Chemistry 4.2 - Aromaticity

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Define aliphatic.
When carbon atoms form open chains, e.g. alkanes, alkenes etc.
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Define aromatic.
When carbon atoms form a ring, e.g. arenes, such as BENZENE.
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Describe Kekule's benzene.
Kekule suggested benzene consisted of six carbon atoms in a ring with alternative single and double bonds. There are three issues with this suggested structure.
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Issue 1: addition reactions.
You would expect Kekule's benzene to undergo addition reactions: a double bond would break and its electrons used to bond to additional atoms. However, BENZENE WILL NOT UNDERGO ADDITION REACTIONS.
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Issue 2: the length of bonds.
C-C and C=C bonds are different lengths, which would give benzene an irregular structure. In real benzene all bonds are the same length (0.139nm), and intermediate between the lengths of a C-C and C=C bond.
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Issue 3: stability.
Real benzene is more stable than Kekule's benzene (by 150KjMol-1) as shown by thermochemistry calculations with cyclohexane.
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Thermochemistry calculations with cyclohexane.
Cyclohexane has one double bond. Upon hydrogenation, this breaks and releases -120kjMol-1. If we multiply this by 3 for Kekule's benzene = -360KjMol-1. The actual value for benzene is -208KjMol-1, making it 150kjMol-1 more stable.
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This increase in stability is due to...
Benzene's delocalisation/resonance energy.
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Define resonance energy.
The difference in enthalpy of hydrogenation values for Kekule's benzene and actual benzene. It indicates the extra stability of the delocalised Pi electron system in actual benzene.
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Bond angle in actual benzene?
120 degrees.
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What happens to the remaining p electron of each carbon atom in benzene?
They overlap with other Pi orbitals to form a delocalised ring of electrons above and below the structure.
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Consequence of the Pi cloud?
Increased electron density - attracts electron deficient electrophiles more strongly.
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What is an electrophile?
Electron acceptor.
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In the electrophilic substitution of benzene an unstable intermediate is formed. What is it called?
A Wheland intermediate.
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Conditions for the nitration of benzene.
Concentrated sulfuric acid (catalyst) and nitric acid, 50'C, reflux.
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In the mechanism for the nitration of benzene, HSO4- actually acts as a base. Why?
It removes a proton from the Wheland intermediate.
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If the temperature exceeds 50'C, what can also be produced?
1,3-dinitrobenzene.
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Conditions for the bromination of benzene.
Iron or aluminium bromide (catalyst), bromine, room temperature.
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What is the purpose of the iron/aluminium bromide.
It acts as a catalyst AND helps to polarise the bromine.
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Conditions for the chlorination of bromine.
Aluminium/iron chloride, chlorine, anhydrous.
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If excess chlorine is used, what else can be produced?
1,2 dichlorobenzene and 1,4 dichlorobenzene.
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Define alkylation.
The addition of an alkyl grtoup (CH3, CH2CH3 etc...) onto a molecule.
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Importance of Friedal Crafts alkylation?
Introduces a new C-C bond.
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Conditions for the F.C. alkylation of benzene?
Chloroalkane (e.g. CH3Cl), anhydrous aluminium chloride (catalyst), reflux.
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Problem with F.C. alkylation?
The introduction of the alkyl group onto the ring activates further activation (e.g 1,2- and 1,4- dimethylbenzene).
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How can the chance of this occurring be reduced?
Add the haloalkane slowly.
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Is a C-Cl bond stronger in chlorobenzene (aromatic) or a chloroalkane (aliphatic)?
The C-Cl bond in benzene is STRONGER as the p electron on the chlorine overlaps with the the Pi electron system, making it more stable.
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Other cards in this set

Card 2

Front

Define aromatic.

Back

When carbon atoms form a ring, e.g. arenes, such as BENZENE.

Card 3

Front

Describe Kekule's benzene.

Back

Preview of the front of card 3

Card 4

Front

Issue 1: addition reactions.

Back

Preview of the front of card 4

Card 5

Front

Issue 2: the length of bonds.

Back

Preview of the front of card 5
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