Why is phenol more acidic than ethanol?

Phenoxide is stabilised by resonance — the negative charge delocalises into the ortho and para positions of the ring. Ethoxide has no such stabilisation, so its conjugate acid (ethanol) is a weaker acid.

How does PCC differ from K2Cr2O7 oxidation of primary alcohols?

PCC (pyridinium chlorochromate) in dichloromethane stops at the aldehyde because no water is present to form the gem-diol. K2Cr2O7/H2SO4 is aqueous and over-oxidises through the gem-diol to the carboxylic acid.

What is the Reimer-Tiemann reaction?

Phenol + CHCl3 + NaOH gives salicylaldehyde (2-hydroxybenzaldehyde). Mechanism: hydroxide deprotonates CHCl3 to CCl3-, which loses Cl- to give dichlorocarbene :CCl2; the electrophilic carbene attacks the ortho position.

Why does conc H2SO4 at 170 C dehydrate ethanol to ethene but at 140 C give diethyl ether?

At higher T, intramolecular E1 elimination dominates, giving the alkene. At lower T, an SN2-like attack of a second ethanol on protonated ethanol produces the ether.

What is the violet colour with FeCl3 used for?

It is the standard qualitative test for phenols. The iron forms a coloured complex with the phenoxide; alcohols and most other compounds give no colour.

Which alcohol gives a positive iodoform test?

2-propanol. The iodoform test is positive for CH3-CH(OH)-R (methyl carbinol) and CH3-CO-R; 2-propanol fits.

Lucas test (HCl/ZnCl2) gives instant turbidity with:

3° alcohols. 3° alcohols form stable carbocations and react instantly via SN1 to give insoluble RCl.

Phenol reacts with bromine water to give:

2,4,6-tribromophenol. The strongly activating -OH makes phenol so reactive that all three ortho/para positions get brominated; a white precipitate forms.

Oxidation of 2-butanol with K2Cr2O7/H2SO4 gives:

2-butanone. Secondary alcohol -> ketone; further oxidation requires breaking C-C bonds.

Which is the most acidic?

p-nitrophenol. p-NO2 is strongly electron-withdrawing (-M and -I), stabilising the phenoxide; p-nitrophenol pKa ~7.2 vs phenol 10.

Alcohols and Phenols MDCAT 2026 — Notes, Key Concepts & MCQs (Chemistry)

Alcohols and Phenols

Alcohols and Phenols averages 2 MCQs per MDCAT paper, dominated by oxidation outcomes, Lucas test, esterification, and electrophilic substitution on phenol.

Alcohols and Phenols is a Chemistry chapter on the official PMDC MDCAT 2026 syllabus, contributing roughly 2 MCQs to the 45-MCQ Chemistry section. Mastering the core concepts below typically secures the full chapter weightage.

Classification and acidity

Alcohols R-OH are 1° (RCH₂OH), 2° (R₂CHOH), or 3° (R₃COH) depending on the carbon bearing -OH. Phenol C₆H₅OH has -OH attached to an sp² aromatic carbon. Acidity: phenol (pK_a 10) > water (15.7) > alcohols (16-18) because the phenoxide is resonance-stabilised by delocalisation of the negative charge into the ring. Electron-withdrawing substituents (NO₂) strengthen acidity; donors (-CH₃, -OCH₃) weaken it. Picric acid (2,4,6-trinitrophenol) has pK_a ≈ 0.4. The FSc Punjab Textbook Chemistry XII Chapter 12 catalogues these.

Preparation routes

Alcohols come from: hydration of alkenes (acid-catalysed, Markovnikov; or hydroboration-oxidation, anti-Markovnikov); reduction of carbonyls (NaBH₄ or LiAlH₄ reduces aldehydes to 1° alcohols, ketones to 2°); Grignard addition to carbonyls (1°, 2°, 3° depending on substrate); SN2 hydrolysis of alkyl halides. Phenol is industrially made by the cumene process: benzene + propene → cumene; cumene + O₂ → cumene hydroperoxide; H₃O⁺ rearrangement → phenol + acetone (a cherished by-product). Alternative: fusion of sodium benzene sulphonate with NaOH; or hydrolysis of chlorobenzene at 350 °C/300 atm (Dow process).

Oxidation outcomes

1° alcohols → aldehydes (PCC, pyridinium chlorochromate, in CH₂Cl₂) or all the way to carboxylic acids (KMnO₄ or K₂Cr₂O₇/H₂SO₄). 2° alcohols → ketones (Jones' reagent CrO₃/H₂SO₄/acetone). 3° alcohols resist oxidation under normal conditions because there is no α-H on the carbon bearing -OH. The Lucas test (conc HCl + ZnCl₂) distinguishes: 3° alcohols turn cloudy immediately; 2° within 5-10 min; 1° not at room temperature. Iodoform test (I₂/NaOH) is positive for ethanol and any 2° methyl carbinol R-CH(OH)-CH₃, giving yellow CHI₃.

Esterification, dehydration, and ether formation

Esterification: R-COOH + R'OH ⇌ R-COOR' + H₂O catalysed by H₂SO₄ (Fischer esterification). Equilibrium is pushed by removing water or using excess alcohol. Dehydration: hot conc H₂SO₄ (170 °C) eliminates water to give an alkene (E1, Saytzeff product). Lower T (140 °C) gives an ether by intermolecular dehydration: 2 ethanol → diethyl ether + water. Williamson ether synthesis (RO⁻ + R'X → ROR') is more general and avoids rearrangement.

Reactions specific to phenol

Phenol's -OH is a strong activator and ortho/para director. Bromination with bromine water gives 2,4,6-tribromophenol instantly (a white precipitate — diagnostic test). Nitration with dilute HNO₃ gives ortho and para mononitrophenols; conc HNO₃/H₂SO₄ gives picric acid. Reimer-Tiemann reaction: phenol + CHCl₃/NaOH → salicylaldehyde (a :CCl₂ carbene mechanism). Kolbe-Schmitt: sodium phenoxide + CO₂ at 125 °C, 5 atm → sodium salicylate → salicylic acid (precursor to aspirin). Phenol turns violet with neutral FeCl₃ — the standard identification test for phenols. Morrison & Boyd Chapters 17-18 cover this in encyclopedic detail.

Key Concepts

Worked MCQs

Frequently Asked Questions

How Alcohols and Phenols Is Tested

MDCAT questions on Alcohols and Phenols are a mix of recall (definitions, classifications), application (predict outcomes, interpret diagrams), and basic numerical/analytical reasoning. PMDC papers from 2020–2025 emphasized the concepts above; older UHS papers (2008–2019) tested them too, with slight variations in question framing.