Chapter 12 Halogen Compounds Flashcards
Halogenoarenes are arenes which are
- bonded to halogen atoms
- They can be prepared from substitution reactions of arenes with chlorine or bromine in the presence of an anhydrous catalyst
Substitution of benzene to form halogenoarenes
- 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
Halogenoarenes can be formed from the electrophilic substitution reaction of arenes with halogens
Substitution of methylbenzene to form halogenoarenes
- 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
The methyl group on methylbenzene directs the incoming halogen on the 2 and 4 position
- In the presence of excess chlorine, substitution on the 6 position will also occur
Difference in Reactivity of Halogenoalkanes & Halogenoarenes
- Halogenoarenes are very unreactive compared to halogenoalkanes
- The difference in reactivity between the two compounds is because of the carbon-halogen bond strengths
Halogenoarenes, such as chlorobenzene, do not readily undergo
-
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
The halogenoalkane chloroethane can take part in
- 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
Halogenoalkanes readily undergo nucleophilic substitution reactions
The carbon-chlorine bond is very strong, as it has partial double-bond character
The unreactivity of halogenoarenes can therefore be explained by the
- 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