Chapter: Alcohols, Phenols, and Ethers – Detailed Notes for NEET/JEE Mains

1. Introduction and Classification

A. Alcohols

• Definition: Organic compounds containing one or more hydroxyl (−OH) groups attached to an alkyl group (aliphatic carbon chain).
• General Formula: R−OH

Classification of Alcohols:

1. Based on the number of -OH groups:
   • Monohydric alcohols: Contain one -OH group (e.g., ethanol, CH3​CH2​OH).
   • Dihydric alcohols (Glycols): Contain two -OH groups (e.g., ethylene glycol, HO-CH2​CH2​-OH).
   • Trihydric alcohols (Glycerols): Contain three -OH groups (e.g., glycerol, HO-CH2​CH(OH)CH2​-OH).
   • Polyhydric alcohols: Contain more than three -OH groups.
2. Based on the hybridization of carbon atom to which -OH group is attached:
   • Compounds containing sp3 C-OH bond:
     • Alkyl Alcohols: -OH group attached to an alkyl group.
       • Primary (1∘): -OH attached to a primary carbon atom (e.g., CH3​CH2​OH).
       • Secondary (2∘): -OH attached to a secondary carbon atom (e.g., CH3​CH(OH)CH3​).
       • Tertiary (3∘): -OH attached to a tertiary carbon atom (e.g., (CH3​)3​COH).
     • Allylic Alcohols: -OH group attached to an sp3 hybridized carbon atom next to a carbon-carbon double bond (e.g., CH2​=CH−CH2​OH).
     • Benzylic Alcohols: -OH group attached to an sp3 hybridized carbon atom next to an aromatic ring (e.g., C6​H5​CH2​OH).
   • Compounds containing sp2 C-OH bond:
     • Vinylic Alcohols (Enols): -OH group attached directly to an sp2 hybridized carbon atom of a carbon-carbon double bond (e.g., CH2​=CH−OH). These are generally unstable and tautomerize to aldehydes or ketones.

B. Phenols

• Definition: Organic compounds in which the hydroxyl (−OH) group is directly attached to an sp2 hybridized carbon atom of an aromatic ring.
• General Formula: Ar−OH
• Classification:
  • 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, phloroglucinol).

C. Ethers

• Definition: Organic compounds in which an oxygen atom is bonded to two alkyl or aryl groups.
• General Formula: R−O−R′ or Ar−O−Ar′ or R−O−Ar
• Classification:
  • Symmetrical (Simple) Ethers: Both alkyl/aryl groups attached to oxygen are identical (e.g., diethyl ether, CH3​CH2​−O−CH2​CH3​).
  • Unsymmetrical (Mixed) Ethers: The two alkyl/aryl groups attached to oxygen are different (e.g., ethyl methyl ether, CH3​−O−CH2​CH3​).

2. Nomenclature

A. Alcohols

• Common names: Alkyl group name followed by “alcohol” (e.g., CH3​OH is methyl alcohol).
• IUPAC names: “Alkanol.” The longest carbon chain containing the -OH group is selected, and the -e of alkane is replaced by -ol. Position of -OH group is indicated by number. (e.g., CH3​OH is methanol, CH3​CH2​OH is ethanol, CH3​CH(OH)CH3​ is propan-2-ol).

B. Phenols

• Common names: Derived from parent phenol (e.g., phenol, cresol).
• IUPAC names: Substituted phenols. The -OH group is assigned position 1. (e.g., 2-methylphenol (o-cresol), 3-chlorophenol).

C. Ethers

• Common names: Alkyl/aryl groups are named alphabetically followed by “ether” (e.g., CH3​OCH3​ is dimethyl ether, CH3​OCH2​CH3​ is ethyl methyl ether).
• IUPAC names: Named as “alkoxyalkanes” or “alkoxyarenes”. The larger alkyl group forms the parent alkane/arene, and the smaller alkyl-oxygen part is named as an “alkoxy” substituent (e.g., CH3​OCH3​ is methoxymethane, CH3​OCH2​CH3​ is methoxyethane).

3. Structure

• The oxygen atom in alcohols, phenols, and ethers is sp3 hybridized.
• The C-O-H bond angle in alcohols and phenols (e.g., 108.9∘ in methanol) and C-O-C bond angle in ethers (e.g., 111.7∘ in diethyl ether) are slightly less than the tetrahedral angle (109.5∘) due to the repulsion between lone pairs of electrons on oxygen.
• The C-O bond length in phenols is slightly shorter than in alcohols due to partial double bond character between carbon and oxygen (sp2 hybridized carbon of benzene ring and oxygen lone pair resonance) and also due to the sp2 hybridized carbon of phenol being smaller.

4. Methods of Preparation

A. Preparation of Alcohols

1. From Alkenes:
   • Acid-catalyzed hydration: Follows Markovnikov’s rule.
     • CH2​=CH2​+H2​OH+​CH3​CH2​OH
     • CH3​CH=CH2​+H2​OH+​CH3​CH(OH)CH3​ (Propan-2-ol, major)
   • Hydroboration-oxidation: Anti-Markovnikov addition of water.
     • CH3​CH=CH2​(i)BH3​/THF(ii)H2​O2​/OH−​CH3​CH2​CH2​OH (Propan-1-ol)
   • Oxymercuration-demercuration: Markovnikov addition of water.
     • CH3​CH=CH2​(i)Hg(OAc)2​,H2​O(ii)NaBH4​​CH3​CH(OH)CH3​
2. From Carbonyl Compounds (Aldehydes, Ketones, Carboxylic Acids, Esters):
   • Reduction:
     • Aldehydes give 1∘ alcohols: RCHOLiAlH4​ or NaBH4​ or H2​/Ni, Pt, Pd​RCH2​OH
     • Ketones give 2∘ alcohols: RCOR′LiAlH4​ or NaBH4​ or H2​/Ni, Pt, Pd​RCH(OH)R′
     • Carboxylic acids give 1∘ alcohols (strong reducing agents like LiAlH4​ needed): RCOOHLiAlH4​​RCH2​OH
     • Esters give 1∘ alcohols (via reduction): RCOOR′LiAlH4​​RCH2​OH+R’OH
   • Using Grignard Reagents (RMgX):
     • Formaldehyde (HCHO) gives 1∘ alcohols: HCHO+RMgX→adductH2​O​RCH2​OH
     • Aldehydes (other than formaldehyde) give 2∘ alcohols: R’CHO+RMgX→adductH2​O​R’CH(OH)R
     • Ketones give 3∘ alcohols: R’COR”+RMgX→adductH2​O​R’C(OH)R”R
3. From Alkyl Halides (Hydrolysis):
   • R-X+aq. KOH/NaOH→R-OH+KX (Nucleophilic substitution)

B. Preparation of Phenols

1. From Haloarenes (Dow’s Process):
   • C6​H5​Cl+NaOH623K,300atm​C6​H5​ONaH+​C6​H5​OH
2. From Benzene Sulphonic Acid:
   • Benzene sulphonic acid (i)NaOH, heat(ii)H+​Phenol
3. From Diazonium Salts: (Common laboratory method)
   • Benzene diazonium chloride H2​O, Warm​Phenol+N2​+HCl
4. From Cumene (Isopropylbenzene): (Industrial method)
   • Cumene (i)O2​,heat(ii)H+/H2​O​Phenol+Acetone

C. Preparation of Ethers

1. From Alcohols by Dehydration:
   • By heating excess alcohol with concentrated H2​SO4​ at 413 K (140∘C).
   • 2R-OHConc. H2​SO4​,413K​R-O-R+H2​O
   • This is an SN​2 reaction (nucleophilic attack by one alcohol molecule on a protonated alcohol molecule).
   • Limitations: Only effective for symmetrical ethers from 1∘ alcohols. For 2∘ or 3∘ alcohols, or at higher temperatures (443 K/170∘C), elimination (alkene formation) predominates.
   • For unsymmetrical ethers, it gives a mixture of products.
2. Williamson Synthesis: (Best method for preparing symmetrical and unsymmetrical ethers)
   • Reaction of an alkyl halide with sodium alkoxide.
   • R-X+NaOR′→R-O-R′+NaX
   • Mechanism: SN​2 reaction.
   • Important Note: For unsymmetrical ethers, a primary alkyl halide must be used, and a tertiary alkoxide can be used. If a tertiary alkyl halide is used with a primary alkoxide, elimination predominates (E2 reaction).
     • Correct: CH3​Br+(CH3​)3​C-ONa→CH3​−O−C(CH3​)3​ (tert-butyl methyl ether)
     • Incorrect (gives alkene): (CH3​)3​C-Br+CH3​ONa→CH2​=C(CH3​)2​ (major, elimination)

5. Physical Properties

A. Alcohols

• Boiling Points: Higher than corresponding alkanes, haloalkanes, and ethers of comparable molecular mass. This is due to the presence of intermolecular hydrogen bonding between hydroxyl groups. Boiling point increases with increasing number of -OH groups.
• Solubility: Lower alcohols (up to 3 carbon atoms) are miscible with water due to their ability to form hydrogen bonds with water molecules. Solubility decreases with increasing alkyl chain length (hydrophobic part increases).

B. Phenols

• Boiling Points: Higher than corresponding hydrocarbons and haloarenes due to intermolecular hydrogen bonding.
• Solubility: Sparingly soluble in water, but more soluble than alcohols of comparable molecular mass due to their ability to form stronger hydrogen bonds with water. Also soluble in organic solvents.
• Acidity: Phenols are more acidic than alcohols but less acidic than carboxylic acids. This is due to the resonance stabilization of the phenoxide ion (negative charge delocalized over the benzene ring). Electron-withdrawing groups (e.g., −NO2​) increase acidity, while electron-donating groups (e.g., −CH3​) decrease acidity.

C. Ethers

• Boiling Points: Much lower than alcohols of comparable molecular mass (due to the absence of hydrogen bonding) but comparable to alkanes or haloalkanes of similar molecular mass.
• Solubility: Slightly soluble in water (can form H-bonds with water through oxygen’s lone pairs, but no −OH group to donate H-bond). Solubility decreases with increasing size of alkyl groups. Readily soluble in organic solvents.
• Polarity: Ethers have a net dipole moment due to the bent structure and polar C-O bonds.

6. Chemical Reactions

A. Reactions of Alcohols

1. Reactions involving cleavage of O-H bond (Acidic nature):
   • Reaction with active metals: Alcohols are weakly acidic.
     • 2R-OH+2Na→2R-ONa (Sodium alkoxide)+H2​↑
   • Esterification: Reaction with carboxylic acids or acid derivatives to form esters.
     • R-OH+R’COOHConc. H2​SO4​ (catalyst)​R’COOR (Ester)+H2​O
   • Acidity Order: 1∘>2∘>3∘ (due to +I effect of alkyl groups increasing electron density on oxygen, making O-H bond less polar). Methanol is slightly more acidic than ethanol.
2. Reactions involving cleavage of C-O bond:
   • Reaction with Hydrogen halides (HX): (already covered in preparation of haloalkanes)
     • R-OH+HX→R-X+H2​O (Reactivity: HI > HBr > HCl; Alcohol reactivity: 3∘>2∘>1∘)
   • Reaction with Phosphorus halides (PX3​,PX5​): (already covered)
   • Dehydration to Alkenes:
     • R-CH2​−CH2​OHConc. H2​SO4​,443K​R-CH=CH2​+H2​O (Elimination)
     • Reactivity: 3∘>2∘>1∘ (due to stability of carbocation/alkene).
   • Dehydration to Ethers: (already covered in preparation of ethers)
     • 2R-OHConc. H2​SO4​,413K​R-O-R+H2​O
3. Oxidation Reactions:
   • Primary alcohols: Oxidize to aldehydes, then to carboxylic acids.
     • RCH2​OHPCC (Pyridinium Chlorochromate)​RCHO (aldehyde) (mild oxidant, stops at aldehyde)
     • RCH2​OHKMnO4​ or K2​Cr2​O7​/H+​RCOOH (carboxylic acid) (strong oxidant)
   • Secondary alcohols: Oxidize to ketones.
     • RCH(OH)R′PCC or K2​Cr2​O7​/H+​RCOR′ (ketone)
   • Tertiary alcohols: Do not undergo oxidation under normal conditions (no H on the carbon bearing -OH). Require drastic conditions (strong oxidizing agents, high temperature) to undergo C-C bond cleavage, yielding a mixture of carboxylic acids with fewer carbon atoms.
4. Dehydrogenation: (Using Cu at 573 K)
   • 1∘ alcohol Cu, 573K​Aldehyde
   • 2∘ alcohol Cu, 573K​Ketone
   • 3∘ alcohol Cu, 573K​Alkene (undergoes dehydration)

B. Reactions of Phenols

1. Acidic Nature: Phenols are acidic due to the resonance stabilization of the phenoxide ion formed after losing a proton.
   • Phenols are stronger acids than alcohols but weaker than carboxylic acids.
   • They react with active metals and strong bases (NaOH), but generally not with weaker bases like NaHCO3​.
   • Effect of substituents on acidity:
     • Electron-withdrawing groups (EWG, like −NO2​, −CN, −COOH, halogens) at ortho and para positions increase acidity by delocalizing the negative charge of the phenoxide ion. p-Nitrophenol is more acidic than o-nitrophenol due to lesser steric inhibition of resonance.
     • Electron-donating groups (EDG, like −CH3​, −OCH3​) decrease acidity by intensifying the negative charge on the phenoxide ion.
2. Electrophilic Aromatic Substitution Reactions: -OH group is a strong activating and ortho-para directing group due to resonance (lone pair on oxygen donates electron density to the ring).
   • Nitration:
     • With dilute HNO3​: Phenol Dil. HNO3​,298K​o-nitrophenol (steam volatile)+p-nitrophenol (higher boiling)
     • With concentrated HNO3​: Phenol Conc. HNO3​,Conc. H2​SO4​​2,4,6-trinitrophenol (Picric acid)
   • Halogenation (Bromination):
     • With Br2​/CS2​ (non-polar solvent, low temperature): Forms mono-substituted product (o- and p-bromophenol).
     • With Br2​/H2​O (polar solvent, phenol ionizes to phenoxide ion, which is highly activated): Forms 2,4,6-tribromophenol (white precipitate).
       • C6​H5​OH+3Br2​H2​O​2,4,6-tribromophenol (white ppt)
   • Sulphonation:
     • Phenol Conc. H2​SO4​,288K​o-Phenol sulphonic acid
     • Phenol Conc. H2​SO4​,373K​p-Phenol sulphonic acid
   • Kolbe’s Reaction (Kolbe-Schmidt Reaction): For salicylic acid preparation.
     • Sodium phenoxide reacts with CO2​ under pressure (4-7 atm) at 398 K, followed by acid hydrolysis.
     • Sodium phenoxide (i)CO2​,398K,4−7atm(ii)H+​Salicylic acid (o-hydroxybenzoic acid)
   • Reimer-Tiemann Reaction: For salicylaldehyde preparation.
     • Phenol reacts with chloroform (CHCl3​) in the presence of NaOH(aq) at 340 K, followed by acid hydrolysis.
     • Phenol (i)CHCl3​/NaOH(aq),340K(ii)H+​Salicylaldehyde (o-hydroxybenzaldehyde)
   • Reaction with Zinc Dust:
     • Phenol Zn dust, heat​Benzene+ZnO
   • Oxidation:
     • Phenol Na2​Cr2​O7​/H2​SO4​​Benzoquinone (conjugated diketone)

C. Reactions of Ethers

• Ethers are relatively unreactive due to the stable C-O-C bond.

1. Cleavage of C-O bond by Hot Concentrated HX:
   • Reactivity of HX: HI > HBr > HCl.
   • Excess HX: Both alkyl groups are converted to alkyl halides.
     • R-O-R′+HX→R-X+R’-OH
     • R’-OH+HX→R’-X+H2​O
     • Overall: R-O-R′+2HX→R-X+R’-X+H2​O
   • Mechanism:
     • For 1∘/2∘ alkyl groups: SN​2 mechanism. The smaller alkyl group forms the halide, and the larger forms alcohol (due to steric hindrance).
       • Example: CH3​OCH2​CH3​+HI→CH3​I+CH3​CH2​OH
     • For 3∘/Benzylic/Allylic alkyl groups: SN​1 mechanism. The 3∘/Benzylic/Allylic group forms the halide (due to stable carbocation formation).
       • Example: (CH3​)3​C-O-CH3​+HI→(CH3​)3​C-I+CH3​OH
   • Cleavage of Alkyl aryl ethers: The alkyl-oxygen bond breaks, forming phenol and alkyl halide, as the aryl-oxygen bond is stronger due to resonance.
     • Ar-O-R+HX→Ar-OH+R-X
2. Electrophilic Substitution in Aromatic Ethers: The alkoxy group (−OR) is an activating and ortho-para directing group (due to resonance).
   • Halogenation (e.g., Bromination of Anisole):
     • C6​H5​OCH3​+Br2​Ethanoic acid​p-bromoanisole (major)+o-bromoanisole (minor)
   • Nitration:
     • C6​H5​OCH3​Conc. HNO3​/Conc. H2​SO4​​o-nitroanisole+p-nitroanisole
   • Friedel-Crafts Reactions (Alkylation/Acylation):
     • C6​H5​OCH3​+CH3​ClAnhy. AlCl3​​o-methoxytoluene+p-methoxytoluene

7. Important Commercial Compounds

A. Methanol (CH3​OH / Wood Spirit)

• Preparation: From synthesis gas (CO+H2​) using catalysts.
  • \text{CO} + \text{2H}_2 \xrightarrow{\text{ZnO-Cr}_2\text{O}_3, 573-673K, 200-300 atm}} \text{CH}_3\text{OH}
• Uses: Solvent for paints, varnishes, in formaldehyde and other chemical synthesis, antifreeze.
• Harmful effects: Highly poisonous. Can cause blindness and death even in small quantities.

B. Ethanol (CH3​CH2​OH / Grain Alcohol)

• Preparation:
  • From ethene (hydration): Industrial method.
  • By fermentation of molasses/starch: Industrial method.
• Uses: Alcoholic beverages, solvent, in the synthesis of many organic compounds, fuel (gasohol).
• Denatured alcohol: Ethanol mixed with small amounts of copper sulphate (color) and pyridine (foul smell) to make it unfit for drinking.
• Harmful effects: Consumption leads to liver damage, nervous system problems. Acts as a depressant.

C. Diethyl Ether (CH3​CH2​OCH2​CH3​)

• Preparation: Williamson synthesis, dehydration of ethanol.
• Uses: Solvent for fats, oils, resins, etc., anesthetic (historically).
• Storage: Stored in dark brown bottles to prevent peroxide formation (explosive) on exposure to light and air.