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) > 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) + 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 + H2O ⇌ C6H5O– + H3O+
- Ka: 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 (FeCl3(aq)) – yellowish brown
- Observation: violet colour obtained
Test 2:
- Reagent: bromine water (Br2(aq)) – orange
- Observation: white ppt. obtained
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