Alkanes - Alkanes: Properties and Reactivity (A-Level Chemistry)
Alkanes: Properties and Reactivity
The Alkanes
Key Terms
Alkanes – a group of compounds which have the general formula CnH2n+2 e.g. Butane. Alkanes are saturated hydrocarbons, because they have the maximum number of hydrogens for a given number of carbons.
Saturated – compounds which contain only single bonds. They have the maximum number of hydrogens for a given number of carbons.
Hydrocarbon – compounds which are made up of only Carbon and Hydrogen.
Cycloalkane – a special type of alkane which is still saturated but has the general formula CnH2n, e.g. cyclopentane C5H10.
Properties of Alkanes
Shape
- Alkane molecules have a tetrahedral shape around carbon atoms. Carbon atoms in alkane molecules form sigma bonds with surrounding carbon and hydrogen atoms. As a result, carbon atoms in alkanes are surrounded by 4 pairs of bonding electrons which equally repel each other to form 109.5o bond angles.
- Alkanes show structural isomerism. C-C carbon bonds can give rise to straight, branched or cyclic molecules so that an alkane with the same molecular formula can have different structural formula.
Melting and Boiling Points
- Alkane molecules have weak Van der Waals forces between them. As C-C bonds are non-polar and C-H bonds are practically non-polar too, alkanes are non-polar molecules which can therefore only be attracted to each other by induced dipole-dipole interactions.
- The stronger the Van der Waals forces between its molecules, the higher the melting and boiling points of an alkane. This is because greater amounts of energy are required to break them apart.
- The strength of the Van der Waals forces depends on the size and of the alkane molecules. Longer carbon chains have larger surfaces area and hence can form more contact points than shorter carbon chains. As a result, longer carbon chains form more Van der Waals interactions and have greater melting and boiling points.
- The strength of the Van der Waals forces depends on and shape of the alkane molecules. Straight carbon chains can be stacked closely together and have large surface areas for interacting with neighboring alkane molecules, unlike branched carbon chains, which cannot be packed closely together and hence can form less contact points between them. As a result, straight carbon chains form more Van der Waals interactions and have greater melting and boiling points.
Reactivity
Alkanes are relatively unreactive because C-C and C-H bonds are very strong and have very low polarities. However, alkanes can still be involved in some
Combustion of Alkanes
Alkanes are used as fuels to produce energy by combustion with or without oxygen.
Complete Combustion
Complete combustion: alkanes and other organic compounds are burnt fully in excess oxygen to produce CO2 and H2O. This produces lots of energy, alongside water and carbon dioxide.
Example: Complete Combustion of Butane
Butane + Oxygen —> Carbon Dioxide + Water
The reaction is very exothermic.
Incomplete Combustion
Incomplete combustion: Alkanes are burnt in an insufficient supply of oxygen and so do not react fully and so make the toxic products carbon monoxide and soot.
Example: Incomplete Combustion of Butane
Butane + (less) oxygen —> Carbon Monoxide + Water
Butane + (less) oxygen —> Carbon (soot) + Water
Pollutants From Combustion
Carbon monoxide – carbon monoxide is toxic. It binds to haemoglobin in red blood cells more strongly than oxygen. This reduces the ability of the blood to carry oxygen in the body.
Soot – soot, or particulates, cause respiratory problems including asthma. Soot also causes global dimming in which light from the sun is reflected from the particles in the atmosphere back into space. Less light reaches the Earth, so there is less photosynthesis and cooler temperatures.
Smog – As engines are not 100% efficient, unburnt hydrocarbons are passed through the exhaust and into the atmosphere. These can react with nitrogen oxides and ozone to create photochemical smog. Respiratory problems are produced from this type of smog.
Nitrogen oxides – formed when high pressure and temperatures in the engine cause nitrogen and oxygen gases from the atmosphere to react. Nitrogen oxides react with rain water to produce nitric acid.
Sulphur dioxide – small amounts of sulphur are present in hydrocarbon fuels. They burn with oxygen when the hydrocarbons are burnt, to make sulphur dioxide. The sulphur dioxide reacts with rainwater to produce sulphuric acid.
Acid rain – nitrogen oxides and sulfur dioxide gas (produced when sulfur impurities in fuel are burnt in the presence of oxygen) can also react with water in the air to form acid rain which can corrode limestone buildings and statues; harm crops and other plants, and kill aquatic life by acidifying ponds and lakes.
Catalytic Converters and Slurry
We can limit the damage caused by these pollutants using catalytic converters and desulfurisation.
Catalytic converters are devices placed in the exhaust pipes of cars. They are made up of a thin layer of a variety of costly transition metals such a palladium, rhodium or platinum spread over a ceramic honeycomb shape. The honeycomb shape greatly increases the surface area for the reaction to occur.
Catalytic converters work by catalysing the reaction between carbon monoxide and nitrous oxide. They also catalyse the reaction between hydrocarbons and nitrogen monoxide. Less polluting waste products, CO2, N2 and H2O are produced.
Flue Gas Desulfurisation
Flue gas desulfurisation occurs to remove sulfur dioxide from flue gases from coal-fired power stations. This prevents them from entering the atmosphere and causing acid rain. The waste gases are sprayed with a slurry containing calcium oxide and calcium carbonate to produce calcium sulfate (gypsum). The gypsum is used to make plasterboard.
Halogenation of Alkanes
Alkanes can have one or more of the hydrogen atoms in their structure replaced by halogen atoms to form halogenalkanes by the process of free-radical substitution. We looked at this reaction mechanism already in chapter 75. As a recap, it involves 3 steps:
1. Initiation – The bond in diatomic halogen molecules is broken by homolytic fission under UV light to form two free radicals.
2. Propagation – Free radicals attack alkane molecules generating more free radicals. These free radicals go on to attack more alkane molecules forming even more free radicals . It is a chain reaction.
3. Termination – Free radicals react with each other to produce a stable molecules and no new free radicals. The chain reaction ends.
However, this method of halogenoalkane synthesis is inefficient because you get a mixture of products. These unwanted products form because:
- More than one hydrogen atoms in the alkane can be substituted by halogen atoms. In the above example, if substitution continued, you could get dichloromethane, trichloromethane and tetrachloromethane which would then have to be separated from the chloromethane.
- Free-radical substitution can happen at different points along the chain to form a mixture of isomers. If instead of reacting with methane like in the above example, chlorine was reacting with propane, we could get would get both 1-chloropropane and 2-chloropropane.
Alkanes are a type of organic compound that are composed of only carbon and hydrogen atoms and are characterized by their single bonds between the carbon atoms. Alkanes are commonly referred to as “saturated hydrocarbons” because they contain the maximum number of hydrogen atoms possible for their carbon skeleton.
The properties of alkanes in A-Level Chemistry include:
Non-polarity: Alkanes are non-polar compounds, meaning they do not have a positive or negative charge and therefore do not dissolve in water.
Low reactivity: Alkanes are relatively unreactive, meaning they do not easily react with other compounds.
Boiling Point: The boiling point of alkanes increases with the number of carbon atoms in the molecule. This is due to the increase in intermolecular forces as the molecule becomes larger.
Flammability: Alkanes are highly flammable, meaning they easily catch fire and burn.
The reactivity of alkanes in A-Level Chemistry is low because of the strong carbon-carbon and carbon-hydrogen bonds in the molecules. Alkanes typically only react with other compounds under certain conditions, such as high temperatures or the presence of strong catalysts.
Alkanes react with oxygen in A-Level Chemistry through a process called combustion, which is the burning of the alkane in the presence of oxygen to produce carbon dioxide and water vapor. This reaction is exothermic, meaning it releases energy in the form of heat and light.
The equation for the combustion of alkanes in A-Level Chemistry can be represented as:
CnHm + (n + (m/2))O2 -> nCO2 + (m/2)H2O
The uses of alkanes in A-Level Chemistry include:
Fuel: Alkanes are a common source of fuel, such as gasoline and diesel fuel.
Raw Materials: Alkanes are used as raw materials for the production of other chemicals, such as plastics, synthetic rubber, and other organic compounds.
Pharmaceuticals: Alkanes are used as solvents in the pharmaceutical industry and are also used as raw materials for the production of certain drugs.
The structure of alkanes in A-Level Chemistry is characterized by a carbon skeleton composed of single bonds between the carbon atoms. The hydrogen atoms are attached to the carbon atoms through single bonds. The arrangement of the carbon and hydrogen atoms in an alkane molecule determines its properties and reactivity.
Alkanes react with halogens in A-Level Chemistry through a substitution reaction, in which a halogen atom takes the place of a hydrogen atom in the alkane molecule. This reaction is typically performed under controlled conditions, such as in the presence of a catalyst or at high temperatures, to increase the reactivity of the alkane.
As the size of an alkane increases, its boiling point and melting point increase, as do its viscosity and density. This is because the intermolecular forces between larger alkane molecules are stronger than those between smaller molecules.
Structural isomers have the same molecular formula but different structural arrangements of their atoms. Stereoisomers have the same molecular formula and structural arrangement of atoms, but differ in the spatial arrangement of their atoms.
Complete combustion of alkanes occurs when there is enough oxygen present to convert all of the carbon and hydrogen in the molecule into carbon dioxide and water, respectively. Incomplete combustion occurs when there is not enough oxygen present, resulting in the production of carbon monoxide and/or soot in addition to carbon dioxide and water. Incomplete combustion can also release harmful pollutants such as nitrogen oxides.
Alkanes are used as fuels for heating and transportation, such as gasoline and propane. They are also used as solvents and in the production of plastics, synthetic fibers, and other materials.
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