Alcohol & Phenol — Intuition Note
1. Definition — What Are They?
Alcohol: R-OH. The -OH is attached to an sp3 hybridised carbon. General formula: $C_nH_{2n+2}O$ for saturated monohydric alcohols.
Phenol: Ar-OH. The -OH is attached directly to a benzene ring.
Crucial: Phenol is NOT an alcohol and NOT an aromatic alcohol. They are different functional groups. Exam trick: benzyl alcohol ($C_6H_5CH_2OH$) is an aromatic alcohol, NOT a phenol — the -OH is on an sp3 carbon.
Alcohols are functional group isomers of ethers. Same formula ($C_nH_{2n+2}O$), different connectivity. Example: ethanol ($CH_3CH_2OH$) vs dimethyl ether ($CH_3OCH_3$).
Alcohol Classification — The Carbon Counts
| Class | Structure | Oxidation product |
|---|---|---|
| Primary (1°) | $R-CH_2OH$ | Aldehyde $\rightarrow$ Carboxylic acid |
| Secondary (2°) | $R_2CHOH$ | Ketone |
| Tertiary (3°) | $R_3COH$ | Resistant to oxidation |
The classification is about the carbon bearing the -OH, not the number of R groups on oxygen. Contrast with amines (classified by R groups on N — different system).
MM: Think of the alcoholic carbon as the "busy-ness" of that carbon. Primary has 1 carbon neighbour, secondary has 2, tertiary has 3. More neighbours = more alkyl groups donating electrons = more stable carbocation if it forms.
2. Physical Properties — What They're Like
Boiling Point — The Hierarchy
Compare compounds in this order (most dominant first):
H-bonding > Dipole-dipole > Exposed surface area > RMM
| Compound | Type | BP | Why? |
|---|---|---|---|
| Propan-1-ol | Alcohol | 97 °C | H-bonding |
| Propanone | Ketone | 56 °C | Dipole-dipole only |
| Dimethyl ether | Ether | −24 °C | Dipole-dipole only |
| Propane | Alkane | −42 °C | vdW only |
The chasm between alcohol and ether of same formula is pure H-bonding. That ~120 °C gap is what $-$OH hydrogen bonding is worth.
Trend within alcohols for same RMM: Primary > Secondary > Tertiary. Branching reduces exposed surface area, less vdW, lower BP.
Intramolecular vs Intermolecular H-bonding (phenols):
- 2-nitrophenol (intramolecular H-bond): BP 217 °C
- 4-nitrophenol (intermolecular H-bond): BP 245 °C
MM: Intramolecular H-bonding is like holding hands with yourself — it doesn't help you grip others. Intermolecular is everyone holding hands, forming a chain — much harder to pull apart.
Solubility
| Chain length | Solubility |
|---|---|
| 1–3 C | Fully soluble in water |
| 4–10 C | Oily, decreasing solubility |
| >11 C | Almost insoluble solids |
Solubility drops as the hydrophobic tail gets longer. More -OH groups → higher solubility.
Phenol: partially soluble below 66 °C, completely soluble above. The aromatic ring is compact enough and forms strong H-bonds with water.
Intuition Summary
Alcohols: H-bond with everything → high BP, good solubility (short chain)
Phenols: H-bond + ring stacking → complex solubility, higher BP than aliphatic analogues
Ethers: Dipole-dipole only → moderate BP, moderate solubility
Alkanes: vdW only → low BP, insoluble
3. Acidity — The pKa Story
The Big Picture
| Compound | pKa | Notes |
|---|---|---|
| Carboxylic acid | ~4.8 | Strongest of the organic acids |
| Phenol | ~10.0 | Resonance stabilised phenoxide |
| Water | 14.0 | Reference point |
| Methanol | 15.5 | Most acidic alcohol |
| Ethanol | 15.9 | |
| Isopropyl alcohol | 16.5 | |
| tert-Butyl alcohol | 18.0 | Least acidic |
| Cyclohexanol | ~16–18 | Typical 2°/3° alcohol |
Order: Carboxylic acid > Phenol > Water > Alcohol
Phenol is ~100 million times more acidic than cyclohexanol.
Why Phenol Is More Acidic Than Alcohol
Phenoxide ion ($PhO^-$) is stabilised by resonance delocalisation — the negative charge spreads over the oxygen and three ring carbons. An alkoxide ($RO^-$) has nowhere to dump the charge; it sits entirely on oxygen.
The phenoxide ion has resonance forms that put negative charge on ortho and para carbons. This is why substituents at ortho/para positions have the strongest effect on acidity.
Alcohol Acidity Trend
Methanol > Primary > Secondary > Tertiary
Two effects that push in the same direction:
- Inductive effect: Alkyl groups are electron-donating (+I). More alkyl groups → more electron density pushed toward O → less willing to lose H⁺.
- Solvation effect: Bulkier R groups physically block water from stabilising the alkoxide ion.
MM: Think of the alkoxide ion as a hot potato. More alkyl groups = thicker gloves = harder to pass the potato (proton) to water.
Phenol Acidity — Substituent Effects
EWG (electron-withdrawing): Pull electron density from ring → stabilise phenoxide → more acidic (lower pKa). Most effective at ortho/para (resonance can place + charge on carbon adjacent to -OH).
| Compound | pKa |
|---|---|
| Phenol | 10.00 |
| 2-nitrophenol | 7.20 |
| 4-nitrophenol | 7.20 |
| 3-nitrophenol | 8.40 |
| 2,4,6-trinitrophenol (picric acid) | ~0.4 |
EDG (electron-donating): Push electron density to ring → destabilise phenoxide → less acidic (higher pKa).
| Compound | pKa |
|---|---|
| Phenol | 10.00 |
| 4-aminophenol | 10.30 |
| 4-methylphenol | ~10.2 |
The key exam skill: Draw resonance structures. If the resonance form puts positive charge on the carbon next to -OH (EWG at ortho/para), acidity increases. If it puts negative charge there (EDG at ortho/para), acidity decreases.
4. Identification Tests — How to Tell Them Apart
Lucas Test (Distinguishes 1°/2°/3° Alcohols)
Reagent: Conc. HCl + ZnCl2 What happens: Zn2+ complexes with O lone pairs, weakens C-O bond. Alcohol dissolves in reagent. Alkyl chloride is insoluble → cloudy/turbid.
| Alcohol | Time to cloudiness | What it means |
|---|---|---|
| 3° | Immediate | Forms stable carbocation instantly |
| 2° | ~5 minutes | Moderate |
| 1° | No cloudiness at RT | Unreactive without heat |
Limit: Only works for alcohols < 6 carbons (need complete solubility in the reagent).
MM: The Lucas test is a race. Tertiary alcohols are Usain Bolt (instant finish). Primary alcohols are a tortoise (won't finish in the exam time).
Chromic Acid Test (Distinguishes Oxidisable Alcohols)
Reagent: Orange $K_2Cr_2O_7/H^+$ (or Jones reagent) Positive: Orange → green/blue (Cr3+)
| Type | Result |
|---|---|
| 1° alcohol | Positive (green/blue) |
| 2° alcohol | Positive (green/blue) |
| 3° alcohol | Negative (stays orange) |
| Ketone | Negative (stays orange) |
| Aldehyde | Positive (green/blue) |
Iodoform Test (Methyl Ketones + Ethanol)
Reagent: I2 + NaOH Positive: Yellow precipitate of CHI3 (triiodomethane)
Positive for:
- Ethanal ($CH_3CHO$) — the only aldehyde
- Methyl ketones ($CH_3COR$)
- Ethanol — the only 1° alcohol (oxidises to ethanal then to CHI3)
- 2° alcohols that oxidise to methyl ketones (e.g. propan-2-ol)
Negative for:
- Methanol, 3° alcohols, other 1° alcohols
MM: The iodoform test is looking for a CH3CO- group (or something that can become one). Ethanol is the only alcohol that can be oxidised to ethanal without breaking the CH3-C=O unit.
Phenol-Specific Tests
| Test | Reagent | Positive result |
|---|---|---|
| Bromine water | $Br_2(aq)$ | Decolourises + white precipitate (2,4,6-tribromophenol) |
| Iron(III) chloride | $FeCl_3(aq)$ | Light purple complex |
Acidity-Based Distinction
Phenol dissolves in NaOH but does NOT produce CO2 with NaHCO3 or Na2CO3.
- Carboxylic acid + NaHCO3 → CO2 bubbles (effervescence)
- Phenol + NaHCO3 → no reaction (not acidic enough)
- Phenol + NaOH → dissolves (phenoxide formed)
MM: This is the Goldilocks of acidity. Carboxylic acids are too strong (fizz with bicarb). Alcohols are too weak (won't even dissolve in NaOH). Phenol is just right — dissolves in NaOH but no bicarb fizz.
5. Preparation — Where They Come From
Alcohol Preparations
1. Fermentation
$$C_6H_{12}O_6 \xrightarrow{\text{yeast}} 2C_2H_5OH + 2CO_2$$
- Yield limited to 12–15% (yeast dies in higher alcohol)
- Distillation → 40–50% (hard liquor)
- 95% azeotrope is the maximum via simple distillation
- Absolute (100%) ethanol requires CaO or other dehydrating agent
2. Hydration of Alkene
$$RCH=CH_2 + H_2O \xrightarrow{\text{dilute } H_2SO_4} RCH(OH)CH_3$$
- Markovnikov addition (OH goes to more substituted carbon)
- Dilute acid (excess water) pushes equilibrium toward alcohol (Le Châtelier)
- Concentrated acid with little water → favours alkene instead
Condition note: High temp, high pressure for industrial ethanol from ethylene.
3. Nucleophilic Substitution of Haloalkane
$$R-X + NaOH/KOH \xrightarrow{\text{aq/acetone}} R-OH + X^-$$
| Haloalkane | Mechanism | Product |
|---|---|---|
| 1° R-X | SN2 | 1° alcohol (clean) |
| 2° R-X | SN2 + E2 compete | 2° alcohol + some alkene |
| 3° R-X | E2 dominates | Alkene (too hindered for SN2) |
4. Grignard Synthesis (Carbon-Carbon Bond Formation)
First make the Grignard: $R-X + Mg \xrightarrow{\text{dry ether}} RMgX$
| Carbonyl | Product alcohol |
|---|---|
| Formaldehyde ($HCHO$) | Primary alcohol ($RCH_2OH$) |
| Aldehyde ($R'CHO$) | Secondary alcohol ($RR'CHOH$) |
| Ketone ($R'_2CO$) | Tertiary alcohol ($RR'_2COH$) |
Critical condition: ANHYDROUS. Grignard reagents are destroyed by water: $RMgX + H_2O \rightarrow R-H + Mg(OH)X$. That's just alkane — useless.
5. Reduction of Carbonyls
- Aldehydes → Primary alcohols (NaBH4, LiAlH4, H2/catalyst)
- Ketones → Secondary alcohols (same reagents)
Phenol Preparations
1. Cumene Process (Industrial)
Benzene $\xrightarrow{+C_3H_6}$ Cumene $\xrightarrow{O_2}$ Cumene hydroperoxide $\xrightarrow{H^+}$ Phenol + Acetone
All industrial phenol is made this way. Requires demand for both phenol AND acetone.
2. Laboratory Method
Aromatic amine $\xrightarrow{HNO_2}$ Diazonium salt $\xrightarrow{H_2O}$ Phenol + N2
6. Reactions — What They Do
Alcohol Reactions
6a. With Active Metals
$$2ROH + 2Na \rightarrow 2RONa + H_2\uparrow$$
Reactivity: Methanol > Ethanol > 2° > 3° 3° reacts very slowly — use K or NaH in THF.
6b. Conversion to Haloalkane
| Reagent | Product | Notes |
|---|---|---|
| HX | R-X | Reactivity: $HI > HBr > HCl$; phenol < 1° < 2° < 3° |
| HCl + ZnCl2 | R-Cl | Lucas test conditions |
| PX3 | R-X | Good for 1°/2°, no rearrangement, poor for 3° |
| SOCl2 (+ pyridine) | R-Cl | Gaseous byproducts (SO2 + HCl), retention of configuration |
6c. Dehydration to Alkene (E1)
$$R_2CHCR_2OH \xrightarrow{\text{conc } H_2SO_4,\ 180^\circ C} R_2C=CR_2 + H_2O$$
| Temp | Product |
|---|---|
| ~140 °C | Symmetrical ether ($ROR$) |
| ~180 °C | Alkene (Zaitsev — most substituted alkene is major) |
Ease: 3° > 2° > 1° (carbocation stability). E1 mechanism. Rearrangements possible.
MM: Heat drives elimination. Lower temp = ether (two alcohols meet). Higher temp = alkene (water leaves one molecule). The temperature dial chooses the path.
6d. Oxidation
| Alcohol | Mild oxidant (PCC/CH2Cl2) | Strong oxidant (K2Cr2O7/H+) |
|---|---|---|
| 1° | Aldehyde | Carboxylic acid |
| 2° | Ketone | Ketone |
| 3° | No reaction | No reaction (needs C-C cleavage) |
PCC is the only reagent that stops at the aldehyde. Strong oxidants barrel through to the carboxylic acid because the aldehyde is even easier to oxidise than the alcohol.
6e. Esterification
$$ROH + R'COOH \xrightarrow{H^+} R'COOR + H_2O$$
- Equilibrium — use excess reagent or remove water to push forward
- Acyl chloride route is irreversible: $ROH + R'COCl \rightarrow R'COOR + HCl$
6f. Iodoform Reaction
Only ethanol among 1° alcohols. See Identification section above.
Phenol Reactions
6a. O-H Bond Cleavage
- Forms phenoxide with NaOH (more acidic than water)
- Phenoxide + acyl chloride/anhydride → ester
- Phenol + carboxylic acid → poor reaction (phenol is a weak nucleophile — lone pairs delocalised into ring)
6b. C-O Bond — Very Resistant
- No acid-catalysed elimination
- No SN2 (back-side attack impossible on aromatic ring)
- Not easily oxidised (no H on carbon bearing -OH)
6c. Electrophilic Aromatic Substitution (EAS)
-OH is strongly activating and ortho-para directing. Reacts without Lewis acid catalyst.
Halogenation:
- Non-polar solvent, low T: mixture of o- and p-halophenol
- Aqueous, higher T: 2,4,6-trihalophenol
Nitration:
- Dilute HNO3, RT: o- and p-nitrophenol
- Conc. HNO3: 2,4,6-trinitrophenol (picric acid)
MM: The -OH group supercharges the ring. Benzene needs FeBr3 to brominate. Phenol does it in water at room temperature — three bromines, no catalyst needed.
7. Exam Watch — Stereochemistry + Alcohol
From exam leaks: Section B combines Alcohol with Stereochemistry.
Watch for:
- Chiral alcohols (e.g. butan-2-ol has a stereocenter at C2)
- R/S configuration of chiral alcohols
- Fischer projections of alcohols with chiral centres
- Optical isomers of alcohols (enantiomers: (+) and (−) forms)
The Lucas test product (alkyl chloride) has the opposite configuration if SN2, or racemic if SN1. This is how stereochemistry ties into alcohol reactions.
Related
- Carbonyl Compounds — Oxidation products; Grignard partners
- Carboxylic Acids & Derivatives — Further oxidation; esterification
- Stereochemistry — Optical isomerism of chiral alcohols