- Combustion: Alcohols burn in air to give CO₂, H₂O, and heat. Ethanol burns with a pale blue flame.
- Oxidation: Alcohols oxidise to aldehydes and then to acids using acidified K₂Cr₂O₇ or KMnO₄.
- Reaction with Sodium: Forms alkoxides and H₂ gas with effervescence (e.g., sodium ethoxide).
- Esterification: React with acetic acid in the presence of conc. H₂SO₄ to form fruity-smelling esters.
- Dehydration:
- At 170°C → forms alkene (e.g., ethene)
- At 140°C → forms ether (e.g., diethyl ether)
Key Points
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
| Type | —OH Groups | Example |
|---|---|---|
| Monohydric | 1 | Ethanol (CH₃CH₂OH) |
| Dihydric | 2 | Ethylene glycol (CH₂OH–CH₂OH) |
| Trihydric | 3 | Glycerol |
| Polyhydric | More than 3 | Glucose |
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: Commercially Important Alcohols
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.
Key Points: Physical Properties of Alcohols
| Property | Details |
|---|---|
| Nature | Inflammable, volatile, colourless liquids with a faint odour and burning taste |
| Boiling Point | Increases with molecular weight • CH₃OH: 64.5°C • C₂H₅OH: 78.3°C |
| Solubility | Soluble in water and organic solvents |
| Density | Ethanol is lighter than water; specific gravity = 0.79 at 293 K |
| Toxicity | Methyl alcohol is toxic and can cause blindness or death if consumed |
key Points: Chemical Properties of Alcohols
Key Points: Preparation of Ethers
-
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: \[\begin{array}{cc}
\phantom{}\ce{CH3 - CH = CH2 + C2H5OH + Hg(OCOCF3)2 -> CH3 - CH - CH2 - HgOCOCF3 ->[NaBH4/OH^{-}] CH3 - CH - CH3}\\
\phantom{................................................................................}|\phantom{.....................................................................}|\phantom{.}\\
\phantom{............................................................................................}\ce{OC2H5}\phantom{...........................................................}\ce{O-C2H5}\phantom{.}
\end{array}\]
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.
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).
Concepts [21]
- Concept of Alcohols, Phenols and Ethers
- Uses of Alcohols
- Classification of Alcohols, Phenols and Ethers
- Nomenclature of Alcohols, Phenols and Ethers
- Isomerism in Alcohols
- Methods of Preparation of Alcohols
- Commercially Importance Alcohols
- Physical Properties of Alcohols
- Chemical Properties of Alcohols
- Phenols
- Methods of Preparation of Phenols
- Physical Properties of phenol
- Chemical Properties of Phenol
- Acidic Nature of Phenol
- Uses of Phenols
- Ethers
- Isomerism of Ether
- Preparation of Ethers
- Physical Properties of Ethers
- Chemical Properties of Ethers
- Uses of Ethers
