Alcohols, Phenols and Ethers

Class 12 Chemistry Quick Revision Notes

Chapter: Alcohols, Phenols and Ethers

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Introduction & Classification

Alcohols, phenols, and ethers are organic compounds formed by the replacement of hydrogen atom(s) from hydrocarbons.

• Alcohols: Hydroxyl group (-OH) is attached to an aliphatic carbon (R-OH).
• Phenols: Hydroxyl group (-OH) is attached directly to an aromatic carbon (Ar-OH).
• Ethers: An oxygen atom is bonded to two alkyl or aryl groups (R-O-R' or R-O-Ar or Ar-O-Ar).

Classification based on -OH groups:

1. Monohydric: Contain one -OH group.
2. Dihydric: Contain two -OH groups (Glycols).
3. Trihydric: Contain three -OH groups (Glycerol).

Classification of Monohydric Alcohols:

• Primary ($1^\circ$): -OH attached to $1^\circ$ carbon.
• Secondary ($2^\circ$): -OH attached to $2^\circ$ carbon.
• Tertiary ($3^\circ$): -OH attached to $3^\circ$ carbon.
• Allylic & Benzylic Alcohols: -OH attached to allylic ($sp^3$ next to $C=C$) or benzylic ($sp^3$ next to benzene ring) carbon.

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Nomenclature (IUPAC System)

• Alcohols: Alkane - 'e' + ol $\rightarrow$ Alkanol (e.g., Methanol, Ethanol).
• Phenols: Hydroxybenzene (commonly known as Phenol).
• Ethers: Alkoxyalkane (e.g., Methoxyethane, CH_3-O-C_2H_5).

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Preparation of Alcohols

1. From Alkenes:

   • Acid-catalyzed hydration (Markovnikov's rule): CH_3-CH=CH_2 + H_2O \xrightarrow{H^+} CH_3-CH(OH)-CH_3
   • Hydroboration-Oxidation (Anti-Markovnikov's rule): 3R-CH=CH_2 + (BH_3)_2 \rightarrow (R-CH_2-CH_2)_3B \xrightarrow{H_2O_2 / OH^-} 3R-CH_2-CH_2-OH + H_3BO_3

2. From Carbonyl Compounds:

   • Reduction of Aldehydes & Ketones: Using reducing agents like $NaBH_4$, $LiAlH_4$, or catalytic hydrogenation.
     • Aldehydes $\rightarrow$ Primary ($1^\circ$) Alcohols
     • Ketones $\rightarrow$ Secondary ($2^\circ$) Alcohols
   • Reduction of Carboxylic Acids & Esters: Carboxylic acids are reduced to $1^\circ$ alcohols using $LiAlH_4$.
     • R-COOH \xrightarrow{1. LiAlH_4 / 2. H_2O} R-CH_2-OH

3. From Grignard Reagents ($RMgX$):

   • HCHO + $RMgX$ $\rightarrow$ $1^\circ$ Alcohol
   • Other Aldehydes + $RMgX$ $\rightarrow$ $2^\circ$ Alcohol
   • Ketones + $RMgX$ $\rightarrow$ $3^\circ$ Alcohol

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Preparation of Phenols

1. From Haloarenes (Dow's Process):
   • Chlorobenzene + $NaOH$ ($623K$, $300\text{ atm}$) $\rightarrow$ Sodium phenoxide $\xrightarrow{HCl}$ Phenol.
2. From Benzenesulphonic Acid:
   • Benzene $\xrightarrow{H_2SO_4} \text{Benzenesulphonic acid} \xrightarrow{NaOH, \Delta} \text{Sodium phenoxide} \xrightarrow{H^+} \text{Phenol}$.
3. From Diazonium Salts:
   • Aniline $\xrightarrow{NaNO_2 + HCl \text{ at } 273-278K} \text{Benzene diazonium chloride} \xrightarrow{H_2O, \warm} \text{Phenol}$.
4. From Cumene (Industrial Method):
   • Cumene (Isopropylbenzene) is oxidized in the presence of air to cumene hydroperoxide, which on treatment with dilute acid gives Phenol and Acetone (byproduct).

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Physical Properties

• Boiling Points: Alcohols and phenols have higher boiling points than hydrocarbons/ethers of comparable molecular mass due to Intermolecular Hydrogen Bonding.
• Solubility: Lower alcohols are soluble in water due to the formation of hydrogen bonds with water molecules. Solubility decreases with an increase in alkyl chain size (hydrophobic part).

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Chemical Reactions of Alcohols

Reactions involving cleavage of $O-H$ bond (Acidity of Alcohols & Phenols)

• Reaction with Metals: Alcohols and phenols react with active metals (like Na, K) to form alkoxides/phenoxides and hydrogen gas.
  • $2R-OH + 2Na \rightarrow 2R-ONa + H_2$
• Acidity Order:
  • Alcohols: $1^\circ > 2^\circ > 3^\circ$ (due to $+I$ effect of alkyl groups increasing electron density on oxygen).
  • Phenol is more acidic than water and alcohols because the phenoxide ion is stabilized by resonance.
  • Electron-withdrawing groups ($-\text{NO}_2$, $-\text{X}$) increase phenol acidity; Electron-donating groups ($-\text{CH}_3$, $-\text{OCH}_3$) decrease it.
• Esterification: Alcohols react with carboxylic acids, acid chlorides, or acid anhydrides in the presence of an acid catalyst to form esters.

Reactions involving cleavage of $C-O$ bond

• Reaction with Hydrogen Halides ($HX$): Alcohols react with $HX$ to form alkyl halides.
  • R-OH + HX \xrightarrow{ZnCl_2} R-X + H_2O (Lucas Test: $3^\circ$ reacts immediately, $2^\circ$ in 5 mins, $1^\circ$ very slowly).
• Reaction with Phosphorus Trihalides ($PX_3$):
  • 3R-OH + PX_3 \rightarrow 3R-X + H_3PO_3
• Dehydration: Alcohols undergo dehydration in the presence of acid catalysts ($H_2SO_4$, $H_3PO_4$) to form alkenes.
  • CH_3CH_2OH \xrightarrow{con. H_2SO_4, 443K} CH_2=CH_2 + H_2O
  • Ease of dehydration: $3^\circ > 2^\circ > 1^\circ$
• Oxidation: Oxidation of alcohols involves the formation of a $C=O$ bond with cleavage of an $O-H$ and $C-H$ bond.
  • $1^\circ$ Alcohol $\xrightarrow{\text{PCC / CrO}_3}$ Aldehyde $\rightarrow$ Carboxylic Acid
  • $2^\circ$ Alcohol $\xrightarrow{\text{CrO}_3 / \text{KMnO}_4}$ Ketone
  • $3^\circ$ Alcohols do not undergo oxidation easily (undergo elimination under drastic conditions).

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Chemical Reactions of Phenols

1. Electrophilic Aromatic Substitution: The -OH group in phenol is strongly activating and ortho/para-directing.
   • Halogenation:
     • With Br-water $\rightarrow$ 2,4,6-tribromophenol (white precipitate).
     • With Br in $CS_2$ (low temp) $\rightarrow$ Mixture of ortho and para bromophenol.
   • Nitration:
     • With dil. $HNO_3$ at low temp $\rightarrow$ ortho and para nitrophenol.
     • With conc. $HNO_3$ $\rightarrow$ 2,4,6-trinitrophenol (Picric acid).
2. Kolbe's Reaction: Phenate ion (formed by treating phenol with $NaOH$) undergoes electrophilic substitution with $CO_2$ (a weak electrophile) to give Salicylic Acid (2-hydroxybenzoic acid).
3. Reimer-Tiemann Reaction: Treating phenol with chloroform ($CHCl_3$) in the presence of aqueous $NaOH$ introduces an aldehyde group ($-\text{CHO}$) at the ortho position, forming Salicylaldehyde.
4. Reaction with Zinc Dust: Phenol is reduced to benzene on heating with zinc dust.
   • $\text{C}_6\text{H}_5\text{OH} + Zn \xrightarrow{\Delta} \text{C}_6\text{H}_6 + ZnO$
5. Oxidation: Oxidation of phenol with chromic acid ($\text{CrO}_3$) gives conjugated diketone known as benzoquinone.

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Ethers

Preparation of Ethers

1. Dehydration of Alcohols:
   • 2R-OH \xrightarrow{con. H_2SO_4, 413K} R-O-R + H_2O (Favoured at lower temperature for primary alcohols).
2. Williamson Synthesis (Most Important):
   • An alkyl halide is reacted with sodium alkoxide to give ether.
   • R-X + R'-ONa \rightarrow R-O-R' + NaX
   • Note: Best results are obtained when the alkyl halide is primary. If tertiary halide is used, alkene is formed as the major product due to elimination.

Physical Properties

• Miscibility: Miscibility of ethers with water resembles that of alcohols of the same molecular mass due to hydrogen bonding between ether oxygen and water.
• Boiling Point: Ethers have much lower boiling points than alcohols of comparable molecular masses due to the absence of intermolecular hydrogen bonding.

Chemical Reactions of Ethers

1. Cleavage of $C-O$ bond in Ethers: Ethers are relatively stable compounds. Cleavage of the $C-O$ bond takes place with excess of hydrogen halides ($HX$) at high temperatures.
   • R-O-R' + HX \rightarrow R-X + R'-OH
   • Order of reactivity of $HX$: $HI > HBr > HCl$.
   • Note: With mixed ethers containing one primary and one tertiary alkyl group, the halide is formed from the tertiary alkyl group (due to stability of $3^\circ$ carbocation).
2. Electrophilic Substitution (in Aromatic Ethers like Anisole):
   • The -OCH_3 group is ortho/para-directing and activates the benzene ring.
   • Halogenation: e.g., Bromination gives p-bromoanisole as major product.
   • Friedel-Crafts Reaction: Alkylation and acylation occur at ortho and para positions in the presence of anhydrous $AlCl_3$.
   • Nitration: Gives a mixture of ortho and para nitroanisole with conc. $HNO_3$ and $H_2SO_4$.