Chapter 7 / 7.4 Alcohols and Phenols

Alcohols & Phenols – Quick, Friendly Notes 📚

Alcohols, phenols and their reactions pop up everywhere in organic chemistry questions. Here’s a chat-style summary that keeps the maths neat with KaTeX, sprinkles of emojis for fun, and just the facts you need.:contentReference[oaicite:0]{index=0}

1 ️⃣ Structure & Bonding 🔬

• In simple alcohols the C–O–H angle is a bit under the perfect tetrahedral value (109° 28′) because the two lone-pairs on oxygen push bonds closer together.:contentReference[oaicite:1]{index=1}
• Phenol’s C–O bond is shorter (136 pm) than methanol’s thanks to partial double-bond character (lone-pair ↔ ring conjugation) and the sp² carbon on the ring.:contentReference[oaicite:2]{index=2}

2 ️⃣ Making Alcohols 🧪

2.1 From alkenes

1. Acid-catalysed hydration
   \( \text{RCH}\!=\!\text{CH}_2 + \mathrm{H_2O} \xrightarrow[\text{H}_2\mathrm{O}]{\mathrm{H}^+} \text{RCH(OH)CH}_3 \)
   Markovnikov addition via protonation → carbocation → water attack → deprotonation.:contentReference[oaicite:3]{index=3}
2. Hydroboration–oxidation
   \( (\mathrm{BH_3})_2 + \text{alkene} \rightarrow \text{trialkyl-B} \xrightarrow[\mathrm{NaOH}]{\mathrm{H_2O_2}} \text{anti-Markovnikov alcohol} \) 🎯 Excellent yields!:contentReference[oaicite:4]{index=4}

2.2 From carbonyl compounds

• Reduce aldehydes/ketones with H₂/Pt, NaBH₄ or LiAlH₄ → primary or secondary alcohol.:contentReference[oaicite:5]{index=5}
• Reduce acids/esters using LiAlH₄ (lab) or convert ester → catalytic H₂ (industry) → primary alcohol.:contentReference[oaicite:6]{index=6}

2.3 With Grignard reagents

\( \text{R–MgX} + \text{C=O} \xrightarrow{\text{dry ether}} \) adduct → \( \xrightarrow{\text{H}_2\text{O}} \) alcohol.

• Methanal → 1° alcohol
• Other aldehydes → 2° alcohol
• Ketones → 3° alcohol

Handy memory: aldehyde Adds one carbon; ketone Keeps carbon count. 😉:contentReference[oaicite:7]{index=7}

3 ️⃣ Making Phenols 🌿

1. Fused NaOH on haloarene (623 K, 320 atm) → sodium phenoxide → acidify.🔧
2. From benzenesulphonic acid: sulphonate benzene → melt with NaOH → acidify.🌀
3. Diazonium salt hydrolysis: warm \(\mathrm{ArN_2^+Cl^-}\) with water → phenol + N₂.🎈
4. Cumene process: air-oxidise cumene → cumene-hydro­peroxide → dilute acid gives phenol + acetone (bonus co-product 💸).:contentReference[oaicite:8]{index=8}

4 ️⃣ Physical Properties 🌡️

• Boiling point rises with chain length and falls with branching. Hydrogen bonding makes alcohols/phenols boil much higher than ethers or alkanes of similar mass.🔥:contentReference[oaicite:9]{index=9}
• Solubility comes from hydrogen-bonding with water. Small chains = fully miscible; big hydrophobic groups break the party.💧:contentReference[oaicite:10]{index=10}

5 ️⃣ Reactions of Alcohols 🧩

5.1 Cleaving the O–H bond (acidity) ⚡

• Metals: \( 2\,\text{ROH} + 2\,\text{Na} \rightarrow 2\,\text{RONa} + \text{H}_2 \uparrow \)
• Esterification: \( \text{ROH} + \text{R’COOH} \xrightarrow[\text{conc H_2SO_4}]{} \text{R’COOR} + \text{H}_2\text{O} \) (reversible, remove water). Aspirin forms by acetylating salicylic acid.💊
• Acid strength order: water > alcohols (3° < 2° < 1°). Phenols beat alcohols thanks to resonance-stabilised phenoxide ion.🔋:contentReference[oaicite:11]{index=11}

5.2 Cleaving the C–O bond 🛠️

Tip: passing vapours over hot Cu (573 K) dehydrogenates 1°/2° alcohols.🔥:contentReference[oaicite:12]{index=12}

6 ️⃣ Reactions of Phenols 🎨

6.1 Electrophilic aromatic substitution

• Nitration (dil HNO₃, 298 K): mix of o- and p- nitrophenols (separate by steam distillation). Concentrated acid gives picric acid (2,4,6-trinitrophenol).💥
• Halogenation:
  • Br₂ in CHCl₃/CS₂ (cold) → o/p-bromophenol.
  • Br₂ water → 2,4,6-tribromophenol (white ppt).⬜

6.2 Name reactions to remember

• Kolbe: \( \text{C}_6\text{H}_5\text{ONa} + \text{CO}_2 \xrightarrow{473\,\text{K}} \)
  salicylic acid (o--hydroxybenzoic acid).🌿
• Reimer–Tiemann: Phenol + CHCl₃ + NaOH → salicyl­aldehyde (o--hydroxybenzaldehyde).🍒
• Zn dust strips –OH → benzene.🌀
• Oxidation with chromic acid → benzoquinone; slow air oxidation darkens stored phenol.🎨

Important Concepts for NEET ⭐

• Hydroboration–oxidation gives anti-Markovnikov alcohols with almost 100 % yield.🔄
• Lucas test speed ranks 3° > 2° > 1°; trusty for classifying alcohols in viva.⏱️
• Kolbe (→ salicylic acid) and Reimer–Tiemann (→ salicylaldehyde) turn phenol into ortho-products tested often.🎯
• Electron-withdrawing –NO₂ groups boost phenol acidity; pK_a drops dramatically (phenol ≈ 10, 2,4,6-trinitrophenol ≈ 7).📉
• Order of ease for dehydration & carbocation formation: 3° > 2° > 1° — use it to predict alkene products quickly.🚦

✨ Happy studying & good luck! ✨