Halogenoalkanes
Nucleophilic substitution by an ammonia molecule
Ammonia is a nucleophile because it can donate a pair of electrons. It is a neutral nucleophile. bromoethane + ammonia → ethylamine + ammonium bromide Conditions: warm with excess ethanolic ammonia in a sealed container under pressure. The amine group in the product still has a lone pair of electrons. This means that it can also act as a nucleophile, so it may react with halogenoalkane molecules, giving a mixture of products.
Nucleophilic substitution reactions of halogenoalkanes
For any reaction to occur the carbon-halogen bond needs to break. The C-F bond is the strongest - it has the highest bond enthalpy. So fluoroalkanes undergo nucleophilic substitution reactions more slowly than the other halogenoalkanes. The C-I bond enthalpy is the lowest, so it is easier to break. This means that iodoalkanes are substituted more quickly. The 𝛿+ carbon atom in the carbon-halogen bond is susceptible to nucleophilic attack. A nucleophile is an electron pair donor. Halogenoalkanes undergo substitution reactions with the nucleophiles OH-, CN- and NH3. The lone pair of electrons on the nucleophile is attracted to the 𝛿+ carbon attached to the halogen atom. The nucleophile makes a bond with the carbon atom and the C-X bond breaks, with both electrons in the bond moving to the X atom.
Nucleophilic substitution by a cyanide ion
Nitriles are formed by reacting halogenoalkanes with cyanides. bromoethane + potassium cyanide → propanenitrile + potassium bromide Conditions: heat gently under reflux with ethanolic potassium cyanide. The initial organic molecule has two carbons, the organic product has three. This reaction increases the length of the carbon chain by one.
Ozone depletion
Ozone, formed naturally in the upper atmosphere, is beneficial because it absorbs ultraviolet radiation, preventing a proportion of these harmful rays from reaching the surface of the earth. O2 → 2O∙ (UV) O2 + O∙ → O3 Chlorine free radicals cause the greatest destructuion due to the widespread use of chlorofluoroalkanes (CFCs) which were valued both for their lack of toxicity and non-flammability. They were used as coolant gas in fridges, as solvents, and as propellants in aerosols. Unfortunately, because CFCs are stable in the lower atmosphere they do not degrade. They diffuse into the stratosphere where UV light causes homolysis of the C-Cl bond and produces Cl•. Free-radical substitution mechanism of the ozone layer: The chlorine free radical reacts with the ozone O3, decomposing it to form oxygen and an intermediate: Cl• + O3 → ClO• + O2 The chlorine free radical is reformed by reacting with more ozone molecules. These free radicals act as a catalyst: ClO• + O3 → 2O2 + Cl• Overall equation: 2O3 → 3O2 Chemists have developed safer alternatives to CFCs which contain no chlorine such as HFCs (hydrofluorocarbons), for example, trifluoromethane, CHF3.
Physical properties of the halogenoalkanes
The C-X bond in halogenoalkanes is polar because the halogen atom is more electronegative than the C atom. The electronegativity of the halogen atom decreases as Group 7 is descended and thus the C-X bond becomes less polar. There are two separate types of intermolecular forces between halogenoalkane molecules; van der Waals' forces and permanent dipole-dipole interactions. The intermolecular force of attraction due to van der Waals' forces increases as the relative molecular mass increases, therefore the boiling point of the chloroalkanes increases as the chain length increases. This pattern is repeated for the bromoalkanes and the iodoalkanes. Now consider the trend in boiling point when the carbon chain length is kept the same and the halogen atom is changed. Although the permanent dipole-dipole interactions are greater the more polar the carbon-halogen bond, the changing van der Waals' forces have a greater effect on the boiling point of the molecule. As the relative molecular mass of the halogen increases the boiling point increases, thus the boiling point of an iodoalkane is greater than the boiling point of a bromoalkane, which is greater than the boiling point of a chloroalkane, for compounds with identical carbon chains. Solubility in water The halogenoalkanes, despite the polar nature of the carbon-halogen bond, are insoluble or only very slightly soluble in water. They are soluble in organic solvents and due to their ability to mix with other hydrocarbons are used extensively as dry cleaning fluids and degreasing agents.
Preparation of 1-bromobutane
The following method can be used to prepare 1-bromobutane. 1) Place 30 cm3 of water, 40 g of powdered sodium bromide and 21.8 g of butan-1-ol in a 250 cm3 round bottomed flask. 2) Allow 25 cm3 of concentrated sulfuric acid to drop into this flask from a tap funnel, cooling the flas in an ice bath. 3) Reflux the reaction mixture for 45 minutes. 4) Distill off the crude 1-bromobutane. 5) Wash the distillate with water before washing with concentrated sulfuric acid. 6) Finally wash with sodium carbonate solution. 7) Remove the 1-bromobutane layer and add a spatula of anhydrous sodium sulfate, swirl and filter. 8) Distill and collect the 1-bromobutane at 99-102°C.
Nucleophilic substitution by a hydroxide ion
The reaction between aqueous sodium or potassium hydroxide and a halogenoalkane can take place at room temperature but it is slow and is often refluxed gently to improve the rate and the yield. Reflux is the continuous boiling and condensing of a reaction mixture. A condenser is placed in a vertical position of the reaction vessel. Any vapourised reactants will condense and return to the reaction flask. The halogenoalkane is dissolved in a little ethanol as it is insoluble in water. The aqueous solution of the hydroxide ions and the ethanolic solution of the halogenoalkane are miscible. The organic product is an alcohol. Conditions: heat gently under reflux with aqueous sodium or potassium hydroxide. This reaction can also be classified as a hydrolysis reaction. Hydrolysis is the breaking of chemical bonds with water. The term also refers to the process of breaking bonds with hydroxide ions. The rate of the nucleophiic substitution/hydrolysis of the halogenoalkanes depends on the ease of breaking the C-X bond. The stronger the carbon-halogen bond the more difficult it will be to break and the slower the reaction.
Elimination reactions of halogenoalkanes
When potassium hydroxide is dissolved in ethanol, the hydroxide ions act as a base and accept a proton to form water. The consequence is that the halogenoalkane molecule loses a hydrogen atom and a halogen atom. The organic product is an alkene. An elimination reaction is one in which a small molecule is removed from the organic compound. Conditions: heat under reflux with concentrated ethanolic potassium hydroxide. bromoethane + potassium hydroxide → ethene + water + potassium bromide The formation of isomers in the elimination product mixture When 2-chloropentane is refluxed in hot ethanolic potassium hydroxide two isomeric alkenes, pent-1-ene and pent-2-ene, are formed. Pent-2-ene exists as two stereoisomers. Methylpropene gas can be prepared by heating 2-chloro-2-methylpropane by heating mineral wool soaked in 2-chloro-2-methylpropane and ethanolic potassium hydroxide. The concurrent substitution and elimination reactions of a halogenoalkane In practice, both elimination and substitution reactions occur simultaneously. By manipulating the conditions the chemist can ensure a high yield of the nucleophilic substitution product or the elimination product. The position of the functional group can also have an effect on the nature of the product. Elimination is more likely for a tertiary halogenoalkane and substitution is more likely for a primary halogenoalkane
