Revision: Std XII >> Alcohols, Phenols, and Ethers MAH-MHT CET (PCM/PCB) Alcohols, Phenols, and Ethers

Definition: Esterification

Alcohols and phenols form esters by reaction with carboxylic acid, acid halides and acid anhydrides. This reaction is called esterification.

Key Points: Alcohols, Phenols and Ethers

Alcohols: Compounds with one or more –OH groups attached directly to a carbon chain. General formula: C₂H₂ₙ₊₁OH.
Phenols: Compounds where –OH group is directly bonded to an aromatic (benzene) ring.
Ethers: Compounds with general formula R–O–R'. If R = R', it is a symmetrical ether; if R ≠ R', it is an unsymmetrical ether.

Types of Alcohols

Type	Meaning	Position of —OH
Allylic Alcohol	—OH attached to sp³-hybridised carbon next to C=C double bond	Carbon next to C=C
Benzylic Alcohol	—OH attached to sp³-hybridised carbon next to aromatic ring	Benzylic carbon
Vinylic Alcohol	—OH attached directly to a vinylic carbon (CH₂=CH—) or aryl carbon	On C=C bond

Key Points: Classification of Alcohols, Phenols and Ethers

Classification of Alcohols

Based on number of —OH groups

Based on hybridisation of carbon bearing —OH (Monohydric only):

Type	Description	Example
Primary (1°)	—OH on primary carbon	R–CH₂–OH
Secondary (2°)	—OH on secondary carbon	R–CH(OH)–R
Tertiary (3°)	—OH on tertiary carbon	R–C(OH)(R)–R
Allylic	—OH on sp³ carbon next to C=C	CH₂=CH–CH₂OH
Vinylic	—OH directly on sp² carbon of C=C	CH₂=CH–OH
Benzylic	—OH on sp³ carbon next to aromatic ring	C₆H₅–CH₂–OH

Classification of Phenols

Type	—OH Groups	Example
Monohydric	1	Phenol
Dihydric	2	Catechol (Benzene-1,2-diol)
Trihydric	3	Phloroglucinol (Benzene-1,3,5-triol)

Classification of Ethers

Type	Description	Example
Simple / Symmetrical	Same alkyl/aryl groups on both sides of O	CH₃–O–CH₃ (Dimethyl ether), C₆H₅–O–C₆H₅ (Diphenyl ether)
Mixed / Unsymmetrical	Different alkyl/aryl groups on both sides of O	CH₃–O–C₂H₅ (Ethyl methyl ether), C₂H₅–O–C₆H₅ (Ethyl phenyl ether)

Key Points: Nomenclature of Alcohols, Phenols and Ethers

Alcohol names are derived from alkanes by replacing ‘e’ with ‘ol’ (e.g., methane → methanol).
In alcohols, the longest chain containing –OH is selected and numbered to give the lowest locant to the –OH group.
Phenol is the simplest aromatic alcohol; substituted phenols use ortho (1,2), meta (1,3), and para (1,4) positions.
Ethers are named as alkoxyalkanes in IUPAC; the smaller group becomes the alkoxy prefix.
Common names: Alcohol → alkyl + alcohol, Ether → alkyl groups + ether

Key Points: Preparation of Alcohols and Phenols

Hydroboration–Oxidation — This method of preparing alcohols from alkenes follows Anti-Markovnikov's rule; though the product seems as Markovnikov's, it is equivalent to anti-Markovnikov's addition.
Lucas Reagent Test — On adding Lucas reagent: a primary alcohol turns turbid only on heating, a secondary alcohol turns turbid slowly without heating, and a tertiary alcohol turns turbid immediately without heating.
Grignard Reagent Preparation of Alcohols — Formaldehyde (HCHO) + RMgX → 1° alcohol (R–CH₂OH); aldehyde (R'CHO) + RMgX → 2° alcohol; ketone (R'COR'') + RMgX → 3° alcohol.
Preparation of Phenol — Phenol can be prepared from chlorobenzene (NaOH/623 K/150 atm – Dow Process), benzene sulphonic acid (NaOH/573 K), cumene (O₂/cobalt naphthenate/423 K – commercial method), and aniline (NaNO₂–HCl/273 K, then H₂O/Δ – diazotisation).
Acidic Character — In aqueous medium, phenols show a weak acidic character, while alcohols are neutral.

Key Points: Physical Properties of Alcohols and Phenols

Intermolecular Forces — Alcohols and phenols are polar; –OH groups form strong hydrogen bonding.
Boiling Point — Increases with molecular mass; decreases with branching (n-butyl > isobutyl > sec-butyl > tert-butyl).
Solubility — Phenols and lower alcohols (≤3 C) are water-soluble via H-bonding with water.
Phenol is less soluble than alcohols due to the large hydrophobic benzene ring.
Lower alcohols are colourless liquids, higher ones become waxy solids.
Phenols are crystalline solids with a characteristic odour and higher boiling points.
Kolbe's & Reimer–Tiemann — Phenoxide + CO₂/H⁺ → salicylic acid; + CHCl₃/NaOH → salicylaldehyde (electrophile: CCl₂).
Oxidation/Reduction of Phenol — Na₂Cr₂O₇/H₂SO₄ → p-benzoquinone; 3H₂/Ni/433 K → cyclohexanol; Zn → benzene.

Key Points: Chemical Properties of Alcohols and Phenols

Litmus Test — Aqueous alcohols are neutral to litmus, while aqueous phenols turn blue litmus red, confirming the acidic character of phenols.
Reaction with Bases — Phenols react with NaOH to form water-soluble sodium phenoxide (regenerated on acidification with HCl) but do not react with NaHCO₃, since phenol is a weak acid.
Esterification — Alcohols/phenols react with carboxylic acids (conc. H₂SO₄ catalyst), acid anhydrides (H⁺ catalyst), or acid chlorides (in pyridine) to form esters; Aspirin is the acetyl derivative of salicylic acid formed using acetic anhydride.
Reactivity with Hydrogen Halides — Order of alcohol reactivity: 3° > 2° > 1°; order of HX reactivity: HI > HBr > HCl (HCl needs anhydrous ZnCl₂ catalyst).
Oxidation of Alcohols — 1° alcohol → aldehyde (with PCC, best reagent) → further to carboxylic acid (with KMnO₄/K₂Cr₂O₇/HNO₃); 2° alcohol → ketone (with CrO₃); 3° alcohols resist oxidation and break C–C bonds only at high temperature.

Key Points: Physical Properties of Ethers

Methods of preparation of ethers: Acid-catalysed dehydration of alcohols (conc. H₂SO₄, 443 K); catalytic dehydration (Al₂O₃, 250°C); Williamson synthesis (alkyl halide + sodium alkoxide, Sₙ2); reaction of alkyl halides with dry Ag₂O.
Preparation of Diethyl Ether (Simple Ether): From ethanol using conc. H₂SO₄ / H₃PO₄ at 413 K; or by Williamson's synthesis from C₂H₅ONa + BrCH₂CH₃ under heat.
Reactions of Diethyl Ether: O₂ (long contact) → peroxide; dil. H₂SO₄ → 2 C₂H₅OH; PCl₅ → C₂H₅OH + C₂H₅Cl; hot HI → C₂H₅I + C₂H₅OH; excess HI → 2 C₂H₅I.
Preparation of Anisole (Mixed Ether): CH₃Br + sodium phenoxide (C₆H₅ONa) → Methyl phenyl ether (Anisole) on heating.
Reactions of Anisole: HI (398 K) → phenol + CH₃I; Br₂/CH₃COOH → p-bromoanisole (major) + o-bromoanisole (minor); conc. HNO₃ + conc. H₂SO₄ → 4-nitroanisole (major) + 2-nitroanisole (minor); CH₃Cl/AlCl₃ → 4-methoxytoluene (major) + 2-methoxytoluene (minor); CH₃COCl/AlCl₃ → 4-methoxyacetophenone (major) + 2-methoxyacetophenone (minor).

Key Points: Physical Properties of Ethers

Colourless liquids (except dimethyl ether and diethyl ether, which are gases).
Polar due to bent structure (like a water molecule).
Low boiling point due to the absence of H-bonding between ether molecules.
Slightly soluble in water due to H-bonding with water; more soluble in organic solvents.
Structure: O is sp³ hybridised; two sp³ orbitals form O–C sigma bonds; two sp³ orbitals have lone pairs.