AS AQA CHEM 14.3 ELIMINATION REACTIONS IN HALOALKANES Flashcards Preview

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Flashcards in AS AQA CHEM 14.3 ELIMINATION REACTIONS IN HALOALKANES Deck (23):
0

How to haloalkanes typically react?

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

1

Elimination reaction summary

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

2

Affect of an OH⁻ ion acting as a base.

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

3

Conditions of reaction when OH⁻ acts as a base.

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

4

Product of elimination reaction:

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

5

Mechanism of elimination reaction.

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.

6

Uses of elimination reactions

To make molecules with carbon-carbon double bonds.

7

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

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

8

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

1. Reaction conditions - aqueous / ethanolic solution

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

9

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

Substitution reactions.

10

Hydroxide ions at high temperature, dissolved in ethanol favour:

Elimination reactions.

11

Effect of the type of haloalkane: Primary haloalkanes

Tend to react by substitution.

12

Effect of the type of haloalkane: Tertiary haloalkanes

Tend to react by elimination.

13

Effect of the type of haloalkane: Secondary haloalkanes

Will react by elimination and substitution.

14

Classification - Primary

At the end of a chain.

15

Classification - Secondary

In the body of the chain.

16

Classification - Tertiary

At a branch in the chain.

17

Chlorofluorocarbons

Haloalkanes containing both chlorine and fluorine atoms but no hydrogen.

18

Reactions of CFCs

Unreactive under normal conditions

19

Uses of chlorofluorocarbons: Short chain CFCs

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

20

Uses of chlorofluorocarbons: Long chain CFCs

Used as dry-cleaning and de-greasing solvents.

21

Problems with CFCs

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.

22

What will CFCs be replaced with?

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