NCERT Solutions for Class 12 Chemistry Chapter 7 Alcohols, Phenols and Ethers
NCERT Solutions for Class 12 Chemistry Chapter 7 Alcohols, Phenols and Ethers provides comprehensive explanations for classification, IUPAC nomenclature, acidic strength, named reactions (Kolbe's, Reimer-Tiemann, Williamson Synthesis), and reaction mechanisms like dehydration of alcohols.
Overview of Topics & Subtopics (Click to Jump)
1. Classification, IUPAC Nomenclature & Structure
Short Note:
• Classification:
- Alcohols & Phenols: Monohydric, dihydric, or polyhydric based on number of -OH groups. Primary (1°), Secondary (2°), and Tertiary (3°) alcohols based on carbon hybridization.
- Ethers: Symmetrical (R-O-R) and Unsymmetrical (R-O-R').
• Structure: Oxygen atom is sp3 hybridized with two lone pairs. Bond angle in alcohols (C-O-H is ~108.9°) is slightly less than tetrahedral angle due to lone pair-lone pair repulsion. In ethers, C-O-C angle (~111.7°) is greater due to steric hindrance between bulky alkyl groups.
2. Alcohols & Phenols: Preparation, Physical Properties & Acidic Strength
Short Note:
• Methods of Preparation:
- Alcohols: Acid-catalysed hydration of alkenes, Hydroboration-oxidation (anti-Markovnikov product), Reduction of aldehydes/ketones using NaBH4 or LiAlH4, and Grignard reagent additions.
- Phenols: From Cumene (isopropylbenzene by oxidation followed by H+ hydrolysis), Chlorobenzene (Dow's process), or Benzene diazonium chloride.
• Physical Properties & Boiling Point: High boiling points compared to hydrocarbons/haloalkanes due to intermolecular hydrogen bonding. Boiling point decreases with branching.
• Acidic Strength: Phenols are much more acidic than alcohols because the phenoxide ion is resonance-stabilised, whereas alkoxide ions are destabilised by +I effect of alkyl groups. Electron-withdrawing groups (-NO2) increase acidity, while electron-releasing groups (-CH3, -OCH3) decrease acidity.
3. Ethers & Important Named Reactions
Short Note:
• Key Named Reactions:
- Kolbe's Reaction: Phenol + NaOH + CO2 → Salicylic acid (2-hydroxybenzoic acid).
- Reimer-Tiemann Reaction: Phenol + CHCl3 + aq. NaOH → Salicylaldehyde (2-hydroxybenzaldehyde).
- Williamson Synthesis: Alkyl halide + Sodium alkoxide → Ether (SN2 mechanism, best yields with 1° alkyl halides).
• Cleavage of Ethers by HX: R-O-R' + HI → R-I + R'-OH. With unsymmetrical ethers, alkyl iodide forms with the smaller alkyl group via SN2, unless a 3° alkyl group is present (which forms 3° iodide via SN1).
Intext & Conceptual Questions
(i) CH3-CH(OH)-CH(CH3)-CH3
(ii) CH3-O-CH2-CH(CH3)2
(iii) 2,3-diethylphenol (Structure to Name verification)
1. CH3-CH(OH)-CH(CH3)-CH3: 3-Methylbutan-2-ol
2. CH3-O-CH2-CH(CH3)2: 1-Methoxy-2-methylpropane
3. 2,3-diethylphenol: 2,3-Diethylphenol
Lucas Reagent is a mixture of concentrated HCl and anhydrous ZnCl2.
| Alcohol Type | Observation with Lucas Reagent | Reaction Rate |
|---|---|---|
| Tertiary Alcohol (3°) | Turbidity (cloudiness) appears immediately at room temperature. | Very Fast (Carbocation is very stable). |
| Secondary Alcohol (2°) | Turbidity appears within 5 minutes. | Moderate rate. |
| Primary Alcohol (1°) | Solution remains clear at room temp; turbidity appears only on heating. | Extremely Slow. |
Textbook Chapter End Exercise Questions
Phenol is significantly more acidic than Ethanol due to two main reasons:
• Resonance Stabilisation of Phenoxide Ion: When phenol loses a proton (H+), it forms a phenoxide ion (C6H5O-). The negative charge on oxygen is delocalised over the benzene ring through resonance, making the phenoxide ion highly stable.
• Alkoxide Ion Instability: When ethanol loses H+, it forms an ethoxide ion (C2H5O-). The ethyl group shows a +I (inductive) effect, which increases the electron density on oxygen and destabilises the ethoxide ion.
Therefore, phenol releases H+ ions much more readily than ethanol.
Dehydration of ethanol to ethene in presence of conc. H2SO4 at 443 K occurs in 3 major steps:
• Step 1: Protonation of Alcohol (Formation of Oxonium Ion)
CH3-CH2-OH + H+ ⇌ CH3-CH2-OH2+
• Step 2: Formation of Carbocation (Slow / Rate Determining Step)
CH3-CH2-OH2+ → CH3-CH2+ + H2O
• Step 3: Elimination of Proton to Form Ethene
CH3-CH2+ → CH2=CH2 + H+
Read More & Chapter Resources
These short notes are designed for quick revision. Read the main points once, then use the related material below to practise the same chapter. Revising a concept and checking it with questions can make your preparation more focused.