Aromatic Chemistry (AQA A Level Chemistry): Flashcards
Exam code: 7405
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- FrontStructure of Benzene
What is the delocalised model of benzene?
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What is the delocalised model of benzene?
Delocalised model of benzene describes benzene as a planar hexagonal ring of six carbon atoms, each forming three σ bonds, with the remaining p electrons delocalised into a π system above and below the plane of the ring.
This model explains benzene's equal bond lengths and resistance to addition reactions.
True or False?
All C–C bond lengths in benzene are equal.
True.
The delocalisation of π electrons means all six C–C bonds are identical (0.140 nm), intermediate between a single bond (0.154 nm) and a double bond (0.134 nm). This contradicts the alternating bonds in the Kekulé structure.
Benzene preferentially undergoes .......... reactions rather than .......... reactions because the delocalised π system is highly .......... and addition would destroy it.
Benzene preferentially undergoes substitution reactions rather than addition reactions because the delocalised π system is highly stable (aromatic) and addition would destroy it.
What is electrophilic substitution in benzene?
An optical isomer is one of two or more forms of a compound that differ only in the spatial arrangement of groups around a chiral centre. The two forms are non-superimposable mirror images called enantiomers.
True or False?
The Kekulé structure of benzene correctly predicts that benzene undergoes electrophilic addition reactions.
False.
The Kekulé structure predicts benzene should react like an alkene (electrophilic addition). In fact, benzene undergoes electrophilic substitution, preserving the stable π system. This is evidence against the Kekulé model.
In electrophilic substitution of benzene: step 1, the delocalised π electrons attack the .......... to form a non-aromatic positive .......... ; step 2, a .......... leaves to restore aromaticity.
In electrophilic substitution of benzene: step 1, the delocalised π electrons attack the electrophile to form a non-aromatic positive intermediate; step 2, a proton (H+) leaves to restore aromaticity.
What bond angles are found in the benzene ring, and why?
120°, because benzene is a regular planar molecule with all six carbon atoms in equivalent positions. Each carbon forms three bond pairs arranged symmetrically, giving bond angles of 120°.
What is the electrophile in the nitration of benzene and how is it generated?
The electrophile is the nitronium ion (NO2+).
It is generated when concentrated H2SO4 protonates concentrated HNO3:
HNO3 + H2SO4 → NO2+ + HSO4- + H2O
The nitration of benzene uses a mixture of concentrated .......... and concentrated .......... heated under reflux at .......... °C. The product is .......... and the by-product is .......... .
The nitration of benzene uses a mixture of concentrated HNO3 and concentrated H2SO4 heated under reflux at 50 °C. The product is nitrobenzene and the by-product is water (H2O).
True or False?
Nitration of benzene is an example of electrophilic addition.
False.
Nitration is electrophilic substitution. A hydrogen atom is replaced by the –NO2 group, preserving the aromatic π system. Addition would permanently destroy aromaticity.
Why do arenes prefer electrophilic substitution over electrophilic addition?
Substitution restores aromaticity in the product, preserving the stable delocalised π system. Addition would permanently destroy aromaticity, making it energetically less favourable.
Nitration of benzene proceeds in three steps: (1) generation of the electrophile .......... ; (2) electrophilic .......... on the benzene ring forming a cationic intermediate; (3) loss of a .......... to regenerate aromaticity.
Nitration of benzene proceeds in three steps: (1) generation of the electrophile NO2+; (2) electrophilic attack on the benzene ring forming a cationic intermediate; (3) loss of a proton (H+) to regenerate aromaticity.
True or False?
Hydrogenation of benzene is an example of an electrophilic substitution reaction.
False.
Hydrogenation of benzene is an addition reaction that converts benzene to cyclohexane and destroys the delocalised π system. It is not electrophilic substitution.
Nitronium ion
The nitronium ion (NO2+) is the electrophile in the nitration of benzene. It is generated when concentrated H2SO4 protonates concentrated HNO3, causing it to lose a water molecule: HNO3 + H2SO4 → NO2+ + HSO4- + H2O.
What is a Friedel–Crafts acylation reaction?
A σ (sigma) bond is a covalent bond formed by the direct (head-on) overlap of orbitals along the internuclear axis. All single bonds are σ bonds; they allow free rotation about the bond axis.
In Friedel–Crafts acylation, AlCl3 reacts with the acyl chloride to generate the .......... ion. This ion attacks the .......... π system of benzene to form a non-aromatic .......... .
In Friedel–Crafts acylation, AlCl3 reacts with the acyl chloride to generate the acylium ion. This ion attacks the delocalised π system of benzene to form a non-aromatic positive intermediate.
True or False?
AlCl3 is consumed and not regenerated in Friedel–Crafts acylation.
False.
AlCl3 is regenerated in step 3 when [AlCl4]- removes the H+ from the intermediate to restore aromaticity, releasing AlCl3 and HCl.
What is the overall equation for the Friedel–Crafts acylation of benzene with ethanoyl chloride?
The overall equation for the Friedel–Crafts acylation of benzene with ethanoyl chloride is:
C6H6 + CH3COCl → C6H5COCH3 + HCl
The product is phenylethanone (methyl phenyl ketone).
An acyl group consists of an .......... group with a .......... group (C=O) attached. In Friedel–Crafts acylation, arenes are usually .......... so direct reactions are difficult without generating an electrophile.
An acyl group consists of an alkyl group with a carbonyl group (C=O) attached. In Friedel–Crafts acylation, arenes are usually unreactive so direct reactions are difficult without generating an electrophile.
True or False?
Friedel–Crafts acylation is an electrophilic substitution reaction that preserves the aromaticity of the benzene ring in the product.
True.
After electrophilic attack, aromaticity is restored in step 3 by loss of a proton. The final product retains the stable delocalised π system.
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