Chemistry: Class 12 — Chapter 2: Alcohols, Phenols and Ethers

• Based on the number of hydroxyl groups:
  • Monohydric: Contains one $-OH$ group (e.g., $C_2H_5OH$).
  • Dihydric: Contains two $-OH$ groups (e.g., ethylene glycol, $CH_2(OH)-CH_2(OH)$).
  • Trihydric: Contains three $-OH$ groups (e.g., glycerol, $CH_2(OH)-CH(OH)-CH_2(OH)$).
• Classification of Monohydric Alcohols (based on hybridization of carbon):
  • Compounds containing $sp^3$ C-OH bond:
    • Primary (1°), Secondary (2°), and Tertiary (3°) alcohols depending on whether the $-OH$ group is attached to a primary, secondary, or tertiary carbon.
    • Allylic alcohols: $-OH$ group is attached to an $sp^3$ hybridized carbon adjacent to a carbon-carbon double bond (allylic carbon).
    • Benzylic alcohols: $-OH$ group is attached to an $sp^3$ hybridized carbon atom next to an aromatic ring.
  • Compounds containing $sp^2$ C-OH bond:
    • Vinylic alcohols: $-OH$ group attached directly to a carbon-carbon double bond (e.g., $CH_2=CH-OH$).
    • Phenols: $-OH$ group attached directly to an $sp^2$ hybridized carbon of an aromatic ring.

• Simple or Symmetrical Ethers: Alkyl or aryl groups attached to the oxygen atom are the same (e.g., diethyl ether, $C_2H_5OC_2H_5$).
• Mixed or Unsymmetrical Ethers: Alkyl or aryl groups attached to the oxygen atom are different (e.g., ethylmethyl ether, $C_2H_5OCH_3$).

• Alcohols: Common names are derived from the alkyl group followed by the word "alcohol". IUPAC names are derived by replacing the ending '-e' of the parent alkane with the suffix '-ol'.
• Phenols: The simplest hydroxy derivative of benzene is phenol. Substituted phenols use prefixes like ortho (1,2-), meta (1,3-), and para (1,4-) in common names, and numbers in IUPAC names.
• Ethers: Common names list the alkyl/aryl groups in alphabetical order followed by "ether". IUPAC names designate the smaller alkyl group as an alkoxy substituent (e.g., methoxymethane).

In alcohols, the oxygen of the $-OH$ group is attached to carbon by a $\sigma$ bond formed by the overlap of an $sp^3$ hybridized orbital of carbon with an $sp^3$ hybridized orbital of oxygen.

• The $C-O-H$ bond angle in methanol is $108.9^\circ$, slightly less than the tetrahedral angle ($109^\circ 28'$) due to repulsion between unshared electron pairs of oxygen.
• In phenol, the $C-O$ bond length ($136\text{ pm}$) is shorter than in methanol ($142\text{ pm}$) due to partial double bond character from conjugation of oxygen's lone pair with the aromatic ring, and the $sp^2$ hybridized state of carbon.
• In ethers, the $C-O-H$ bond angle is slightly greater than the tetrahedral angle (e.g., $111.7^\circ$ in methoxymethane) due to repulsive interactions between bulky alkyl groups.

1. From Alkenes:
   • Acid catalysed hydration: Alkenes react with water in the presence of an acid catalyst to form alcohols following Markovnikov's rule.

     $$R-CH=CH_2 + H_2O \xrightarrow{H^+} R-CH(OH)-CH_3$$

   • Hydroboration-Oxidation: Diborane reacts with alkenes to yield trialkylboranes, which are oxidised by alkaline hydrogen peroxide to give anti-Markovnikov alcohols.

     $$3R-CH=CH_2 + (BH_3)_2 \rightarrow (R-CH_2-CH_2)_3B \xrightarrow{H_2O_2, OH^-} 3R-CH_2-CH_2-OH$$

2. From Carbonyl Compounds:
   • Reduction of Aldehydes and Ketones: Catalytic hydrogenation (using $Pd$, $Pt$, or $Ni$) or reduction with $NaBH_4$ or $LiAlH_4$ yields primary alcohols from aldehydes and secondary alcohols from ketones.
   • Reduction of Carboxylic Acids and Esters: Carboxylic acids are reduced to primary alcohols using the strong reducing agent $LiAlH_4$. Commercially, acids are converted to esters and then hydrogenated.
3. From Grignard Reagents: Nucleophilic addition of Grignard reagents ($R-MgX$) to carbonyl compounds followed by hydrolysis yields alcohols.
   • Methanal yields primary alcohols.
   • Other aldehydes yield secondary alcohols.
   • Ketones yield tertiary alcohols.

1. From Haloarenes (Dow's Process): Chlorobenzene is fused with $NaOH$ at $623\text{ K}$ and $320\text{ atm}$ to form sodium phenoxide, which yields phenol upon acidification.
2. From Benzenesulphonic Acid: Benzene is sulphonated with oleum, and the resulting benzenesulphonic acid is fused with molten $NaOH$ and acidified.
3. From Diazonium Salts: Aniline is treated with nitrous acid ($NaNO_2 + HCl$) at $273-278\text{ K}$ to form benzene diazonium chloride, which is hydrolysed by warming with water.
4. From Cumene (Industrial Method): Cumene (isopropylbenzene) is oxidised by air to cumene hydroperoxide, which is converted to phenol and acetone (valuable by-product) using dilute acid.

• Boiling Points: Alcohols and phenols have higher boiling points than hydrocarbons, ethers, and haloalkanes of comparable molecular masses due to intermolecular hydrogen bonding. Boiling points increase with carbon chain length and decrease with branching.
• Solubility: Lower alcohols are highly 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.

• Acidity of Alcohols and Phenols:
  • Alcohols and phenols react with active metals (like $Na$, $K$, $Al$) to release hydrogen gas and form alkoxides/phenoxides.
  • Phenols are stronger acids than alcohols because the phenoxide ion is resonance-stabilized, whereas the alkoxide ion is not.
  • Electron-withdrawing groups (like $-NO_2$) increase the acidity of phenols, especially at ortho and para positions. Electron-releasing groups (like alkyl groups) decrease acidity.
• Esterification: Alcohols and phenols react with carboxylic acids, acid anhydrides, and acid chlorides to form esters. Acetylation of salicylic acid produces aspirin.
• Reactions involving C-O bond cleavage in Alcohols:
  • Reaction with HX: Alcohols react with hydrogen halides to form alkyl halides. The Lucas test (conc. $HCl$ and $ZnCl_2$) distinguishes 1°, 2°, and 3° alcohols based on turbidity formation rates.
  • Dehydration: Alcohols undergo dehydration to form alkenes when treated with protic acids (like conc. $H_2SO_4$ or $H_3PO_4$). Ease of dehydration order: $3^\circ > 2^\circ > 1^\circ$.
  • Oxidation: Primary alcohols are oxidised to aldehydes (using PCC) and further to carboxylic acids (using strong oxidants like $KMnO_4$). Secondary alcohols yield ketones (using $CrO_3$). Tertiary alcohols resist oxidation.
• Reactions of Phenols:
  • Electrophilic Aromatic Substitution:
    • Nitration: Dilute $HNO_3$ at $298\text{ K}$ yields a mixture of o- and p-nitrophenols. Concentrated $HNO_3$ yields picric acid (2,4,6-trinitrophenol).
    • Halogenation: Phenol reacts with $Br_2$ in $CS_2$ at low temperature to give mono-bromophenols, and with bromine water to give a white precipitate of 2,4,6-tribromophenol.
  • Kolbe's Reaction: Phenoxide ion reacts with $CO_2$ followed by acidification to yield salicylic acid (2-hydroxybenzoic acid).
  • Reimer-Tiemann Reaction: Phenol reacts with chloroform ($CHCl_3$) in the presence of $NaOH$ to introduce a $-CHO$ group at the ortho position, yielding salicylaldehyde.
  • Reaction with Zinc Dust: Phenol is reduced to benzene when heated with zinc dust.
  • Oxidation: Oxidation of phenol with chromic acid produces benzoquinone.

1. Dehydration of Alcohols: Dehydration of ethanol at $413\text{ K}$ in the presence of protic acids yields ethoxyethane. (At $443\text{ K}$, ethene is the major product).
2. Williamson Synthesis: An alkyl halide reacts with a sodium alkoxide via an $S_N2$ mechanism. Best yields are obtained with primary alkyl halides; tertiary halides undergo elimination to yield alkenes instead of ethers.

• Cleavage of C-O bond: Ethers are cleaved by strong hydrogen halides ($HX$, especially $HI$ or $HBr$) under drastic conditions.

  $$R-O-R' + HX \rightarrow R-X + R'-OH$$

  If one of the alkyl groups is tertiary, the reaction proceeds via an $S_N1$ mechanism to yield a tertiary halide.
• Electrophilic Substitution: The alkoxy group ($-OR$) is ortho, para-directing and activates the aromatic ring. Ethers like anisole undergo halogenation, Friedel-Crafts alkylation/acylation, and nitration.