module 6 - aromatic compounds Flashcards
What is the evidence that disproves Kekules model?
lack of reactivity of benzene:
- if benzene contained C=C then it should decolourise bromine under normal conditions in an electrophilic addition reaction
but benzene doesn’t undergo electrophilic addition nor does benzene decolourise bromine water under normal conditions
the length of the carbon-carbon bonds in benzene
- using x-ray diffraction it has been found that all the bonds in benzene were the same length
-it is known that double bonds are shorter than single bonds so this disproves Kekules
hydrogenation enthalpies
- the enthalpy changes of Kekulues has a bond enthalpy of -360KJmol-1 whilst the actual Benzene has enthalpy change of hydrogenation of -120KJmol-1
Describe the nitration of benzene (and practise drawing out the mechanism)
- benzene reacts slowly with nitric acid to form nitrobenzene
- reaction catalysed by sulphuric acid and heated to 50 degrees (if temp goes above this then further substitution reactions may occur leading to production of dinitrobenzene)
- The electrophile used is NO2+ so this needs to be formed:
HNO3 + H2SO4 -> NO2+ + HSO4- + H20 - then the mechanism happens and finally nitrobenzene and a hydrogen ion is formed
- the H2SO4 has the be reformed so H+ + HSO4- -> H2SO4
Describe the delocalised model of benzene
Benzene is a cyclic hydrocarbon containing 6 carbon atoms and 6 hydrogen atoms
each carbon uses 3/4 of the available electros to bond to 2 other carbon atoms and to one hydrogen
the final electron is in a p-orbital at right angles to the plane of the C-H bond
the adjacent p-orbitals overlap sideways in both directions, above and below the plane to form a ring of high electron density
this creates a system of pi-bonds
Describe the halogenation of benzene (and practise drawing out the mechanism)
the only time when halogens can react with benzene is when a catalyst called a halogen carrier is present e.g. ALCL3 and FeCl3 and FeBr3 and AlBr3.
Bromination:
Br2 + FeBr3 -> FeBr4- + Br+
then the mechanism reaction takes place -> Br+ is used as the electrophile
bromobenzene and H+ are formed
step 3:
H+ + FeBr4- -> FeBr3 + HBR
Chlorination
the same as bromination but using a different halogen carrier: either FeCl3 or AlCl3, with chlorobenzene and HCl forming as a result
Describe Alkylation reactions
substitution of a hydrogen atom in the benzene ring by an alkyl group - carried out by reacting benzene with a haloalkane in the presence of AlCL3 which acts as a halogen carrier catalyst, generating the electrophile
Describe Alcylation reactions
when benzene react with an acyl chloride in the presence of AlCL3 catalyst and an aromatic ketone is formed
What are phenols?
phenols are a type of organic chemistry containing an -OH functional group directly bonded to the aromatic ring
the simplest phenol C6H5OH is called phenol -> the same as the name of the group
Why is phenol a weak acid?
- less soluble in water than alcohols due to presence of the non-polar benzene ring
- when it does dissolve in water, phenol partially dissociated forming the phenoxide ion and a proton making it a weak acid
- it is more acidic than alcohols but less than carboxylic acids
Describe the reaction of phenol with sodium hydroxide
phenol reacts with sodium hydroxide to form the salt, sodium phenoxide and water in a neutralisation reaction
Describe and draw electrophilic substitution reacts of phenol
bromination of phenol:
- phenol reacts with an aqueous solution of bromine to form a white precipitate of 2,4,6 tribromophenol
- a halogen carrier is not required and the reaction is carried out at room temp
nitration of phenol:
- phenol reacts readily with dilute nitric acid at room temp and a mixture of 2-nitrophenol and 4-nitrophenol is formed
Describe the reactivities of phenol vs benzene
bromine and nitric acid react more readily with phenol than they do with benzene - phenol is nitrated with dilute nitric acid rather than needing concentrated nitric acid and sulphuric acids like benzene
the increased reactivity is caused by a lone pair of electrons from the oxygen p-orbital of the -OH group being donated in the pi system of phenol. This results in the electron density of benzene ring in phenol being increased which attracts the electrophiles more
Describe activation and deactivation
a group activates the ring if the aromatic ring reacts more readily with electrophiles
e.g. bromines requires a halogen carrier catalyst to react with benzene whilst bromine will react rapidly with phenyl amine
- the -NH2 group directs the second substituent to positions 2 or 4
a group deactivates the aromatic ring if the ring reacts less readily with electrophiles
e.g. nitrobenzene reacts slowly with bromine, requiring both a halogen carrier catalyst and a high temperature (the benzene ring in nitrobenzene is less susceptible to electrophilic substitution than benzene itself)
- the -No2 group directs the second substituent to position 3
What are directing effects?
different groups can have a directing effect on any second substituent on the benzene ring
-all 2 nd 4 directing groups (ortho and para) are activating groups (apart from halogens) and all 3 directing groups (meta) are deactivating groups
what are the 2 and 4 directing groups
-NH2 or -NHR
-OH
-OR
-R or -C6H5
-F, -Cl, -Br, -I
what are the 3 directing groups
RCOR
-COOR
-SO3H
-CHO
-COOH
-CN
-NO2
-NR3+
