Chapter 12 Halogen Compounds Flashcards

1
Q

Halogenoarenes are arenes which are

A
  • bonded to halogen atoms
  • They can be prepared from substitution reactions of arenes with chlorine or bromine in the presence of an anhydrous catalyst
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2
Q

Substitution of benzene to form halogenoarenes

A
  • Chlorine gas is bubbled into benzene at room temperature and in the presence of an anhydrous AlCl3 catalyst to form chlorobenzene
  • The AlCl3 catalyst is also called a halogen carrier and is required to generate the electrophile (Cl+)
  • This electrophile attacks the electron-rich benzene ring in the first stage of the reaction which disrupts the delocalised π system in the ring
  • To restore the aromatic stabilization, a hydrogen atom is removed in the second stage of the electrophilic substitution reaction to form chlorobenzene
  • When this happens, the delocalised π system of the ring is restored
  • The same reaction occurs with benzene and bromine in the presence of an AlBr3 catalyst to form bromobenzene
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3
Q

Halogenoarenes can be formed from the electrophilic substitution reaction of arenes with halogens

A
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4
Q

Substitution of methylbenzene to form halogenoarenes

A
  • The electrophilic substitution of methylbenzene with halogens results in the formation of multiple halogenoarenes as products
  • This is because the methyl group (which is an alkyl group) in methylbenzene is electron-donating and pushes electron density into the benzene ring
  • This makes the benzene ring more reactive towards electrophilic substitution reactions
  • The methly group is said to be 2,3-directing and as a result, the 2 and 4 positions are activated
  • Electrophilic substitution of methylbenzene with chlorine and anhydrous AlCl3 catalyst, therefore, gives 2-chloromethylbenzene and 4-chloromethylbenzene
  • The reaction mechanism is the same as the substitution mechanism of benzene
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5
Q

The methyl group on methylbenzene directs the incoming halogen on the 2 and 4 position

A
  • In the presence of excess chlorine, substitution on the 6 position will also occur
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6
Q

Difference in Reactivity of Halogenoalkanes & Halogenoarenes

A
  • Halogenoarenes are very unreactive compared to halogenoalkanes
  • The difference in reactivity between the two compounds is because of the carbon-halogen bond strengths
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7
Q

Halogenoarenes, such as chlorobenzene, do not readily undergo

A
  • nucleophilic substitution reactions
    • Only under extremely harsh conditions, such as temperatures of 200 oC and a pressure of 200 atmospheres, will the chlorine in chlorobenzene get replaced by a nucleophile such as a hydroxide (OH) ion
  • This is because the carbon-chlorine bond is very strong and cannot be easily broken
    • One of the lone pairs of electrons on the chlorine will interact with the π system of the ring
    • This causes the carbon-chlorine bond to have a partial double-bond character, which strengthens the bond
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8
Q

The halogenoalkane chloroethane can take part in

A
  • nucleophilic substitution reactions
  • A nucleophile, such as a hydroxide (OH) ion, will attack the slightly positive carbon atom
  • A covalent bond is formed between that carbon atom and the nucleophile which causes the carbon-halogen bond to break
  • Overall, the halogen is replaced by the nucleophile
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9
Q

Halogenoalkanes readily undergo nucleophilic substitution reactions

A
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10
Q

The carbon-chlorine bond is very strong, as it has partial double-bond character

A
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11
Q

The unreactivity of halogenoarenes can therefore be explained by the

A
  • delocalisation of a lone pair on the halogen over the benzene
  • This causes additional stabilisation of the system and strengthens the carbon-halogen bond, which affects the reactions that halogenoarenes will undergo
  • It gets harder to break the carbon-halogen bond in halogenoarenes, which decreases reactivity
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