Alcohols, Carbonyls & Carboxylic Acids

Concept Core

Alcohols, phenols, aldehydes, ketones, and carboxylic acids — their tests and reactions.

Alcohols — Classification & Tests

Classification: 1° (one alkyl group on C−OH), 2° (two), 3° (three).

Test	Reagent	1°	2°	3°
Lucas test	Conc. HCl + ZnCl₂	No turbidity (cold)	Turbidity after ~5 min	Immediate turbidity
Victor Meyer	P + I₂ → PI₃; then HNO₂; then FeSO₄/NaOH	Red	Blue	Colourless

Oxidation of alcohols: 1° → aldehyde (mild oxidant like PCC) → carboxylic acid (strong oxidant like KMnO₄). 2° → ketone. 3° → resistant to oxidation (no H on α-carbon).

Phenol — Special Properties

Phenol (C₆H₅OH) is much more acidic than aliphatic alcohols (pKa ≈ 10 vs ~16 for EtOH) because the phenoxide ion (C₆H₅O⁻) is stabilised by resonance with the ring.

FeCl₃ test: Phenol gives violet/purple colour with FeCl₃ (phenol → ferric phenoxide).

Kolbe-Schmidt reaction: Phenol + CO₂ (under pressure, NaOH) → salicylic acid (o-hydroxybenzoic acid)

Coupling with diazonium salt: Phenol → azo dye (orange-red colour)

Aldehydes & Ketones — Tests

Test	Reagent	Aldehydes	Ketones
Tollens' test	AgNO₃/NH₃ (silver mirror)	Silver mirror ✓	No reaction ✗
Fehling's test	Cu²⁺ alkaline	Red ppt ✓	No reaction ✗
2,4-DNP test	2,4-Dinitrophenylhydrazine	Yellow/orange ppt ✓	Yellow/orange ppt ✓
Iodoform test	I₂/NaOH	Only CH₃CHO ✓	CH₃COR (methyl ketone) ✓

Aldehydes & Ketones — Key Reactions

Nucleophilic addition: C=O + Nu⁻ → alkoxide → add H⁺ → product. Aldehydes more reactive than ketones (less steric, less electron donation).

Aldol condensation: In presence of dilute base, two carbonyl compounds (with α-H) condense.

Cannizzaro reaction: HCHO or ArCHO (no α-H) + conc. NaOH → alcohol + carboxylate (disproportionation).

Reduction: LiAlH₄ → alcohol; NaBH₄ → alcohol (milder); Clemmensen (Zn/Hg/HCl) → CH₂.

Carboxylic Acids — Reactions

Carboxylic acids (RCOOH) are stronger than alcohols/phenols (pKa ≈ 4–5 for aliphatic acids).

Reagent	Product
SOCl₂ (thionyl chloride)	Acid chloride (RCOCl) + SO₂ + HCl
PCl₅	Acid chloride (RCOCl) + POCl₃ + HCl
NH₃ → heat	Amide (RCONH₂)
LiAlH₄	Primary alcohol (RCH₂OH)
Decarboxylation (heat)	Alkane (R−H) + CO₂

Iodoform Test — Scope and Limitations

Iodoform test (I₂/NaOH) gives yellow precipitate (CHI₃) with:

CH₃CHO (acetaldehyde) — only aldehyde that gives positive test CH₃COR (methyl ketones, e.g., acetone CH₃COCH₃) CH₃CH(OH)R (secondary alcohols with CH₃ group, oxidised to methyl ketone in situ) CH₃CH₂OH (ethanol — oxidised to acetaldehyde then reacts)

HCHO (formaldehyde) does NOT give iodoform test despite being an aldehyde — it lacks the CH₃−C=O pattern.

Formula Vault

Tests, oxidation patterns, and key reagents for functional groups.

Lucas Test Order

3° > 2° > 1° (rate of turbidity)

Conc. HCl + ZnCl₂; tests alcohol class

Tollens' Reagent

Ag(NH₃)₂⁺ + RCHO → RCOOˉ + Ag↓

Silver mirror = aldehyde (not ketone)

Fehling's Test

Cu²⁺ (blue) → Cu₂O ↓ (red-brick)

Aldehydes only (not aromatic)

Iodoform Test

CH₃COR + I₂/NaOH → CHI₃ + RCOO⁻

Yellow ppt; tests CH₃CO− group

Alcohol Oxidation

1° → RCHO → RCOOH; 2° → R₂C=O

3° resists oxidation

Acid Chloride Formation

RCOOH + SOCl₂ → RCOCl + SO₂ + HCl

SOCl₂ is preferred reagent

Phenol Acidity

C₆H₅OH: pKa ≈ 10

Resonance stabilises phenoxide ion

FeCl₃ Test

Phenol + FeCl₃ → violet colour

Distinguishes phenol from alcohol

Worked Examples

5 problems — Lucas test, Tollens', iodoform, carboxylic acid, and a phenol trap.

EasyWhat is the result of Tollens' test on ethanal (CH₃CHO)?▾

Describe the result of Tollens' test when applied to ethanal (acetaldehyde).

Tollens' reagent: Ag(NH₃)₂⁺ (ammoniacal silver nitrate).

Ethanal is an aldehyde → gets oxidised to ethanoic acid → silver is reduced to Ag metal.

Silver deposits on the glass wall as a shiny mirror.

CH₃CHO + 2Ag(NH₃)₂⁺ + 2OH⁻ → CH₃COO⁻ + 2Ag↓ + 4NH₃ + H₂O

✓ Silver mirror forms (silver deposits on inner wall of test tube)

EasyWhich gives a positive iodoform test: CH₃CHO, HCHO, or CH₃CH₂OH?▾

Identify which compounds give a positive iodoform test: formaldehyde (HCHO), acetaldehyde (CH₃CHO), ethanol (CH₃CH₂OH).

Iodoform test requires CH₃−C=O group (or CH₃CHOH− which gets oxidised to it).

HCHO: H−CHO, no CH₃ group → negative.

CH₃CHO: CH₃−CHO has CH₃−CO− pattern → positive (CHI₃ yellow precipitate).

CH₃CH₂OH: oxidised to CH₃CHO by NaOI in situ → positive.

✓ CH₃CHO and CH₃CH₂OH give positive; HCHO gives negative

MediumConvert ethanol to acetic acid using oxidation — reagents needed▾

What reagents convert ethanol (CH₃CH₂OH) to acetic acid (CH₃COOH) stepwise?

Ethanol is a 1° alcohol.

Step 1: CH₃CH₂OH → CH₃CHO (mild oxidation): PCC (pyridinium chlorochromate) in CH₂Cl₂. OR: K₂Cr₂O₇/H₂SO₄ (dilute, controlled)

Step 2: CH₃CHO → CH₃COOH (further oxidation): KMnO₄/H⁺ or K₂Cr₂O₇/H₂SO₄ (strong conditions).

Direct: ethanol → acetic acid in one step using excess KMnO₄/H⁺.

✓ Stepwise: PCC (for aldehyde), then KMnO₄/H⁺ (for acid). Or KMnO₄/H⁺ directly.

EAPCET LevelWhy is phenol more acidic than ethanol despite both having −OH group?▾

Explain why phenol (pKa ≈ 10) is more acidic than ethanol (pKa ≈ 16).

Acid strength = stability of conjugate base after proton loss.

Ethanol loses H⁺ → ethoxide (C₂H₅O⁻): charge localised on one O atom. Not particularly stable.

Phenol loses H⁺ → phenoxide (C₆H₅O⁻): negative charge delocalised into the benzene ring by resonance.

Phenoxide has 4–5 resonance structures distributing the negative charge → much more stable conjugate base.

More stable conjugate base → more acidic compound. Phenol pKa ≈ 10 vs ethanol pKa ≈ 16 (lower pKa = more acidic).

✓ Phenol more acidic because phenoxide ion is resonance-stabilised by benzene ring; ethoxide is not

Trap QuestionFehling's test positive means the compound is definitely an aldehyde — True or False?▾

A sample gives a positive Fehling's test (red precipitate). A student concludes it must be an aldehyde. Evaluate.

Fehling's test detects aliphatic aldehydes (reducing aldehydes).

Aromatic aldehydes (like benzaldehyde, PhCHO) do NOT give positive Fehling's test — they are not strong enough reducing agents.

Also, formic acid (HCOOH) gives positive Fehling's test despite being a carboxylic acid (has an aldehyde-like H on C).

Tollens' test is more reliable — it detects both aliphatic AND aromatic aldehydes.

Conclusion: Positive Fehling's indicates aliphatic aldehyde. But negative Fehling's doesn't rule out aromatic aldehydes.

✓ Not completely true — aromatic aldehydes (PhCHO) don't give Fehling's; Tollens' test is more reliable for all aldehydes

Mistake DNA

5 functional group errors from EAPCET distractor analysis.

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Iodoform: All Aldehydes Give Positive Test

Only CH₃CHO (acetaldehyde) gives iodoform. Other aldehydes (HCHO, C₂H₅CHO, PhCHO) do not — they lack the CH₃−CO pattern.

❌ Wrong

HCHO gives iodoform (it's an aldehyde) ✗

✓ Correct

HCHO: H−CHO; no CH₃−CO ✓ Only CH₃CHO among aldehydes gives iodoform ✓

The iodoform test detects CH₃−CO− pattern specifically. HCHO has no methyl group. For the test to work, there must be at least one CH₃ attached to the carbonyl carbon.

🔵

Tollens' Test and Fehling's Test Give Same Results

Tollens' detects both aliphatic AND aromatic aldehydes. Fehling's detects only aliphatic (reducing) aldehydes — not aromatic ones.

❌ Wrong

PhCHO + Fehling's → red precipitate ✓ ✗ (benzaldehyde does not reduce Fehling's)

✓ Correct

PhCHO + Tollens' → Ag mirror ✓ PhCHO + Fehling's → no reaction ✓ Aromatic: Tollens' positive ✓

Benzaldehyde: Tollens' positive (detects all aldehydes). Fehling's negative (aromatic aldehydes are insufficient reducing agents). Use Tollens' for complete aldehyde detection.

🔴

Victor Meyer Test: 3° Alcohol Gives Red Colour

Victor Meyer test: 1° alcohol → red, 2° → blue, 3° → colourless. Students often remember the 1° and 2° correctly but get 3° wrong.

❌ Wrong

3° alcohol gives red or blue colour in Victor Meyer test ✗

✓ Correct

3° alcohol → colourless ✓ 1° → red (RCH₂NO₂) ✓ 2° → blue (R₂CHNO₂) ✓

Victor Meyer final step: add FeSO₄/NaOH. Primary nitro compound (RCH₂NO₂) → red. Secondary (R₂CHNO₂) → blue. Tertiary has no α-H → no reaction with HNO₂ → no colour (colourless).

⚡

3° Alcohol Can Be Oxidised Easily

Tertiary alcohols are resistant to oxidation because there is no α-H on the carbon bearing the OH group for the oxidant to abstract.

❌ Wrong

3° alcohol + KMnO₄ → oxidation product ✓ ✗ (resistant to most oxidants)

✓ Correct

3° alcohol: no α-H on C bearing OH ✓ Resistant to PCC, K₂Cr₂O₇ under normal conditions ✓

Oxidation of alcohols requires an α-H (hydrogen on the carbon bonded to OH). For tertiary alcohols, no such H exists. Under drastic conditions (very concentrated KMnO₄), C−C bonds can break.

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Phenol Gives Blue Colour with FeCl₃

Phenol gives violet/purple colour with FeCl₃ (not blue). Blue would suggest enol form of a 1,3-diketone.

❌ Wrong

Phenol + FeCl₃ → blue colour ✗

✓ Correct

Phenol + FeCl₃ → violet/purple ✓ (ferric phenoxide complex) 1,3-diketones (enol form) also give colours with FeCl₃

FeCl₃ test: phenol → characteristic violet/purple. This distinguishes phenol from aliphatic alcohols (which do not give colour). The colour arises from coordination of phenoxide oxygen to Fe³⁺.

Chapter Intelligence

This chapter cluster has the highest organic chemistry EAPCET weightage — master all functional group tests.

EAPCET Weightage (2019–2024)

Aldehyde/ketone identification tests

Alcohol class tests (Lucas, Victor Meyer)

Carboxylic acid reactions

Nucleophilic addition mechanism

Phenol special reactions

High-Yield PYQ Patterns

Tollens' vs Fehling's: which is positive?Iodoform test: which compounds qualify?Lucas test: order of reactivityOxidation product of 1°/2°/3° alcoholVictor Meyer: 1°/2°/3° coloursPhenol vs ethanol aciditySOCl₂ converts RCOOH to RCOCl

Exam Strategy

Iodoform test: requires CH₃−CO− pattern. Only CH₃CHO among aldehydes. Methyl ketones (CH₃COR). Ethanol and secondary methyl alcohols.
Tollens' = aldehydes (all, including aromatic). Fehling's = aliphatic aldehydes only (not aromatic).
Lucas test speed: 3° (immediate) > 2° (5 min) > 1° (requires heating). Turbidity indicates alkyl chloride formation.
Phenol tests: violet with FeCl₃, reacts with NaOH (acidic), forms azo dye, undergoes Kolbe-Schmidt, Reimer-Tiemann reactions.
Carboxylic acid to acid chloride: SOCl₂ is the cleanest (gaseous byproducts escape). PCl₅ also works but harder to separate POCl₃.