Chap 15D - Arenes (Methylbenzene)

Describe addition to hydrogen (reduction) 

• Do not use this reaction unless specified by the question

• The resonance-stabilised benzene ring is to be preserved at all times

Describe free radical substitution (reagents and conditions, excess methylbenzene VS X2)

• Methylbenzene has two reactive sites

  • Benzene ring, which undergoes electrophilic substitution

  • Methyl side-chain, which can undergo free-radical substitution and oxidation to benzoic acid

• Reagents and conditions: X2 (Cl2 or Br2), UV light at room temperature

• In excess methylbenzene, mono-substituted product is obtained

• In excess X2, di- or tri- substituted products are obtained

• All the products formed can be hydrolysed using aqueous NaOH and heat to give alcohols, carbonyl compounds or carboxylic acids

Describe possible produces when methylbenzene reacts with limited CI2 VS excess CI2 → then NaOH + heat is added

Draw free radical substitution mechanism (between methylbenzene + limited CI2)

Describe methylbenzene reacting with weak oxidation agents + reagents

Describe methylbenzene reacting with strong oxidation agents + reagents + observations

• Acidic conditions: benzoic acid produced 

• Alkaline conditions: benzoate ions obtained 

• This is a chemical test to distinguish between benzene and methylbenzene

• Potassium dichromate(VI), K2Cr2O7, is unable to oxidise the side chain of benzene

Describe these reactions

• Position of the carboxylic acid group gives information on the location of the original alkyl side-chain

• Eg. Benzene-1,4-dicarboxylic acid = alkyl groups were originally at the 1st and 4th carbons of the benzene ring

Describe these reactions + products

• Strong oxidising agents like hot acidified KMnO4(aq) is capable of oxidising the longer side chains with a benzylic hydrogen (H atom bonded to C atom adjacent to the benzene ring) to form benzoic acid (white solid) with effervescence of CO2

• The rest of the side chain is destroyed, forming carbon dioxide and water

Describe reaction between methylbenzene + hot alkaline KMnO4

• Hot alkaline KMnO4 gives the salt of carboxylic acid which must be acidified before solid benzoic acid can be obtained

Describe this reaction