· 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…