Unit 2.5

• The general formula of the homologous series of alkanes is CnH2n+2
• Alkane are saturated hydrocarbons (no double bonds)
• Small alkane are gases at room temperature, larger ones are liquid and even larger ones are solids (eg wax candles)
• Alkane are non polar as hydrogen and carbon have similar electronegative values. They only have sigma (single) bonds and so are generally unreactive.
• Some products of the combustion of fossil fuels contribute to the formation of acid rain. Many fossil fuels contain sulfur, which produces sulfur dioxide when burnt. Sulfur dioxide can react with water to form sulfuric (IV) acid, which can then be oxidised to form sulfuric (VI) acid, a contributor of acid rain.
• If fossil fuels are burnt with a shortage of oxygen, incomplete combustion occurs and produces carbon monoxide which is toxic. It combines with heamoglobin in the blood and so it is then unavailable to carry oxygen around the body.
• Heterocyclic bond fission describes when bothe electrons from a covalent bond go to one atom forming a positive and negative ion.
• Homolytic bond fission describes when one electron from the covalent bond go to each atom, forming radicals (they will have the same charge).
• An electrophile is an electron deficient species that can accept a lone pair of electrons.
• An addition reaction is a reaction in which the reactants combine to give only one product.
• To test for alkenes bromine is used. If alkenes are present, the bromine water will go from brown to colourless. Bromine water is often used instead of liquid bromine as it is highly corrosive
• Alkenes can be tested by acidified potassium manganese. If alkenes are present, the purple acidified potassium manganate will turn colourless.
• A nucleophile has a lone pair of electrons on an electronegative atom, which they use to form new bonds. They are negatively charged or electron rich
• CO2 is a greenhouse gas which absorbs infrared radiation from the earth's surface before emitting it from all different directions. Some of this radiation goes back towards Earth's surface and therefore the surface temperature rises.
• A double bond is made up of a sigma bond and a pi bond
• A sigma bond is the overlap of horizontal orbitals. The electrons are between the atoms.
• A pi bond is the overlap of 2 vertical orbitals. The electrons are above and below the plane of the bonded atoms.
• A pi bond creates a region of high electron density, so the molecule is more susceptible to electrophilic attack.
• A double bond (in alkenes) is weaker than 2 single bonds (in alkanes) ao the double bond breaks more easily to form a single bond. This means that alkenes are more reactive than alkanes.
• Crude oil is a mix of hydrocarbons. It is separated by fractional distillation which uses boiling points to differentiate between the different molecules.
• Hydrocarbons are broken down into smaller, useful molecules via cracking.
• Photochlorination, or free radical substitution, requires UV light for homolytic bond fission of Cl2 to take place.
• The combustion of alkanes produces CO2 and H2O
• Incomplete combustion occurs when there is a limited supply of oxygen. Less oxygen atoms lead to carbon monoxide or even carbon to be produced.
• CO2 production is a major drawback of fossil fuels. As well as this, acidic gases, such as SO2 and toxic NOx may be produced from the combustion of impurities in the fuels.
• The polymerisation of ethene requires a high temperature (200°),a high pressure (2000atm) and oxygen inhibition. A ziegler catalyst may be used to create different polymer structures.
• Addition polymerisation is the process of breaking carbon double bonds to create monomers, which may bond togther to form polymers.
• Hydrogenation of alkenes will create alkanes. This mechanism is the addition of hydrogen. Heat is needed, as well as a nickel catalyst.
• Free radicals are species with a lone pair of electrons, making it very reactive.
• Electrophilic addition is the opposite of elimination reactions as alkenes form halogenoalkanes.
• When Hbr is added to an alkene, the electron rich double bond is attracted to the delta positive hydrogen (electrophile) in the polar HBr molecule. The double bond breaks to form an intermediate. The HBr bind also breaks so that the hydrogen bonds to a carbon. The Br atom becomes an ion as it recieves both electrons, and is attracted to the positive carbon. They bond, forming a halogenoalkane.
• When Br2 us added to an alkene, it is very similar to the HBr reaction. The Br2 bond is only polar when in the region of the carbon double bond, unlike HBr as the 2 atoms have a large difference in electronegativity.
• A priority group is assigned to the atom bonded to the carbon with the highest atomic number.
• When the 2 priority groups are on the same side, it is a Z-isomer.
• When the 2 priority groups are on opposite sides, an E-isomer is formed.
• A double bond restricts the movement of atoms, so there is no free rotation about the double bond. This means that the position of the atoms matters, giving rise to E Z isomers.