Class 12 Chemistry Notes Chapter 2 (Alcohols, Phenols and Ethers) – Chemistry-II Book
Detailed Notes with MCQs of Chapter 2: Alcohols, Phenols, and Ethers from your NCERT Chemistry-II textbook. This is a crucial chapter, not just for your board exams but also forms the basis for many questions in competitive government exams. Pay close attention to the structures, reactions, and underlying concepts.
Chapter 2: Alcohols, Phenols and Ethers - Detailed Notes for Government Exam Preparation
1. Introduction
- Alcohols: Organic compounds containing one or more hydroxyl (-OH) groups directly attached to a saturated carbon atom (sp³ hybridized). General formula: R-OH.
- Phenols: Organic compounds containing one or more hydroxyl (-OH) groups directly attached to a benzene ring or other arene ring (sp² hybridized carbon). General formula: Ar-OH.
- Ethers: Organic compounds derived from water or alcohols/phenols where both hydrogen atoms are replaced by alkyl (R) or aryl (Ar) groups. Functional group: Ether linkage (-O-). General formula: R-O-R', R-O-Ar, or Ar-O-Ar'.
2. Classification
- Alcohols & Phenols:
- Based on the number of -OH groups:
- Mono-hydric (e.g., Ethanol, Phenol)
- Di-hydric (e.g., Ethane-1,2-diol (Glycol), Benzene-1,2-diol (Catechol))
- Tri-hydric (e.g., Propane-1,2,3-triol (Glycerol), Benzene-1,3,5-triol (Phloroglucinol))
- Poly-hydric (more than three -OH groups)
- Monohydric Alcohols (Based on hybridization of C bearing -OH):
- Compounds containing C(sp³)-OH bond:
- Primary (1°): -OH attached to a primary carbon (e.g., Ethanol, CH₃CH₂OH)
- Secondary (2°): -OH attached to a secondary carbon (e.g., Propan-2-ol, CH₃CH(OH)CH₃)
- Tertiary (3°): -OH attached to a tertiary carbon (e.g., 2-Methylpropan-2-ol, (CH₃)₃COH)
- Allylic Alcohols: -OH attached to sp³ carbon next to a C=C double bond (e.g., CH₂=CH-CH₂OH). Can be 1°, 2°, or 3°.
- Benzylic Alcohols: -OH attached to sp³ carbon next to an aromatic ring (e.g., C₆H₅CH₂OH). Can be 1°, 2°, or 3°.
- Compounds containing C(sp²)-OH bond:
- Vinylic Alcohols: -OH attached directly to a C=C double bond carbon (generally unstable, tautomerize to aldehydes/ketones).
- Phenols: -OH attached directly to an aromatic ring carbon.
- Compounds containing C(sp³)-OH bond:
- Based on the number of -OH groups:
- Ethers:
- Symmetrical (Simple): Both R/Ar groups are identical (e.g., Diethyl ether, C₂H₅-O-C₂H₅).
- Unsymmetrical (Mixed): The two R/Ar groups are different (e.g., Ethyl methyl ether, CH₃-O-C₂H₅).
3. Nomenclature
- Alcohols:
- Common: Alkyl alcohol (e.g., Ethyl alcohol, Isopropyl alcohol).
- IUPAC: Replace '-e' of parent alkane with '-ol'. Indicate position of -OH group with lowest possible number. Use di-, tri- prefixes for multiple -OH groups. (e.g., Ethanol, Propan-2-ol, Ethane-1,2-diol).
- Phenols:
- Common: Phenol is the common and IUPAC accepted name for C₆H₅OH. Substituted phenols are named as derivatives (e.g., o-Cresol, m-Nitrophenol) or using IUPAC rules (e.g., 2-Methylphenol, 3-Nitrophenol). Special names: Catechol, Resorcinol, Hydroquinone/Quinol.
- Ethers:
- Common: Name the two alkyl/aryl groups alphabetically followed by 'ether' (e.g., Diethyl ether, Ethyl methyl ether, Diphenyl ether).
- IUPAC: Named as 'Alkoxyalkanes'. The smaller alkyl group forms the alkoxy part, and the larger alkyl group forms the parent alkane. (e.g., Methoxyethane, Ethoxybenzene (Anisole), Methoxybenzene).
4. Structure of Functional Groups
- Alcohols: C-O-H bond angle slightly less than tetrahedral (≈108.9° in methanol) due to repulsion between lone pairs on oxygen. Oxygen is sp³ hybridized. C-O and O-H bonds are polar.
- Phenols: C-O bond length is shorter than in alcohols due to partial double bond character (resonance) and sp² hybridization of carbon. C-O-H bond angle ≈109°. O-H bond is polar.
- Ethers: C-O-C bond angle is slightly greater than tetrahedral (≈111.7° in methoxymethane) due to repulsion between bulky R groups. Oxygen is sp³ hybridized. C-O bonds are polar, resulting in a net dipole moment (bent structure).
5. Preparation Methods
- Alcohols:
- From Alkenes:
- Acid-catalysed hydration: (Markovnikov addition, rearrangement possible). Alkene + H₂O/H⁺ → Alcohol.
- Hydroboration-Oxidation: (Anti-Markovnikov addition, syn-addition, no rearrangement). Alkene + (i) B₂H₆/THF (ii) H₂O₂/OH⁻ → Alcohol.
- From Carbonyl Compounds:
- Reduction of Aldehydes and Ketones:
- Aldehyde + H₂/Pd (or NaBH₄ or LiAlH₄) → Primary Alcohol.
- Ketone + H₂/Pd (or NaBH₄ or LiAlH₄) → Secondary Alcohol.
- (Note: NaBH₄ doesn't reduce esters, carboxylic acids, C=C).
- Reduction of Carboxylic Acids and Esters:
- RCOOH + LiAlH₄/ether → RCH₂OH (Strong reducing agent needed).
- RCOOR' + LiAlH₄/ether → RCH₂OH + R'OH.
- Reduction of Aldehydes and Ketones:
- From Grignard Reagents (RMgX): (Very important for C-C bond formation)
- Formaldehyde (HCHO) + RMgX → (Addition product) --H₃O⁺→ Primary Alcohol (RCH₂OH).
- Other Aldehydes (R'CHO) + RMgX → (Addition product) --H₃O⁺→ Secondary Alcohol (R'CH(OH)R).
- Ketones (R'COR'') + RMgX → (Addition product) --H₃O⁺→ Tertiary Alcohol (R'C(OH)(R'')R).
- Esters (R'COOR'') + 2 RMgX → (Addition product) --H₃O⁺→ Tertiary Alcohol (R'C(OH)R₂) + R''OH (if R=R).
- From Alkenes:
- Phenols:
- From Haloarenes (Dow's Process): Chlorobenzene + (i) NaOH (623K, 320 atm) (ii) H⁺ → Phenol. (Requires drastic conditions). Electron withdrawing groups (like -NO₂) at o/p positions facilitate the reaction.
- From Benzene Sulphonic Acid: Benzene --Oleum→ Benzene sulphonic acid + (i) NaOH (fusion) (ii) H⁺ → Phenol.
- From Diazonium Salts: Aniline --NaNO₂ + HCl (273-278K)→ Benzene diazonium chloride --H₂O/Warm (or H⁺)→ Phenol + N₂ + HCl.
- From Cumene (Industrial Method): Cumene (Isopropylbenzene) --O₂ (air oxidation)→ Cumene hydroperoxide --H⁺/H₂O→ Phenol + Acetone (Propanone). (Economical).
- Ethers:
- Dehydration of Alcohols:
- Intermolecular (forms Ethers): 2 ROH --H₂SO₄ (conc.), 413K→ R-O-R + H₂O. (Suitable for primary alcohols only, forms symmetrical ethers. At higher temp (443K), elimination to alkene dominates). Mechanism: SN2.
- Intramolecular (forms Alkenes): ROH --H₂SO₄ (conc.), 443K→ Alkene + H₂O. Mechanism: E1/E2.
- Williamson Synthesis: (Very important, versatile lab method). Alkoxide (RONa) + Alkyl halide (R'X) → R-O-R' + NaX.
- Mechanism: SN2. Best results with primary alkyl halides. Secondary/Tertiary alkyl halides tend to undergo E2 elimination with strong base (alkoxide).
- Phenoxides (ArONa) can also be used. ArONa + RX → Ar-O-R.
- Cannot prepare diaryl ethers or ethers with tertiary alkyl groups easily using this if the tertiary group comes from the halide. (e.g., (CH₃)₃CONa + CH₃Br → (CH₃)₃COCH₃ is feasible, but CH₃ONa + (CH₃)₃CBr → Isobutene (elimination)).
- Dehydration of Alcohols:
6. Physical Properties
- Boiling Points:
- Alcohols and Phenols have much higher boiling points than corresponding hydrocarbons, ethers, and haloalkanes of comparable molecular mass due to intermolecular hydrogen bonding.
- Boiling point order: Alcohols/Phenols > Ethers > Hydrocarbons.
- Among isomeric alcohols, boiling point decreases with increased branching (decreased surface area, weaker van der Waals forces): 1° > 2° > 3°.
- Solubility:
- Lower alcohols and phenols are soluble in water due to their ability to form hydrogen bonds with water molecules.
- Solubility decreases as the size of the hydrophobic alkyl/aryl group increases.
- Ethers are slightly soluble in water (oxygen can accept H-bonds from water) but much less soluble than alcohols. Solubility comparable to alkanes of similar molar mass.
- Acidity:
- Acidity of Alcohols: Weakly acidic (weaker than water). Acidity order: H₂O > 1° ROH > 2° ROH > 3° ROH (due to +I effect of alkyl groups destabilizing the alkoxide ion). They react with active metals (Na, K, Al) to liberate H₂.
- Acidity of Phenols: Significantly more acidic than alcohols and water (pKa ≈ 10). This is due to:
- Resonance stabilization of the phenoxide ion (negative charge delocalized into the benzene ring).
- Polarization of O-H bond due to sp² hybridized carbon of the ring.
- Effect of Substituents on Phenol Acidity:
- Electron Withdrawing Groups (EWG) like -NO₂, -CN, -X (at o/p positions) increase acidity by stabilizing the phenoxide ion.
- Electron Donating Groups (EDG) like -CH₃, -OCH₃, -NH₂ decrease acidity by destabilizing the phenoxide ion.
- Order: Nitrophenols > Phenol > Cresols. p-Nitrophenol > m-Nitrophenol > o-Nitrophenol (due to intramolecular H-bonding in ortho) > Phenol.
- Ethers: Chemically quite inert due to the absence of active sites like the -OH group. Do not react with bases, active metals, reducing/oxidizing agents (under normal conditions).
7. Chemical Reactions
-
Reactions of Alcohols:
- Reactions involving cleavage of O-H bond (Acidity):
- Reaction with active metals: 2ROH + 2Na → 2RONa + H₂
- Esterification (with Carboxylic Acids, Acid Chlorides, Acid Anhydrides):
- ROH + R'COOH ⇌(H⁺) R'COOR + H₂O (Reversible, Fischer Esterification)
- ROH + R'COCl --Pyridine→ R'COOR + HCl
- ROH + (R'CO)₂O --H⁺ or Pyridine→ R'COOR + R'COOH
- Mechanism: Nucleophilic acyl substitution. Reactivity order: 1° > 2° > 3° alcohol.
- Reactions involving cleavage of C-O bond (Nucleophilic Substitution):
- Reaction with Hydrogen Halides (HX): ROH + HX → RX + H₂O.
- Reactivity order of ROH: 3° > 2° > 1°.
- Reactivity order of HX: HI > HBr > HCl.
- Mechanism: SN1 for 3° and 2° alcohols (via carbocation); SN2 for 1° alcohols.
- Lucas Test: Distinguishes 1°, 2°, 3° alcohols using Lucas Reagent (anhyd. ZnCl₂ + conc. HCl). Turbidity appears immediately (3°), after 5-10 min (2°), only on heating (1°).
- Reaction with Phosphorus Halides:
- 3ROH + PCl₃ → 3RCl + H₃PO₃
- ROH + PCl₅ → RCl + POCl₃ + HCl
- 3ROH + PX₃ (X=Br, I; P + X₂) → 3RX + H₃PO₃
- Reaction with Thionyl Chloride (SOCl₂): ROH + SOCl₂ --Pyridine→ RCl + SO₂↑ + HCl↑ (Darzen's process - preferred method for RCl as byproducts are gases).
- Dehydration (Elimination): Formation of Alkenes. Requires acid catalyst (conc. H₂SO₄ or H₃PO₄) and heat.
- Ease of dehydration: 3° > 2° > 1°.
- Mechanism: E1 (usually for 2°, 3°) or E2 (for 1°).
- Follows Saytzeff's Rule (more substituted alkene is the major product).
- Reaction with Hydrogen Halides (HX): ROH + HX → RX + H₂O.
- Oxidation: Product depends on the type of alcohol and oxidizing agent.
- Primary Alcohols:
- ROH --Strong Oxidizing Agent (KMnO₄/H⁺, K₂Cr₂O₇/H⁺)→ RCOOH (Carboxylic Acid).
- RCH₂OH --Mild Oxidizing Agent (PCC - Pyridinium Chlorochromate, CrO₃ in anhyd.)→ RCHO (Aldehyde).
- Secondary Alcohols:
- R₂CHOH --Any Oxidizing Agent (KMnO₄, K₂Cr₂O₇, CrO₃, PCC)→ R₂C=O (Ketone). Further oxidation requires drastic conditions (C-C bond cleavage).
- Tertiary Alcohols:
- R₃COH --No reaction with mild agents. Strong agents/heat → Mixture of carboxylic acids/ketones with fewer carbons (C-C cleavage).
- Primary Alcohols:
- Dehydrogenation: Passing alcohol vapors over heated Copper (Cu) at 573 K.
- 1° Alcohol → Aldehyde (+ H₂)
- 2° Alcohol → Ketone (+ H₂)
- 3° Alcohol → Alkene (Dehydration) (+ H₂O)
- Reactions involving cleavage of O-H bond (Acidity):
-
Reactions of Phenols:
- Reactions involving cleavage of O-H bond (Acidity):
- Reaction with active metals: 2ArOH + 2Na → 2ArONa + H₂
- Reaction with NaOH: ArOH + NaOH → ArONa + H₂O (Phenols are acidic enough to react with strong bases, unlike alcohols).
- Esterification: ArOH + (RCO)₂O / RCOCl --Pyridine/H⁺→ ArOCOR + RCOOH / HCl. (Requires stronger acylating agents than alcohols). Aspirin preparation: Salicylic acid + Acetic anhydride/H⁺ → Acetylsalicylic acid (Aspirin).
- Electrophilic Aromatic Substitution: -OH group is strongly activating and ortho-, para- directing.
- Nitration:
- Phenol + Dil. HNO₃ (298K) → Mixture of o-Nitrophenol and p-Nitrophenol (separable by steam distillation; o-isomer is steam volatile due to intramolecular H-bonding).
- Phenol + Conc. HNO₃ → 2,4,6-Trinitrophenol (Picric Acid). (Yield is low due to oxidation). Better prepared by sulphonating first, then nitrating.
- Halogenation:
- Phenol + Br₂ (in CS₂ or CCl₄, low temp) → Mixture of o-Bromophenol and p-Bromophenol (major).
- Phenol + Br₂ (aq) → 2,4,6-Tribromophenol (white precipitate). (Test for phenol).
- Kolbe's Reaction: Phenol --(i) NaOH→ Sodium Phenoxide --(ii) CO₂, 400K, 4-7 atm→ Sodium Salicylate --H⁺→ Salicylic Acid (2-Hydroxybenzoic acid).
- Reimer-Tiemann Reaction: Phenol --(i) CHCl₃ + aq. NaOH (340K)→ Intermediate --(ii) H⁺→ Salicylaldehyde (2-Hydroxybenzaldehyde). (Electrophile: Dichlorocarbene :CCl₂). If CCl₄ is used instead of CHCl₃, Salicylic acid is formed.
- Nitration:
- Reaction with Zinc Dust: Phenol + Zn (dust) --Heat→ Benzene + ZnO. (Reduction).
- Oxidation: Phenol --Air or Chromic acid (Na₂Cr₂O₇/H₂SO₄)→ Benzoquinone (conjugated diketone).
- Reactions involving cleavage of O-H bond (Acidity):
-
Reactions of Ethers:
- Cleavage of C-O bond by Hydrogen Halides (HX):
- R-O-R' + HX --Heat→ RX + R'OH → (if excess HX) → RX + R'X + H₂O.
- Reactivity order: HI > HBr >> HCl.
- Mechanism: SN1 or SN2 depending on the ether structure.
- Unsymmetrical Ethers: Halide ion (X⁻) attacks the smaller alkyl group if both are primary/secondary (SN2). If one group is tertiary, halide attacks the tertiary group (SN1 mechanism favored via stable 3° carbocation).
- Alkyl Aryl Ethers (e.g., Anisole): Cleavage occurs at Alkyl-Oxygen bond because Aryl-Oxygen bond is strong (partial double bond character). Products: Phenol + Alkyl halide. Ar-O-R + HX → Ar-OH + RX.
- Electrophilic Substitution in Aromatic Ethers (e.g., Anisole): Alkoxy group (-OR) is activating and ortho-, para- directing.
- Halogenation: Anisole + Br₂/Ethanoic acid → p-Bromoanisole (major) + o-Bromoanisole.
- Friedel-Crafts Reaction:
- Anisole + CH₃Cl/Anhyd. AlCl₃ → p-Methoxytoluene (major) + o-Methoxytoluene. (Alkylation)
- Anisole + CH₃COCl/Anhyd. AlCl₃ → p-Methoxyacetophenone (major) + o-Methoxyacetophenone. (Acylation)
- Nitration: Anisole + Conc. H₂SO₄ + Conc. HNO₃ → p-Nitroanisole (major) + o-Nitroanisole.
- Peroxide Formation: Ethers (especially diethyl ether) form explosive peroxides on standing in contact with air and light. Store in dark, airtight bottles.
- Cleavage of C-O bond by Hydrogen Halides (HX):
8. Uses
- Methanol: Solvent, antifreeze, formaldehyde production.
- Ethanol: Solvent, beverages, fuel (gasohol), antiseptic.
- Phenol: Antiseptic (dilute solution), disinfectant, production of polymers (Bakelite), drugs (aspirin, salol).
- Diethyl Ether: Anaesthetic (largely replaced), solvent for Grignard reagents, fats, oils.
Practice Multiple Choice Questions (MCQs)
-
Which of the following reagents is used to distinguish between primary, secondary, and tertiary alcohols?
(a) Fehling's solution
(b) Tollen's reagent
(c) Lucas reagent (Anhyd. ZnCl₂ + Conc. HCl)
(d) Schiff's reagent -
The reaction of phenol with chloroform in the presence of aqueous NaOH at 340 K, followed by acidification, yields:
(a) Salicylic acid
(b) Salicylaldehyde
(c) Anisole
(d) Benzoquinone -
Williamson synthesis is used for the preparation of:
(a) Alcohols
(b) Aldehydes
(c) Ethers
(d) Ketones -
Which of the following compounds is the most acidic?
(a) Ethanol
(b) Phenol
(c) p-Cresol (p-Methylphenol)
(d) p-Nitrophenol -
The product formed when ethanol is heated with concentrated H₂SO₄ at 443 K is:
(a) Ethoxyethane (Diethyl ether)
(b) Ethene
(c) Ethanal
(d) Ethanoic acid -
Identify the product 'Z' in the following reaction sequence:
Cumene --(i) O₂, (ii) H⁺/H₂O→ X + Y
X --Zn dust, Heat→ Z
(a) Benzene
(b) Toluene
(c) Phenol
(d) Acetone -
When anisole (methoxybenzene) is treated with HI, the products formed are:
(a) Iodobenzene and Methanol
(b) Phenol and Iodomethane
(c) Iodobenzene and Iodomethane
(d) Phenol and Methanol -
Primary alcohols can be oxidized to aldehydes using which of the following reagents?
(a) Acidified KMnO₄
(b) Acidified K₂Cr₂O₇
(c) Pyridinium Chlorochromate (PCC)
(d) Concentrated HNO₃ -
The IUPAC name for the compound CH₃-CH(OH)-CH₂-CH=CH₂ is:
(a) Pent-4-en-2-ol
(b) Pent-1-en-4-ol
(c) 4-Hydroxypent-1-ene
(d) 2-Hydroxypent-4-ene -
Which reaction condition is NOT suitable for preparing tert-butyl ethyl ether using Williamson synthesis?
(a) Sodium ethoxide + tert-butyl bromide
(b) Sodium tert-butoxide + ethyl bromide
(c) Ethanol + tert-butyl bromide + NaH
(d) tert-Butyl alcohol + ethyl bromide + NaH
Answers to MCQs:
- (c)
- (b)
- (c)
- (d) (Due to the electron-withdrawing -NO₂ group)
- (b) (Elimination dominates at higher temperature)
- (a) (X is Phenol, Y is Acetone. Phenol + Zn dust -> Benzene)
- (b) (Cleavage occurs at the alkyl-oxygen bond)
- (c) (PCC is a mild oxidizing agent specific for this conversion)
- (a) (Numbering starts from the end nearer to the double bond, but -OH gets priority if equidistant or gives lower locant set. Here, double bond gets priority for lowest number, -OH position is 2. So, Pent-4-en-2-ol) Correction: Priority goes to functional group (-OH) over double bond. Numbering starts from right. Pent-1-en-4-ol. Let me recheck IUPAC rules. Rechecking: Functional group (-OH) gets priority over double bond for lowest number. Numbering starts from right: CH₃(5)-CH(OH)(4)-CH₂(3)-CH(2)=CH₂(1). Name: Pent-1-en-4-ol. So the answer is (b).
- (a) (Tert-butyl bromide is a 3° halide and will undergo elimination with the strong base ethoxide)
Remember to thoroughly revise these concepts, especially the named reactions, reaction mechanisms, and the reasons behind acidity/boiling point trends. These are frequently tested areas. Good luck with your preparation!