Halogenoalkanes: Elimination Reactions

Question 1

What are elimination reactions?

Answer 1

• Elimination reactions involve removing the halogens from halogenoalkanes with hydroxide ions
• This forms alkenes, water and a halide salt
• The reaction between a halogenoalkane and potassium hydroxide usually results in the formation of a mixture of substitution and elimination products

Question 2

Describe the mechanism of an elimination reaction between 2-bromopropane and potassium hydroxide

Answer 2

• OH- acts as a base and takes a proton, H+ from a carbon that is next to a carbon bonded to the halogen (bromine)
• This carbon has a lone pair, so it forms a double bond with the carbon bonded to the halogen
• The carbon bonded to the halogen has to let go of the Br
• This forms propene, water and a Br- ion
• Br- reacts with potassium to form KBr

Question 3

Which conditions favour elimination reactions?

Answer 3

• Higher temperatures (under reflux)
• Concentrated solution of sodium or potassium hydroxide
• Pure ethanol as the solvent
• Increased branching

Question 4

Which conditions favour nucleophilic reactions?

Answer 4

• Lower temperatures
• More dilute solutions of sodium or potassium hydroxide
• Aqueous solvent
• Little/no branching

Question 5

What role does the reagent play in nucleophilic substitution reactions?

Answer 5

• Reagent
- Potassium hydroxide, potassium cyanide, ammonia

• Role
- Nucleophile

Question 6

What role does the reagent play in elimination reactions?

Answer 6

• Reagent
- Potassium/sodium hydroxide

• Role
- Base