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Benzene

Kekule thought that it was made up of 3 double bonds however xray electron density maps have shown this not to be the case and that it is actually made up of a delocalised ring structure evidence for this is  :

  • the bonds lengths of the molecule are all the same
  •  the energy needed for hydrogenation is not equal to that of 3 bonds becoming saturated - the actual value which is needed represents the energy required to delocalise the 6 electrons ( one for eaxh carbon atom) the difference between the predicted and the actual value of this reaction represents the aromatic stabilisation energy of the molecule - benzene is stable
  • the same idea from the above can be applied to the enthalpy of formation of the molecule - it is less than expected
  • the value for a double bonded carbon is different to that of an Alkene in this molecule
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Bonding - the delocalised ring structure

the ring is formed from electrons which have come from the pi bonds in the molecule from p orbitals the electrons are free to move about the entire molcule

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Reactions of benzene

Combustion very sooty flame Oxidation N/A Bromination N/A H2SO4 conc

N/A

during the oxidation reaction there is very little change and in the bromination reaction 2 layers are formed and decolourisation occurs

with the sulfuric acid there is no reaction and two layers are formed.

 these reactions are done with methyl benzene as it is not carciogenic ( cancer causing)

 

 

 

 

 

 

 

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Reactions of methyoxybenzene

Bromination- occured and the mixture was decolourised

sulfoniation - layersd form and the solution gets hot

Nitration- browny/purple layers form

friedel craft reaction- when the fumes formed where tested with ammonia they formed white smoke - NH4Cl is formed in this reaction

the reason that this molecule is far more reactive is becuase the oxygen in the molecule donates 2 more electrons to the delocalised ring structure. The reactions of the methyl benzene and alkenes in general are quite similar due to the the unsaturated nature of the the molecules.

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Nomenclature- naming things

the naming of molcules like this is very easy as you can count around the ring to find out the number of the carbon that is being used it is best to count from the top and work your way around... 

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Electrophlic substitution reactions

  • these are more common reaction simply because it will maintain the stablity of the molecule  examples inculde friedel crafts reactions 

for example - Alkylation this is where something like a methyl group is added to the molecule 

you will need a catalyst - anhydrous AlCl3 and a a temp of 30 degrees under reflux and finally a chloroalkane

formation of the electrophile - AlCl3 +CH3Cl = AlCl4-+ CH3+ ( the electrophile)

 another reaction of a similar nature is Acylation which uses the same cataylst but instead of a chloralkane you need an acid chloride

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Bromanation and sulfonation

Bromination/ or chlorination uses uv light and solutions of Br2 / Cl2 to subsitute all of the hydrogens on Benezene or you need a catalyst of FeBr3 and heat under reflux at room temp  to change just one of the hydrogens to a bromine

for sulfonation you need fuming H2SO4 which provides the electrophile SO3  - heat under reflux and this will add HSO3 to the molecule

for nitration you need Concetrated HNO3 and H2SO4 to form the NO2 electrophile p this is done at low temperatures to reduce the chances of  multiple nitrations 

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Phenol

This is far more reactive than Benzene because the delocalised ring structure takes in the lone pairs of electrons on the oxygen atom and this makes the molecule more attractive to electrophiles so the same reactions that happen with benzene occur at low temperatures and often with out cataysts.As the electrons on the oxygen have been taken in to the delocalised ring structure the molcule is harder to esterify 

phenols are acidic compounds and less acidic than carboxylic acids so they are very weakly acidic

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