AS AQA CHEM 14.3 ELIMINATION REACTIONS IN HALOALKANES

Question 0

Elimination reaction summary

Answer 0

A hydrogen halide is eliminated from the molecule leaving a double bond in its place so that an alkene is formed.

Question 1

How to haloalkanes typically react?

Answer 1

Haloalkanes typically react by nucleophilic substitution but under different conditions may react by elimination.

Question 2

Affect of an OH⁻ ion acting as a base.

Answer 2

OH- ion can act as a base, removing an H+ ion from the haloalkane.

Question 3

Conditions of reaction when OH⁻ acts as a base.

Answer 3

Sodium/potassium hydroxide is dissolved in ethanol an mixed with the haloalkane.
No water is present.

Question 4

Product of elimination reaction:

Answer 4

Product is ethene. Ethen burns and also decolourises bromine solution, showing that it has a carbon-carbon double bond.

Question 5

Mechanism of elimination reaction.

Answer 5

1. OH- ion uses its lone pair to form a bond with one of the hydrogen atoms on the carbon next to the C-Br bond. These hydrogen atoms are very slightly δ+.
2. Electron pair from the C-H bond now becomes part of a carbon-carbon double bond.
3. Bromide takes the pair of electrons in the C-Br bond and becomes and ion.

Question 6

Uses of elimination reactions

Answer 6

To make molecules with carbon-carbon double bonds.

Question 7

Result of hydrogen ions reaction with haloalkanes as a nucleophile or as a base.

Answer 7

There is competition between elimination and substitution reactions. A mixture of an alcohol and an alkene is formed.

Question 8

Reaction that predominates when hydrogen ions act as nucleophiles and bases depend on:

Answer 8

1. Reaction conditions - aqueous / ethanolic solution

2. Type of haloalkane (primary, secondary or tertiary).

Question 9

Hydroxide ions at room temperature dissolved in water (aqueous) favour:

Answer 9

Substitution reactions.

Question 10

Hydroxide ions at high temperature, dissolved in ethanol favour:

Answer 10

Elimination reactions.

Question 11

Effect of the type of haloalkane: Primary haloalkanes

Answer 11

Tend to react by substitution.

Question 12

Effect of the type of haloalkane: Tertiary haloalkanes

Answer 12

Tend to react by elimination.

Question 13

Effect of the type of haloalkane: Secondary haloalkanes

Answer 13

Will react by elimination and substitution.

Question 14

Classification - Primary

Answer 14

At the end of a chain.

Question 15

Classification - Secondary

Answer 15

In the body of the chain.

Question 16

Classification - Tertiary

Answer 16

At a branch in the chain.

Question 17

Chlorofluorocarbons

Answer 17

Haloalkanes containing both chlorine and fluorine atoms but no hydrogen.

Question 18

Reactions of CFCs

Answer 18

Unreactive under normal conditions

Question 19

Uses of chlorofluorocarbons: Short chain CFCs

Answer 19

Cases and were used as aerosol propellants, refrigerants and blowing agents for foams like expanded polystyrene.

Question 20

Uses of chlorofluorocarbons: Long chain CFCs

Answer 20

Used as dry-cleaning and de-greasing solvents.

Question 21

Problems with CFCs

Answer 21

They eventually end up in the atmosphere where they decompose to give chlorine atoms.

Chloride atoms decompose ozone causing a hole in the Earth’s ozone layer which will take years to recover.

Question 22

What will CFCs be replaced with?

Answer 22

Scientists influenced politicians who under international agreement are phasing out CFCs and replacing them with HCFCs (Hydrochlorofluorocarbons).