Electrophilic Aromatic Substitution

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  • Created by: LBCW0502
  • Created on: 03-01-18 15:45
Describe the general reaction equation between a nucleophile and an electrophile
Nucleophile attacks electrophile to produce a product and a new nucleophile/leaving group
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How many resonance forms does benzene have?
2
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What is the C-C bond lengths in benzene?
139 pm (between 154 pm for C-C and 134 pm for C=C)
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How is the delocalisation of electrons represented in a benzene ring?
Draw a circle within the benzene ring
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Describe Kekule's structure for benzene
Alternating single and double C-C bonds - no resonance
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Why is it difficult to react anything with benzene?
Benzene is stable with low internal energy. Providing high energy to the system leads to less stability
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Is benzene aromatic?
Yes (4n + 2 pi electrons). Also conjugation with more than one resonance form
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Describe the 2p orbitals in the structure of benzene
All 2p orbitals overlap equally (on each C atom)
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Why can benzene not undergo addition reactions?
Leads to electrons being lost in benzene ring (contains a positive charge) which results in less stability, no resonance, lose symmetry and less delocalisation. Benzene wants to maintain stability
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Describe the determination of resonance energy
Enthalpy change of hydrogenation is predicted to be -360 kJ but the actual value if -208 kJ (benzene has high internal energy)
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Name the positions on a benzene ring from 1-4
Ipso, ortho, meta and para (labelled from variable group attached to benzene ring)
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What is toluene?
Methylbenzene
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What is aniline?
Phenylamine
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What is anisole?
Methoxybenzene
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What is phenol?
Benzene group with OH attached
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What is benzoic acid?
Benzene group with COOH attached
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Electron density in anisole is increased in which positions?
Ortho and para positions (see 5 resonance structures). Electron donating
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What are the two kinds of resonance in operation for anisole?
Kekule structures 'native' resonance of benzene. Interaction of the substituent with the benzene ring (these interactions cause changes in electron densities in the ring)
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Electron density in nitrobenzene is decreased in which positions?
Ortho and para positions (see 5 resonance structures). Electron withdrawing
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What are the three kinds of resonance in operation for nitrobenzene?
Kekule structures 'native' resonance of benzene. Interaction of substituent with benzene ring . 'Native' resonance of nitro group (interactions change electron densities in the ring)
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Describe features of 1-methoxy-4-nitrobenzene
Has Kekule resonance, interaction of substituents and nitro group resonance (push/pull interaction of substituents)
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Do double bonds in the benzene ring react like other double bonds?
No
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What does benzene require to undergo a substitution reaction?
A strong electrophile and a catalyst
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What is a catalyst?
A substance which speeds up the rate of reaction by providing an alternative route with a lower activation energy
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Give examples of substitution reactions of benzene
Halogenation, friedel-crafts alkylation, friedel-crafts acylation, nitration, sulfonation
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Describe the energy profile for aromatic substitution
Two energy barriers (two transition states). Slow step involves formation of intermediate. Fast step involves nucleophile attacking intermediate
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What are activators?
Substituents that make the ring react faster (than benzene) with electrophiles. They stabilise the carbocation intermediate and speed up the reaction. Direct electrophiles to ortho/para
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What are deactivators?
Substituents that make the ring react slower (than benzene). They raise the energy for the carbocation and slow the reaction. Most direct to meta (for halogens - ortho/para)
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Give examples of strongly activating substituents (5)
Amine, primary amine, secondary amine, hydroxy, alkoxy groups (ortho/para)
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Give examples of moderately activating substituents (5)
NHCOR, COOR, R, Ar, CH=CHR (ortho/para)
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Give examples of weakly activating substituents (3)
R, Ar, CH=CHR (ortho/para)
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Give examples of weakly deactivating substituents (4)
F, Cl, Br, I (ortho/para)
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Give examples of moderately deactivating substituents (3)
COR, COOR, COOH (meta)
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Give examples of strongly deactivating substituents (4)
CN, NOO, SO3H, N+R3
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Describe the effect of halide substituents
They are deactivating/electron withdrawing inductive effect. They are ortho/para directors. They have an electron donating mesomeric effect
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With bulky directing groups, which substitution is preferred
P-substitution
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Other cards in this set

Card 2

Front

How many resonance forms does benzene have?

Back

2

Card 3

Front

What is the C-C bond lengths in benzene?

Back

Preview of the front of card 3

Card 4

Front

How is the delocalisation of electrons represented in a benzene ring?

Back

Preview of the front of card 4

Card 5

Front

Describe Kekule's structure for benzene

Back

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