Organic Chemistry


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  • Hydrocarbon - compound of carbon and hydrogen only.
  • Crude oil = source of hydrocarbons.
  • Complex mixture of hydrocarbons - is seperated into fractions.
  • Each fraction contains a mixture of hydrocarbons with the similar boiling points..
  • Tre fractions are obtained by fractional distillations.
  • Short chains - low Boiling point ... condense near the top.
  • long chains - high Bp ... condense near the bottom.
  • Gases do not condense and pass through an outlet at the top as petroleum jelly.
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Alkanes And cycloalkanes

  • Saturated molecules.
  • will not decolourise bromine water.
  • Alkanes - single chained hydrocarbons.
  • single C-C bond.
  • Alkanes - each carbon is bonded to 4 other atoms.
  • Each carbon has a tetrahedral shape.
  • Each bond angle = 109.5.
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Boiling points Of Alkanes

  • Crude oil has many fractions.
  • There are weak intermolecular forces (Wan der waals forces) in order to boil these forces have to be broken.
  • As the chains length increases the bp increases...more points of contact.
  • therefore, more points of contact and so more wan der waals forces of attraction.
  • It takes more energy and therefore a higher temperature to seperate the bonds.
  • This is why alkanes in crude oil can be seperated into fractions.
  • Effects of Branching
  • Isomers of alkanes have the same Mr value.
  • A branched isomer has a lower bp than a unbranched isomer.
  • Because in a branched isomer there are fewer points of contact between molecules and therefore fewer wan der waals forces of attraction.
  • Branched molecules can not get as close to each other unbranched molecules ... and therefore fewer WDW's...
  • So less heat is required to seperate the molecules.
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The combustion of Alkenes

  • Short chain alkanes = valuable ... as they = clean fuel.
  • They burn in plentiful supply of oxygen and they for CO2 and H2O.
  • Methane - main constituent for natural gas.
  • CH4 + 2 O2 -----> CO2 + 2 H2O.
  • This shows that methan burns cleanly with oxygen.
  • Propane and butane are easily liquified and are used as liquified petroluem gas or LPG.
  • Insuffiecient supply of oxygen.
  • E.e internal combustion engine in most cars - limited supply of O2.
  • Octane (fuel) undergo's incomplete combustion  ... to for CO.
  • CO= colourless, poisonous gas.
  • Produced when a fossil fuel is combusted in insufficient oxygen.
  • CO = poisonous.
  • prevent RBC's carrying O2 and so the body is starved of oxygen.
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  • After fraction distillation there is a surpluss of long chained hydrocarbons.
  • BUT... short chained hydrocarbons are in demand.
  • short chained alkanes = great fuels.
  • short chained alkenes = for polymer production.
  • Cracking - breakdown of long chained hydrocarbons to form a mixture of short chain alkanes and alkenes.
  • Catalytic cracking ... first used mid 1930's.
  • Initially Al2O3 and Silicondioxide were used as the catalysts.
  • But now there are more effiecient catalytic systems.
  • IN CRACKING - long chained alkanes are broken down randomly!!!
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Producing Cyclic hydrocarbons

  • Aliphatic hydrocarbons = carbon atoms attached in straight unbranched chains.
  • Can be converted to cyclic and aromatic hydrocarbons.
  • H2 gas is always produced as a result of reforming.

Improving Fuels

  • Research octon number rates how well fuels burn.
  • Fuels with high octane rating burn close to 100% (efficiently).
  • Straight chain - heptane = poor fuel and it has an actane rating of 0.
  • Branched and cyclic alkanes = important petrol additives.
  • They promote more efficient combustion.
  • consequently branched chain alkanes = used in fuel.
  • H2 gas which is produced in reforming is then recycled in the production of ammonia and margarine.
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Fossil fuels - fuels of the future

  • Crude oil --- and the material produced from it play a big part in our life.
  • sf impact on standard of living.
  • We rely on crude oil for power for electrical generations.
  • Use of crude oil = apparent ... produce alkanes.
  • Branched and cyclic alkanes = best of fuel ...
  • GOOD FUEL = readily available, inexpensive, and easily transportable.
  • Oil price -- increasing.
  • Now need an alternative!
  • Burning hydrocarbons - increase atmospheric pollution.
  • CO - incomplete combustion in the internal of a combustion engine.
  • CO2 - contributes to global warming and the greenhouse effect...
  • Nitrogen oxides - contribute to aciod rain and the destruction of rain forests
  • Hence, increase temp on earth.
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Future fuels

  • Biofuel - derived from recently living materials - like plants and animal waste.
  • Rapeseed - Agricultural crops... specially grown for energy use ... another eg... sugar cane (biodiesel).
  • Ethanol - fermenting sugar and other carbs.
  • burns efficiently in plentiful supply of oxygen.
  • Ehtanol and petroleum can be blended together to make the fuel burn more efficiently.
  • Ethanol can be used (when blended with petroluem ) in engines without any need for majour modifications ...
  • sf reduced harmful gases that are produced.
  • Bioethanol - getting a big boost.
  • First plant located in somerset.
  • Capable of producing 105000 tonnes of bioethanol.
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  • Fuel oil.
  • Derived from rapeseeds.
  • Compatible with engines - can be used 100 % pure.
  • However, usually plended with normal diesel.
  • Motor fuel being produced from sugar can = long term interest ...
  • Because crude oil = finite.
  • crude oil - millions of year to produce.
  • Ethanol - alternative source for fuel (produced from sugar cane).
  • sugar cane - annual crop.
  • BRAZIL - produces ethanol from sugar cane... climate favour this means of production.
  • And it has few oil reserves of its own.
  • Fossil fuel = non renewable.
  • Ethanol - produced from hydration of ethene and fermentation
  • Increased CO2 lvls from combustion of fossil fuels - leads to global warming and climate change.
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Substitution reactions of Alkanes

  • Using alkanes and UV radiation.
  • by Cl2 and Br2 to form halogenoalkanes.
  • Halogen react with alkanes in the presence of UV radiation or at temp above 300degs .
  • Methane reacts with chlore = a readical substitution.
  • Radical substitution - reaction where radical replaces ... different atom.
  • A hydrogen atom in the alkane is substitued with a halogen.
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  • The Cl-Cl bonds are broken by homolytic fission ... forming 2 free radicals Cl* and Cl*.
  • UV RADIATION - provides the enrgy for the bond fission.
  • Cl-Cl -------------> Cl* + Cl* (The * represents the free radical... normall it is a fullstop).
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  • Alkenes and cycloalkenes = unsaturated.
  • They have atleast one double bond.
  • A sigma bond is formed between the 2 carbon atoms (from the s-subshell).
  • Each carbon contributes 1 electron.
  • Pi - bonds form above and below the plane of the carbon atoms.
  • There is an overalp of the P-orbitals.
  • Each carbon atom contributes 1 electron from a p-orbital to the pi bond.
  • Pi-bond fixes the carbon atoms in a fixed position - at either end of the double bond.
  • This prevent rotation.
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Shape Of Alkene molecules

  • 3 regions of electron density surround each carbon in the double bond.
  • The Pair of electrons repel each other far apart to minimise repulsion.
  • Gives a trigonal planar shape.
  • 120 degrees..
  • Ethene = flat trigonal planar.
  • Cyclic Alkenes.
  • Closed rings of carbon atoms - containing one or more double bonds.
  • Most common cyclic alkene - cyclohexene.
  • Don't follow the same formula as alphatic alkenes.
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