Alcohols Phenols and Ethers revision notes

• Markovnikov's Rule: Applies to the acid-catalyzed hydration of unsymmetrical alkenes to form alcohols; the nucleophile (water) attaches to the more substituted carbon.
• Anti-Markovnikov's Addition (Effective): Hydroboration-oxidation of alkenes yields alcohols that look exactly as if water was added in opposition to Markovnikov's rule. JEE Tip No carbocation is formed here, yielding excellent alcohol production without rearrangements.
• Brønsted Acid-Base Principle: Alcohols and phenols act as Brønsted acids (donating protons to stronger bases). Alcohols also act as Brønsted bases due to unshared electron pairs on oxygen making them proton acceptors.
• Zymase Inhibition Rule: During fermentation, the action of the zymase enzyme is naturally inhibited once the alcohol percentage exceeds 14%.

• Alcohols: The oxygen is attached to carbon via a sigma bond formed by overlapping $sp^3$ hybridized orbitals of carbon and oxygen. The C-O-H bond angle is slightly less than the tetrahedral angle ($109^\circ 28'$) due to repulsion between the unshared electron pairs on oxygen.
• Phenols: The –OH group attaches to an $sp^2$ hybridized carbon. The C-O bond length (136 pm) is slightly less than in methanol due to: (i) partial double bond character from conjugation of oxygen's lone pair with the aromatic ring, and (ii) the $sp^2$ hybridized state of the carbon.
• Ethers: The oxygen is $sp^3$ hybridized. The C-O-C bond angle is slightly greater than the tetrahedral angle due to the repulsive interaction between the two bulky (–R) groups. The C-O bond length (141 pm) is almost the same as in alcohols.

• Boiling Points of Ethers: Weak polarity does not appreciably affect their boiling points, making them comparable to alkanes of similar mass and much lower than isomeric alcohols.
• Solubility: Alcohols, phenols, and ethers are soluble in water because their oxygen atoms form hydrogen bonds with water molecules. Solubility decreases as the size of the hydrophobic alkyl/aryl group increases. Ethers and alcohols of comparable molecular mass have almost the exact same solubility in water (e.g., ethoxyethane 7.5g/100mL vs butan-1-ol 9g/100mL).

1. From Alkenes (Acid Catalyzed Hydration): Alkenes react with water + acid catalyst following Markovnikov's rule. Mechanism involves protonation to form a carbocation, nucleophilic attack by water, and deprotonation.
2. From Alkenes (Hydroboration-Oxidation): Reaction of alkenes with diborane $(BH_3)_2$ followed by oxidation with $H_2O_2$ in aqueous NaOH. Yields anti-Markovnikov alcohol products in excellent yield.
3. From Carbonyl Compounds (Reduction):
   • Catalytic hydrogenation (using Pt, Pd, or Ni).
   • Treatment with $NaBH_4$ or $LiAlH_4$.
   • Aldehydes yield $1^\circ$ alcohols; ketones yield $2^\circ$ alcohols.
   • Carboxylic acids are reduced to $1^\circ$ alcohols by $LiAlH_4$. Because $LiAlH_4$ is expensive, acids are usually first converted to esters, then catalytically hydrogenated.
4. From Grignard Reagents: Nucleophilic addition of RMgX to carbonyls, followed by hydrolysis.
   • Methanal (Formaldehyde) + RMgX $\rightarrow$ $1^\circ$ Alcohol.
   • Other Aldehydes + RMgX $\rightarrow$ $2^\circ$ Alcohol.
   • Ketones + RMgX $\rightarrow$ $3^\circ$ Alcohol.

1. From Haloarenes (Dow Process context): Chlorobenzene fused with NaOH at 623 K and 320 atm forms sodium phenoxide, which is acidified to phenol.
2. From Benzenesulphonic acid: Benzene sulfonated with oleum forms benzene sulphonic acid. Heated with molten NaOH to form sodium phenoxide, followed by acidification.
3. From Diazonium salts: Aniline treated with $NaNO_2 + HCl$ at 273-278 K forms benzene diazonium chloride. Warmed with water or treated with dilute acids to yield phenol.
4. From Cumene (Commercial Method): Cumene (isopropylbenzene) is oxidized in air to cumene hydroperoxide. Treated with dilute acid to yield phenol and acetone. JEE Tip Acetone is a highly valuable by-product produced in large quantities here.

1. By Dehydration of Alcohols: Heating ethanol with conc. $H_2SO_4$ at 413 K yields ethoxyethane (at 443 K, ethene forms via elimination).
   • Mechanism: $S_N2$ attack of an alcohol molecule on a protonated alcohol.
   • Limitations: Only suitable for $1^\circ$ unhindered alkyl groups. For $2^\circ$ and $3^\circ$ alcohols, dehydration to alkenes strongly competes and dominates.
2. Williamson Synthesis: Reaction of an alkyl halide with sodium alkoxide ($R-X + NaOR' \rightarrow R-O-R' + NaX$).
   • Involves an $S_N2$ attack of the alkoxide ion on the primary alkyl halide.
   • Phenols can also be converted to ethers via sodium phenoxide + alkyl halide.

Cleavage of O-H Bond (Acidity and Nucleophilic Behavior)

• Reaction with Metals: Alcohols/phenols react with active metals (Na, K, Al) to form alkoxides/phenoxides and $H_2$ gas.

Cleavage of C-O Bond (Alcohols Only)

Phenols only show C-O cleavage when reacting with zinc dust.

• Reaction with Phosphorus Trihalides: $ROH + PBr_3 \rightarrow RBr$.

Electrophilic Aromatic Substitution of Phenols

The –OH group is strongly activating and ortho/para directing due to resonance.

• Kolbe's Reaction: Phenoxide ion + $CO_2$ (weak electrophile) + $H^+$ $\rightarrow$ Ortho-hydroxybenzoic acid (Salicylic acid).
• Reimer-Tiemann Reaction: Phenol + $CHCl_3$ + aq. NaOH $\rightarrow$ intermediate benzal chloride $\rightarrow$ hydrolysis yields Salicylaldehyde (–CHO introduced at ortho position).
• Reaction with Zinc Dust: Phenol + Zn (heat) $\rightarrow$ Benzene + ZnO.
• Oxidation of Phenol: With chromic acid ($Na_2Cr_2O_7 / H_2SO_4$), phenol oxidizes to benzoquinone (a conjugated diketone). In air, it slowly forms dark mixtures containing quinones.

• Alcohol Hydration: $Alkene + H_2O \xrightarrow{H^+} Alcohol$
• Hydroboration-Oxidation: $Alkene \xrightarrow{(i) (BH_3)_2 \ (ii) H_2O_2/OH^-} Alcohol$
• Grignard Synthesis: $R-Mg-X + C=O \rightarrow R-C-O-MgX \xrightarrow{H_2O} R-C-OH + Mg(OH)X$
• Esterification: $ROH + R'COOH \xrightleftharpoons{H^+} R'COOR + H_2O$
• Lucas Reaction: $ROH + HCl \xrightarrow{ZnCl_2} RCl + H_2O$
• Dehydration: $CH_3CH_2OH \xrightarrow[443 K]{conc. H_2SO_4} CH_2=CH_2 + H_2O$
• Williamson Synthesis: $R-X + Na^+O^--R' \rightarrow R-O-R' + NaX$
• Ether Cleavage: $R-O-R' + HI \rightarrow R-I + R'-OH$

• Bond Angle Anomaly: The C-O-C bond angle in ethers ($>109.5^\circ$) is greater than the tetrahedral angle, whereas the C-O-H bond angle in alcohols ($<109.5^\circ$) is less. Why: Ethers experience intense steric repulsion between two bulky alkyl groups, overriding lone-pair repulsion. Alcohols lack the second bulky group, so lone-pair repulsion dominates.
• Boiling Point vs Branching Exception: For isomeric alcohols, as branching increases, boiling point decreases. Why: Branching makes the molecule more spherical, decreasing the surface area and thereby weakening the van der Waals dispersion forces.
• Halogenation Catalyst Anomaly: Bromination of benzene strictly requires a Lewis acid catalyst ($FeBr_3$). Bromination of phenol occurs without any Lewis acid. Why: The –OH group is so strongly activating that it polarizes the bromine molecule directly.
• Ether Cleavage Mechanism Flip (The $3^\circ$ Exception): Cleavage of mixed ethers with HI normally follows $S_N2$ (the halogen attacks the smaller, less hindered alkyl group). However, if one group is tertiary ($3^\circ$), the mechanism entirely flips to $S_N1$, and the halogen attacks the bulky $3^\circ$ group. Why: The stability of the intermediate $3^\circ$ carbocation completely overrides the $S_N2$ steric preference.
• Alkyl Aryl Ether Cleavage Exception: Ethers normally cleave into two alkyl halides with excess HI. Alkyl aryl ethers (like anisole) never cleave at the aryl-oxygen bond, only the alkyl-oxygen bond, yielding Phenol + Alkyl Halide. Why: The aryl-oxygen bond has partial double-bond character due to resonance and $sp^2$ hybridization, making it too strong to break.
• Williamson Synthesis Reagent Reversal: Mixing a $3^\circ$ alkyl halide + $1^\circ$ alkoxide yields $0\%$ ether and $100\%$ alkene. Why: Alkoxides are strong bases. Steric hindrance in the $3^\circ$ halide prevents $S_N2$ substitution, so elimination dominates. To make the ether, you must use a $1^\circ$ halide and a $3^\circ$ alkoxide.
• Temperature-Dependent Dehydration Anomaly: Reacting ethanol with conc. $H_2SO_4$ gives completely different functional groups depending on a slight temperature change. Why: At 413 K, substitution ($S_N2$) occurs forming ethoxyethane (ether). At 443 K, elimination occurs forming ethene (alkene).
• Heated Copper ($3^\circ$ Alcohol) Exception: Passing $1^\circ$ and $2^\circ$ alcohol vapors over Cu at 573 K causes dehydrogenation (oxidation) to aldehydes and ketones. Passing $3^\circ$ alcohols over the same catalyst causes dehydration. Why: $3^\circ$ alcohols do not have an alpha-hydrogen to lose for oxidation, so they lose a water molecule to form an alkene instead.
• Acid vs. Base Anomaly for Alcohols: Alcohols are amphoteric. They act as Brønsted acids (donating a proton to active metals) AND as Brønsted bases (accepting a proton due to lone pairs on oxygen).
• Solubility vs. Boiling Point Disconnect in Ethers: Ethers have very low boiling points (similar to alkanes because they can't H-bond with themselves), but they have high solubility in water (similar to alcohols because they can H-bond with water).

• Acidic Strength Comparisons: Specifically substituent effects on phenol (e.g., p-nitrophenol vs m-nitrophenol vs p-cresol).
• Regioselectivity in Ether Cleavage by HI: Differentiating between $1^\circ/2^\circ$ (forms smaller alkyl iodide) and $3^\circ$ (forms $3^\circ$ alkyl iodide via carbocation).
• Cumene Process: Identifying the intermediates (cumene hydroperoxide) and the economically vital by-product (acetone).
• Lucas Test: Time taken for turbidity appearance ($3^\circ$ immediate, $1^\circ$ none at room temp) to distinguish structural isomers of alcohols.
• Reimer-Tiemann & Kolbe's Reaction: Intermediate formation (benzal chloride in R-T) and specific final products (salicylaldehyde vs. salicylic acid).