Elimination Reactions in Halogenolalkanes Flashcards
How do halogenoalkanes typically react?
Halogenoalkanes typically react by nucleophilic substitution
Under what conditions do halogenoalkanes undergo elimination reactions?
Under different conditions, a hydrogen halide is eliminated, forming an alkene and water molecule
What is the role of the OH- ion, when acting as a base in elimination reactions?
The OH- ion removes an H+ ion from the halogenoalkane, leading to the elimination of a hydrogen halide (HX) and the formation of an alkene
What are the products when bromoethane undergoes elimination with potassium hydroxide?
The reaction produces ethene, bromine and water
What are the conditions for elimination reactions of halogenoalkanes?
The sodium (or potassium) hydroxide is dissolved in ethanol and mixed with the halogenoalkane.
There is no water present, and the mixture is heated
Why must there be no water present in elimination reactions of halogenoalkanes?
The presence of water would lead to nucleophilic substitution instead of elimination
What happens to the C-H bond during elimination?
The electron pair from the C-H bond moves to form a carbon - carbon double bond (C=C)
Why is elimination useful in organic synthesis?
Elimination reactions are useful for making molecules with carbon - carbon double bonds, which are important in alkene synthesis
What are the two factors that determine whether substitution or elimination predominates?
1/ The reaction conditions - aqueous or ethanolic solution
2/ The type of halogenoalkane - whether primary, secondary or tertiary
Why is there competition betwen substitution and eliminatiojn in halogenoalkane reactions?
The OH- ion can act as either a nucleophile (substitution) or a base (elimination), leading to a mixture of alcohol and alkene being produced
Which reaction conditions favor elimination?
Ethanolic sodium/potassium hydroxide (with no water) and heat favor elimination, forming an alkene
Which reaction conditions favour nucleophilic substitution?
Aqueous sodium/potassium hydroxide favours substitution, forming an alcohol
How does the type of halogenoalkane affect the reaction?
Primary halogenoalkanes - Favours substitution
Secondary halogenoalkanes - Can undergo both reactions (elimination + substitution)
Tertiary halogenoalkanes - Favour elimination
Why can elimination reactions produce a mixture of isomers?
In some cases, different hydrogen atoms can be removed, leading to the formation of isomeric alkenes
