Organic Chemistry

about the organic chemistry needed for OCR chemistry as level

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introduction to organic chemistry


  • molecular formula: the number of each atom in a molecule
  • structural formula: shows how many atoms are joined together in a molecule
  • displayed/graphical formula:show all of the bonds and spacing of atoms in a molecule
  • skeletal formula:shows the bonds but not the carbon and hydrogen atoms.

2)functional group: the functional group is an atom or group of atoms joined to a carbon atom which makes the substace have it's own characteristic set of reactions

3)homologous series: these are groups of molecules in the same functional group and shows a trend in physical properties.

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naming of organic compounds

first find the number of carbon atoms in the longest chain... this is the base name.

then find the position of the branches on the longest chain

then find the functional group

number of carbon=longest chain=branch

1 carbons= methane= methyl

2 carbons= ethane= ethyl

3 carbons= propane= propyl

4carbons= butane= butyl

5 carbons= pentane pentyl

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Isomers: compounds with the same molecular formula but the atoms are arranged in a different way

  • structural isomers: same molecular formula but different structural formula, e.g Pentane and 2,2-dimethyl propane.
  • stereo isomers:these are molecules with the same molecular formula but different arrangement of the atoms.e.g geometric isomers-these occur when there is a double bond which does not allow rotation i.e trans-but-2-ene and cis-but-2-ene..... and optical isomers- the central carbon should be bonded to 4 different groups. 1 arrangement is a mirror image of the other and the central carbon is the chiral centre.
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  • they have single bonds between carbon-hydrogen and carbon- carbon atoms.
  • carbon contain the maximum number of hydrogen atoms so they are saturated
  • 4-bonds radiate from the carbon atom in the tetra-hedral pattern (109.5).

General physical properties

  • they are non-polar molecules
  • they are unreactive
  • they have weak dipole-dipole attraction(vander Waals) which increase with the number of electrons and therefore size of the molecules.
  • branched alkanes do not pack easily so they tend to have lower melting and boiling points then molecules of the same mass.
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chemical reactions of alkanes

1)combustion- they burn in oxygen to form carbon dioxide and water... incompete combustion produces soot (carbon), carbon monoxide and water. the reaction is very exothermic and the ease of of burning depends on the size of the molecule.

2)substitution reactions with Cl/Br-alkanes are unreactive but they react slowly with halogens in the presence of uv/light/sunlight. this type of reaction is free radical substitutution where hydrogen in alkane is removed and substituted with a halogen. the alkanes slowly decolourise bromine water in the presence of sunlight. this process happens in three stages

  • Initiation- chlorine/bromine molecules absorb UV energy and break to form free radicals
  • propagation-the free radicals are very reactive and will react with other molecules to form other free radicals.
  • termination-this occurs when 2 free radicals react together eliminating themselves.
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chemical reactions of alkenes

alkenes have a high electron density at the C=C bonds. and so they can be attacked by substances we call electrophites (electron deficit species) some substances are added to the carbon atoms with the double bond.

1) Hydrogenation/ addition of Hyrogen- alkenes react with hydrogen in the presence of a nickel catalyst at about 152'c to form alkanes. this type of reaction is useull in converting unsaturated oils into margarine.

2)With steam- they react with steam in the presence of phosphoric acid to form alcohol

3)with hydrogen halide-they react with hydrogen halide to form halogenoalkanes. the reactivity decrease with the bond strength of the hydrogen halide.

4)with halogens-they react very fast with halogens even in darkness to form (di)halogenoalkane. the reaction between bromine and alkenes is used as a test for unsaturation. and also potassium promanganate.

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continuation of chemical reactions of alkenes

5)with dilute manganate 7- alkenes are quickly oxidised by dilute MnO4 to form diol (contains OH) in this reaction an OH group is added to the carbon which had the C=C.

6)with hot concentrated manganate 7- this reaction breaks the C=C double bond to form oxidised products as follows

  • CH2- forms carbon dioxide and water
  • CHR-where R is an alkyl-forms acid
  • CRN-where R and N are alkyl groups- forms N-C=O-R

7)self addition polymerisation- In the presence of a catalyst, right temperature and pressure they under go polymerisation to form polyalkenes. Some compounds containing C=C may also undergo polymerisaiton to form polymers.

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Electrophilic addition

Electrophile-a species/molecule that is attracted to a high electron density site.

Electrophiles attack the site and form covalent bonds (by accepting a pair of electrons)e.g addition of Bromine to alkene:

  • as Bromine molecules approach the double bond it becomes polarised.
  • the bromine molecules undergo heterolytic fission. where the double bond breaks. As this happens the electron in the pie bond attack the bromine particles to form a covalent bond.
  • the carbo cation accepts a pair of electrons from the bromide ion to form a dative bond and a stable compound.

other electrophiles include: water, hydrogen halide and other halogens.

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