Benzene & Aromatic Compounds (OCR A Level Chemistry A): Flashcards
Exam code: H432
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Define delocalised π-system
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Define delocalised π-system
A delocalised π-system is a continuous ring of electron density formed when the p-orbitals of all six carbon atoms in benzene overlap sideways above and below the plane of the ring. The six π-electrons are spread over the entire ring rather than fixed between specific atoms.
True or False?
The Kekulé model of benzene predicts that all C–C bond lengths are identical at 0.140 nm.
False.
The Kekulé model predicts alternating short C=C bonds (0.134 nm) and long C–C bonds (0.154 nm). The identical intermediate bond length of 0.140 nm supports the delocalised model.
Why is the experimental enthalpy of hydrogenation of benzene less exothermic than the value predicted by the Kekulé model?
Benzene is more stable than the Kekulé structure predicts because the delocalised π-system lowers its energy. The experimental value is -208 kJ mol-1, compared to the predicted -360 kJ mol-1.
X-ray diffraction shows that all C–C bond angles in benzene are .......... and the molecule is a perfectly .......... regular hexagon.
X-ray diffraction shows that all C–C bond angles in benzene are 120° and the molecule is a perfectly planar regular hexagon.
Why does benzene not react with bromine water under normal conditions, unlike cyclohexene?
The delocalised electron density in benzene is spread over the entire ring, so it is not concentrated enough to polarise the Br2 molecule. Without polarisation, no electrophilic addition can occur.
What type of reaction does benzene prefer over addition reactions, and why?
Benzene preferentially undergoes electrophilic substitution reactions because these restore the stable delocalised π-system, whereas addition reactions would disrupt it.
True or False?
The accepted IUPAC name for C6H5OH is phenol.
True.
The preferred IUPAC name for C6H5OH is phenol.
It can also be known as hydroxybenzene.
Define electrophilic substitution
An electrophilic substitution is a reaction in which a hydrogen atom on a benzene ring is replaced by an electrophile, preserving the aromaticity of the ring.
Why must electrophiles for benzene reactions usually be generated in situ rather than added directly?
The delocalised π-system of benzene is very stable and not reactive enough to be attacked by weak electrophiles. Stronger electrophilic species must be generated from appropriate reagents before they can attack the ring.
True or False?
The nitrating electrophile in the nitration of benzene is the NO2+ ion.
True.
The nitronium ion (NO2+) is generated by reacting concentrated HNO3 with concentrated H2SO4 and acts as the electrophile in the nitration of benzene.
Why does benzene undergo electrophilic substitution rather than electrophilic addition?
Substitution restores the delocalised π-system and the associated aromatic stabilisation, whereas addition would permanently disrupt it and produce a less stable product.
In the nitration of benzene, the reaction mixture of HNO3 and H2SO4 is refluxed with benzene at .......... to .......... °C.
In the nitration of benzene, the reaction mixture of HNO3 and H2SO4 is refluxed with benzene at 25 to 60 °C.
What is the role of AlBr3 in the halogenation of benzene?
AlBr3 acts as a halogen carrier catalyst, reacting with Br2 to generate the Br+ electrophile needed to attack the benzene ring. It is regenerated at the end of the reaction.
True or False?
In the halogenation of benzene, one hydrogen atom and one halogen atom are both added to the ring.
False.
Halogenation is an electrophilic substitution: one hydrogen atom is replaced by one halogen atom, producing HX as a byproduct. No atoms are added to the ring.
What reagents are required for Friedel-Crafts acylation of benzene, and what functional group is introduced?
An acyl chloride and an AlCl3 catalyst are required. An acyl group (containing a C=O) is substituted into the benzene ring.
Define phenoxide ion
A phenoxide ion is the ion C6H5O-, formed when phenol loses a hydrogen ion from its -OH group. It is stabilised by delocalisation of a lone pair from oxygen into the benzene ring.
Why is phenol a weak acid rather than a strong acid?
Although the phenoxide ion is stabilised by delocalisation, oxygen is strongly electronegative and still holds much of the charge, making it likely to re-attract the hydrogen ion. This means the equilibrium lies to the left, so phenol only partially ionises.
True or False?
Phenol reacts with sodium hydroxide to form a soluble salt and water.
True.
Phenol undergoes an acid-base reaction with NaOH to form sodium phenoxide (a soluble salt) and water, consistent with its behaviour as a weak acid.
Why does phenol react more readily with electrophiles than benzene does?
The lone pair on the oxygen atom of the -OH group overlaps with the π bonding system, increasing the electron density of the ring and making it more susceptible to electrophilic attack.
When phenol reacts with bromine water at room temperature, the orange solution is .......... and a .......... precipitate of 2,4,6-tribromophenol forms.
When phenol reacts with bromine water at room temperature, the orange solution is decolourised and a white precipitate of 2,4,6-tribromophenol forms.
What product forms when phenol is nitrated with dilute HNO3 at room temperature?
A mixture of 2-nitrophenol and 4-nitrophenol is formed. The -OH group directs the incoming nitro group to the 2 and 4 positions on the ring.
True or False?
Phenol requires a halogen carrier catalyst to react with bromine.
False.
Unlike benzene, phenol reacts with bromine water at room temperature without a catalyst. The activating -OH group increases ring electron density sufficiently for direct reaction.
Define electron-donating group
An electron-donating group is a substituent that donates electron density into the π system of the benzene ring, making it more reactive towards electrophiles and directing attack to the 2 and 4 positions.
Why does a methyl group on a benzene ring direct incoming electrophiles to the 2 and 4 positions?
The methyl group is an electron-donating group that increases the electron density at the 2 and 4 positions, making those sites more attractive to incoming electrophiles.
Electron-withdrawing substituents such as -NO2 .......... attack by electrophiles and direct them to the .......... position.
Electron-withdrawing substituents such as -NO2 deactivate attack by electrophiles and direct them to the 3 position.
What products form when methylbenzene is brominated, and why?
The products are 2-bromomethylbenzene and 4-bromomethylbenzene, because the electron-donating methyl group directs the bromine electrophile to the 2 and 4 positions.
True or False?
An electron-withdrawing group makes the benzene ring more reactive towards electrophilic substitution.
False.
Electron-withdrawing groups remove electron density from the π system, making the ring less reactive towards electrophilic attack and directing electrophiles to the 3 position.
How can the directing effect of substituents be used in organic synthesis?
Knowing whether a substituent is electron-donating or electron-withdrawing allows chemists to predict and control the position of substitution, enabling the targeted synthesis of specific isomers.
True or False?
The -OH and -NH2 groups are both electron-donating groups that activate the benzene ring.
True.
Both -OH and -NH2 are electron-donating groups that donate electron density into the π system, activating the ring and directing electrophiles to the 2 and 4 positions.
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