2.6 - Halogenoalkanes

In this unit we will learn about the structure of halogenoalkanes, including hydrolosis of halogenaolkanes and their uses.

Halogenoalkane

An alkane with at least one halogen in place of a hydrogen atom.

Halogenoalkane general formula.

CnH2n+1X

Nucleophile

A species with a lone pair of electrons that can be donated to an electron deficient centre to form a new covalent bond, E.g. halogen anions, OH- ions.

Test for halogen in haloalkane.

• Add NaOH(aq)

• Add HNO3 and AgNO3(aq)

• Observe colour change of precipitate

Chloride precipitate colour

White

Bromide precipitate colour

Cream

Iodide precipitate colour

Yellow

Nucleophillic substitution

Preparation of alcohols by substitution reactions of halogenoalkanes.

Conditions for nucleophillic substitution

Heat under reflux.

Electronegativity

The ability of an atoms, when in a covalent bond to pull the lone pair of electrons towards itself.

Bond strength

To substitute the halogen you need to break the C-X bond. the strongest is C-Cl so it is the hardest to break.

Test for the rate of hydrolysis (Nucleophilic substitution)

Measure time taken for silver halide precipitate to form.

Elimination reactions (No water)

A small molecule is lost to form a double bond. (alkene)

Conditions for elimination reaction.

• Concentrated sodium hydroxide.

• Ethanol (E for elimination, E for ethanol)

• Heat.

Uses of halogenoalkanes.

• Solvents

• Anaesthetics

• Refrigerants

Halogenoalkanes disadvantages

• CFC’s - cause holes in ozone layer

• In ozone layer CFC’s encounter UV light which breaks the C-Cl bond, making a chlorine free radical.