it is secondary, and it is tertiary if it is on a branch)
What conditions of the reaction favour a nucleophilic substitution reaction
What conditions of the reaction favour an elimination reaction
. Hydroxide ions at room temperature, dissolved in water (aqueous) favour substitution
. Hydroxide ions at a high temperature, dissolved in ethanol favour elimination
How does the type of halogenoalkane impact what reaction is predominant
How are halogenoalkanes classified
. Primary halogenoalkanes tend to react by substitution
. Secondary react by both substitution and elimination
. Tertiary react by elimination
Halogenoalkanes are classified based on where the halogen atom is (eg if its at the end of hydrocarbon chain it is primary, if it is in the body of the chain it is secondary, and it is tertiary if it is on a branch)
What are chlorofluorocarbons and what are their properties
Eg reactivity
. They are halogenoalkanes containing both chlorine and fluorine atoms but NO hydrogen
Eg trichlorofluoromethane
CCl3F
. They are also called CFCs
. Very unreactive under normal conditions
What is ozone and why is it important
Molecule O3 found in upper atmosphere
It absorbs harmful UV rays to stop them reaching us
Uses of short chain and long chain CFCs
. Short chain CFCs are used as refridgerants, aerosol propellants, and blowing agents for foams
. Long chain CFCs are used in dry cleaning, and de-greasing solvents
What is the problem with CFCs
. They eventually end up in the atmosphere where they decompose to give chlorine atoms in equation:
CF2Cl2 –> .CF2Cl + .Cl
. Chlorine atoms decompose ozone (O3) in the stratosphere which has caused a hole in the Earths ozone layer
. Now CFCs are phased out and replaced by safer compounds eg hydrochlorofluorocarbons
HCFCs
How are .Cl free radicals formed in atmosphere (give equation)
What are the equations of .Cl free radical with ozone
CF2Cl2 —> .CF2Cl + .Cl
.Cl + O3 –> .ClO + O2
.ClO + O —> .Cl + O2
Overall equation for depletion of ozone:
2O3 —> 3O2
