Introduction
Alcohols, phenols, and ethers are important classes of organic compounds that play a significant role in chemistry due to their wide range of applications and unique properties. This study note will cover their structures, nomenclature, preparation methods, physical and chemical properties, and reactions relevant to the JEE Advanced Chemistry syllabus.
Alcohols
Structure and Classification
Alcohols are organic compounds that contain one or more hydroxyl ($-OH$) groups attached to a carbon atom. They can be classified based on the number of hydroxyl groups and the nature of the carbon atom to which the $-OH$ group is attached:
- Monohydric Alcohols: Contain one $-OH$ group.
- Dihydric Alcohols (Glycols): Contain two $-OH$ groups.
- Trihydric Alcohols (Glycerol): Contain three $-OH$ groups.
- Primary (1°) Alcohols: The $-OH$ group is attached to a carbon atom that is bonded to only one other carbon atom.
- Secondary (2°) Alcohols: The $-OH$ group is attached to a carbon atom that is bonded to two other carbon atoms.
- Tertiary (3°) Alcohols: The $-OH$ group is attached to a carbon atom that is bonded to three other carbon atoms.
Nomenclature
- IUPAC Naming: Replace the suffix "-e" of the parent alkane with "-ol". Number the carbon chain such that the $-OH$ group gets the lowest possible number.
For $CH_3CH_2OH$, the IUPAC name is ethanol.
- Common Naming: Use the name of the alkyl group followed by "alcohol".
For $CH_3OH$, the common name is methyl alcohol.
Preparation Methods
- Hydration of Alkenes: $$ CH_2=CH_2 + H_2O \xrightarrow{H_2SO_4} CH_3CH_2OH $$
- Reduction of Aldehydes and Ketones: $$ RCHO + 2[H] \xrightarrow{NaBH_4} RCH_2OH $$ $$ RCOR' + 2[H] \xrightarrow{NaBH_4} RCH(OH)R' $$
- Hydrolysis of Alkyl Halides: $$ R-X + KOH \xrightarrow{H_2O} ROH + KX $$
Physical Properties
- Boiling Point: Higher than corresponding alkanes due to hydrogen bonding.
- Solubility: Lower alcohols are miscible with water due to hydrogen bonding with water molecules.
Chemical Properties
- Oxidation:
- Primary alcohols to aldehydes and then to carboxylic acids.
- Secondary alcohols to ketones.
- Tertiary alcohols resist oxidation.
Use mild oxidizing agents like PCC for controlled oxidation of primary alcohols to aldehydes.
- Dehydration: $$ CH_3CH_2OH \xrightarrow{H_2SO_4, \Delta} CH_2=CH_2 + H_2O $$
- Reaction with Sodium: $$ 2ROH + 2Na \rightarrow 2RONa + H_2 $$
Phenols
Structure and Classification
Phenols are compounds in which a hydroxyl group ($-OH$) is directly bonded to an aromatic ring. They are classified based on the number of hydroxyl groups:
- Monohydric Phenols: One $-OH$ group (e.g., phenol).
- Dihydric Phenols: Two $-OH$ groups (e.g., catechol, resorcinol, hydroquinone).
- Trihydric Phenols: Three $-OH$ groups (e.g., pyrogallol).
Nomenclature
- IUPAC Naming: Use "phenol" as the parent name. Number the ring such that the $-OH$ group gets the lowest number.
For $C_6H_5OH$, the IUPAC name is phenol.
Preparation Methods
- From Benzene Sulfonic Acid: $$ C_6H_5SO_3H + 2NaOH \xrightarrow{300^\circ C} C_6H_5OH + Na_2SO_3 $$
- From Diazonium Salts: $$ C_6H_5N_2^+Cl^- + H_2O \rightarrow C_6H_5OH + N_2 + HCl $$
Physical Properties
- Boiling Point: Higher than alcohols due to stronger hydrogen bonding.
- Solubility: Phenols are sparingly soluble in water but soluble in organic solvents.
Chemical Properties
- Acidity: $$ C_6H_5OH \rightleftharpoons C_6H_5O^- + H^+ $$ Phenols are more acidic than alcohols due to the resonance stabilization of the phenoxide ion.
Phenols react with bases to form phenoxide salts.
- Electrophilic Aromatic Substitution:
- Nitration: $$ C_6H_5OH + HNO_3 \rightarrow C_6H_4(NO_2)OH + H_2O $$
- Halogenation: $$ C_6H_5OH + Br_2 \rightarrow C_6H_4BrOH + HBr $$
Ethers
Structure and Classification
Ethers are compounds with an oxygen atom connected to two alkyl or aryl groups. They can be classified as:
- Simple (Symmetrical) Ethers: Both alkyl or aryl groups are the same.
- Mixed (Asymmetrical) Ethers: The alkyl or aryl groups are different.
Nomenclature
- IUPAC Naming: The longer alkyl group is the parent chain, and the shorter alkyl group is named as an alkoxy substituent.
For $CH_3OCH_2CH_3$, the IUPAC name is methoxyethane.
- Common Naming: Name the alkyl groups alphabetically followed by "ether".
For $CH_3OCH_3$, the common name is dimethyl ether.
Preparation Methods
- Williamson Ether Synthesis: $$ RONa + R'X \rightarrow ROR' + NaX $$
- Dehydration of Alcohols: $$ 2ROH \xrightarrow{H_2SO_4, 140^\circ C} ROR + H_2O $$
Physical Properties
- Boiling Point: Lower than alcohols due to the absence of hydrogen bonding.
- Solubility: Ethers are generally less soluble in water compared to alcohols but are good solvents for many organic compounds.
Chemical Properties
- Cleavage by Acids: $$ ROR' + HX \rightarrow RX + R'OH $$
- Autooxidation: Ethers can form explosive peroxides when exposed to air for prolonged periods.
Avoid storing ethers for long periods due to the risk of peroxide formation.
Conclusion
Alcohols, phenols, and ethers are fundamental organic compounds with diverse structures, properties, and reactions. Understanding their preparation, physical and chemical properties, and reactivity is crucial for mastering organic chemistry concepts in the JEE Advanced syllabus.
