what is benzene
it is a naturally occurring aromatic compound which is a very stable ring structure with delocalised electrons
Describe Kekules structure of Benzene
Kekule suggested that benzene was a 6 carbon compound with alternating double and single bonds between the carbon atoms
- he discovered that when one group was added it only made one isomer but when two groups were added it made 3 isomers this was the evidence for his structure of benzene
- contains alternating double and single bond
Describe problems with Kekules model
- unlike alkenes benzene is resistant to addition reactions
- enthalpy of hydrogenation of benzene shows that benzene is much more stable than was predicted
- all 6 carbon bonds in benzene were the same length
Describe resistance to reaction
using kekules model you would expect benzene to undergo similar reactions to alkenes for example ethene readily undergoes substitution of a hydrogen atom whereas benzene tends to undergo substitution reaction of a hydrogen atom rather than addition reactions
- Kekule tried to explain this by saying that the double and single bonds changed positions in a fast equilibrium
Describe Hydrogenation
Hydrogenation is the addition of hydrogen to an unsaturated chemical
- the enthalpy change of cyclohexene which has one double bond is -120 kjmol-1 therefore the enthalpy change of benzene should be -360kjmol-1
- in fact the enthalpy change of benzene is -208kjmol-1 therefore it is -152kjmol-1 more energetically stable than predicted
Describe bond lengths
- x ray diffraction techniques have shown that all 6 bonds in benzene are 0.140nm if Kekules structure was right than the single bonds would be 0.147nm and the double bonds would be 0.135nm therefore 3 bonds would be shorter than the other 3 and an irregular hexagon would form
Describe the delocalised structure of benzene
3 electrons from each C used to form sigma bonds to 2c and a H atom
1 electron from each C in a p-orbital overlaps with electrons from adjacent C p-orbitals
- creates a delocalised system of electrons
- rings of charge/ pie bodns are above and below the plane of carbon atoms
- the electrons in the pie system are delocalised and spread out
- ring is planar with bonds of 120
- the carbon bonds are of equal length
- sigma bonds are betweed C-C or C-H
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What is electrophillic substitution
this is a substitution reaction where an electrophile is attracted to an electron rich atom or part of a molecule and a new covalent bond is formed by the electrophile accepting an electron pair
What are the conditions needed for nitration with benzene
- concentrated sulfuric acid acting as a catalyst
- concentrated nitric acid
- mixed in a flask and held in an ice bath
- benzene is then added and a reflux condenser is set up keeping the mixture at 50 degrees to prevent further substitution reactions
- Sulfuric acid is needed to generate NO2+ electrophile from the nitric acid so is therefore a catalyst
What are the conditions needed for halogenation with benzene
- Benzene does not react with halogens as the aromatic compound is too stable
- uses a halogen carrier such as AlCl3, FeCl3, FE or AlBr3, FeBr3, of FE
- bromine is generated in situ
What is a friedel-crafts reaction
this is a substitution reaction where hydrogen is exchanged for an alkyl or acyl chain
what happens when benzene is in bromine water
When benzene is shaken with bromine water no reaction occurs, this is because benzene has a lower electron density between carbon atoms than an alkene
therefore when non polar molecules like bromine approach the benzene rings there is not enough electron density between carbon atoms to induce a dipole and start a reaction
- also the case when substituiting alkyl halides like haloalkanes as by using a hydrogen carrier a stronger electrophile can be generated and alkylation can occur
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What are the conditions needed for a benzene reaction with haloalkanes
- mixed with a halogen carrier such as Iron chloride, this acts as a catalyst and is regenerated at the end of the reaction
- makes a reactive carbocation which undergoes electrophillic substitution
- multiple substitutions are likely therefore a mixture of products are made so separate using fractional distillation and chromatography
What is the increase in reactivity due to when a haloalkane is added to a benzene
- the increase reactivity is due to the alkyl chain donating electrons to the aromatic ring which can lead to multiple substitutions
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Describe the reaction conditions needed for a reaction of benzene with acyl chloride
- AlCl3 catalyst
- reflux
- anhydrous conditions
- reaction is held at 60 degrees for 30 minutes under reflux for the reaction to occur
What are phenols
this is a class of aromatic compounds where a hydroxyl group is directly attached to the aromatic ring
Describe the acidity of phenols
- phenol is a weak acid, partially dissociates in water
- it will react with strong bases such as NaOH to form a salt and water
- weak acid as it does not react with carbonates or weak bases such as sodium carbonate
Describe the reactivity of phenols
phenol is more reactive than benzene
- due to P-orbital electrons from the oxygen and hydroxyl group adding to the delocalised electrons of the aromatic ring so the pie system of the aromatic ring becomes more nucleophilic this increases electron density allowing the aromatic ring in phenol to be more susceptible to electrophilic attacks as it can induce a dipole in non polar molecules
- therefore it can undergo direct halogenation unlike benzene
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