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Chemistry · Unit 16

Alcohols and Phenols: MDCAT Notes and MCQs

Alcohols (R–OH)

  • IUPAC suffix -ol: CH3OH methanol, C2H5OH ethanol, CH3CH(OH)CH3 propan-2-ol, (CH3)3COH 2-methylpropan-2-ol.
  • Classified as 1°, 2°, 3°; monohydric, dihydric (glycol), trihydric (glycerol).
  • Structure: O is sp3; C–O–H angle ≈ 105–109°. H-bonding → high b.p. and water solubility of lower alcohols.
  • Methanol is "wood spirit" (toxic — blindness); ethanol made by fermentation of molasses/starch (yeast enzyme zymase). Rectified spirit 95%; absolute alcohol 100%.

Reactivity

  • O–H cleavage (acidic, very weak): 2ROH + 2Na → 2RONa + H2; esterification.
  • C–O cleavage: with HX, PCl5, SOCl2 → RX; dehydration.
  • Reactivity towards HX: 3° > 2° > 1°. Lucas test (ZnCl2 + conc. HCl): 3° → turbidity immediately; 2° → in ≈ 5 min; 1° → no turbidity at room temperature.
  • Oxidation (K2Cr2O7/H2SO4): 1° → aldehyde → carboxylic acid; 2° → ketone; 3° resist oxidation. Iodoform test positive for ethanol and CH3CH(OH)– alcohols.

Ethers and esters from alcohols

  • Ethers: (a) dehydration of excess ethanol with conc. H2SO4 at 140 °C → diethyl ether; (b) Williamson synthesis: RONa + R'X → R–O–R' + NaX.
  • Esters (esterification): RCOOH + R'OH ⇌ RCOOR' + H2O (conc. H2SO4 catalyst, reversible). CH3COOH + C2H5OH → ethyl acetate (fruity smell).

Phenol (C6H5OH, carbolic acid)

  • –OH directly attached to the benzene ring. Lone pair of O is delocalized into the ring.
  • More acidic than alcohols (pKa ≈ 10 vs ≈ 16) because the phenoxide ion is resonance-stabilized; less acidic than carboxylic acids (does not liberate CO2 from NaHCO3). Reacts with NaOH (alcohols don't).
  • –OH strongly activates the ring and is o/p-directing:
    • Bromine water → 2,4,6-tribromophenol (white ppt) without catalyst.
    • Dilute HNO3 → o- and p-nitrophenol; conc. HNO3 → 2,4,6-trinitrophenol (picric acid).
    • Sulphonation: o-product at low temperature, p-product at high temperature.
  • FeCl3 test: phenol gives a violet colour.

Alcohol vs phenol

AlcoholPhenol
Neutral to litmusWeakly acidic
No reaction with NaOHForms sodium phenoxide with NaOH
No colour with FeCl3Violet colour with FeCl3
No ppt with bromine waterWhite ppt (tribromophenol)
Oxidized to aldehyde/ketoneNot oxidized that way
PMDC learning outcomes for this unit
  • Alcohols and phenols: 16.1 Explain nomenclature and structure of alcohols.
  • Alcohols and phenols: 16.2 Explain the reactivity of alcohols.
  • Alcohols and phenols: 16.3 Describe the chemistry of alcohols by preparation of ethers and esters.
  • Alcohols and phenols: 16.4 Explain the nomenclature, structure and reactivity of phenol
  • Alcohols and phenols: 16.5 Discuss the reactivity of phenol and their chemistry by electrophilic aromatic substitution.
  • Alcohols and phenols: 16.6 Differentiate between an alcohol and phenol.

Alcohols and Phenols MCQs with answers

Practise and track your score →
Q1
Immediate turbidity in the Lucas test is given by:
ASecondary alcohol
BTertiary alcohol
CPrimary alcohol
DPhenol
Show answer
Answer: B (Tertiary alcohol). Tertiary alcohols form stable carbocations and react instantly.
Q2
Lucas reagent is:
AK₂Cr₂O₇ + H₂SO₄
BNaOH + I₂
CConc. HCl + anhydrous ZnCl₂
DAmmoniacal AgNO₃
Show answer
Answer: C (Conc. HCl + anhydrous ZnCl₂). Lucas reagent = conc. HCl with anhydrous ZnCl2.
Q3
Oxidation of a secondary alcohol gives:
AAldehyde
BCarboxylic acid
CKetone
DEther
Show answer
Answer: C (Ketone). 2° alcohols are oxidized to ketones.
Q4
Williamson synthesis is used to prepare:
AAlkenes
BEsters
CAldehydes
DEthers
Show answer
Answer: D (Ethers). Sodium alkoxide + alkyl halide → ether.
Q5
Phenol is more acidic than ethanol because:
APhenol is a gas
BPhenol has more H-bonding
CPhenoxide ion is resonance stabilized
DEthanol has a benzene ring
Show answer
Answer: C (Phenoxide ion is resonance stabilized). Delocalization of the negative charge over the ring stabilizes phenoxide.
Q6
Phenol reacts with bromine water to give:
A2,4,6-Tribromophenol
Bo-Bromophenol only
CBromobenzene
DNo reaction
Show answer
Answer: A (2,4,6-Tribromophenol). The strongly activating –OH allows trisubstitution without catalyst (white precipitate).
Q7
Which reagent distinguishes phenol from ethanol?
AWater
BNeutral FeCl₃
CSodium metal
DConc. H₂SO₄
Show answer
Answer: B (Neutral FeCl₃). Phenol gives a violet colour with FeCl3; ethanol does not.
Q8
Reaction of acetic acid with ethanol in presence of conc. H₂SO₄ gives:
AEthyl acetate
BDiethyl ether
CAcetaldehyde
DEthene
Show answer
Answer: A (Ethyl acetate). Esterification gives ethyl ethanoate (ethyl acetate).
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