Halogen Compounds (Cambridge (CIE) A Level Chemistry): Flashcards
Exam code: 9701
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Define halogenoarene.
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Define halogenoarene.
A halogenoarene is an aromatic compound in which one or more halogen atoms are bonded directly to the benzene ring. They are also called aryl halides.
What reagents and conditions are needed to produce chlorobenzene from benzene?
Chlorine gas is bubbled into benzene at room temperature in the presence of anhydrous AlCl3 catalyst. The AlCl3 generates the Cl+ electrophile, which attacks the benzene ring in an electrophilic substitution reaction.
True or False?
The chlorination of benzene requires UV light to generate the electrophile.
False.
The chlorination of benzene uses an anhydrous AlCl3 catalyst (halogen carrier) to generate the Cl+ electrophile. UV light is used for free-radical side-chain halogenation, not ring halogenation.
In the chlorination of benzene, the AlCl3 catalyst generates the .......... electrophile. After electrophilic attack, a .......... atom is eliminated to restore aromaticity.
In the chlorination of benzene, the AlCl3 catalyst generates the Cl+ electrophile. After electrophilic attack, a hydrogen atom is eliminated to restore aromaticity.
Why does chlorination of methylbenzene give two products rather than one?
The methyl group is a 2,4-directing electron-donating group that activates the 2 and 4 positions on the ring. Electrophilic substitution therefore occurs at both positions, giving 2-chloromethylbenzene and 4-chloromethylbenzene.
True or False?
In the presence of excess chlorine, chlorination of methylbenzene can also occur at the 6 position.
True.
With excess chlorine, substitution can additionally occur at the 6 position, since the methyl group activates the 2, 4 and 6 positions by electron donation.
Define bromobenzene.
A bromobenzene is a halogenoarene produced by the electrophilic substitution of benzene with bromine (Br2) in the presence of anhydrous AlBr3 catalyst.
The role of the AlCl3 catalyst in the chlorination of benzene is to generate the .......... by reacting with Cl2, and it is described as a .......... .
The role of the AlCl3 catalyst in the chlorination of benzene is to generate the electrophile (Cl+) by reacting with Cl2, and it is described as a halogen carrier.
What is restored at the end of the electrophilic substitution of benzene with chlorine, and how does this happen?
Aromaticity is restored. A hydrogen atom is removed in the second stage of the reaction (heterolytic cleavage of the C–H bond), returning the bonding electrons to the benzene π system and regenerating the delocalised ring.
Define partial double bond character.
A partial double bond character is a property of a bond that has intermediate character between a single and a double bond, arising from electron delocalisation. In halogenoarenes, the C–X bond has partial double bond character due to lone pair overlap with the benzene π system.
Why are halogenoarenes much less reactive than halogenoalkanes?
In halogenoarenes, a lone pair on the halogen overlaps with the benzene π system, giving the C–X bond partial double bond character. This strengthens the bond and makes it very difficult to break, so nucleophilic substitution does not readily occur.
True or False?
Chlorobenzene undergoes nucleophilic substitution readily at room temperature.
False.
Chlorobenzene does not readily undergo nucleophilic substitution. Extremely harsh conditions (200°C and 200 atm pressure) are needed to replace the chlorine with a nucleophile such as OH-.
In halogenoarenes, delocalisation of a lone pair on the halogen over the benzene ring causes additional .......... of the system and .......... the carbon–halogen bond.
In halogenoarenes, delocalisation of a lone pair on the halogen over the benzene ring causes additional stabilisation of the system and strengthens the carbon–halogen bond.
How does chloroethane (a halogenoalkane) undergo nucleophilic substitution, and why is this different for chlorobenzene?
In chloroethane, the carbon bonded to chlorine is slightly positive, so a nucleophile such as OH- attacks and forms a covalent bond, displacing Cl-. In chlorobenzene, lone pair delocalisation into the ring strengthens the C–Cl bond so much that a nucleophile cannot break it under normal conditions.
True or False?
The lone pair on the halogen in a halogenoarene interacts with the σ framework of the benzene ring.
False.
The lone pair on the halogen interacts with the π system of the benzene ring, not the σ framework. This overlap gives the C–X bond partial double bond character.
Define chlorobenzene.
A chlorobenzene is a halogenoarene formed by the electrophilic substitution of benzene with chlorine using an AlCl3 catalyst. It is very unreactive towards nucleophilic substitution due to the partial double bond character of its C–Cl bond.
To replace the chlorine in chlorobenzene by nucleophilic substitution, conditions of .......... °C and .......... atmospheres are required.
To replace the chlorine in chlorobenzene by nucleophilic substitution, conditions of 200 °C and 200 atmospheres are required.
Explain why the unreactivity of halogenoarenes can be attributed to delocalisation.
A lone pair on the halogen delocalises into the benzene π system, providing additional stabilisation of the whole molecule and giving the C–X bond partial double bond character. This strengthens the bond, making it harder to break. The result is that halogenoarenes are much less reactive than halogenoalkanes.
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