4.2 Aromaticity
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- Created on: 30-09-19 19:16
What is benzene?
A simple aromatic hydrocarbon
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What can long term exposure to benzene cause?
Cancer
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What is the kekulé structure?
Over 100 years ago kekulé proposed a structure for benzene. He suggested that the carbons were arranged in a hexagon with alternating double and single bonds and each carbon had 1 hydrogen attached to it.
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What was the problems of the chemistry of kekulé's structure?
Due to the three double bonds you'd expect benzene to react as an alkene and undergo addition reaction. Benzene rarely does this and instead, it usually undergoes substitution reactions.
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What did kekulé propose to explain the discrepancy of the chemistry behind the structure of benzene?
He suggested that there were two forms and one form changed to the other so quickly that an approaching molecule would have no time to react with it by addition.
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What was the problem with the shape of the Kekulé structure?
c-c and c=c are different lengths. This would mean if Kekulé was right, the hexagon would be irregular with alternating sorter and longer sides. In real benzene, all the bonds are exactly the same and benzene is perfect regular hexagon.
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What is the problem with the stability of the kekulé structure?
Real benzene is more stable than the kekulé structure suggested. If you calculate the thermochemistry of the kekulé structure it is -360kl/mol based on the three double bonds and when fully hydrogenated it is -208 kj/mol. However, the actual thermoch
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What is the difference in energy values called?
Resonance energy.
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What is the delocalised bonding of benzene?
Each carbon atom is bonded to two other carbon atoms and a hydrogen atom by sigma bonds. The fourth outer shell electron of a carbon is in a 2p orbital, above and below the plane of carbon atoms. The p orbitals overlap and give a delocalise electron
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Why does benzene resist addition reaction?
If benzene underwent addition reactions it would disrupt the stability of the delocalised electron system.
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What conditions are required to cause benzene to undergo an addition reaction?
Forcing conditions are required to cause benzene to undergo addition reactions. This means higher temperatures are needed and the presence of a nickel or platinum catalyst.
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What is an electrophile?
An electron-deficient species that can accept a lone pair of electron.
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What happens in the electrophilic substitution reaction of benzene?
Benzene has a delocalised ring of electrons above and below the plane of the carbon atoms. This area of high electron density makes it susceptible for attack by an electrophile. A hydrogen atom(H+) is replaced by the incoming electrophile. To replace
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What happens in the nitration of benzene?
A hydrogen atom is replaced by a nitro group. Th electrophile is the nitric cation and this is produced by the reaction of concentrated nitric acid and concentrated sulphuric acid(VI). The nitryl cation reacts with benzene giving nitrobenzene.
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What is nitrobenzene?
Nitrobenzene is a yellow liquid that is reduced to phenyl amine by using a metal and hydrochloric acid.
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What happens if the temperature of the nitration exceed 50 degrees.
1,3 - dinitrobenzene is also produced.
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What catalyst is for the reaction between benzene and bromine?
Iron fillings, iron (III) bromide or aluminium bromide.
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How does the presence of iron(III) bromide impact the polarisation of bromine?
The electron pie system is not sufficiently nucleophilic to polarise the bromine molecule. So the presence of iron(III) bromide causes the polarisation of bromine to be more pronounced.
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What are the catalysts for the chlorination of benzene?
anhydrous aluminium chloride or iron(III) chloride.
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What is the role of iron(III) bromide?
iron(III) bromide is catalytic, encouraging greater polarisation of the Br-Br bond so that attack by the the ring electrons can occur but being regenerated at the end of the reaction.
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What happens if excess chlorine is used in the chlorination of benzene?
A mixture of 1,2- dichlorobenzene and 1,4 dichlorobenezene is produced?
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Why is chlorination a continuous process in industry?
So any polychlorination is kept to a minimum.
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How is chlorobenzene used in industry?
Chlorobenzene is an important industrial chemical and is nitrated and the nitro-derivatives converted to 2-nitorphenol and 2-nitrophenylamine. The insecticide DDT can be manufactured by the reaction of chlorobenzene with trichloroethanal.
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Why is the use of DDT heavily restricted?
DDT has problems linked to its toxicity, its persistence in the environment and its presence in the food chain.
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What is Friedel-Crafts alkylation?
This method provides a way of a way of producing a new c-c bond, giving an alkyl derivatives of benzene such as , methylbenzene. The reaction is similar to the halogenation of benzene but uses a halogenalkane instead of a halogen.
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What is the catalyst for Friedel-Crafts alkylation?
Anhydrous aluminium chloride
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What is one problem with Friedel-Crafts alkylation?
The introduction of the alkyl group on the ring activates the ring towards further alkylation. To reduce the chance of polyalkylation, the halogenalkane is added slowly to the benzene and the catalyst.
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What is the second problem with Friedel-Crafts alkylation?
A primary carbocation produced in the reaction may rearrange to a second carbocation.
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What is an important industrial use of Friedel-Crafts alkylation?
Manufacture of phenylethene.
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Why does benzene tend to not react with nucleophiles?
Nucleophiles would be repelled by the stable pie system of electrons.
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What is a aliphatic compound?
is a hydrocarbon compound containing carbon and hydrogen joined together in straight chains, branched trains or non-aromatic rings.
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What is a aromatic compound?
property of cyclic, planar structures with a ring of resonance bonds that gives increased stability compared to other geometric or connective arrangements with the same set of atoms.
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What is the bond length and energy of a C-Cl bond in an aliphatic compound?
Length = 0.177 Energy= 346
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What is the bond length and energy of a C-Cl bond in an aromatic compound?
Length= 0.169 Energy= 399
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What is the bond strength of chlorobenzene?
The stronger and shorter bond between carbon and chlorination in chlorobenzene results from a non-bonding p electron pair on chlorine overlapping with the ring pie system of electrons. This bond needs much more energy to be broken and is stronger tha
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Other cards in this set
Card 2
Front
What can long term exposure to benzene cause?
Back
Cancer
Card 3
Front
What is the kekulé structure?
Back
Card 4
Front
What was the problems of the chemistry of kekulé's structure?
Back
Card 5
Front
What did kekulé propose to explain the discrepancy of the chemistry behind the structure of benzene?
Back
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