Organic Compounds Containing Oxygen

Phenol (C₆H₅OH): OH directly attached to a benzene ring.

Why phenols are more acidic than alcohols (pKa ~10 vs ~16 for alcohols):

After losing H⁺, phenoxide ion (C₆H₅O⁻) is stabilized by resonance — the negative charge delocalizes into the aromatic ring (3 ortho/para resonance structures). Alkoxide (RO⁻) has no such delocalization → much less stable → poorer acid.

Effect of substituents on phenol acidity:

Acidity order: 2,4,6-trinitrophenol (picric acid, pKa 0.4) > p-nitrophenol > phenol > p-methylphenol.

Distinguishing tests for phenols:

1. Litmus test. Phenol is weakly acidic → turns blue litmus red. Alcohols don't.

2. Neutral FeCl₃ test. Phenol + FeCl₃ → violet/blue/purple colour. Most alcohols don't react. Diagnostic for phenolic -OH.

3. Bromine water. Phenol + Br₂(aq) → white ppt of 2,4,6-tribromophenol. Phenol is so activated that polysubstitution occurs even without Lewis acid catalyst.

4. Sodium hydroxide test. Phenol dissolves in NaOH (forms sodium phenoxide); alcohols don't (NaOH is too weak a base for ROH).

5. Sodium bicarbonate test. Phenol does NOT react with NaHCO₃ (weak base, not strong enough). But carboxylic acids do. This distinguishes phenols (no reaction) from carboxylic acids (CO₂ fizz).

Key reactions of phenol:

1. Electrophilic aromatic substitution. OH is strongly activating and ortho/para-directing.

• Nitration: phenol + dilute HNO₃ → o-nitrophenol + p-nitrophenol.
• Bromination (in water): tribromophenol (as above).

2. Reimer-Tiemann reaction. Phenol + CHCl₃ + NaOH → salicylaldehyde (ortho hydroxybenzaldehyde).

3. Kolbe-Schmitt reaction. Sodium phenoxide + CO₂ (high P, T) → sodium salicylate → salicylic acid (precursor to aspirin).

4. Phenol + Zinc dust → benzene + ZnO (industrial dehydroxylation).

5. Phenol + acid anhydride → phenyl acetate (Fischer esterification, but more difficult than alcohols).

Industrial importance:

• Phenol-formaldehyde resin (Bakelite — first synthetic plastic, 1907).
• Disinfectants (carbolic acid).
• Salicylic acid → aspirin, methyl salicylate (wintergreen).
• Picric acid — formerly used as an explosive.

Alcohol = R-OH where R is alkyl. The -OH on a carbon classifies:

• Primary (1°): -OH on a carbon attached to 1 other C. e.g., CH₃CH₂OH (ethanol).
• Secondary (2°): -OH on a carbon attached to 2 other Cs. e.g., (CH₃)₂CHOH (isopropanol).
• Tertiary (3°): -OH on a carbon attached to 3 other Cs. e.g., (CH₃)₃COH (t-butanol).
• Methanol (CH₃OH) is sometimes treated as 0°.

(Note: this 1°/2°/3° classification differs from amines, where the count is on N, not on C.)

PREPARATION METHODS

1. From alkenes (hydration):
CH₂=CH₂ + H₂O (H⁺ catalyst) → CH₃CH₂OH.
Markovnikov: H to C with more H; OH to more substituted C.

2. From alkenes (hydroboration-oxidation):
Alkene + BH₃, then H₂O₂/OH⁻ → anti-Markovnikov alcohol.

3. From alkyl halides:
R-X + aq KOH → R-OH (SN reactions; works for 1° better).

4. Reduction of aldehydes/ketones:

• Aldehyde + H₂/Ni (or LiAlH₄, NaBH₄) → 1° alcohol.
• Ketone + same reagents → 2° alcohol.

5. Reduction of esters:
R-COOR' + LiAlH₄ → R-CH₂OH + R'-OH (2 alcohols formed!).

6. Reduction of carboxylic acids:
R-COOH + LiAlH₄ → R-CH₂OH. (NaBH₄ doesn't reduce carboxylic acids; LiAlH₄ does.)

7. Grignard reaction:

• HCHO + RMgBr → R-CH₂OH (1°)
• R'CHO + RMgBr → R'-CHR-OH (2°)
• R'₂CO + RMgBr → R'₂C(R)-OH (3°)

This is THE most flexible method to make alcohols of any class.

PHYSICAL PROPERTIES

• Boiling points higher than alkanes of similar mass (H-bonding).
• Solubility in water: low molecular weight alcohols (CH₃OH, C₂H₅OH) fully miscible. Solubility decreases as carbon chain grows (hydrophobic tail dominates).

KEY REACTIONS

1. With sodium (acidic H):
2 R-OH + 2 Na → 2 R-ONa + H₂↑. (Like with water but slower.)

2. Esterification:
R-OH + R'-COOH ⇌ R'-COOR + H₂O. (Need conc H₂SO₄.)

3. Conversion to alkyl halides:

• R-OH + HX (HCl with ZnCl₂ catalyst = Lucas reagent) → R-X + H₂O.
• R-OH + SOCl₂ → R-Cl + SO₂ + HCl. (Cleanest method.)
• R-OH + PCl₅ → R-Cl + POCl₃ + HCl.

4. Oxidation:

• 1° alcohol → aldehyde → carboxylic acid.
• 2° alcohol → ketone.
• 3° alcohol → no reaction (no α-H attached to carbinol).

Reagents:

• Mild (gives aldehyde from 1°): PCC (pyridinium chlorochromate), PDC, Swern oxidation.
• Strong (gives acid from 1°): KMnO₄/H⁺, K₂Cr₂O₇/H⁺.

5. Dehydration to alkene:
R-CH(OH)-CH₃ → CH₂=CH-CH₃ + H₂O (with conc H₂SO₄, 170°C).

Ease of dehydration: 3° > 2° > 1° (carbocation stability).

6. Williamson synthesis (to ethers):
R-OH + R'-X (after R-OH → R-ONa) → R-O-R' + NaX.

DISTINGUISHING 1°, 2°, 3° ALCOHOLS

1. Lucas test (HCl + ZnCl₂):

• 3° alcohol: turbidity within seconds (instant).
• 2° alcohol: turbidity in 5-10 minutes.
• 1° alcohol: no turbidity at room temperature.

Rate matches carbocation formation (SN1) — most stable for 3°.

2. Victor Meyer test (multi-step: convert to alkyl iodide → nitroalkane → react with HNO₂ → NaOH):

• 1° alcohol: blood-red color.
• 2° alcohol: blue color.
• 3° alcohol: no color.

3. Oxidation:

• 1°: oxidized to acid (changes Cr₂O₇²⁻ orange to green).
• 2°: oxidized to ketone.
• 3°: no easy oxidation.

IMPORTANT ALCOHOLS:

• Methanol (CH₃OH): "wood alcohol" — toxic; causes blindness if ingested. Industrial solvent, fuel.
• Ethanol (C₂H₅OH): beverage alcohol; antiseptic; fuel additive (E10, E20 — 10%/20% blend in petrol).
• Glycol (HOCH₂CH₂OH): ethylene glycol — antifreeze, brake fluid, PET precursor.
• Glycerol (CH₂OH-CHOH-CH₂OH): sweet, viscous; humectant in cosmetics; explosives precursor (nitroglycerin).

Aldehydes (R-CHO) have a hydrogen on the carbonyl carbon; ketones (R-CO-R') don't. This makes aldehydes more easily oxidized — the basis of distinguishing tests.

1. Tollens' test (silver mirror).
Aldehyde + ammoniacal AgNO₃ (Tollens' reagent) → silver mirror on test tube.

R-CHO + 2[Ag(NH₃)₂]OH → R-COOH + 2Ag↓ + 4NH₃ + H₂O.

Ketones: no reaction.

2. Fehling's test (red ppt).
Aldehyde + Fehling's solution (Cu²⁺ tartrate complex, blue) → red Cu₂O precipitate.

R-CHO + 2Cu²⁺ + 5OH⁻ → R-COO⁻ + Cu₂O↓ + 3H₂O.

Ketones: no reaction. Aromatic aldehydes (e.g., benzaldehyde): also no reaction — Fehling's only works for aliphatic aldehydes.

3. Benedict's test. Similar to Fehling's but more stable (used for glucose detection in urine).

4. Schiff's test.
Aldehyde + Schiff's reagent (decolorized magenta with SO₂) → magenta color restored.

Ketones: only certain ketones (e.g., methyl ketones) restore colour, slowly.

5. Iodoform test (CHI₃, yellow ppt).
Methyl ketones (R-CO-CH₃) + I₂ + NaOH → CHI₃ (yellow ppt) + R-COONa.

Acetaldehyde and ethanol also give positive iodoform (since both contain CH₃CHO/CH₃CHOH structure). Not a method to distinguish acetaldehyde from acetone — both give yellow CHI₃.

Summary table:

JEE/NEET trap: "Distinguish acetone and acetaldehyde." Use Tollens' or Fehling's — both are positive only for acetaldehyde (the aldehyde). Don't use iodoform — both give CHI₃ since acetone is also a methyl ketone.

Carboxylic acid: R-COOH. The -COOH (carboxyl) group is the functional group.

Examples: formic acid (HCOOH), acetic acid (CH₃COOH, vinegar), benzoic acid (C₆H₅COOH), stearic acid (palm soaps), oleic acid (olive oil).

ACIDITY

Carboxylic acids are stronger acids than alcohols, phenols (pKa ~3-5 vs ~10 for phenols, ~16 for alcohols).

Why? After losing H⁺, the carboxylate anion (RCOO⁻) is stabilized by resonance — negative charge delocalized over BOTH oxygens (symmetric).

Phenoxide has only partial delocalization into the ring; alkoxide has none.

Substituent effects (electron-withdrawing makes acid stronger):

• Cl-CH₂-COOH (pKa 2.86) > CH₃-COOH (4.76) — Cl pulls electron density.
• F > Cl > Br > I in inductive effect.
• More halogens → more acidic: Cl₃C-COOH (pKa 0.7) > Cl₂CH-COOH (1.25) > ClCH₂-COOH (2.86) > CH₃-COOH (4.76).
• Distance matters: ClCH₂CH₂COOH > ClCH₂CH₂CH₂COOH (closer Cl is more effective).

Substituents on benzoic acid ring:

• Electron-withdrawing groups (-NO₂, -Cl, -CHO) increase acidity.
• Electron-donating (-OCH₃, -CH₃) decrease acidity.

PREPARATION

1. Oxidation of primary alcohols:
R-CH₂OH → R-CHO → R-COOH (using KMnO₄/H₂SO₄ or K₂Cr₂O₇).

2. Oxidation of alkenes (KMnO₄):
Cleaves the double bond; R-CH=CH-R' → R-COOH + R'-COOH.

3. Hydrolysis of nitriles:
R-CN + H₂O (H⁺ or OH⁻) → R-COOH + NH₃.

4. Carboxylation of Grignard:
R-MgX + CO₂ → R-COOMgX → R-COOH (after acid workup).

5. From aldehydes:
Oxidation by Tollens', Fehling's, or KMnO₄.

REACTIONS

1. As acid:

• With NaOH: R-COOH + NaOH → R-COONa + H₂O.
• With NaHCO₃: R-COOH + NaHCO₃ → R-COONa + H₂O + CO₂↑. (Distinguishes from alcohols and phenols — they DON'T fizz with bicarbonate.)
• With metal: 2 R-COOH + Zn → (R-COO)₂Zn + H₂.

2. Esterification (Fischer):
R-COOH + R'-OH ⇌ R-COO-R' + H₂O (catalyst: conc H₂SO₄ or HCl, heat).
Reversible — use excess alcohol or remove water to drive forward (Le Chatelier).

3. Conversion to acid halides:
R-COOH + SOCl₂ → R-COCl + SO₂↑ + HCl↑. (SOCl₂ is the preferred reagent — byproducts are gases that escape.)
Also: R-COOH + PCl₅ or PCl₃.

4. Conversion to anhydrides:
2 R-COOH → R-CO-O-CO-R + H₂O (heated with P₂O₅).

5. Conversion to amides:
R-COOH + NH₃ → R-COONH₄ (heat) → R-CONH₂ + H₂O.

6. Reduction:
R-COOH + LiAlH₄ → R-CH₂OH (strong reducer; NaBH₄ doesn't work on COOH).

7. Decarboxylation:
R-COONa + NaOH/CaO (heat) → R-H + Na₂CO₃ (Kolbe reaction).

8. HVZ (Hell-Volhard-Zelinsky) reaction:
R-CH₂-COOH + Cl₂ + P (red) → R-CHCl-COOH. α-halogenation.

CARBOXYLIC ACID DERIVATIVES (in order of reactivity for nucleophilic acyl substitution):

Acid chloride > Acid anhydride > Ester > Amide

You can convert from a more reactive to a less reactive one (e.g., acid chloride → ester) but not in reverse without forcing conditions.

TESTS

1. Sodium bicarbonate (positive for carboxylic acids only; negative for alcohols and phenols).

2. Litmus (turns blue litmus red).

3. Ester formation: with R'-OH + conc H₂SO₄ gives a pleasant smell (fruity).

4. Esters of long-chain acids: soaps (sodium / potassium salts).