Topic 7 - Organic chemistry

• A hydrocarbon is any compound that contains carbon and hydrogen atoms only
• Alkanes have the general formula CnH2n+2
• A homologous series e.g. alkanes is a group of organic compounds that react in a similar way with the same general formula and functional group
• Alkanes are saturated compounds - each carbon has (four) SINGLE covalent bonds
• The first four alkanes are methane, ethane, propane, butane (Monkeys Eat Peanut Butter)
• As alkanes become bigger molecules:

  • Their boiling point increases (less volatile)
  • They become less flammable
  • They become more viscous (less runny)
  • Their flame becomes more smoky
• The equation for the complete combustion of a hydrocarbon is:

  hydrocarbon + oxygen -> carbon dioxide+ water (+lots of energy)
• During combustion, both carbon and hydrogen are oxidised
• Hydrocarbons (especially alkanes) are used as fuels due to the amount of energy they release
• Crude oil is a fossil fuel, formed from the remains of plants and animals that died millions of years ago and were buried in mud. Over millions of years, with high temperature and pressure, the remains turn to crude oil
• Fossil fuels like coal, oil and gas are non-renewable because they're being used up much faster than they're being formed (they're finite)
• Crude oil is a mixture of lots of different hydrocarbons (mostly alkanes)
• Fractional distillation is used to separate hydrocarbon fractions in crude oil
• During fractional distillation:

  • The oil is heated until most of it evaporates
  • The gases enter a fractionating column which is hotter at the bottom and cooler at the top
  • The vapour rises and then condenses at different temperatures
  • The temperature depends on the size of the molecule as different sized molecules have different sized boiling points
  • The crude oil mixture is separated into different fractions with similar numbers of carbon atoms and boiling points
• Each fraction obtained from fractional distillation contains a mixture of hydrocarbons with similar numbers of carbon atoms
• Crude oil is essential for modern civilization, providing fuel for most modern transport - cars, trains, planes, the lot. Diesel, oil, kerosene, heavy fuel oil and LPG (liquid petroleum gas) all come from crude oil
• The petrochemical industry uses hydrocarbons from crude oil as a feedstock to make new compounds like polymers, solvents, lubricants, and detergents
• All products derived from crude oil are organic compounds (compounds containing carbon atoms), forming different groups called homologous series
• Cracking is a process that splits up long-chain hydrocarbons into smaller, more useful molecules, including alkenes
• Cracking is a thermal decomposition reaction - catalytic cracking over a catalyst, and steam cracking by mixing with steam and heating to high temperatures are the two main methods
• During cracking, long-chain hydrocarbons (that form gloopy liquids like tar) are broken down into shorter-chain hydrocarbons including alkanes and alkenes. Often the products are used for fuel
• Alkenes can often be used not for fuel but as a starting material when making lots of other compounds and can be used to make polymers
• Catalytic cracking:

  • Heat long chain hydrocarbons to vaporise them
  • Pass the vapour over a hot powdered aluminium oxide catalyst
  • The long-chain molecules split apart on the surface of the specks of catalyst
• Steam cracking:

  • Heat long chain hydrocarbons to vaporise them
  • Mix the long chain hydrocarbons with steam and heat them to a very high temperature
• The order of types of hydrocarbons that form during fraction distillation (from smallest molecule to largest molecule) goes:

  • LPG for some cars
  • Petrol for vehicles
  • Naphtha for making chemicals
  • Kerosene as jet fuel
  • Diesel oil for cars and lorries
  • Lubricating oil for lubricating
  • Heavy fuel oil for ships and central heating
  • Bitumen for roads and roofing
• Alkenes are unsaturated hydrocarbons with a double carbon=carbon bond
• Alkenes have two fewer hydrogens compared to alkanes with the same number of carbon atoms
• The C=C double bond in alkenes can open up to make a single bond, allowing the two carbon atoms to bond with other atoms - making them reactive (more than alkanes)
• The first four alkenes are ethene (C₂H₄), propene (C₃H₆), butene (C₄H₈), and pentene (C₅H₁₀)
• Incomplete combustion of alkenes results in a smoky yellow flame and the production of less energy compared to complete combustion
• In a large amount of oxygen, alkenes combust completely to produce ONLY water and carbon dioxide (and of course lots of energy)
• Incomplete combustion of alkenes in the air can produce carbon and or carbon monoxide (CO) as well as carbon dioxide and water
• Whether complete or incomplete combustion happens depends on the amount of oxygen present. In air there isn't enough oxygen for complete combustion of alkenes. The products of incomplete combustion depend on the amount of oxygen present
• Most of the time, alkenes react via addition reactions
• In alkenes, the carbon-carbon double bond opens up to leave a single bond, and a new atom is added to each carbon (addition reactions)
• Alkenes have the functional group 'C=C', which determines how they typically react. Due to this you can suggest the products of a reaction based on your knowledge of alkenes
• All alkenes react in similar ways due to the functional group 'C=C'
• Ethanol can be made by mixing ethene with steam and passing it over a catalyst
• Hydrogenation is the addition of hydrogen (in the presence of a catalyst) to the double-bonded carbons in alkenes to form saturated alkanes. Here the double bonded carbons 'open up' to bond with the hydrogen
• Bromine and ethene react to form dibromoethane, where two bromine atoms are added across the double bond