Arenes Flashcards
Benzenes physical properties
Description of state
Boiling temp
Solubility
Where is it found
- colourless liquid
- boiling temp = 80°C
- insoluble in water
Found in crude oil
Benzene molecular formula
C6H6
Is benzene saturated or unsaturated
Unsaturated
How was the empirical formula of CH of benzene found
Through combustion analysis
Uses of benzene
- organic synthesis
- manufacture of detergents, explosives
- was used for decaffeinated coffee - dissolves caffeine
Describe and draw Kekules structure
Six member ring, alternate single and double bonds
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List the problems with Kekules structure
- Bromine test and reactivity
- Isomers
- Bond length
- Enthalpy changes
Describe the issues with reactivity and the bromine test with kekules structure
If benzene had 3x C=C it should decolourise bromine, it does not suggesting no double bonds
Benzene is much less reactive than equivalent alkenes - doesn’t take part in electrophilic addition.
Describe the issues with isomers with kekules structure
If Kekules model is correct there should be 4 isomers however there are only 3 found to exist.
Suggests bonds in benzene are the same and not different
Describe the issues with bond length with kekules structure
What was used to discover this
C-C bond length = 0.153nm
C=C bond length = 0.134nm
In Benzene all bond lengths = 0.139nm
They are all the same (intermediate bonding) and this was determined using x-ray crystallography.
Describe the issues with enthalpy changes with kekules structure
Use numbers
The model predicts an enthalpy change of hydrogenation to the equivalent for 2x C=C: -360 KJ mol-1
Actual enthalpy change = -208 kj mol-1
Meaning the change is lower by 152kj mol-1
Describe the new benzene model
Draw it
After all sigma bonds have formed there is one electron in each p orbital.
One large pi bond forms from all 6 electrons, three above and three below the atom.
6 electrons form a delocalised pi bond.
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How does the new model agree with the bromine test and reactivity
There are no individual C=C bonds so less reactivity or addition reaction with bromine
How does the new model agree with isomers
3 not 4 isomers as there is now no difference in the arrangement of electrons between these atoms
How does the new model agree with bond length
The carbon-carbon bonds are now all the same length as they are not individual c-c or c=c
How does the new model agree with enthalpy
As charge is spread around the species there is an increased stability explaining why benzene has a greater stability.
2 electron clouds, delocalised electrons cause charge to spread around the species.
Why does a reaction with Br2 only let one Br bond to benzene
As it preserves the stability of the delocalised electrons in the pi bonds.
What does aromatic mean
And does it apply
Draw the new structure
A cyclical structure in chemistry
Yes
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List the general reactions of benzene
Hydrogenation
Combustion
Equation for the hydrogenation of benzene
What is the product called
Draw the new structure
C6H6 + 3H2 —> C6H12
Cyclohexane
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Conditions for the hydrogenation of benzene
Benzene and hydrogen react under reflux with a nickel catalyst
Equation for the combustion of benzene
C6H6 + 7 1/2 O2 —> 6CO2 + 3H2O
Conditions and observations - why
Burning in oxygen
Burns with a smoky flame due to high C to H ratio
List the substitution reactions of benzene
Bromination
Nitration
Sulfonation
Equation to form the electrophile for the bromination of benzene
AlCl3 + Br2 —> Br+ + AlCl3Br-
Equation for the reaction between bromine and benzene
C6H6 + Br2 —> C6H5Br + HBr
Equation to form the inorganic product within the bromination of benzene
Which product can be reused
AlCl3Br- + H+ —> AlCl3 + HBr
AlCl3
Conditions for the bromination of benzene
In the presence of UV light, heated under reflux in the presence of a halogen carrier catalyst (AlCl3)
Examples of halogen carrier catalysts
AlCl3, AlBr3, FeCl3, FeBr3
Draw the mechanism for the bromination of benzene
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Nitration equation for the formation of the nucleophile
HNO3 + 2H2SO4 —> NO2+ + 2HSO4- + H3O+
Nitration equation for the formation of the organic product
C6H6 + NO2+ —> C6H5NO2 + H2O
Nitration equation for the formation of the inorganic product
What is done with the product of this equation
HSO4- + H+ —> H2SO4
It is reused
Conditions for the nitration of benzene
Warming benzene with a mixture of concentrated nitric and sulfuric acids.
Sulfuric acid acts as a catalyst.
Draw the mechanism for the nitration reaction with benzene
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Write the equation for the sulfonation of benzene
C6H6 + SO3 —> C6H5SO3H
Conditions for the sulfonation of benzene
Why
Fuming sulfur acid.
It is a solution of sulfur trioxide (SO3) dissolved in sulfuric acid
Draw the mechanism for the sulfonation of benzene
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What type of reactions are alkylation and acylation of benzene
Friedel-crafts reactions
What are the features of friedel-crafts reactions
- One hydrogen atom in benzene is substituted by a reagent and the other is a HX product
- A catalyst is needed - often aluminium chloride, iron chloride or iron bromide.
- Anhydrous conditions are needed because water would react with the catalyst and sometimes with the organic product
What are the two friedel-crafts reactions with benzene
Alkylation
Acylation
What is the equation for the formation of the electrophile in alkylation
AlCl3 + CH3Cl —> CH3 + + AlCl4 -
What is the equation for the alkylation of benzene
C6H6 + CH3+ —> C6H5CH3 + H+
What is the reaction for the formation of the inorganic products in the alkylation of benzene
AlCl4- + H+ —> AlCl3 + HCl
What are the conditions for alkylation of benzene
A halogenoalkane
Anhydrous conditions
Catalyst of aluminium chloride
Equation for the formation of the electrophile for the acylation of benzene
AlCl3 + CH3COCl —> CH3CO + + AlCl4 -
Equation for the acylation of benzene
C6H6 + RCO+ —> C6H5RCO + HCl + AlCl3
Equation for the formation of the inorganic products in the acylation of benzene
AlCl4 - + H+ —> AlCl3 + HCl
Conditions for the acylation of benzene
anhydrous
Acyl chloride
AlCl3 catalyst
Mechanism for the alkylation of benzene
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Mechanism for the acylation of benzene
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What is reused at the end of acylation and alkylation of benzene
AlCl3
Formula for phenol
C6H5OH
Draw phenol
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Equation for the bromination of phenol
C6H5OH + 3Br2 —> C6H2OHBr3 + 3HBr
Conditions for the bromination of phenol
Room temperature
Without a catalyst
Giver the name of the organic product form the bromination of phenol
2,4,6 - tribromophenol
Why does the bromination of phenol occur more readily than the bromination of benzene
- The oxygen in the OH has lone pairs of electrons which can merge with the electrons in the delocalised pi bonds.
- The electron density above and below the ring of atoms is increased - ‘activation’ occurs because the molecule is now much more reactive towards the electrophile.
- Bromine molecules are polarised as they approach the benzene ring. The Br-Br bond breaks and the Br+ electrophile attacks the benzene rings.
Draw a mechanism for the bromination of phenol
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What are 2 ways in which phenol is used
Manufacture of polymers and pharmaceuticals
