3.3.3 Halogenoalkanes

Introduction Halogenoalkanes are much more reactive than alkanes. They have many uses, including as refrigerants, as solvents and in pharmaceuticals. The use of some halogenoalkanes has been restricted due to the effect of chlorofluorocarbons (CFCs) on the atmosphere.

3.3.3.1 Nucleophilic substitution Halogenoalkanes contain polar bonds, as a result of the electronegativity difference between the carbon and halogen atoms. Definition: A nucleophile is a species which can donate a lone pair of electrons Nucleophiles you need to know are: Hydroxide ion :OH- Cyanide ion :CN- Ammonia :NH3

The lone pair of the nucleophile (:OH-, :CN- or :NH3) can attack the δ+ carbon atom of the C-X bond. When the nucleophile attacks the carbon atom of the C-X bond it replaces (substitutes) the halogen atom Name of mechanism: Nucleophilic substitution

Mechanisms You need to be able to outline (draw) the reaction mechanism: A curly arrow indicates the movement of a pair of electrons. The direction of the arrow is important. Curly arrows always start at either a pair of electrons or the centre of a covalent bond.

CH3CH2CH2Br + NaOH → CH3CH2CH2OH + NaBr Hydroxide ions Name of mechanism: Nucleophilic substitution Reagents/ conditions: Warm with aqueous NaOH or KOH Product: Alcohol Example: CH3CH2CH2Br + NaOH → CH3CH2CH2OH + NaBr

CH3CH2CH2Br + KCN → CH3CH2CH2CN + KBr Cyanide ions Name of mechanism: Nucleophilic substitution Reagents/ conditions: Warm with aqueous alcoholic NaCN or KCN Product: Nitrile Example: CH3CH2CH2Br + KCN → CH3CH2CH2CN + KBr

CH3CH2CH2Br + 2NH3 → CH3CH2CH2NH2 + NH4Br Ammonia Name of mechanism: Nucleophilic substitution Reagents/ conditions: Warm with excess ammonia (:NH3) Product: Primary amine Example: CH3CH2CH2Br + 2NH3 → CH3CH2CH2NH2 + NH4Br

Halogenoalkanes react with excess ammonia to produce primary amines. The excess of ammonia reduces the chance of further nucleophilic substitution.

Rate of nucleophilic substitution reactions The rate of the nucleophilic substitution reaction. depends on the strength of the carbon-halogen bond (C-X bond). The C-F bond is very strong making fluoroalkanes unreactive. The C-I bond is the weakest and so iodoalkanes are the most reactive. C-X Bond Bond enthalpy kJ mol-1 Bond strength Reactivity C-F 484 Very strong Unreactive C-Cl 338 Strong Very slow C-Br 276 Moderate Reasonable C-I 238 Weakest Most reactive

3.3.3.2 Elimination The hydroxide ion, :OH- can act as a nucleophile and a base (accepting an H+ ion). When the hydroxide ion acts as a base an elimination reaction takes place. In elimination the halogenoalkane is converted into an alkene.

CH3CHBrCH3 + NaOH → CH3CH=CH2 + H2O + NaBr Name of mechanism: Elimination Reagents/ conditions: Hot ethanolic KOH or NaOH Product: Alkene Example: 2-bromopropane and hot ethanolic KOH CH3CHBrCH3 + NaOH → CH3CH=CH2 + H2O + NaBr

Only one elimination product (alkene) is possible for a halogenoalkane with 3 carbons. However, for halogenoalkanes with 4 carbons, two elimination products (alkenes) are possible. A mixture of the two alkenes would be produced.

Substitution or elimination? The hydroxide ion, :OH- can act as a nucleophile or base The reaction conditions can be changed to favour nucleophilic substitution or elimination Substitution is favoured by warm aqueous NaOH or KOH Elimination is favoured by hot ethanolic NaOH or KOH.

3.3.3.3 Ozone depletion Hydrocarbons in which all hydrogen atoms have been substituted by chlorine and fluorine atoms are called chlorofluorocarbons (CFCs) Examples:

Ozone, formed naturally in the upper atmosphere, is beneficial because it absorbs ultraviolet radiation Ozone, O3 is formed in the atmosphere by free radical reactions in the presence of UV light O3 ⇌ O2 + •O• The rate of ozone production = rate of ozone decomposition. Therefore amount of ozone in the atmosphere stays constant.

CFCs and the ozone layer CFCs can get into the atmosphere because they are so unreactive and volatile. There the UV light causes the C-Cl bond in CFCs to break, forming chlorine radicals (Cl•) CF2Cl2 → Cl• + •CF2Cl The chlorine radical reacts with ozone and decomposes it: Cl• + O3 → ClO• + O2 ClO• + O3 → 2O2 + Cl• Overall: 2O3 →3O2 The chlorine radical (Cl•) is not used up and it acts as a catalyst.

Because of this, small amounts of chlorine radicals (Cl•) can decompose a large amount of ozone causing a ‘hole’ in the ozone layer. The hole in the ozone layer exists over the Antarctic The concentration of ozone above the Antarctic is much lower than expected.

At a meeting in Montreal in 1987, 24 countries signed the ‘Montreal Protocol on Substances that Deplete the Ozone Layer’. The Montreal protocol banned the use of CFCs. The results of research by different groups in the scientific community provided evidence for legislation to ban the use of CFCs as solvents and refrigerants. Chemists have now developed alternative chlorine-free compounds.

Hydrofluorocarbons, HFCs (containing hydrogen, fluorine and carbon) have replaced CFCs. HFCs are less likely to form free radicals in the presence of UV light. Example: 1,1,1,2-tetrafluoroethane, CF3CH2F The C-F bond is stronger than the C-Cl bond so it is less likely to be broken by UV light.