Alcohols, Phenols and Ethers — Revision Notes

⚡ 30-Second Revision

Alcohols: — R-OH. $sp^3$ C-OH. Higher BP (H-bonding). Weakly acidic.

- Prep: Hydration of alkenes, reduction of carbonyls ($LiAlH_4, NaBH_4$), Grignard reagents. - Rxns: Acidity ($2ROH + 2Na \rightarrow 2RONa + H_2$), Esterification, Dehydration (alkene at $443K$, ether at $413K$), Rxn with HX ($3^circ > 2^circ > 1^circ$), Oxidation ($1^circ \rightarrow$ aldehyde (PCC) $ightarrow$ acid; $2^circ \rightarrow$ ketone; $3^circ$ resistant).

Phenols: — Ar-OH. $sp^2$ C-OH. More acidic than alcohols (resonance stabilized phenoxide).

- Prep: Dow's process, Cumene process, Diazonium salts. - Rxns: Acidity ($ArOH + NaOH \rightarrow ArONa + H_2O$), Electrophilic substitution (o,p-directing, activating), Kolbe's (salicylic acid), Reimer-Tiemann (salicylaldehyde), Rxn with $Zn$ dust (benzene).

Ethers: — R-O-R'. No H-bonding between molecules (lower BP than alcohols). Relatively inert.

- Prep: Williamson Synthesis ($R-X \text{ (primary)} + R'-O^-Na^+$), Dehydration of alcohols (symmetrical, $413K$). - Rxns: Cleavage by HI/HBr ($S_N2$ for $1^circ$, $S_N1$ for $3^circ$/benzylic), Peroxide formation (explosive).

Distinguishing Tests: — Lucas (alcohols), $FeCl_3$ (phenols), Iodoform (alcohols with $CH_3CH(OH)$ group).

2-Minute Revision

Alcohols (R-OH) have a hydroxyl group on an aliphatic carbon. They are polar, form strong hydrogen bonds, leading to high boiling points and water solubility. They are weakly acidic. Key preparations include hydration of alkenes (Markovnikov/anti-Markovnikov), reduction of aldehydes/ketones/esters, and Grignard reactions.

Reactions involve the O-H bond (acidity, esterification) and C-O bond (dehydration to alkenes or ethers, reaction with HX, oxidation). Primary alcohols oxidize to aldehydes (PCC) or carboxylic acids (strong agents), secondary to ketones, and tertiary are resistant.

Phenols (Ar-OH) have a hydroxyl group directly on an aromatic ring. They are more acidic than alcohols due to resonance stabilization of the phenoxide ion. They undergo electrophilic aromatic substitution (ortho-para directing, activating) and important name reactions like Kolbe's (forming salicylic acid) and Reimer-Tiemann (forming salicylaldehyde). They give a characteristic color with neutral $FeCl_3$.

Ethers (R-O-R') have an oxygen atom bridging two alkyl or aryl groups. They lack an acidic hydrogen, so they cannot form intermolecular hydrogen bonds, resulting in lower boiling points than alcohols. They are relatively unreactive but can be cleaved by hot concentrated HI/HBr (mechanism depends on alkyl group type) and form explosive peroxides on standing in air.

5-Minute Revision

Let's quickly review the core aspects of Alcohols, Phenols, and Ethers. Alcohols (R-OH) are characterized by an -OH group on an $sp^3$ hybridized carbon. Their polarity and ability to form strong intermolecular hydrogen bonds lead to higher boiling points and solubility in water compared to hydrocarbons.

They are weak acids, reacting with active metals to form alkoxides. Preparation methods include acid-catalyzed hydration of alkenes (Markovnikov), hydroboration-oxidation (anti-Markovnikov), reduction of carbonyl compounds (aldehydes to $1^circ$, ketones to $2^circ$ alcohols using $LiAlH_4$ or $NaBH_4$), and reaction with Grignard reagents (formaldehyde for $1^circ$, other aldehydes for $2^circ$, ketones for $3^circ$).

Chemical reactions include esterification, dehydration (to alkenes at $443K$ or ethers at $413K$), reaction with HX (Lucas test distinguishes $1^circ, 2^circ, 3^circ$), and oxidation ($1^circ \rightarrow$ aldehyde (PCC) $ightarrow$ carboxylic acid; $2^circ \rightarrow$ ketone; $3^circ$ resistant).

Phenols (Ar-OH) have the -OH group directly attached to an aromatic ring. This direct attachment allows for resonance stabilization of the phenoxide ion, making phenols significantly more acidic than alcohols, though less acidic than carboxylic acids.

They react with NaOH. Key preparation methods include Dow's process from haloarenes, from benzene sulfonic acid, from diazonium salts, and industrially from cumene. Phenols are highly reactive towards electrophilic aromatic substitution, with the -OH group being strongly activating and ortho-para directing (e.

g., nitration, bromination, Friedel-Crafts). Important name reactions are Kolbe's reaction (with $CO_2$ to form salicylic acid) and Reimer-Tiemann reaction (with $CHCl_3/NaOH$ to form salicylaldehyde).

Phenols give a characteristic color with neutral $FeCl_3$ solution.

Ethers (R-O-R') feature an oxygen atom bonded to two alkyl or aryl groups. They are less polar than alcohols and, crucially, cannot form intermolecular hydrogen bonds with each other (no O-H bond). This results in lower boiling points compared to alcohols of similar molecular mass.

Ethers are relatively unreactive, making them good solvents. The primary synthetic route is Williamson synthesis, an $S_N2$ reaction between a primary alkyl halide and a sodium alkoxide/phenoxide. Dehydration of alcohols can also yield symmetrical ethers.

Their main chemical reaction is cleavage by hot concentrated HI or HBr, yielding alkyl halides and alcohols. The mechanism ($S_N1$ or $S_N2$) depends on the nature of the alkyl groups. Ethers also pose a safety hazard by forming explosive peroxides upon prolonged exposure to air and light.

Quick Example: How to synthesize ethyl methyl ether ($CH_3OCH_2CH_3$)? Use Williamson synthesis: $CH_3Br + CH_3CH_2ONa \rightarrow CH_3OCH_2CH_3 + NaBr$. (Note: $CH_3CH_2Br + CH_3ONa$ would also work, but avoid tertiary alkyl halides).

Prelims Revision Notes

I. Alcohols (R-OH)

Definition: — OH group on $sp^3$ hybridized carbon.
Nomenclature: — IUPAC: '-ol' suffix. Common: Alkyl alcohol.
Classification: — $1^circ, 2^circ, 3^circ$ based on carbon attached to -OH.
Physical Properties:

* Boiling Point: High due to intermolecular H-bonding ($R-O-H cdots O-R$). Increases with MW, decreases with branching. * Solubility: Lower members soluble in water (H-bonding with water). Decreases with increasing hydrocarbon part.

Preparation:

1. From Alkenes: * Acid-catalyzed hydration ($H_2SO_4/H_2O$): Markovnikov addition. Carbocation rearrangements possible. * Hydroboration-oxidation ($BH_3 cdot THF$, then $H_2O_2/OH^-$): Anti-Markovnikov addition, syn-stereochemistry.

2. From Carbonyl Compounds: * Reduction ($LiAlH_4$ or $NaBH_4$): Aldehydes $ightarrow 1^circ$, Ketones $ightarrow 2^circ$. $LiAlH_4$ also reduces acids/esters to $1^circ$. 3. **From Grignard Reagents ($RMgX$):** * Formaldehyde $ightarrow 1^circ$ alcohol.

* Other aldehydes $ightarrow 2^circ$ alcohol. * Ketones $ightarrow 3^circ$ alcohol.

Chemical Reactions:

1. O-H bond cleavage (Acidity): * Weakly acidic (weaker than water). Acidity order: $1^circ > 2^circ > 3^circ$ (due to +I effect). * Reaction with active metals: $2ROH + 2Na \rightarrow 2RONa + H_2$.

* Esterification: $RCOOH + R'OH xrightarrow{H^+} RCOOR' + H_2O$. 2. C-O bond cleavage: * Reaction with HX: $ROH + HX \rightarrow RX + H_2O$. Reactivity: $HI > HBr > HCl$. Alcohol reactivity: $3^circ > 2^circ > 1^circ$.

(Lucas test: $HCl/ZnCl_2$). * Reaction with $PCl_3, PCl_5, SOCl_2$: Convert to alkyl halides. $SOCl_2$ (Thionyl chloride) is preferred (gaseous byproducts). * Dehydration (Elimination): $ROH xrightarrow{H_2SO_4, 443K} \text{Alkene}$ (Zaitsev's rule).

$ROH xrightarrow{H_2SO_4, 413K} \text{Ether}$ (intermolecular). 3. Oxidation: * $1^circ$ alcohol: $RCH_2OH xrightarrow{PCC} RCHO$ (aldehyde). $RCH_2OH xrightarrow{KMnO_4 \text{ or } K_2Cr_2O_7} RCOOH$ (carboxylic acid).

* $2^circ$ alcohol: $RCH(OH)R' xrightarrow{K_2Cr_2O_7 \text{ or } PCC} RCOR'$ (ketone). * $3^circ$ alcohol: Resistant to mild oxidation. Vigorous conditions cause C-C bond cleavage.

II. Phenols (Ar-OH)

Definition: — OH group directly on aromatic ring.
Acidity: — More acidic than alcohols, less than carboxylic acids. Due to resonance stabilization of phenoxide ion. Electron-withdrawing groups increase acidity, electron-donating groups decrease it.
Preparation:

1. Dow's Process: From chlorobenzene ($C_6H_5Cl + NaOH \rightarrow C_6H_5ONa xrightarrow{H^+} C_6H_5OH$). 2. From Benzene Sulfonic Acid: $C_6H_5SO_3H xrightarrow{NaOH, Delta} C_6H_5ONa xrightarrow{H^+} C_6H_5OH$. 3. From Diazonium Salts: $ArN_2^+Cl^- xrightarrow{H_2O, Delta} ArOH + N_2 + HCl$. 4. Cumene Process: $C_6H_5CH(CH_3)_2 xrightarrow{O_2} \text{Cumene hydroperoxide} xrightarrow{H_3O^+} C_6H_5OH + CH_3COCH_3$.

Chemical Reactions:

1. Acidity: React with NaOH ($C_6H_5OH + NaOH \rightarrow C_6H_5ONa + H_2O$). 2. Electrophilic Aromatic Substitution: -OH is activating and ortho-para directing. * Nitration: Dilute $HNO_3 \rightarrow$ o- and p-nitrophenol.

Conc. $HNO_3 \rightarrow$ 2,4,6-trinitrophenol (Picric acid). * Halogenation: $Br_2/CS_2 \rightarrow$ mono-bromophenols. $Br_2/H_2O \rightarrow$ 2,4,6-tribromophenol (white ppt). * Kolbe's Reaction: $C_6H_5ONa + CO_2 \rightarrow$ Salicylic acid.

* Reimer-Tiemann Reaction: $C_6H_5OH + CHCl_3/NaOH \rightarrow$ Salicylaldehyde. 3. Reaction with Zinc Dust: $C_6H_5OH + Zn xrightarrow{Delta} C_6H_6 + ZnO$. 4. Oxidation: To p-benzoquinone with chromic acid.

Test: — Neutral $FeCl_3$ test (violet/blue/green coloration).

III. Ethers (R-O-R')

Definition: — Oxygen atom bonded to two alkyl/aryl groups.
Nomenclature: — IUPAC: Alkoxyalkane. Common: Dialkyl ether.
Physical Properties:

* Boiling Point: Lower than alcohols of comparable MW (no intermolecular H-bonding). Higher than hydrocarbons (dipole-dipole). * Solubility: Slightly soluble in water (can accept H-bonds from water).

Preparation:

1. Williamson Synthesis: $R-X \text{ (primary)} + R'-O^-Na^+ \rightarrow R-O-R' + NaX$. (Avoid $2^circ/3^circ$ alkyl halides to prevent E2 elimination). 2. Dehydration of Alcohols: $2ROH xrightarrow{H_2SO_4, 413K} R-O-R + H_2O$ (for symmetrical ethers).

Chemical Reactions:

1. Cleavage by Hot Concentrated HI/HBr: $R-O-R' + HI \rightarrow R-I + R'-OH$. If excess HI, $R'-OH \rightarrow R'-I$. * Mechanism: $S_N2$ if $1^circ$ or $CH_3$ group (attack on less hindered carbon).

$S_N1$ if $3^circ$ or benzylic group (via stable carbocation). * Aryl-O bond is not cleaved (resonance stabilization). 2. Electrophilic Aromatic Substitution (for aromatic ethers like anisole): -OR is activating and ortho-para directing.

3. Peroxide Formation: Ethers form explosive peroxides on exposure to air and light. Store in dark bottles, test before use.

Vyyuha Quick Recall

Alcohols Phenols Ethers: Acidity, Preparation, Every Reaction.

Alcohols: Oxidation, Hydrogen bonding, Lucas test, Carbocation (dehydration/HX). Phenols: Resonance (acidity), Kolbe's, Reimer-Tiemann, Ferric chloride. Ethers: Williamson, Cleavage (HI/HBr), Peroxides (explosive).