AS/A-level Chemistry - Phenol

Before you start it would be helpful to revisit chapters on functional groups, electrophilic substitution of aromatic rings in AS/A-level Chemistry! 👨‍🏫

Phenol 

Phenol is an aromatic alcohol with formula C₆H₅OH the OH group is attached directly to the benzene ring, it is almost colourless crystalline solid. 

Preparation:

You cannot put the Oh group directly onto a benzene ring by electrophilic substitution, so phenol has to be synthesised in a multi-stage process.

Step 1- Nitration of benzene

• Reagents: conc. nitric acid and conc. sulphuric acid (catalyst) 
• Conditions: reflux at 55°C
• Equation: C₆H₆ + HNO₃ ➔ C₆H₅NO₂ + H₂O
• Mechanism: Electrophilic substitution 

• Reagents: tin and conc. hydrochloric acid
• Conditions: reflux
• Equation: C₆H₅NO₂ + 6[H] ➔ C₆H₅NH₂ + 2H₂O
• Mechanism: Reduction

• Reagent: nitrous acid and hydrochloric acid (use sodium nitrile) 
• Conditions: keep below 10°C
• Equation: C₆H₅NH₂ + HNO₂ +HCl ➔ C₆H₅N₂⁺ + Cl^- + 2H₂O
• Reaction type: diazotisation

• Reagents: water
• Conditions: warm above 10°C
• Equation: C₆H₅N₂⁺ + Cl^- + H₂O ➔ C₆H₅OH + N₂ +HCl
• Reaction type: hydrolysis/ substitution 

Reactions of the OH group

Water

• Phenol is a weak acid
• it dissolves very slightly in water to form a weak acidic solution 
• it is a stronger acid than aliphatic alcohols
• the ring helps weaken the O-H bond and stabilises the resulting anion  

NaOH

• Phenol reacts with sodium hydroxide to form a salt- sodium phenoxide
• it is ionic and water soluble 

Sodium 

• Phenol reacts with sodium to form an ionic salt- sodium ohenoxide hydrogen is also produced 
• This reaction is similar to that with aliphatic alcohols such as ethanol 

Reactions of the Aromatic Ring- Electrophilic Substitution

Ease 

• the OH group is electron releasing 
• it increases the electron density of the delocalised system
• is makes substitution much easier compared to benzene
• a p orbital on the oxygen overlaps with the p orbitals in benzene 

Bromine

• the electron density is greatest at the 2, 4 and 6 positions substitution takes place at those positions
• Phenol reacts readily with bromine water WITHOUT A CATALYST 
• it is so easy that multiple substitution takes place 
• NO⁺ react in a similar way 

Reference:

https://getrevising.co.uk/resources/phenol

This is the end of the topic!!👏👏

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