Chapter 12 Halogen Compounds

Question 1

Halogenoarenes are arenes which are

Answer 1

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

Question 2

Substitution of benzene to form halogenoarenes

Answer 2

• 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

Question 3

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

Answer 3

Question 4

Substitution of methylbenzene to form halogenoarenes

Answer 4

• 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

Question 5

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

Answer 5

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

Question 6

Difference in Reactivity of Halogenoalkanes & Halogenoarenes

Answer 6

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

Question 7

Halogenoarenes, such as chlorobenzene, do not readily undergo

Answer 7

• 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

Question 8

The halogenoalkane chloroethane can take part in

Answer 8

• 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

Question 9

Halogenoalkanes readily undergo nucleophilic substitution reactions

Answer 9

Question 10

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

Answer 10

Question 11

The unreactivity of halogenoarenes can therefore be explained by the

Answer 11

• 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