Hydrocarbons Straight and Branched All Single Bonds Aliphatic Alkanes Cyclic Alkenes At least one Double Bond Alkynes At least one Triple Bond Aromatics Benzene ring Delocalised Pi Bonds

Sources of Hydrocarbons

Structure of Aliphatic Hydrocarbons

PETROCHEMICALS In the past, the most important organic chemicals were derived from coal. Nowadays, natural gas and crude petroleum provide an alternative source. The composition of crude petroleum varies according to its source. It is a dark coloured, viscous liquid It consists mostly of alkanes with up to 40 carbon atoms, plus water, sulphur and sand It can be split up into fractions by fractional distillation. Distillation separates the compounds according to their boiling point

Crude Oil

Hydrocarbons Compounds which contain only carbon and hydrogen bonded together covalently Sources Coal Natural Gas Petroleum [Crude Oil]

General members of a homologous series ALKANES General members of a homologous series general formula is CnH2n+2 - for non-cyclic alkanes saturated hydrocarbons - all carbon-carbon bonding is single bonds are spaced tetrahedrally about carbon atoms. Have the same functional group Have similar chemical properties Show gradation in physical properties

Alkanes Alkanes CnH2n+2 Methane CH4 GAS Ethane C2H6 GAS Propane C3H8 GAS Butane C4H10 GAS Pentane C5H12 LIQUID Hexane C6H14 LIQUID

THE STRUCTURE OF ALKANES In ALKANES, the four orbitals of carbon repel each other into a TETRAHEDRAL arrangement with bond angles of 109.5º. Each orbital in carbon overlaps with the 1s orbital of a hydrogen atom to form a C-H bond. 109.5º

Methane Methane is produced from the remains of marine animals and plants that sank to the bottom of ancient seas and lakes. Methane is produced in the formation of coal. Methane is produced in an uncontrolled way in slurry pits and refuse dumps. Methane also is produced in large quantities in the digestive tracts of animals such as cattle.

Hazards of Methane In coalmines methane can produce an explosive mixture with air. Methane is produced in uncontrolled amounts in slurry pits and refuse dumps and if a source of ignition is provided then fires or explosions can occur. Methane production in slurry pits also causes deaths by suffocation. Contributes to Greenhouse Effect

Greenhouse Effect Result Temperature Rises Long and Short Wave from Sun Soil and Plants Long Short Long Wave reflected by Glass [or atmosphere] Short Wave enters Plants and Soil absorb Short Wave Plants and Soil then emit Long Wave Long Wave Reflected by Glass so Trapped Result Temperature Rises

Enhanced Greenhouse Effect Long and Short Wave from Sun Soil and Plants Long Short Greenhouse Gases Long Wave reflected by greenhouses gases Short Wave enters Plants and Soil absorb Short Wave Plants and Soil then emit Long Wave Long Wave Reflected by greenhouse gases so Trapped Result Temperature Rises

Greenhouse Gases Water Vapour Methane Nitrous Oxides CFCs Carbon Dioxide Methane comes from cattle, landfills and Paddy Fields Nitrous Oxides from exhausts CFCs from aerosols, refrigeration unit, etc. High levels of CO2 comes from burning fossil fuels (home heating, transport)

Greenhouse Effect Greenhouse gases Cause Global Warming Climate Changing Melting Polar Caps More violent weather Growth of deserts Rising sea levels Flooding of low lying cities Spread of tropical pests and diseases such as malaria

PHYSICAL PROPERTIES OF ALKANES Boiling point increases as they get more carbon atoms in their formula more atoms = greater induced dipole-dipole interactions greater intermolecular force = more energy to separate the molecules greater energy required = higher boiling point CH4 (-161°C) C2H6 (-88°C) C3H8 (-42°C) C4H10 (-0.5°C) difference gets less - mass increases by a smaller percentage Straight chains molecules have greater interaction than branched “The greater the branching, the lower the boiling point” STRUCTURAL ISOMERS OF C5H12 HIGHEST BOILING POINT LOWEST BOILING POINT

PHYSICAL PROPERTIES OF ALKANES

PHYSICAL PROPERTIES OF ALKANES Melting point general increase with molecular mass the trend is not as regular as that for boiling point. Solubility alkanes are non-polar so are immiscible with water they are soluble in most organic solvents.

CHEMICAL PROPERTIES OF ALKANES Introduction - fairly unreactive; (old family name, paraffin, meant little reactivity) - have relatively strong, almost NON-POLAR, SINGLE covalent bonds - they have no real sites that will encourage substances to attack them Combustion - make useful fuels - especially the lower members of the series - react with oxygen in an exothermic reaction complete CH4(g) + 2O2(g) ——> CO2(g) + 2H2O(l) combustion incomplete CH4(g) + 1½O2(g) ——> CO(g) + 2H2O(l) the greater the number of carbon atoms, the more energy produced BUT the greater the amount of oxygen needed for complete combustion. Handy tip When balancing equations involving complete combustion, remember... every carbon in the original hydrocarbon gives one carbon dioxide and every two hydrogen atoms gives a water molecule. Put the numbers into the equation, count up the O’s and H’s on the RHS of the equation then balance the oxygen molecules on the LHS.

Alkyl Groups Methyl CH3 Ethyl C2H5 Propyl C3H7 Butyl C4H9 Pentyl C5H11

STRUCTURAL ISOMERS OF C5H12 Isomerism the first example of structural isomerism occurs with C4H10 BUTANE 2-METHYLPROPANE Structural isomers have the SAME MOLECULAR FORMULA BUT DIFFERENT STRUCTURAL FORMULA They possess different physical properties such as boiling point, melting point and density STRUCTURAL ISOMERS OF C5H12 PENTANE 2-METHYL BUTANE 2,2 DIMETHYL PROPANE

Alkenes Alkenes are a homologous series of hydrocarbons with the general formula CnH2n They are named systematically, with a prefix indicating the number of carbon atoms per molecule, and the ending “ene”

Alkenes Ethene C2H4 Propene C3H6 But-1-ene C4H8 But-2-ene C4H8

Ethene C2H4

Propene C3H6

But-1-ene C4H8

But-2-ene C4H8

2,methylpropene C4H8 CH3 CH3 C CH2

Physical properties of alkenes Physical state: Gases Insoluble in water Soluble in non-polar solvents such as cyclohexane

Alkynes Alkynes are a homologous series of hydrocarbons with the general formula CnH2n-2 Ethyne (C2H2) is the first member of the series

Physical properties of ethyne Physical state: Gas Insoluble in water Soluble in non-polar solvents such as cyclohexane

Other Alkanes Hexane Heptane Octane Cyclohexane 2,2,4 trimethylpentane