Elimination

Question 1

What are the conditions for hydroxide ions to react with halogenoalkanes via an elimination reaction?

Answer 1

1.) Warm ethanolic sodium/ potassium hydroxide. Ethanol is solvent rather than water in substituition reaction (hence: ethanolic NaOH rather than aq NaOH.)
2.) Carried out under reflux

() - extra info for clarification

Question 2

Outline mechanism of reaction of 2-bromopropane with hydroxide ions via elimination.

Diagram of this on paper in pink folder (which you need to know!)

Answer 2

1.) OH- will react with/ attack H on the carbon adjacent to the carbon with the halogen.
2.) Pair of electrons move from bond between the C and the H (H that gained two electrons) to form double bond between two C atoms.
3.) Pair of electrons move from the C-X bond to the halogen, C-X bond breaks.

Question 3

What do the OH- ions act as in elimination reaction of halogenoalkane? Why?

Answer 3

• Act as base.
• Because bases will accept protons so will react with H+ ions.

Question 4

What is formed from the elimination reaction between halogenoalkane and OH- ions? What is eliminated?

Answer 4

• Alkene
• H+ ion and Cl- are eliminated.
• The H+ ion will react with the initital OH- ion to form water.

Question 5

True or False

In the elimination of a halogenoalkane using OH- ions, the OH- can attack ANY hydrogen in any position in the halogenoalkane.

Answer 5

• False.
• The OH- ion must attack the H attached to the carbon that is adjacent (ie. directly next to) the carbon with the halogen bonded to it.

Question 6

What is the overall equation for the elimination reaction between a halogenoalkane and OH- ions using potassium hydroxide?

Answer 6

Halogenoalkane + (K)OH- —> alkene + H20 + (K) X-
(R= alkyl/ X = halide ion)
K in brackets is a spectator ion!!

Question 7

What is the solvent in the elimination of halogenoalkane using OH- ions?

Answer 7

• Ethanol.

Question 8

When a halogenoalkane reacts with sodium/ potassium hydroxide, what different products can be formed when we use different solvents?

Answer 8

• Use ethanol as solvent: alkene formed.
• Use water as solvent (ie. aq) : alcohol formed

Question 9

If you react a halogenoalkane with sodium/ potassium hydroxide in elimination reation (where solvent is water AND ethanol), what two products will be formed?

Answer 9

• Alkene and alcohol.

Question 10

What do we need to check for in our alkene product formed from elimination reaction of halogenoalkane using OH- ions?

Answer 10

• Check for E-z isomerism

Question 11

Why does the carbon-halogen (C-X) bond break in elimination reactions using OH- ions?

Answer 11

• Because, after the C-H bond is broken and double bond is formed between the two carbon atoms, the CS+ has 5 bonds around it, when it should only have 4.
• So the weakest bond to break is broken - the C-X bond.

Question 12

With what halogenoalkane can elimination occur?

Answer 12

• Halogenoalkane with hydrogen attached to carbon that is adjacent to carbon attached to halogen.