Elimination reactions in halogenoalkanes

Question 1

What is eliminated?

Answer 1

• a hydrogen halide
• leaving a double bond in its place so an alkene is formed

Question 2

How does the hydroxide ions act as a base?

Answer 2

• acts as a base under different conditions
• removing a H+ ion from the halogenoalkane

Question 3

What are the products for the reaction between bromoethane and postassium hydroxide (KOH)?

Answer 3

• ethene
• potassium bromide
• water

Question 4

What are the conditions for the reaction? KOH + C2H5Br

Answer 4

• sodium (or potassium) hydroxide dissolved in ethanol and mixed with the halogenoalkene - no water
• heated
• mineral wool
• ethene decolorises bromine showing that it is the product

Question 5

What is the mechanism of elimination? draw it

Answer 5

• hydroxide ion uses its lone pair to form a bond with one of the hydrogen atoms on the carbon next to the C-Br bond
• they are slightly δ+
• the electron pair from the C-H bond now becomes part of a carbon-carbon double bond
• the bromine takes the pair of electrons in the C-Br bond and leaves as abromide ion

Question 6

What conditions favour elimination? What condition favour subsititution? for reaction with OH-

Answer 6

• elimination
  
  • higher temperature
  • not aqueous, dissolved in ethanol
• substititution
  
  • cold OH^- in water

Question 7

How does the type of halogenoalkane affect whether it is elimination or substitution?

Answer 7

• primary - tend to react by substitution
• tertiary -elmination
• secondary -both

Question 8

What are chloroflurocarbons?

Answer 8

• halogenoalkanes containing chlorine and fluorine but no hydrogoen -unreactive under normal conditions
• short chains - used as aerosol propellents, refrigerants
• long chains - dry cleaning and de-greasing solvent

Question 9

Why are CFCs a problem?

Answer 9

• decompose in the upper atmosphere
• chlorine atoms decompose ozone