Phenols

Phenol

• Phenol is an aromatic alcohol; a white crystalline solid that is both corrosive and anaesthetic
• OH group is ring activating so phenol more reactive than benzene and substituents attached at positions 2,4 and 6

Solubility and Acidity of Phenol

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• OH group forms H-bonds with water molecules however there are weak VDW forces due to heavy nonpolar benzene ring hence phenol partially soluble in water
• Solution of phenol is slightly acidic because –OH in phenol breaks to form H⁺
• Bond breaks readily because phenoxide ion formed is stabilized by partial delocalization of –ve charge over the benzene ring

• The conjugate base (phenoxide) is stable and less likely to react with H⁺ going back to phenol

Acid-Base Reactions

Reaction with Base

• C₆H₅OH_(l) + NaOH_(aq) > C₆H₅O^–Na⁺_(aq)
  • H₂O
• Type of reaction:
  • Neutralization
• Reagent:
  • sodium hydroxide
• Condition:
  • r.t.p.
• Regenerating phenol:
  • When strong acid (HCl) added to solution of sodium phenoxide, phenol regenerated

C₆H₅O^–Na⁺_(aq) + HCl_(aq) → C₆H₅OH_(l) + NaCl

• Initially dense white emulsion formed then slowly oily liquid layer of phenol separates

Reaction with Sodium

• C₆H₅OH_(l) + Na_(s) >
  C₆H₅O^–Na⁺_(aq) + ½H_2(g)
• Type of reaction:
  • metal and acid, redox
• Condition:
  • r.t.p.
• Reacts with Na liberating H₂ gas
• Note: phenol does not react with metal carbonate to liberate CO₂ hence shows its fairly weaker than other carboxylic acids

Reaction with Benzene Ring

Bromination

• Type of reaction:
  • electrophilic substitution
• Reagent:
  • aq. Bromine
• Condition:
  • r.t.p. (no catalyst)
• Rapid reaction, forms a white ppt.
• No catalyst needed since Br₂ molecule easily polarized by increase e^– density in ring

Nitration

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• Type of reaction:
  • electrophilic substitution
• Reagent:
  • dilute nitric acid
• Condition:
  • r.t.p.
• If concentrated nitric acid used, then trinitrophenol produced

Relative Acidities

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• The stronger the acid:
  • Higher the K_a value
  • More easily H⁺ is donated
  • More stable is the conjugate base

Ethanol: C₂H₅OH + H₂O⇌ C₂H₅O^– + H₃O⁺

• K_a: 10^-18 mol dm^-3
• Ability to donate H⁺ ions:
  • C₂H₅ is an e^– donating gp. which increases charge density on O of OH

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• More attraction between O–H so loss of H⁺ difficult
• Stability of conjugate base:
  • In ethoxide ion, C₂H₅ gp. increases –ve charge
  • Makes ethoxide ion more basic than OH^–
  • Ability to accept H⁺ increases, moving equi. to left

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Water: H₂O + H₂O ⇌ OH^– + H₃O⁺

• K_a: 10^-16 mol dm^-3
• No e^– donating or withdrawing gp. present hence ability to donate and accept H⁺ ion is the same

Phenol: C₆H₅OH + H₂O ⇌ C₆H₅O^– + H₃O⁺

• K_a: 10^-10 mol dm^-3
• Ability to donate H⁺ ions:
  • OH is a ring activating gp. & the lone pair of e^–s on O becomes part of delocalized e^– system
  • Decreases e^– density on O of OH and attraction between O and H decreases so H⁺ lost more easily
• Stability of conjugate base:
  • In phenoxide ion, -ve charge on O delocalized around ring and reduces tendency to attract H⁺
  • Conjugate base stable

Test for Phenol

Test 1:

• Reagent: Iron(III) Chloride (FeCl_3(aq)) – yellowish brown
• Observation: violet colour obtained

Test 2:

• Reagent: bromine water (Br_2(aq)) – orange
• Observation: white ppt. obtained