Chemistry Unit 2

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  • Created on: 08-04-16 19:19


·         Halogenoalkanes have the general formula CnH2n+1X.   X is a halogen.

·         Can also be primary, secondary and tertiary like alcohols.

·         When naming, the halogen part is named first (prefix chloro-, bromo-, iodo-) followed by name of alkane

·         E.g. CH3Cl = Cloromethane

              CH3CH2Br = Bromoethane

·         If there is more than one halogen di- and tri- are used to indicate the number of halogens present, e.g. CH2BrCH2Br = 1,2-dibromoethane


Reactions of Halogenoalkanes:

·         Halogenoalkanes contain polar bonds because the halogen is more electronegative than the carbon. This leaves a carbon with a delta + charge, making it open to attack by nucleophiles.

·         Nucleophiles: attracted to electron deficient atom, d+ and donate a pair of electrons to form a new covalent bond

·         The halogen will be replaced by the nucleophile, which gives a substitution reaction, giving a new functional group.

1.       Halogenoalkanes react with aqueous alkalis to form ALCOHOLS

·         Aqueous hydroxide ions need to substitute the halogen. Sodium hydroxide NaOH(aq) or potassium hydroxide KOH(aq) can be used.

·         The reaction is called hydrolysis and usually carried out under reflux

·         Hydrolysis:  is a reaction with water or aq hydroxide ions that break a chemical compound into two compounds


Water can act as a nucleophile too, but it is a much slower reaction

·         If water with dissolved silver nitrate is used, this can tell us about the reactivities of halogenoalkanes

·         When water and an alcohol react, and an alcohol is formed, the silver nitrate will react with the halide ions when they form giving a silver halide precipitate

·         The precipitate that forms first indicates which halogenoalkanes hydrolyses first:

                      Tertiary halogenoalkanes – precipitate forms immediately

                  Secondary halogenoalkanes – precipitate forms after several seconds

                    Primary halogenoalkanes – precipitate forms after several minutes


·         This shows that the reactivity is tertiary 3o > secondary 2o > primary 1o

2.       Alcoholic alkali to form alkenes

When a halogenoalkane reacts with alcoholic alkali, e.g. potassium hydroxide, KOH in hot ethanol, an alkene is made

·         This is an elimination reaction

·         Heated under reflux

Uses of halogenoalkanes:

·         Halogenoalkanes are used as fire retardants and refrigerants

·         Chlorofluorocarbons (CFCs) used to be used in the past because of their unique properties (non-toxic, non-flammable, unreactive), but it was found that they deplete the ozone layer in the atmosphere, so are being phased out (see notes later)

·         Other halogenoalkanes such as hydrofluorocarbons (HFCs) are now used as safer alternatives.


Free radical – species with an unpaired electron

Electrophile – species that accepts a pair of electrons

Nucleophile – species that donates a pair of electrons

Substitution – one species is replaced by another

Addition – joining two or more


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