Alcohols, Phenols and Ethers - Comprehensive Study Notes (Unit 7)

7.1 Classification of Alcohols, Phenols and Ethers

• Alcohols, phenols and ethers are formed by replacing one or more hydrogen atoms in hydrocarbons by –OH groups (alcohols/phenols) or by an –O– linkage (ethers).

• Alcohols: one or more –OH groups directly attached to carbon atoms of an aliphatic system (e.g., CH3OH).

• Phenols: –OH group directly attached to an aromatic ring (e.g., C6H5OH).

• Ethers: –OR or –OAr group replaces a hydrogen in a hydrocarbon; general structure R–O–R′.

• Important examples: ethanol (CH3CH2OH) as a monohydric alcohol; dimethyl ether (CH3OCH3) as an ether; phenol (C6H5OH) as a phenol.

7.1.1 Alcohols – classification by C–OH environment and functional groups

• Monohydric, dihydric, tri- or polyhydric alcohols depend on the number of –OH groups in the molecule.

• Primary, secondary, tertiary alcohols are classified by the degree of substitution of the carbon to which the –OH is attached (C–OH bond):

  • Primary: –OH on a carbon attached to one other carbon (R–CH2–OH).

  • Secondary: –OH on a carbon attached to two other carbons (R–CH(OH)–R′).

  • Tertiary: –OH on a carbon attached to three other carbons (R3C–OH).

• Allylic alcohols: –OH attached to an sp3 carbon adjacent to a C=C (e.g., allylic benzyl alcohols).

• Benzylic alcohols: –OH on a carbon adjacent to an aromatic ring.

• Vinylic alcohols (sp2): –OH directly attached to a vinyl carbon (e.g., CH2=CH–OH).

• The classification system provides a systematic framework for studying properties and reactivity.

7.1.2 Phenols – classification

• Phenols are mono-, di- and polyhydric depending on the number of –OH groups attached to the benzene ring (e.g., phenol, o-/m-/p-cresol, dihydroxybenzenes).

• Common dihydroxy derivatives: catechol (1,2-benzenediol), resorcinol (1,3-benzenediol), hydroquinone (1,4-benzenediol).

7.1.3 Ethers – classification

• Simple vs symmetrical ethers: both alkyl/aryl groups on the oxygen are the same (e.g., diethyl ether, C2H5OC2H5) or different (asymmetrical, e.g., C2H5OC6H5).

• Diethyl ether is a symmetrical ether; ethyl phenyl ether (anisole) is unsymmetrical.

7.2 Nomenclature

• Alcohols (IUPAC):

  • Name derived from the parent alkane; replace the ending -e with -ol.

  • Number the longest chain to give the –OH the lowest possible locant; substituents get locants as well.

  • For polyhydric alcohols, retain the -e, add -ol, and indicate the number of –OH groups with di-, tri-, etc. (e.g., HO–CH2–CH2–OH is ethane-1,2-diol).

• Alcohols (Common names): derived from alkyl group + alcohol (e.g., methanol, ethyl alcohol).

• Cyclic alcohols: named as cyclo[…]ol with the –OH considered at C-1.

• Phenols: common name often uses phenol as the parent; for substituted phenols, ortho (1,2-), meta (1,3-), para (1,4-) are used in common names; IUPAC naming assigns numbers to substituents on the benzene ring (e.g., 2-methylphenol).

• Ethers: Common names based on the two alkyl/aryl groups in alphabetical order followed by 'ether' (e.g., ethyl methyl ether).

• IUPAC names for ethers regard the larger group as the parent hydrocarbon; the smaller group is named as an alkoxy substituent (e.g., 2-Ethoxypropane, CH3CH(OCH2CH3)CH3).

• For symmetric ethers the prefix di- is used (e.g., diethyl ether is ethenic Ethoxyethane).

7.3 Structures and Bonding (selected points)

• In alcohols, the O–H bond is a sigma bond formed by overlap of an sp3 O orbital with a hydrogen, and the O atom bears two lone pairs.

• Bond angles: the O–H–C geometry is slightly distorted from ideal tetrahedral angle (~109.5°) due to lone-pair repulsion on oxygen.

• In phenols, the –OH group attaches to an sp2-hybridized carbon of the aromatic ring; C–O bond length is around 136 pm in phenol, shorter than in aliphatic alcohols due to conjugation and sp2 carbon.

• In ethers, the C–O–C linkage is sp3 on the oxygen with two lone pairs, giving a roughly tetrahedral arrangement around oxygen; C–O bond length is about 141 pm.

7.4 Preparation of Alcohols

7.4.1 From alkenes

• Acid-catalyzed hydration: alkene + water in the presence of an acid forms alcohols; Markovnikov's rule applies for unsymmetrical alkenes.

  • Mechanism (three steps):
    1) Protonation of the alkene to form the carbocation via H3O+.
    2) Nucleophilic attack by water on the carbocation.
    3) Deprotonation to yield the alcohol.

  • Overall:
    $ext{R-CH=CH2} + H_2O <br>ightarrow ext{R-CH(OH)-CH3} ext{ (Markovnikov product)}$

• Hydroboration–oxidation (anti-Markovnikov hydration): alkene → trialkyl borane, followed by oxidation to give alcohol with –OH installed at the less substituted carbon.

  • Mechanism:
    $ext{R-CH=CH2} <br>ightarrow ext{R-CH2-CH2-BH2} <br>ightarrow ext{R-CH2-CH2–OH}$

  • Advantage: high regioselectivity (anti-Markovnikov) and high yield.

7.4.2 From carbonyl compounds

• Reduction of aldehydes and ketones:

  • Catalytic hydrogenation (H2, Pt/Pd/Ni) reduces aldehydes to primary alcohols and ketones to secondary alcohols.

  • Hydride reagents such as NaBH4 or LiAlH4 reduce aldehydes/ketones:
    $RCHO <br>ightarrow RCH2OH <br>$ RCOR'
    ightarrow RCH(OH)R'$</p></li></ul></li><li><p>Reduction of carboxylic acids and esters:</p><ul><li><p>LiAlH4 reduces carboxylic acids and esters to primary alcohols; however, LiAlH4 is expensive and used selectively.</p></li><li><p>Typical two-step alternative: convert acid to ester, then hydrogenate to alcohols.</p></li></ul></li><li><p>From Grignard reagents (RMgX):</p><ul><li><p>Grignard reagents add to aldehydes and ketones to form alcohols after acidic workup:</p></li><li><p>With formaldehyde (H2C=O): primary alcohol</p></li><li><p>With other aldehydes: secondary alcohol</p></li><li><p>With ketones: tertiary alcohol</p></li><li><p>Overall: RMgX + R'CHO → R-CH2OH (after hydrolysis) depending on substituents and reagents.</p></li></ul></li><li><p>Brown’s hydroboration–oxidation history: first reported by H. C. Brown (1959); awarded Nobel Prize in 1979 with G. Wittig for advances in boron chemistry.</p></li></ul><h3 collapsed="false" seolevelmigrated="true">7.4.3 Preparation of Phenols</h3><ul><li><p>From haloarenes: chlorobenzene fused with NaOH at high temperature and pressure (623 K, 320 atm) → phenol after acidification of sodium phenoxide.</p><ul><li><p>Equation:$ ext{C}6 ext{H}5 ext{Cl} + ext{NaOH}
    ightarrow ext{C}6 ext{H}5 ext{ONa}
    ightarrow ext{C}6 ext{H}5 ext{OH} + ext{NaCl}$</p></li></ul></li><li><p>From benzene sulfonic acids: sulphonation of benzene to benzene sulfonic acid, then fusion with NaOH to sodium phenoxide; acidification yields phenol.</p></li><li><p>From diazonium salts: aryl amines converted to diazonium salts (NaNO2/HCl, 0–5°C); hydrolysis of diazonium salts to phenols upon warming with water or with dilute acids.</p><ul><li><p>General: <br>$ ext{Ar-N}2^+ ext{Cl}^- + H2O
    ightarrow ext{Ar–OH} + N_2 c$</p></li></ul></li><li><p>From cumene: cumene (isopropylbenzene) is oxidized to cumene hydroperoxide and then treated with dilute acid to yield phenol and acetone as a by-product.</p><ul><li><p>This method is a major industrial route to phenol.</p></li></ul></li></ul><h3 collapsed="false" seolevelmigrated="true">7.5 Physical Properties</h3><ul><li><p>Boiling points: rise with increasing carbon chain due to larger van der Waals forces; mono- to polyhydric alcohols have higher boiling points than hydrocarbons with similar molar mass because of hydrogen bonding.</p></li><li><p>Branching reduces boiling point in alcohols due to decreased surface area and weaker van der Waals forces.</p></li><li><p>Hydrogen bonding: OH groups enable extensive intermolecular hydrogen bonding in alcohols and phenols, contributing to high boiling points and water solubility.</p></li><li><p>Ethers: though they have polar C–O bonds, their boiling points lie closer to alkanes of similar molecular mass because they cannot form H-bonds with themselves as readily as alcohols; however, ethers can form hydrogen bonds with water, giving some miscibility.</p></li><li><p>Solubility in water decreases with increasing size of the alkyl/aryl substituents due to hydrophobic effects.</p></li><li><p>pKa values (selected): phenols are more acidic than alcohols because the phenoxide anion is resonance-stabilized. Example data:</p><ul><li><p>Phenol: pK_a ≈ 10.0</p></li><li><p>Ethanol: pK_a ≈ 15.9</p></li><li><p>o-/m-/p-nitrophenol: pK_a around 7–9 depending on substituents (e.g., o-nitrophenol ≈ 7.2; m-nitrophenol ≈ 8.3; p-nitrophenol ≈ 7.9).</p></li></ul></li></ul><h3 collapsed="false" seolevelmigrated="true">7.6 Ethers – Preparation, Properties and Reactions</h3><h4 collapsed="false" seolevelmigrated="true">7.6.1 Preparation of Ethers</h4><ul><li><p>Dehydration of alcohols (acidic conditions): R–OH + R′–OH ⇌ R–O–R′ + H2O, typically with H2SO4/H3PO4; favored for primary alcohols under controlled temperatures; higher substitution (secondary/tertiary) gives alkenes predominantly due to elimination rather than substitution.</p></li><li><p>SN2 (Williamson) synthesis: alkoxide ion (R–O−) deprotonates with Na to give sodium alkoxide, which then reacts with primary alkyl halides (R–X) to form ethers:<br>$ R–X + ext{Na}R'–O^-
    ightarrow R–O–R' + ext{Na}X $</p></li><li><p>Scope: symmetrical ethers (R = R′) as well as unsymmetrical ethers; conditions: primary halides give best yields; secondary/tertiary halides lead to side-elimination.</p></li><li><p>Example: Diethyl ether historically produced by Williamson synthesis; diethyl ether used as anesthetic but largely replaced due to slower action and recovery.</p></li><li><p>Ethers can also be formed by methoxylation or alkylation of phenol derivatives (phenyl ethers) via Williamson conditions.</p></li></ul><h4 collapsed="false" seolevelmigrated="true">7.6.2 Physical Properties of Ethers</h4><ul><li><p>C–O–C bonds are polar; ethers generally have boiling points closer to alkanes of similar molar mass and lower than alcohols due to lack of extensive intermolecular H-bonding between molecules.</p></li><li><p>Ethers can still form hydrogen bonds with water, giving some miscibility with water that depends on molecular size.</p></li><li><p>Example comparison: ethoxyethane (diethyl ether) vs butan-1-ol; bp ≈ 34–35 °C for diethyl ether vs ~117 °C for butan-1-ol.</p></li></ul><h4 collapsed="false" seolevelmigrated="true">7.6.3 Reactions of Ethers</h4><ul><li><p>Cleavage of C–O bond (acidic conditions with HI or HBr at high temperature): R–O–R′ + HI/HBr → RI + R′–OH or RI + ROH depending on substitution; the reaction may proceed via SN1 (tertiary) or SN2 (primary) depending on the alkyl groups.</p></li><li><p>With HI, the cleavage tends to favor the formation of the more substituted alkyl iodide via SN1, whereas primary systems favor SN2 (less substituted carbon is attacked by I−).</p></li><li><p>Anisole (methoxybenzene) under strong acids forms a benzyloxonium-type intermediate; cleavage of O–CH3 is favored, producing methyl iodide (CH3I) as one product and phenol or related by-products depending on conditions.</p></li></ul><h3 collapsed="false" seolevelmigrated="true">7.7 Reactions Involving Alcohols, Phenols and Ethers (Selected Highlights)</h3><ul><li><p>Alcohols as acids (Bronsted acids): reaction with active metals (Na, K, Al) yields alkoxides/phenoxides and H2; phenol especially forms sodium phenoxide with NaOH.</p><ul><li><p>Example: <br>$ ext{C}6 ext{H}5 ext{OH} + ext{NaOH}
    ightarrow ext{C}6 ext{H}5 ext{ONa} + ext{H}_2 ext{O}

• Acidity trend: electron-donating groups (e.g., –CH3) reduce acidity of alcohols; phenols are more acidic than alcohols due to stabilization of the phenoxide anion by resonance.

• Acidity of phenols vs water and alcohols: phenols are stronger acids than alcohols and water because the negative charge in phenoxide is delocalized over an aromatic ring.

• Esterification (acylation): alcohols and phenols react with carboxylic acids, acid chlorides, or anhydrides to give esters; for phenols, electron-withdrawing substituents (e.g., –NO2) increase acidity and facilitate phenoxide formation, favoring esterification.

• Acetylation: introduction of acetyl (–COCH3) group into alcohols/phenols forms acetates (e.g., acetylation of salicylic acid to aspirin).

• Reactions of the C–O bond in alcohols (ROH): conversion to alkyl halides with HX; reaction with POCl3 to form alkyl halides; dehydration to alkenes under strong acid; oxidation to carbonyl species.

  • Primary alcohols oxidize to aldehydes (mild conditions) and then to carboxylic acids under strong oxidation.

  • Secondary alcohols oxidize to ketones; tertiary alcohols are resistant to oxidation under mild conditions (dehydration may occur instead).

  • Strong oxidizing agents (e.g., KMnO4) can cleave C–C bonds under severe conditions, yielding mixed carboxylic acids.

• Grignard reagents with aldehydes/ketones: preparation of higher-order alcohols following hydrolysis (examples given in the text).

• Phenols: electrophilic aromatic substitution (EAS) occurs on the ring; –OH activates the ring and directs ortho/para substituents due to resonance donation; typical reagents include nitration (HNO3/H2SO4), halogenation (Br2 in nonpolar solvents), and other EAS reactions.

• Kolbe’s reaction: phenoxide ion with CO2 yields ortho-hydroxybenzoic acid after subsequent steps; Reimer–Tiemann reaction introduces a formyl group at the ortho position using chloroform in base to yield salicylaldehyde; phenols easily oxidize to benzoquinone under certain conditions.

7.8 Reactions of Phenols – Electrophilic substitutions

• Phenols undergo nitration with dilute HNO3 to yield o-/p-nitrophenols; with concentrated HNO3, trinitro derivatives (picric acid) can be formed, though yields vary.

• Halogenation: phenol reacts readily with bromine without Lewis acid catalysts to form mono-, di-, or tri-bromophenols depending on reaction conditions; in presence of bromine water, 2,4,6-tribromophenol precipitates as a solid.

• Kolbe reaction, Reimer–Tiemann, and benzene ring activations are discussed with representative schemes.

7.9 Reactions of Ethers – Further details

• Ethers are relatively unreactive towards electrophiles under mild conditions but can be cleaved by strong acids or strong hydrogen halides under vigorous conditions.

• Williamson ether synthesis details: primary alkyl halides react with sodium alkoxides to form the corresponding ethers; unsymmetrical ethers can be prepared if conditions favor SN2 displacement.

• Differences in reactivity: tertiary alkyl halides lead to elimination rather than substitution with alkoxide bases.

• The reaction of ethers with HI is used to illustrate SN1 vs SN2 pathways and the formation of alkyl iodides and alcohols depending on substituent patterns and reaction conditions.

7.10 Industrial and Practical Notes

• Phenol is widely produced from cumene via cumene hydroperoxide oxidation, followed by acid-catalyzed cleavage to phenol and acetone.

• Methanol and ethanol are commercially important; methanol is produced mainly by catalytic hydrogenation of CO; ethanol is produced by fermentation of sugars or by catalytic hydration of ethene.

• Ethanol denaturation is used for industrial or consumer-grade alcohols to prevent drinking; denatured ethanol contains additives like copper sulfate and pyridine.

7.11 Summary of Key Comparisons

• Alcohols vs. Ethers vs. Phenols:

  • Hydrogen bonding: alcohols > phenols > ethers; this largely explains boiling points and solubility trends.

  • Acidity: phenols > alcohols; phenoxide ions are resonance-stabilized, making phenols more acidic.

  • Nucleophilicity/electrophilicity: alcohols can act as nucleophiles at the O–H and C–O bonds; ethers are relatively poor nucleophiles but can participate in SN2/SN1 under appropriate conditions; phenols participate in electrophilic aromatic substitution due to activation by the –OH group.

• Reactions of interest include hydration and hydroboration of alkenes to alcohols, reductions of carbonyl compounds to alcohols, Grignard additions to aldehydes/ketones, and the various pathways to prepare phenols and ethers.

• The interconnections among these classes underpin the synthesis of detergents, antiseptics, and fragrances, as highlighted in the Unit objectives.

7.12 Practice and Examples (Representative)

• Name the following according to IUPAC rules and predict products of standard reactions:

  • Predict major products in dehydration of alcohols (tertiary > secondary > primary order).

  • Predict oxidation products of primary, secondary, and tertiary alcohols under various oxidants (e.g., PCC vs. KMnO4).

  • Explain why propanol has a higher boiling point than butane.

  • Outline the Williamson synthesis for unsymmetrical ethers and identify limitations.

• Example reactions to practice:

  • Hydration of propene in dilute H2SO4 gives a secondary alcohol (prop-2-ol).

  • Ethanol dehydration with concentrated H2SO4 at 443 K yields ethene; at ~413 K, diethyl ether is the major product.

  • Ethanol + H2SO4 at high T leads to dehydration to ethene; conversely, ethanol + NaH yields ethoxide for Williamson synthesis.

  • Phenol nitration under dilute conditions yields ortho/para-nitrophenols; stronger nitrating conditions produce picric acid (2,4,6-trinitrophenol).

7.13 Key Equations (LaTeX)

• Hydration of alkenes (acid-catalyzed): ext{R-CH=CH}2 + ext{H}2 ext{O}
  ightarrow ext{R-CH(OH)-CH}_3$</p></li><li><p>Hydroboration–oxidation (anti-Markovnikov):$ ext{R-CH=CH}2 ightarrow ext{R-CH}2 ext{CH}_2 ext{OH}$</p></li><li><p>Reduction of aldehydes/ketones:$RCHO
  ightarrow RCH_2OH ext{ (primary alcohol)}
  ewline RCOR'
  ightarrow RCH(OH)R' ext{ (secondary alcohol)}$</p></li><li><p>Reduction of carboxylic acids/esters (LiAlH4):$RCOOH
  ightarrow RCH2OH ext{ and } RCOOR' ightarrow RCH2OH + R'OH$</p></li><li><p>Grignard additions to aldehydes/ketones (hydrolysis):$ RMgX + R'CHO
  ightarrow R-CH(OMgX)-R'
  ightarrow R-CH_2OH-R'$</p></li><li><p>Halo-arene to phenol:$ ext{C}6 ext{H}5 ext{Cl} + ext{NaOH}
  ightarrow ext{C}6 ext{H}5 ext{OH} ext{ (after workup)} {

• Williamson ether synthesis: R-X + ext{NaOR'}
  ightarrow R–O–R' + NaX$</p></li><li><p>Acid-catalyzed esterification of phenol with carboxylic acid (or acid chloride/anhydride):$ ext{Ph–OH} + ext{RCOOH}
  ightleftharpoons ext{Ph–O–COR} + ext{H}_2 ext{O}$</p></li><li><p>Reimer–Tiemann reaction (ortho-formylation of phenol):$ ext{Ph–OH} + ext{CHCl}_3 + ext{NaOH}
  ightarrow ext{Salicaldehyde (o-hydroxybenzaldehyde)}$</p></li><li><p>Nitration of phenol:$ ext{Ph–OH} + ext{HNO}3 ightarrow ext{o-/p-NO}2 ext{Ph–OH}$$

7.14 Real-world relevance

• Alcohols, phenols and ethers are foundational in detergents, antiseptics, and fragrances.

• The industrial production routes (e.g., cumene process for phenol) link chemical structure and reactivity to large-scale manufacture and safety considerations.