Organic Chemistry – Chapter 9: Alcohols, Ethers, and Epoxides

Alcohols, Ethers, and Epoxides – Key Definitions

• Alcohol (ROH): Contains a hydroxy ((\ce{–OH})) bonded to an (sp^3)-hybridized C.
  • Generic formula: $R{-}OH$
• Ether (ROR'): Two alkyl (or aryl) groups bonded to the same O.
  • Generic formula: $R{-}O{-}R'$
• Epoxide (Oxirane): A cyclic ether in a three-membered ring; significant ring strain drives reactions.

Intermolecular Forces & Physical Properties

• Molecules are bent and possess two polar C–O (and possibly O–H) bonds ⇒ permanent dipole.
• Dipole–dipole (DD) interactions for all three classes.
• Hydrogen bonding (HB) possible only for alcohols ((\ce{O–H}) donor).
  • Ranking of polarity: Alcohols (>) Ethers (\approx) Epoxides.
• Boiling/Melting Points (bp/mp)
  • At equal molecular weight: \text{VDW} < \text{VDW+DD} < \text{VDW+DD+HB}
  • Example series ((M_w \approx 60)–74):
  • Butane (\ce{CH3CH2CH2CH3})
    • Forces: VDW only • bp $0^\circ\mathrm C$
  • Diethyl ether (\ce{CH3OCH2CH3})
    • Forces: VDW + DD • bp $11^\circ\mathrm C$
  • 1-Propanol (\ce{CH3CH2CH2OH})
    • Forces: VDW + DD + HB • bp $97^\circ\mathrm C$
  • For isomeric alcohols: more H-bonding sites or less steric hindrance ⇒ higher bp.
  • 1° butanol bp $118^\circ\mathrm C$ > 2° (bp $98^\circ\mathrm C$) > 3° (bp $83^\circ\mathrm C$).
• Solubility in water
  • (\le 5) C atoms ⇒ miscible via H-bonding.
  • (> 5) C atoms ⇒ hydrophobic chain dominates ⇒ insoluble.
  • Always soluble in most organic solvents.

IUPAC Nomenclature of Alcohols

• Parent chain: longest carbon chain containing the C attached to (\ce{–OH}).
• Suffix: replace “-e” of alkane with “-ol”.
• Numbering: lowest possible locant for (\ce{–OH}); other substituents follow usual rules.
• Example walkthrough
  1. Molecule: (\ce{CH3CH(CH3)CH2CH(CH3)CH3}) with (\ce{–OH}) on third carbon.
  2. Longest chain = 6C ⇒ “hexan”.
  3. Position of (\ce{–OH}): C-3 ⇒ “3-hexanol”.
  4. Substituent: methyl on C-5 ⇒ 5-methyl-3-hexanol.
• Cyclic alcohols: ring numbered starting at the carbon bearing OH; the “1” is usually omitted (e.g.
  2-methylcyclohexanol).
• Common names: name alkyl group + space + “alcohol” (e.g. tert-butyl alcohol).
• Polyols (diols/triols)
  • Two OH: “diol”; three OH: “triol”.
  • Positions required unless every carbon bears OH.
  • Common examples: ethylene glycol (1,2-ethanediol), glycerol (1,2,3-propanetriol).

Nomenclature of Ethers

• Common system (simple ethers): list two alkyl groups alphabetically + “ether”.
  • Symmetrical: prefix “di-” (e.g. diethyl ether).
• IUPAC (complex ethers)
  • Smaller group = alkoxy substituent (replace “-yl” with “-oxy”).
  • Larger group = parent alkane.
  • Example: (\ce{CH3OCH2CH3}) → methoxyethane.

Preparation of Ethers – Williamson Ether Synthesis

• React an alkoxide ((RO^-)) with an alkyl halide ((R'X)) via (\mathrm{S_N2}):
  $RO^- + R'X \longrightarrow R{-}O{-}R' + X^-$
• Key planning points
  • Best when the halide is 1° (or CH(_3)) to avoid elimination.
  • Alkoxide made by deprotonating alcohol with strong base ((\ce{NaH}), (\ce{Na}) metal, etc.).
  • Unsymmetrical ether: choose path giving less hindered halide.

Alcohol Reactivity – Leaving-Group Problems & Solutions

• Native (\ce{OH^-}) is a poor leaving group; must be transformed.
  • Protonation (strong acid) ⇒ (\ce{H2O}), a good LG.
  • Convert to tosylate ((\ce{OTs})), halides ((\ce{Cl}), (\ce{Br}), (\ce{I})), or similar.
• Ethers: (\ce{OR^-}) also poor LG ⇒ generally inert; exceptions: protonated epoxides, cleavage with strong acids.

Dehydration of Alcohols ((\beta)-Elimination)

• Removes (\ce{H2O}) ((\ce{OH}) from (\alpha)-C, (\ce{H}) from (\beta)-C) → forms an alkene.
• Typical reagents: conc. (\ce{H2SO4}), (\ce{H3PO4}); or (\ce{POCl3}/)pyridine.
• Relative reactivity (ease of dehydration):
  3^\circ > 2^\circ > 1^\circ (stability of resulting carbocation or transition state).
• Regioselectivity – Zaitsev Rule
  • When multiple (\beta) carbons, major product = more substituted alkene.
• Mechanisms
  • 3° & 2° alcohols: (E1)
  1. Protonate OH ⇒ (\ce{H2O}) leaves → carbocation.
  2. Base ((\ce{HSO4^-}) or pyridine) removes (\beta)-H.
  • 1° alcohols: (E2) (no stable carbocation).
  • Concerted: proton transfer + C–O cleavage + (\beta)-H removal.
• Carbocation Rearrangements
  • 1,2-hydride or 1,2-alkyl shifts produce a more stable cation → different skeletal product.
  • Migration carries bonding e⁻ pair; migrating carbon becomes cationic.
• Alternative reagent ((\ce{POCl3})/pyridine)
  • Converts (\ce{OH}) → (\ce{OP(O)Cl2}) good LG, followed by (E2); avoids strong acid/cation rearrangements.
• Application: synthesis of patchouli alcohol derivatives.

Conversion of Alcohols to Alkyl Halides

Using Hydrogen Halides ((\ce{HX}); X = Cl, Br, I)

• Overall: $ROH + HX \rightarrow RX + H_2O$
• Reactivity order of HX: HI > HBr > HCl (parallel to acidity).
• Mechanistic dichotomy
  • 3° & many 2°: (S_N1) (carbocation, racemization/inversion).
  • 1° & (\ce{CH3OH}): (S_N2) → inversion at stereocenter.
• HCl with 1° alcohol requires ZnCl(_2) (Lucas reagent); Zn(^ {2+}) coordinates O, enhancing LG ability.
• Stereochemical outcomes
  • (S_N2): inversion.
  • (S_N1): racemic mixture (if stereogenic center created).

Using Thionyl Chloride ((\ce{SOCl2})/pyridine)

• Excellent for 1° & 2° alcohols → RCl.
• Mechanism ((S_N2))
  1. Alcohol attacks (\ce{SOCl2}) → chlorosulfite intermediate.
  2. Pyridine deprotonates ⇒ good LG (\ce{OSOCl}).
  3. Cl¯ (generated in step 1) back-side attacks, expelling (\ce{SO2}) + Cl⁻; net inversion.
• By-products (\ce{SO2}) (gas) + HCl removed – drives equilibrium.

Using Phosphorus Tribromide ((\ce{PBr3}))

• Similar two-step (S_N2) → RBr (+ (\ce{HOPBr2})).
• Retains inversion of configuration.

Summary Table (ROH → RX)

• \ce{HCl}\, (\pm ZnCl2): all ROH; (SN1) for 2°, 3°; (SN2) for 1°.
• \ce{SOCl2}\ (\text{pyridine}): best for 1°, 2°; (S_N2), inversion.
• \ce{HBr} & \ce{PBr3}: analogous rules.
• \ce{HI}: strong acid works for all, similar mechanism trend.

Formation & Reactions of Tosylates

• Tosyl chloride (TsCl) + pyridine → alkyl tosylate ((\ce{ROTs})).
  • Reaction retains stereochemistry at C–O (no bond to C broken).
  • Converts poor LG (OH) to excellent LG ((OTs^--)).
• Subsequent reactions
  • (S_N2) or (E2) with nucleophiles/bases just like alkyl halides.
  • For stereocenters: overall sequence (formation = retention, substitution = inversion) → net inversion.
  • Example: cis- (R,R) diol → tosylate formation (retention) → (S_N2) attack by (\ce{OCH3^-}) (inversion) ⇒ trans product.

Epoxides – Leaving-Group & Ring-Opening Insight

• Though (\ce{OR^-}) is a poor LG, protonated epoxides possess a strained, positively charged three-membered ring.
• Nucleophilic attack opens ring, relieving angle strain → highly exergonic; stereospecific anti-opening.

Conceptual & Practical Connections

• Interconversion logic: Alcohol ⟺ alkyl halide ⟺ tosylate provides modular strategies for C–C and C–heteroatom bond formation.
• Synthetic planning: Choose leaving-group modification (acidic, sulfonyl, halogen) that best aligns with substrate structure (1° vs 3°), desired mechanism ((SN1), (SN2), (E1), (E2)), and stereochemical outcome.
• Safety/Green Chemistry
  • (\ce{SOCl2}) and (\ce{PBr3}) produce toxic, corrosive by-products ((\ce{SO2}), HCl, (\ce{HOPBr2})); proper ventilation & PPE required.
  • Williamson synthesis avoids strong acids but uses reactive alkoxides; moisture exclusion essential.

Worked Example Capsules

• Williamson Synthesis Planning
  1. Target ether: anisole ((\ce{CH3OPh})).
  2. Best path: phenoxide (hindered, stable) + (\ce{CH3I}) (methyl iodide, unhindered) ⇒ avoid aryl halide (S_N) issues.
• Dehydration of 2-methyl-2-butanol
  • 3° alcohol; (E1) with (\ce{H2SO4}) ⇒ mixture dominated by 2-methyl-2-butene (Zaitsev) with minor 2-methyl-1-butene.
• Carbocation Rearrangement
  • 3,3-dimethyl-2-butanol + acid → initial 2° cation → 1,2-hydride shift → 3° cation → elimination → 2,3-dimethyl-2-butene (skeleton changed).