Alcohols, Phenols, and Ethers

1. Classification

Compounds are classified based on the number of hydroxyl (-OH) groups attached:

Monohydric: One -OH group. E.g., Ethanol (C₂H₅OH).
Dihydric: Two -OH groups. E.g., Ethylene glycol.
Trihydric: Three -OH groups. E.g., Glycerol.
Polyhydric: Many -OH groups.

Based on hybridization:

Compounds containing C_sp³-OH bond: Primary (1°), Secondary (2°), Tertiary (3°) alcohols. Allylic alcohols, Benzylic alcohols.
Compounds containing C_sp²-OH bond: Vinylic alcohols, Phenols (OH attached directly to a benzene ring).

2. Nomenclature

Alcohols: IUPAC suffix is -ol. Common names use alkyl alcohol.

Phenols: The simplest hydroxy derivative of benzene is phenol. Substituted phenols use ortho (o), meta (m), para (p) in common names, and numbers in IUPAC relative to the -OH group.

Ethers: Common names use alkyl alkyl ether. IUPAC names treat them as derivatives of alkanes: alkoxyalkane. The larger alkyl group forms the parent hydrocarbon.

3. Structures of Functional Groups

Alcohols: The oxygen is sp³ hybridized. The C-O-H bond angle is slightly less than the tetrahedral angle (109°28') due to repulsion between the unshared electron pairs of oxygen (e.g., 108.9° in methanol).
Phenols: The C-O bond length in phenol (136 pm) is slightly less than in methanol (142 pm) because of (i) partial double bond character due to resonance and (ii) sp² hybridized state of carbon to which oxygen is attached.
Ethers: The four electron pairs on oxygen (two bond pairs, two lone pairs) are roughly tetrahedral. The R-O-R bond angle (111.7°) is slightly greater than the tetrahedral angle due to the repulsive interaction between the two bulky alkyl groups.

4. Alcohols and Phenols

Preparation of Alcohols

From Alkenes: Acid catalyzed hydration (Markovnikov). Hydroboration-oxidation (Anti-Markovnikov, yields 1° alcohol mostly).
From Carbonyls: Reduction of aldehydes (yields 1°), ketones (yields 2°), and carboxylic acids/esters (yields 1°) using LiAlH₄ or NaBH₄.
From Grignard Reagents: Formaldehyde (1°), Other aldehydes (2°), Ketones (3°).

Preparation of Phenols

From Haloarenes (Dow's Process): Chlorobenzene + NaOH at 623K and 300atm → Sodium phenoxide, followed by acidification.
From Cumene (Isopropylbenzene): (Commercially important method) Cumene is oxidized to cumene hydroperoxide, which is cleaved with dilute acid to yield phenol and acetone (valuable byproduct).

Acidity: Phenols vs Alcohols

Phenols are stronger acids than alcohols. This is a critical comparative concept frequently tested in the IAT.

Feature	Alcohols	Phenols
Ion Formed	Alkoxide ion (R-O^-)	Phenoxide ion (C₆H₅-O^-)
Stability	Unstable. Alkyl group (ERGs) destabilizes the negative charge via +I effect.	Stable. The negative charge is delocalized over the aromatic ring via resonance.
Relative Acidity	Weaker than water.	Stronger than alcohols and water, but weaker than carboxylic acids.
Substituent Effect	Alkyl groups (+I) decrease acidity.	EWG (-NO₂) increases acidity (stabilizes ion). ERG (-CH₃) decreases acidity.

Chemical Reactions

Reactions involving cleavage of O-H bond (Acidity): React with active metals (Na, K) to form alkoxides/phenoxides and H₂ gas. Esterification (reaction with carboxylic acids, acid chlorides, anhydrides).
Reactions involving cleavage of C-O bond (Alcohols only): Reaction with HX (Lucas test distinguishes 1°, 2°, 3° alcohols based on turbidity speed), PCl₃, SOCl₂, dehydration to alkenes.
Oxidation:
1° Alcohol → Aldehyde → Carboxylic acid. (PCC stops at aldehyde).
2° Alcohol → Ketone.
3° Alcohol → Resists oxidation (undergoes cleavage under drastic conditions).

Important Name Reactions (Phenols)

Reimer-Tiemann Reaction: Phenol + Chloroform + aq. NaOH → Salicylaldehyde (o-hydroxybenzaldehyde). The electrophile is dichlorocarbene (:CCl₂).
Kolbe's Reaction: Sodium phenoxide + CO₂ (under pressure) followed by acidification → Salicylic acid (o-hydroxybenzoic acid).
Oxidation of Phenol: Phenol oxidized with chromic acid (Na₂Cr₂O₇ / H₂SO₄) produces benzoquinone (a conjugated diketone).

5. Commercially Important Alcohols

Methanol (CH₃OH): Also known as 'wood spirit'. Produced commercially by catalytic oxidation of carbon monoxide (CO + 2H₂ → CH₃OH) using ZnO-Cr₂O₃ catalyst. It is highly poisonous (causes blindness or death).
Ethanol (C₂H₅OH): Obtained commercially by fermentation of sugars (using invertase and zymase enzymes). Used as an excellent solvent. Denatured spirit is ethanol made unfit for drinking by adding methanol, pyridine, or copper sulphate.

6. Ethers

Preparation

Dehydration of Alcohols: Ethanol heated with conc. H₂SO₄ at 413K gives diethyl ether (S_N2 mechanism). Higher temperatures (443K) favor alkene formation via elimination.
Williamson Synthesis: (Very Important) Important laboratory method for the preparation of symmetrical and unsymmetrical ethers. R-X + R'-O^-Na⁺ → R-O-R' + NaX. Mechanism is S_N2. Therefore, the alkyl halide should be primary (1°). If 3° alkyl halide is used, elimination dominates to give an alkene.

Chemical Reactions

Cleavage by Acids: Ethers are cleaved by strong acids (HX) at high temperatures. R-O-R + HX → RX + ROH. With excess HX, ROH becomes RX. Reactivity: HI > HBr > HCl. For mixed ethers, if one group is tertiary, the halide forms the tertiary carbocation (S_N1). Otherwise, the halide attaches to the smaller alkyl group (S_N2).
Anisole Cleavage: Cleavage of alkyl aryl ethers (like anisole) yields phenol and alkyl halide, NEVER aryl halide and alcohol, because the Oxygen-Aryl bond has partial double bond character.
Electrophilic Substitution: The alkoxy group (-OR) is ortho, para directing and activates the aromatic ring. Anisole undergoes halogenation, nitration, and Friedel-Crafts reactions.

7. Practice Mock Test

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Take a quick assessment specifically designed for Alcohols, Phenols, and Ethers. Secure those crucial marks by practicing IAT-level problems on acidity orders and Williamson synthesis.

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