topic 7 - organic chemistry

• Hydrocarbons are compounds that contain hydrogen and carbon atoms only
• Crude oil is a finite resource that is found in the Earth’s crust. It is the remains of organisms that lived and died millions of years ago - mainly plankton which was buried in mud
• Crude oil is a complex mixture of hydrocarbons. The carbon atoms in these molecules are joined together in chains and rings

  
• Crude oil is an important source of fuels and feedstock for the petrochemical industry. A petrochemical is a substance made from crude oil using chemical reactions. A feedstock is a raw material used to provide reactants for an industrial reaction
• Other useful substances made from compounds found in crude oil are: solvents, lubricants and detergents
• The alkanes form a homologous series. They:

  • have the same general formula
  • differ by CH2 in molecular formulae from neighbouring compounds
  • show a gradual variation in physical properties, such as their boiling points
  • have similar chemical properties
• The general formula for alkanes is CnH2n+2 where n is the number of carbon atoms in the molecule
• The table shows 4 alkanes: methane, ethane, propane and butane

  
• The alkanes are saturated hydrocarbons:

  • hydrocarbons, because they are compounds containing hydrogen and carbon only
  • saturated, because their carbon atoms are joined by C-C single bonds
• Fractional distillation is the process used to separate crude oil into simpler, more useful mixtures. This method can be used because different hydrocarbons have different boiling points
• During the fractional distillation of crude oil:

  • heated crude oil enters a tall fractionating column, which is hot at the bottom and gets cooler towards the top
  • vapours from the oil rise through the column
  • vapours condense when they become cool enough
  • liquids are led out of the column at different heights
• Small hydrocarbon molecules have weak intermolecular forces so have low boiling points. Long hydrocarbon molecules have stronger intermolecular forces so have higher boiling points
• The different, useful mixtures are called fractions because they are only part of the original crude oil

  
• Each crude oil fraction contains a mixture of hydrocarbons. They are mostly alkanes and have similar (but not identical):

  numbers of hydrogen and carbon atoms in their molecules

  • boiling points
  • ease of ignition
  • viscosity
• The gases fraction contains hydrocarbons with one to four carbon atoms. These have:

  • boiling points below room temperature
  • they are very flammable
  • a low viscosity
• The bitumen fraction contains hydrocarbons with more than 35 carbon atoms. These have:

  • boiling points well above room temperature
  • are very difficult to ignite
  • have a high viscosity
• Hydrocarbon fuels can undergo complete combustion or incomplete combustion depending on the amount of oxygen available
• Complete combustion of a hydrocarbon fuel happens when there is a good supply of air. Carbon and hydrogen atoms in the fuel react with oxygen in an exothermic reaction:

  • carbon dioxide and water are produced
  • the maximum amount of energy is given out
• Incomplete combustion happens when the supply of air or oxygen is poor. Water is still produced but carbon monoxide and carbon are produced . Less energy is released
• Cracking is a reaction in which larger saturated hydrocarbons are broken down into smaller, useful hydrocarbons. The original starting hydrocarbons are alkanes and the products include alkanes and alkenes
• Hexane can be cracked to form butane and ethene:

  hexane → butane + ethene

  C6H14 → C4H10 + C2H4

  The starting compound will always fit the rule for an alkane, CnH2n+2. The first product will also follow this rule. The second product will contain all the other C and H atoms. The second product is an alkene, so it will follow the rule CnH2n
• Various methods can be used for cracking:

  • Catalytic cracking uses a temperature of approximately 550°C and a catalyst known as a zeolite which contains aluminium oxide and silicon oxide
  • Steam cracking uses a higher temperature of over 800°C and no catalyst
• Cracking is important for two main reasons:

  1. It helps to match the supply of fractions with the demand for them.
  2. It produces alkenes, which are useful as feedstock for the petrochemicalindustry
• Alkanes and alkenes both form homologous series of hydrocarbons, but:

  • alkanes are saturated, their carbon atoms are only joined by C-C single bonds
  • alkenes are unsaturated, they contain at least one C=C double bond
• Alkenes are more reactive than alkanes. Alkenes can take part in reactions that alkanes cannot. For example, ethene molecules can react together to form poly(ethene), a polymer.

  Alkenes will react with bromine water and turn it from orange/brown to colourless. This is the way to test for a double C=C bond in a molecule
• How to remember the order of the alkanes: Monkeys - Methane

  Eat - Ethane

  Purple - Propane

  Bananas - Butane

  Pentane
• The order of the alkenes:

  Ethane

  Propene

  Butene

  Pentene
• The alkenes form a homologous series. Like all homologous series, the alkenes:

  • have the same general formula
  • differ by CH2 in molecular formulae from neighbouring compounds
  • show a gradual variation in physical properties, such as their boiling points
  • have similar chemical properties
• The general formula for the alkenes is CnH2n, where n is the number of carbon atoms in the molecule
• The table shows three alkenes: ethene, propene and butene

  
• The alkenes are unsaturated hydrocarbons:

  • hydrocarbons, because they are compounds containing hydrogen and carbon only
  • unsaturated, because they contain a C=C double bond, which means that they have two fewer hydrogen atoms than the corresponding alkane

  The C=C bond is the functional group in the alkenes. It is responsible for the typical reactions of alkenes
• Like the alkanes, the alkenes undergo combustion. However, alkenes are less likely to combust completely, so they tend to burn in air with a smoky flame due to incomplete combustion
• The functional group, C=C, allows alkenes to undergo addition reactions. For example, ethene reacts with bromine to form 1,2-dibromoethane:

  CH2=CH2 + Br2 → CH2BrCH2Br

  The reaction is an ‘addition’ reaction because one molecule combines with another molecule, forming one larger molecule and no other products.
• An orange-brown solution of bromine dissolved in water, called bromine water, is used to distinguish between alkanes and alkenes:

  • there is no change when bromine water is mixed with an alkane
  • the bromine water becomes colourless when it is mixed with an alkene
• Alkenes can react with different types of chemicals during addition reactions.

  Alkene + hydrogen → alkane

  This is called hydrogenation, and it needs a catalyst.

  For example:

  Propene + hydrogen → propane
• Alkene + water (steam) → alcohol

  This is called hydration, and it needs a temperature of approximately 300°C and a catalyst.

  For example:

  Butene + water → butanol
• Chlorine, bromine or iodine can be added to an alkene. These reactions are usually spontaneous. Here are some examples:

  Ethene + chlorine → dichloroethane

  
• The alcohols form a homologous series. Like all homologous series, the alcohols:

  • have the same general formula
  • differ by CH2 in molecular formulae from neighbouring compounds
  • show a gradual variation in physical properties, such as their boiling points
  • have similar chemical properties
• The functional group in the alcohols is the hydroxyl group, -OH. It is responsible for the typical reactions of alcohols
• The table shows four alcohols: methanol, ethanol, propanol, butanol