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  • Created on: 15-05-14 18:57
Preview of Mechanisms

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- Electrophilic addition:
- Alkene + hydrogen halide
- Alkene + halogen
- Elimination: when something is removed, leaving a double bond
- Halogenoalkane - hydrogen halide (e.g. When heated with water) alkene
- Substitution:
- Alcohol + hydrogen halide halogenoalkane
- Alkane + halogen halogenoalkane
- Halogenoalkanes: SN1, SN2 (hydrolysis with HO, NaOH, ammonia)
- Redox:
- Oxidation = when the oxidation number of an element becomes more positive/less negative
- Alcohol + dichromate aldehyde/ketone/carboxylic acid
- Hydrolysis: when water is used to split a compound apart
- Often substitution reactions with OH- ions
- Halogenoalkanes + water alcohols + hydrogen halides (catalysed by alkali as its OH- is
stronger nucleophile due to full negative charge)
- Addition polymerisation: joining together small molecules which have double bonds
- Homolytic fission: when a covalent bond breaks to give 1 electron back to each atom, producing
free radicals (curly half arrow shows movement of only 1 electron to each)
- Heterolytic fission: when covalent bond breaks to give both electrons to one of the atoms (which
becomes therefore a nucleophile), leaving an ionic intermediate (which is an electrophile)(usually
occurs in polar solvents when the bond is polar)- caused by electrophiles or nucleophiles
- Free radical: reactive particle with unpaired electrons- often produced in reactions which take place
in gas phase (at high temperature or in UV light) or in non-polar solvents (with initiator or with UV)
- Electrophile: a reagent attracted to electrons that participates in a chemical reaction by accepting an
electron pair in order to bond to a nucleophile
- Nucleophile: a chemical species that donates an electron pair to an electrophile to form a chemical
bond. All species with a free pair of electrons or at least one pi bond can act as nucleophiles
Investigating mechanisms
- Isotopes of elements involved can be used to see where each element moves to during reactions-
non-radioactive isotopes can be followed by using mass spectrometry, radioactive ones can be
traced by detecting their radiation
- Intermediates can be found with spectroscopy or by adding other chemicals to react with them
- Shapes of products can be studied: for addition reactions with cycloalkenes, the major product has
bromine atoms on opposite sides of the ring, hinting that they are not added simultaneously
- Rate of reaction: if the concentrations of both reactants affects the rate, such as in nucleophilic
substitution with 1ary halogenoalkanes, it shows that both reactants are involved in the first step
The environment
- O absorbs UV radiation from the sun and is split (O + UV 2O·); the free radicals convert oxygen
to ozone (O + O· O3). Ozone also absorbs UV and so is split again, destroying the ozone (O3 +
UV O + O·). Ozone is normally destroyed at the same rate that it is made, absorbing most of the
suns dangerous UV radiation
- NO and NO2 react catalytically with ozone, speeding up the rate of its breakdown: O3 + UV O +
O·; NO + O3 NO2 + O; NO2 + O· NO + O
- Aircrafts (especially supersonic ones in the stratosphere) release nitrogen oxides from their engines
directly into the ozone layer; micro-organisms in the soil release N2O naturally, which reacts with
oxygen in the stratosphere to form NO


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