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

  • 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) > C6H5ONa+(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

C6H5ONa+(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) >
    C6H5ONa+(aq) + ½H2(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

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

Relative Acidities

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

Ethanol: C2H5OH + H2O C2H5O + H3O+

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

  • 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

Water: H2O + H2O OH + H3O+

  • Ka: 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 + H2 C6H5O + H3O+

  • Ka: 10-10 mol dm-3
  • Ability to donate H+ ions:
    • OH is a ring activating gp. & the lone pair of es 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 (FeCl3(aq)) – yellowish brown
  • Observation: violet colour obtained

Test 2:

  • Reagent: bromine water (Br2(aq)) – orange
  • Observation: white ppt. obtained

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