Revision: Alcohols, Phenols and Ethers Chemistry Science (English Medium) Class 12 CBSE

Organic compounds containing one or more hydroxyl (–OH) groups attached to a saturated carbon atom are called alcohols.

Organic compounds containing –OH group directly attached to an aromatic ring are called phenols.

Compounds formed by replacing the hydrogen of hydroxyl group of alcohol or phenol by an alkyl or aryl group are called ethers.

Alcohols containing one hydroxyl group are called monohydric alcohols.

Alcohols containing two hydroxyl groups are called dihydric alcohols.

Alcohols containing three hydroxyl groups are called trihydric alcohols.

An alcohol in which the –OH group is attached to a primary carbon atom is called primary alcohol.

An alcohol in which the –OH group is attached to a secondary carbon atom is called secondary alcohol.

An alcohol in which the –OH group is attached to a tertiary carbon atom is called tertiary alcohol.

The reaction in which phenol reacts with carbon dioxide in presence of sodium hydroxide is called Kolbe’s reaction.

The reaction in which phenol reacts with chloroform and sodium hydroxide to introduce –CHO group at ortho position is called Reimer–Tiemann reaction.

The preparation of ethers by reacting alkyl halide with sodium alkoxide is called Williamson synthesis.

The reaction in which alcohol reacts with carboxylic acid to form ester is called esterification reaction.

• 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

Classification of Alcohols

Based on number of —OH groups

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

• 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

• Alcohols: O atom is sp³ hybridised; two bond pairs + two lone pairs; bent structure.
• Phenols: –OH directly on benzene ring; lone pair on O delocalised into ring → more acidic than alcohols.
• Ethers: O is sp³ hybridised. Two O–C sigma bonds + two lone pairs. Structure similar to water molecule. Bent/angular shape.

• 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.

Methanol (Wood Spirit):

• Produced by catalytic hydrogenation of CO: 

  \[\ce{CO + 2H2  ->[ZnO/Cr2O3, 200-300atm, 573-673K] CH3OH}\]

• Highly poisonous; used as a solvent in paints and varnishes.

Ethanol:

• Produced by fermentation of sugar: 

  \[\ce{C12H22O11 + H2O ->[Invertase] \underset{Glucose}{C6H12O6} + \underset{Fructose}{C6H12O6}}\]

• Used as a solvent and in the preparation of carbon compounds.

Differentiation between Methanol & Ethanol:

• Iodoform test: Ethanol gives yellow ppt (CHI₃); methanol gives no reaction.

• With salicylic acid + H₂SO₄: Methanol forms methyl salicylate (characteristic odour); ethanol gives no specific odour.

• Williamson Synthesis (most important): R–O–Na + X–R' → R–O–R' + NaX. Primary alkyl halide is preferred (SN2 mechanism; 2° or 3° alkyl halide gives elimination).

• Acid-catalysed dehydration of alcohols: 

  \[\ce{2R - OH ->[H2SO4, 413K] R - O - R + H2O}\]

  (works best for symmetrical ethers)

• From alcohols by catalytic dehydration: 

  \[\ce{2C2H5OH ->[Al2O3, 513-523K] C2H5 - O - C2H5 + H2O}\]

• Alkoxy mercuration-demercuration: \[\begin{array}{cc}
  \phantom{}\ce{CH3 - CH = CH2 + C2H5OH + Hg(OCOCF3)2 -> CH3 - CH - CH2 - HgOCOCF3 ->[NaBH4/OH^{-}] CH3 - CH - CH3}\\
  \phantom{................................................................................}|\phantom{.....................................................................}|\phantom{.}\\
  \phantom{............................................................................................}\ce{OC2H5}\phantom{...........................................................}\ce{O-C2H5}\phantom{.}
  \end{array}\]

• 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.

• 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).

• Ethers are generally very unreactive (no H-bonding between ether molecules).
• When excess HX is added → C–O bond cleaves → alkyl halides.
• Reactivity of HX: HI > HBr > HCl
• If 1° or 2° alkyl groups: Smaller alkyl group forms alkyl iodide. (e.g., C₂H₅–O–CH₃ + HI → C₂H₅OH + CH₃I)
• If one alkyl group is 3°: Forms tertiary alkyl halide (SN1 pathway).

Reaction with conc. HI:

• With excess HI: Both groups convert to iodo compounds
• e.g., \[\ce{C2H5OC2H5 + HI ->[Cold] C2H5I + C2H5OH}\]

Substitution Reactions in Aromatic Ether: The alkoxу group in ether activates the aromatic ring at ortho and para positions for electrophilic substitution. Common electrophilic substitution reactions are halogenation, Friedel-Crafts reaction, etc.

Statement:
Ethers are prepared by reacting alkyl halide with sodium alkoxide.

Mechanism:
SN2 reaction.

Best for:
Primary alkyl halides.

Limitation:
Tertiary halides undergo elimination.

Step 1: Protonation of alcohol
Step 2: Formation of carbocation (slow step)
Step 3: Elimination of proton to form alkene

Order:
Tertiary > Secondary > Primary

Due to stability of carbocation.

Statement:
Phenols are more acidic than alcohols due to resonance stabilisation of phenoxide ion.

Reason:

• Negative charge delocalised in phenoxide ion

• sp² hybridised carbon increases O–H polarity

Electron withdrawing groups (–NO₂) increase acidity.

Electron donating groups decrease acidity.

Statement:
Alcohols are weak acids due to polar O–H bond and can donate a proton.

Order of acidity:
Primary > Secondary > Tertiary

Reason:
Electron donating alkyl groups decrease polarity of O–H bond.

Alcohols are weaker acids than water.

Statement:
Alcohols and phenols form hydrogen bonds due to presence of –OH group.

Effect:

• Higher boiling points

• Greater solubility in water

Step 1: Nucleophilic addition to carbonyl carbon.
Step 2: Hydrolysis of adduct.

Products:

• With formaldehyde → Primary alcohol

• With aldehyde → Secondary alcohol

• With ketone → Tertiary alcohol

Statement:
In hydroboration–oxidation, boron attaches to less substituted carbon and finally OH group appears at less substituted carbon.

Result:
Anti-Markovnikov product.

Characteristic:
Occurs without carbocation rearrangement.

Statement:
In addition of HX or water to an unsymmetrical alkene, hydrogen attaches to the carbon atom having greater number of hydrogen atoms.

Example:
Propene + H₂O → Propan-2-ol (major)

Reason:
Formation of more stable carbocation.

Statement:
Ethers undergo cleavage of C–O bond in presence of concentrated HI or HBr to form alkyl halides.

Mechanism:

Step 1: Protonation of ether oxygen
Step 2: Nucleophilic attack by halide ion

• Primary ether → SN2 mechanism

• Tertiary ether → SN1 mechanism

Order of reactivity of HX:
HI > HBr > HCl