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Alcohol, Phenol and Ether
In organic chemistry, alcohols, phenols, and ethers are among the most important functional groups. They are frequently tested in JEE Main and JEE Advanced because they combine concepts of structure, reactivity, preparation, and mechanisms.
- Alcohols: Organic compounds containing a hydroxyl group (–OH) attached to a saturated carbon atom.
- Phenols: Aromatic compounds where the hydroxyl group (–OH) is directly attached to a benzene ring.
- Ethers: Organic compounds with an oxygen atom bonded to two alkyl or aryl groups (R–O–R').
These compounds are highly relevant in both industrial applications (fuels, pharmaceuticals, solvents) and in understanding reaction mechanisms, making them vital for JEE preparation.
1.0What are Alcohols?
Alcohols are organic compounds that contain a hydroxyl (−OH) functional group bonded to a saturated, aliphatic carbon atom. The simplest alcohol is methanol (CH_3OH).
Classification of Alcohols
Depending on the number of hydroxyl groups attached, alcohols can be classified into three types.
- Monohydric alcohols: They contain one -OH group. Example, CH3CH2-OH
- Dihydric alcohols: They contain two -OH groups. Example, 1,2-Ethandiol.
- Trihydric alcohols: They contain three -OH groups. Example 1,2,3-Propantriol.
Depending on the number of carbon atoms which are directly attached to the carbon that is bonded with the -OH group, alcohols can be classified into three types.
- Primary The carbon with the −OH group is bonded to only one other carbon atom (e.g., ethanol).
- Secondary (2^∘): The carbon with the −OH group is bonded to two other carbon atoms (e.g., isopropanol).
- Tertiary (3^∘): The carbon with the −OH group is bonded to three other carbon atoms (e.g., tert-butyl alcohol).
Nomenclature of Alcohols
Alcohols (IUPAC rules):
- Longest carbon chain containing –OH group is selected.
- The –OH group gets the lowest possible number.
- The suffix –ol is used.
- Example: CH₃CH₂OH → Ethanol.
Preparation of Alcohols
- Hydration of Alkenes:
- Ethene + H₂O → Ethanol (using H₂SO₄ catalyst).
- Mechanism follows Markovnikov’s rule.
- Hydrolysis of Alkyl Halides:
- R–X + NaOH (aq) → R–OH.
- Reduction of Carbonyl Compounds:
- Aldehydes + H₂/Ni → Primary Alcohols.
- Ketones + H₂/Ni → Secondary Alcohols.
- Fermentation of Sugars:
- Glucose → Ethanol (using yeast enzyme).
2.0Chemical Reactions of Alcohols
- Oxidation:
- Primary alcohol → Aldehyde → Carboxylic acid.
- Secondary alcohol → Ketone.
- Tertiary alcohol → No oxidation (under mild conditions).
- Dehydration:
- Alcohol → Alkene (conc. H₂SO₄, heat).
- Reaction with Na metal:
- R–OH + Na → R–O⁻Na⁺ + H₂.
Properties of Alcohols
- Boiling point: Higher than corresponding hydrocarbons and ethers due to intermolecular hydrogen bonding.
- Solubility: Small-chain alcohols are very soluble in water because the hydroxyl group can form hydrogen bonds with water molecules. Solubility decreases as the hydrocarbon chain gets longer.
- Reactivity: Alcohols can act as both nucleophiles and electrophiles. They undergo reactions such as oxidation (to form aldehydes or ketones), dehydration (to form alkenes), and esterification.
- Acidity: Weakly acidic, but less so than phenols or water.
Uses of Alcohols
- Ethanol: Found in alcoholic beverages, serves as a fuel, and is used as a solvent and disinfectant.
- Methanol: Used as an industrial solvent and to produce other chemicals like formaldehyde.
3.0What is Phenols (𝐴𝑟OH)?
Phenols are aromatic compounds where a hydroxyl (−OH) group is bonded directly to a carbon atom of a benzene ring. The simplest phenol is hydroxybenzene, which is also commonly called phenol.
Classification of Phenol
Depending on the number of hydroxyl groups attached, phenols can be classified into three types.
- Monohydric phenols: They contain one -OH group.
- Dihydric phenols: They contain two -OH groups. They may be ortho-, meta- or para- derivative.
- Trihydric phenols: They contain three -OH groups.
Nomenclature of Phenols
- Hydroxyl group directly bonded to benzene ring.
- Substituents are named accordingly.
- Example: o-Cresol, p-Nitrophenol.
Preparation of Phenols
- From Chlorobenzene (Dow’s Process):
- C₆H₅Cl + NaOH (at 623 K, 300 atm) → Phenol.
- From Benzene Sulphonic Acid:
- C₆H₅SO₃H + NaOH → Phenol.
- From Diazonium Salts:
- C₆H₅N₂⁺Cl⁻ + H₂O → Phenol + N₂.
Properties of Phenols
- Acidity: Phenols are more acidic than alcohols and water because the phenoxide ion (the conjugate base) is stabilized by the delocalization of the negative charge via resonance throughout the benzene ring.
- Reactivity: Undergo electrophilic aromatic substitution, such as nitration and halogenation, which are directed to the ortho and para positions. They also react with bases to form phenoxide salts.
- Physical properties: White crystalline solids that can become discolored by oxidation. They are moderately soluble in water and form hydrogen bonds.
Reactions of Phenols
- Electrophilic Substitution:
- Phenol + Br₂ → 2,4,6-Tribromophenol.
- Phenol + HNO₃ → Nitro phenols.
- Kolbe’s Reaction:
- Phenol + CO₂/NaOH → Salicylic acid.
- Reimer–Tiemann Reaction:
- Phenol + CHCl₃/NaOH → o-Hydroxybenzaldehyde (Salicylaldehyde).
Uses of Phenols
- Antiseptics: Used in disinfectants and antiseptics (e.g., Dettol).
- Polymers: A precursor in the synthesis of polymers like Bakelite.
- Pharmaceuticals: Used to make pharmaceuticals such as aspirin.
4.0What is Ethers?
Ethers are organic compounds in which an oxygen atom is bonded to two alkyl or aryl groups. The general formula is 𝑅O𝑅′, where the R groups can be the same or different.
Classification
Ethers are classified based on the nature of the two groups attached to the oxygen atom:
- Symmetrical (or simple): Both R groups are the same (e.g., diethyl ether, ).
- Unsymmetrical (or mixed): The R groups are different (e.g., ethyl methyl ether, ).
Nomenclature of Ethers
- The smaller alkyl/aryl group + “oxy” + larger group.
- Example: CH₃OCH₂CH₃ → Methoxyethane.
Preparation of Ethers
- Williamson’s Ether Synthesis:
- R–ONa + R’–X → R–O–R’.
- Example: CH₃ONa + CH₃Br → CH₃OCH₃ (Dimethyl ether).
- Dehydration of Alcohols:
- 2R–OH (conc. H₂SO₄, 413 K) → R–O–R + H₂O.
Properties
- Boiling point: Lower than alcohols of comparable molecular mass, as they cannot form intermolecular hydrogen bonds with themselves.
- Solubility: Only slightly soluble in water, but can act as a hydrogen-bond acceptors.
- Reactivity: Ethers are generally unreactive, making them excellent solvents. They can be cleaved by strong acids at high temperatures.
- Flammability: Highly flammable and can form explosive peroxides when exposed to air.
Reactions of Ethers
- Acidic Cleavage:
- R–O–R’ + HI → R–I + R’–OH.
- Electrophilic Substitution in Aromatic Ethers:
- Anisole (C₆H₅OCH₃) undergoes substitution at ortho and para positions.
Uses
- Solvents: Used widely as solvents for fats, oils, and resins.
- Anesthetics: Diethyl ether was historically used as an anesthetic in surgery.
- Refrigerants: Dimethyl ether is used as a refrigerant.