Oxygen Derivatives: Alcohols, Phenols, Ethers, Carbonyls & Carboxylic Acids

1. Alcohols & Phenols: Classification & C-O/O-H Polarity

• Classification Framework:
  • Alcohols ($sp^3\text{ C-OH}$): Primary ($1^\circ$), secondary ($2^\circ$), and tertiary ($3^\circ$) based on alpha-carbon substitution. Allylic ($\text{CH}_2=\text{CH}-\text{CH}_2\text{OH}$) and benzylic ($\text{Ar}-\text{CH}_2\text{OH}$) alcohols can be $1^\circ, 2^\circ$, or $3^\circ$.
  • Vinylic Alcohols ($sp^2\text{ C-OH}$): Hydroxyl group directly attached to an alkene carbon ($\text{CH}_2=\text{CH}-\text{OH}$).
  • Phenols ($sp^2\text{ C-OH}$): Hydroxyl group bonded directly to an aromatic ring.
• Bond Geometries & Physical Constants:
  • Methanol $\text{C}-\text{O}-\text{H}$ angle is $108.9^\circ$ (compressed from $109.5^\circ$ due to oxygen lone-pair repulsions).
  • Phenol $\text{C}-\text{O}$ bond length is $136\text{ pm}$ versus $142\text{ pm}$ in methanol, shortened by resonance delocalization of oxygen lone pairs and $sp^2$ carbon hybridization.
  • Boiling points follow: $\text{Carboxylic Acids} > \text{Alcohols} > \text{Aldehydes/Ketones} > \text{Ethers} > \text{Alkanes}$.
  • Boiling points decrease with branching due to spherical surface minimization: $\text{Butan-1-ol}\ (390\text{ K}) > \text{Butan-2-ol}\ (373\text{ K}) > \text{2-Methylpropan-2-ol}\ (356\text{ K})$.

2. Preparative Routes & Lucas Distinction

• Hydration Pathways of Alkenes:
  • Acid-Catalyzed Hydration: Reversible Markovnikov addition via free carbocation intermediate susceptible to rearrangements.
  • Hydroboration-Oxidation: Syn-addition of diborane followed by alkaline $\text{H}_2\text{O}_2$ yields net anti-Markovnikov hydration product without rearrangement:

$$3\text{R-CH}=\text{CH}_2 + \frac{1}{2}(\text{BH}_3)_2 \longrightarrow (\text{R-CH}_2-\text{CH}_2)_3\text{B} \xrightarrow{\text{H}_2\text{O}_2,\ \text{OH}^-} 3\text{R-CH}_2-\text{CH}_2\text{OH} + \text{B(OH)}_3$$

• Reduction of Carbonyl Compounds:
  • $\text{NaBH}_4$ reduces aldehydes to $1^\circ$ alcohols and ketones to $2^\circ$ alcohols, leaving esters and acids unaffected.
  • $\text{LiAlH}_4$ reduces aldehydes, ketones, esters, and carboxylic acids to alcohols.
• Grignard Addition to Carbonyls:
  • $\text{HCHO} + \text{RMgX} \xrightarrow{\text{H}_3\text{O}^+} 1^\circ\text{ Alcohol}$
  • $\text{R'CHO} + \text{RMgX} \xrightarrow{\text{H}_3\text{O}^+} 2^\circ\text{ Alcohol}$
  • $\text{R'COR''} + \text{RMgX} \xrightarrow{\text{H}_3\text{O}^+} 3^\circ\text{ Alcohol}$
• Lucas Distinction Reagent ($\text{conc. HCl} + \text{anhyd. ZnCl}_2$):

$$\text{R-OH} + \text{HCl} \xrightarrow{\text{ZnCl}_2} \text{R-Cl}\downarrow (\text{Turbidity}) + \text{H}_2\text{O}$$

• $3^\circ$ Alcohols: Turbidity appears immediately.
• $2^\circ$ Alcohols: Turbidity appears within 5 minutes.
• $1^\circ$ Alcohols: Solution remains clear at room temperature; requires heating.

3. Phenols: Acidity, Kolbe & Reimer-Tiemann

• Acidity Hierarchy:
  • Electron-withdrawing groups ($-M, -I$) stabilize the phenoxide anion and increase acidity ($o/p\text{-NO}_2 > m\text{-NO}_2 > \text{Phenol} > \text{Cresols}$).
  • $\text{p}K_a\text{ metrics: 2,4,6-trinitrophenol (picric acid, 0.38)} < 4\text{-nitrophenol (7.1)} < 2\text{-nitrophenol (7.2)} < \text{phenol (10.0)} < \text{ethanol (15.9)}$.

• Industrial Synthesis from Cumene:
  • Cumene (isopropylbenzene) is oxidized with air to cumene hydroperoxide, then cleaved with dilute $\text{H}_2\text{SO}_4$ to yield phenol and acetone.
• Kolbe Reaction: Sodium phenoxide reacts with $\text{CO}_2$ at 400 K and 4-7 atm followed by acidification to produce salicylic acid (2-hydroxybenzoic acid).
• Reimer-Tiemann Reaction: Phenol reacts with $\text{CHCl}_3$ and aqueous $\text{NaOH}$ through a dichlorocarbene intermediate ($:\text{CCl}_2$) to form salicylaldehyde.

4. Ethers: Williamson Synthesis & Acidic Cleavage

• Williamson Ether Synthesis:

$$\text{R-X} + \text{R'-O}^-\text{Na}^+ \longrightarrow \text{R-O-R'} + \text{NaX}$$

• Alkyl halide must be primary ($1^\circ$). Secondary and tertiary halides undergo E2 elimination to yield alkenes as the major or exclusive product.
• Acidic Cleavage with Concentrated Hydrogen Halides ($\text{HI} > \text{HBr}$):
  • Primary / Secondary Alkyl Groups ( $\text{S}_\text{N}2$ ): Iodide attacks the less hindered alkyl carbon to yield lower alkyl iodide and higher alcohol:

$$\text{CH}_3\text{-O-CH(CH}_3)_2 + \text{HI} \longrightarrow \text{CH}_3\text{I} + (\text{CH}_3)_2\text{CHOH}$$

• Tertiary Alkyl Group Present ( $\text{S}_\text{N}1$ ): Cleavage proceeds via a stable tertiary carbocation to yield tertiary iodide:

$$(\text{CH}_3)_3\text{C-O-CH}_3 + \text{HI} \longrightarrow (\text{CH}_3)_3\text{C-I} + \text{CH}_3\text{OH}$$

• Alkyl Aryl Ethers (Anisole): Protonation forms an oxonium ion; iodide attacks the methyl carbon because the $sp^2$ phenyl-oxygen bond has partial double bond character and resists attack:

$$\text{C}_6\text{H}_5\text{-O-CH}_3 + \text{HI} \longrightarrow \text{C}_6\text{H}_5\text{OH} + \text{CH}_3\text{I}$$

5. Carbonyls: Nucleophilic Addition Dynamics

• Planar Carbonyl Geometry: The carbonyl carbon is $sp^2$-hybridized, planar, and electrophilic due to polarization ($\text{C}^{\delta+} = \text{O}^{\delta-}$). Nucleophiles attack perpendicular to the plane at approximately $107^\circ$ (Bürgi-Dunitz angle).

• Addition Reagents and Products:
  • HCN: Generates cyanohydrins; base catalyst required to generate strong $\text{CN}^-$ nucleophile.
  • $\text{NaHSO}_3$: Bisulphite addition compound (crystalline solid used for aldehyde separation and purification).
  • Alcohols: Aldehydes react with $1\text{ eq. ROH} / \text{dry HCl}$ to form hemiacetals, and $2\text{ eq. ROH}$ to yield acetals. Ketones react with ethylene glycol to form cyclic ketals.
  • Ammonia Derivatives ($\text{H}_2\text{N-Z}$):
    • Hydroxylamine ($\text{NH}_2\text{OH}$) $\longrightarrow$ Oxime
    • Hydrazine ($\text{NH}_2\text{NH}_2$) $\longrightarrow$ Hydrazone
    • 2,4-Dinitrophenylhydrazine (2,4-DNP) $\longrightarrow$ 2,4-DNP hydrazone (orange-red crystalline precipitate)
    • Semicarbazide ($\text{H}_2\text{NNHCONH}_2$) $\longrightarrow$ Semicarbazone

6. Condensation Pathways: Aldol vs Cannizzaro

• Aldol Condensation ($\ge 1\ \alpha\text{-Hydrogen}$):
  • Driven by resonance stabilization of the conjugate enolate carbanion in dilute alkali:

$$2\text{CH}_3\text{CHO} \xrightarrow{\text{dil. NaOH}} \text{CH}_3-\text{CH(OH)}-\text{CH}_2-\text{CHO} \xrightarrow[\Delta]{-\text{H}_2\text{O}} \text{CH}_3-\text{CH}=\text{CH}-\text{CHO}\quad (\text{But-2-enal})$$

• Cannizzaro Reaction (No $\alpha\text{-Hydrogens}$):
  • Disproportionation of aldehydes lacking $\alpha$-hydrogens ($\text{HCHO}, \text{C}_6\text{H}_5\text{CHO}, (\text{CH}_3)_3\text{C-CHO}$) in concentrated base:

$$2\text{C}_6\text{H}_5\text{CHO} + \text{conc. NaOH} \longrightarrow \text{C}_6\text{H}_5\text{CH}_2\text{OH}\ (\text{Benzyl alcohol}) + \text{C}_6\text{H}_5\text{COONa}\ (\text{Sodium benzoate})$$

• Oxidation Tests to Distinguish Aldehydes & Ketones:
  • Tollens' Test: Aldehydes reduce ammoniacal silver nitrate to form a silver mirror:

$$\text{R-CHO} + 2[\text{Ag(NH}_3)_2]^+ + 3\text{OH}^- \longrightarrow \text{R-COO}^- + 2\text{Ag}\downarrow + 4\text{NH}_3 + 2\text{H}_2\text{O}$$

• Fehling's Test: Aliphatic aldehydes reduce alkaline cupric bistartrate to a red-brown cuprous oxide precipitate ($\text{Cu}_2\text{O}$); aromatic aldehydes do not react.
• Iodoform Reaction: Methyl ketones ($\text{RCOCH}_3$) and secondary methyl carbinols react with $\text{I}_2 / \text{NaOH}$ to yield a canary-yellow precipitate of iodoform ($\text{CHI}_3$).

7. Carboxylic Acids: Acidity Hierarchy & HVZ Reaction

• Resonance Stabilization of Carboxylate: The conjugate base is stabilized by two equivalent resonance structures placing negative charge exclusively on electronegative oxygens.
• Substituent Effects on Acidity:
  • Electron-withdrawing groups ($-I, -M$) disperse negative charge and increase acidity.
  • Decreasing acidity order:

$$\text{CF}_3\text{COOH} > \text{CCl}_3\text{COOH} > \text{CHCl}_2\text{COOH} > \text{NO}_2\text{CH}_2\text{COOH} > \text{FCH}_2\text{COOH} > \text{ClCH}_2\text{COOH} > \text{HCOOH} > \text{CH}_3\text{COOH}$$

• Hell-Volhard-Zelinsky (HVZ) Reaction: Carboxylic acids with $\alpha$-hydrogens react with chlorine or bromine in the presence of red phosphorus to form $\alpha$-halocarboxylic acids:

$$\text{R-CH}_2-\text{COOH} \xrightarrow[\text{(ii) }\text{H}_2\text{O}]{\text{(i) }\text{X}_2 / \text{Red P}} \text{R-CH(X)-COOH} \quad (\text{X} = \text{Cl, Br})$$

• Decarboxylation: Heating sodium carboxylates with soda-lime ($\text{NaOH} + \text{CaO}$ in $3:1$ ratio) eliminates carbon dioxide to yield an alkane with one less carbon atom:

$$\text{R-COONa} + \text{NaOH} \xrightarrow[\Delta]{\text{CaO}} \text{R-H} + \text{Na}_2\text{CO}_3$$