🍷 Alcohols, Phenols & Ethers

Oxygen-containing functional groups provide diverse synthetic routes through nucleophilic substitution, dehydration rearrangements, and aromatic substitutions. High-yield competitive exam highlights include Lucas reagent kinetics, Pinacol rearrangement migratory aptitudes, Kolbe vs Reimer-Tiemann phenol carboxylations, and Williamson ether cleavage under hot concentrated $HI$.

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1. 🧪 Alcohols: Classification & Rearrangements

1.1 Lucas Test (Distinction of $1^{c}irc,2^{c}irc,3^{c}irc$ Alcohols)

$$\text{Reagent: Anhydrous }ZnC{l}_2+\text{conc. }HCl(S_{N}1\text{ mechanism forming insoluble }R-Cl)$$

• $3^{c}irc$ Alcohols: Immediate turbidity/cloudiness appears within seconds at room temperature ($3^{c}irc{C}^{+}$ forms instantaneously).
• $2^{c}irc$ Alcohols: Turbidity appears within $5-10\text{ minutes}$.
• $1^{c}irc$ Alcohols: Solution remains clear; turbidity appears only on prolonged heating.

1.2 Pinacol-Pinacolone Rearrangement

Acid-catalyzed dehydration of 1,2-diols (Pinacols) to Ketones with 1,2-alkyl/aryl shift:

$$\text{Pinacol }((C{H}_3{)}_{2}C(OH)-C(OH)(C{H}_3{)}_2)\stackrel{H_{2}S{O}_4}{\to }\text{Pinacolone }((C{H}_3{)}_{3}C-CO-C{H}_3)+H_{2}O$$

Migratory Aptitude Rules:

$$\text{Aryl Groups}\mathbf{>}\text{Alkyl Groups}\mathbf{>}\text{Hydrogen}$$$$\text{Among Aryl Groups: }p\text{-Anisyl }(-C_6{H}_{4}OC{H}_3)>p\text{-Tolyl }(-C_6{H}_{4}C{H}_3)>\text{Phenyl }(-C_6{H}_5)>p\text{-Chlorophenyl}>p\text{-Nitrophenyl}$$

(More electron-rich aryl groups migrate significantly faster!).

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2. 🌸 Phenols: Industrial Synthesis & Named Reactions

2.1 Industrial Synthesis from Cumene (Isopropylbenzene)

$$\text{Cumene}+O_2\stackrel{\text{Air, }95-{135}^{\circ }\text{C}}{\to }\text{Cumene Hydroperoxide }(Ph-C(C{H}_3{)}_2-O-O-H)\stackrel{H^{+}/H_{2}O}{\to }\text{Phenol}\mathbf{+}\text{Acetone (High yield byproduct)}$$

2.2 Kolbe-Schmitt Reaction (Synthesis of Salicylic Acid)

$$\text{Sodium Phenoxide }(Ph{O}^{-}N{a}^{+})+C{O}_2\stackrel{400\text{ K},4-7\text{ atm}}{\to }\text{Sodium Salicylate}\stackrel{H^{+}}{\to }\text{Salicylic Acid }\mathbf{(}\mathbf{o}\text{-Hydroxybenzoic acid}\mathbf{)}$$

• Acetylation of salicylic acid with acetic anhydride gives Aspirin (Acetylsalicylic Acid).

2.3 Reimer-Tiemann Reaction (Synthesis of Salicylaldehyde)

$$\text{Phenol}+CHC{l}_3+3KOH\stackrel{{60}^{\circ }\text{C}}{\to }\text{Salicylaldehyde }\mathbf{(}\mathbf{o}\text{-Hydroxybenzaldehyde}\mathbf{)}+3KCl+2H_{2}O$$

• Active Electrophile: Dichlorocarbene ($:CC{l}_2$, a neutral singlet carbocation-like electrophile with empty $p$-orbital).
• If $CC{l}_4$ is used instead of $CHC{l}_3$, the product is Salicylic Acid.

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3. 🌐 Ethers: Synthesis & Cleavage Mechanisms

3.1 Williamson Ether Synthesis ($S_{N}2$)

$$R-O^{-}N{a}^{+}+R^{\prime }-X\to R-O-R^{\prime }+NaX$$

• Condition for High Yield: Substrate $R^{\prime }-X$ must be strictly $1^{c}irc$ or Methyl halide.
• Trap: If $R^{\prime }-X$ is a $3^{c}irc$ alkyl halide, the alkoxide acts as a strong base, and $100\mathrm{\%}$ E2 elimination occurs to give an alkene!

3.2 Cleavage of Ethers with Hot Concentrated $HI$

$$R-O-R^{\prime }+HI\to \text{Alkyl Iodide}+\text{Alcohol}$$

1. When both $R$ and $R^{\prime }$ are $1^{c}irc$ or $2^{c}irc$ ($S_{N}2$ Mechanism):
   • $I^{-}$ attacks the less hindered alkyl group:$$C{H}_3-O-CH(C{H}_3{)}_2+HI\to {\mathbf{CH}}_{\mathbf{3}}\mathbf{I}\mathbf{+}\mathbf{(}{\mathbf{CH}}_{\mathbf{3}}{\mathbf{)}}_{\mathbf{2}}\mathbf{CHOH}$$
2. When one alkyl group is $3^{c}irc$, Benzyl, or Allyl ($S_{N}1$ Mechanism):
   • Cleavage proceeds via the stable carbocation $ o I^-$ attacks the $3^{c}irc$ or benzyl group:$$(C{H}_3{)}_{3}C-O-C{H}_3+HI\to \mathbf{(}{\mathbf{CH}}_{\mathbf{3}}{\mathbf{)}}_{\mathbf{3}}\mathbf{C}\mathbf{-}\mathbf{I}\mathbf{+}{\mathbf{CH}}_{\mathbf{3}}\mathbf{OH}$$
3. Anisole ($Ph-O-C{H}_3$) with $HI$:
   • The $Ph-O$ bond has partial double bond character $ o$ bond cleavage occurs strictly at $O-C{H}_3$:$$Ph-O-C{H}_3+HI\to \mathbf{Ph}\mathbf{-}\mathbf{OH}\mathbf{(}\text{Phenol}\mathbf{)}\mathbf{+}{\mathbf{CH}}_{\mathbf{3}}\mathbf{I}$$