Reactions of Arenes
- Arenes are very stable compounds due to the delocalisation of π electrons in the ring
- This is because the negative charge is spread out over the molecule instead of being confined to a small area
- During chemical reactions such as substitution reactions, this delocalised ring is maintained
- Addition reactions however, disrupt the aromatic stabilisation
- Arenes undergo a series of reactions including:
- Substitution
- Nitration
- Friedel-Crafts alkylation
- Friedel-Crafts acylation
- Complete Oxidation
- Hydrogenation
Substitution
- Halogenation reactions are examples of electrophilic substitution reactions
- Arenes undergo substitution reactions with chlorine (Cl2) and bromine (Br2) in the presence of anhydrous AlCl3 or AlBr3 catalyst respectively to form halogenoarenes (aryl halides)
- The chlorine or bromine act as an electrophile and replaces a hydrogen atom on the benzene ring
- The catalyst is required for the reaction to take place, due to the stability of the benzene structure
Arenes undergo substitution reactions with halogens to form aryl halides
- Alkylarenes such as methylbenzene undergo halogenation on the 2 or 4 positions
- This is due to the electron-donating alkyl groups which activate these positions
- Phenol (C6H5OH) and phenylamine (C6H5NH2) are also activated in the 2 and 4 positions
- The halogenation of alkylarenes therefore result in the formation of two products
Alkylarenes are substituted on the 2 or 4 position
- Multiple substitutions occur when excess halogen is used
In the presence of excess halogen, multiple substitutions occur
Nitration
- Another example of a substitution reaction is the nitration of arenes
- In these reactions, a nitro (-NO2) group replaces a hydrogen atom on the arene
- The benzene is reacted with a mixture of concentrated nitric acid (HNO3) and concentrated sulfuric acid (H2SO4) at a temperature between 25 and 60 oC
Nitration of benzene
- Again, due to the electron-donating alkyl groups in alkylarenes, nitration of methylbenzene will occur on the 2 and 4 position
Nitration of alkylarenes
Friedel-Crafts reactions
- Friedel-Crafts reactions are also electrophilic substitution reactions
- Due to the aromatic stabilisation in arenes, they are often unreactive
- To use arenes as starting materials for the synthesis of other organic compounds, their structure, therefore, needs to be changed to turn them into more reactive compounds
- Friedel-Crafts reactions can be used to substitute a hydrogen atom in the benzene ring for an alkyl group (Friedel-Crafts alkylation) or an acyl group (Friedel-Crafts acylation)
- Like any other electrophilic substitution reaction, the Friedel-Crafts reactions consist of three steps:
- Generating the electrophile
- Electrophilic attack on the benzene ring
- Regenerating aromaticity of the benzene ring
Examples of Friedel-Crafts alkylation and acylation reactions
Friedel-Crafts alkylation
- In this type of Friedel-Crafts reaction, an alkyl chain is substituted into the benzene ring
- The benzene ring is reacted with a chloroalkane in the presence of an AlCl3 catalyst
- An example of an alkylation reaction is the reaction of benzene with chloropropane (CH3CH2CH2Cl) to form propylbenzene
Example of a Friedel-Crafts alkylation reaction
Friedel-Crafts acylation
- In the Friedel-Crafts acylation reaction, an acyl group is substituted into the benzene ring
- An acyl group is an alkyl group containing a carbonyl, C=O group
- The benzene ring is reacted with an acyl chloride in the presence of an AlCl3 catalyst
- An example of an acylation reaction is the reaction of methylbenzene with propanoyl chloride to form an acyl benzene
- Note that the acyl group is on the 4 position due to the -CH3 group on the benzene
Example of a Friedel-Crafts acylation reaction
Complete oxidation
- Normally, alkanes are not oxidised by oxidising agents such as potassium manganate(VII) (KMnO4)
- However, the presence of the benzene ring in alkyl arenes affect the properties of the alkyl side-chain
- The alkyl side-chains in alkyl arenes are oxidised to carboxylic acids when refluxed with alkaline potassium manganate(VII) and then acidified with dilute sulfuric acid (H2SO4)
- For example, the complete oxidation of ethylbenzene forms benzoic acid
The complete oxidation of alkyl side-chains in arenes gives a carboxylic acid
Hydrogenation
- The hydrogenation of benzene is an addition reaction
- Benzene is heated with hydrogen gas and a nickel or platinum catalyst to form cyclohexane
Hydrogenation of benzene
- The same reaction occurs when ethylbenzene undergoes hydrogenation to form ethylcyclohexane
Hydrogenation of methylbenzene
Summary of Reactions of Arenes Table