Alcohols, Phenols and Ethers
Easy Overview
Alcohols, phenols, and ethers are oxygen-containing organic compounds fundamental to chemistry and biology. Alcohols (R-OH) range from the familiar ethanol in drinks and methanol in fuel to isopropyl rubbing alcohol and glycerol in cosmetics. Phenols (Ar-OH) have hydroxyl groups attached to aromatic rings — they are more acidic than alcohols because the phenoxide ion is stabilized by resonance. Ethers (R-O-R') are the calm, stable cousins used as solvents. The functional group — the -OH group — defines alcohols and phenols. The O-H bond is polar, and the lone pairs on oxygen can form hydrogen bonds. This gives alcohols relatively high boiling points and water solubility (for small alcohols). Going down the series: methanol, ethanol, propanol, and butanol are fully miscible with water. As the carbon chain lengthens, the nonpolar alkyl part dominates, and solubility decreases. Phenol is somewhat soluble in water (~8 g/100 mL) and forms a homogeneous solution with NaOH (phenol is acidic). Ethers (R-O-R') have an oxygen connecting two alkyl or aryl groups. They are relatively unreactive (good solvents!) but can form explosive peroxides on prolonged exposure to air. The C-O-C bond angle is about 110° — close to the tetrahedral angle. Ethers are polar but cannot hydrogen bond with each other (no O-H), so their boiling points are lower than alcohols of similar molecular mass. The most famous ether — diethyl ether — was used as a general anaesthetic for over a century. Williamson's ether synthesis is the standard method for preparing ethers asymmetrically. Understanding the reactions of alcohols, phenols, and ethers is essential — they are the entry point to carbonyl chemistry, and many of these reactions teach you patterns (oxidation, substitution, elimination) that apply to many other functional groups.
Classification and nomenclature of alcohols
Alcohols are classified as primary (1°), secondary (2°), or tertiary (3°) based on how many alkyl groups are attached to the carbon bearing the -OH group. Examples: 1° — ethanol (CH₃CH₂OH), n-butanol; 2° — isopropyl alcohol (2-propanol), 2-butanol; 3° — tert-butyl alcohol (2-methyl-2-propanol). IUPAC nomenclature: replace -e of parent alkane with -ol. Number the carbon chain to give the -OH group the lowest number. Common names: use 'alcohol' after the alkyl group (methyl alcohol, ethyl alcohol, isopropyl alcohol). Diols (-diol) and triols (-triol): ethylene glycol (CH₂OH-CH₂OH, 1,2-ethanediol), glycerol (HOCH₂-CHOH-CH₂OH, 1,2,3-propanetriol). Allyl alcohol (CH₂=CH-CH₂OH) and benzyl alcohol (C₆H₅CH₂OH). The -OH group is the functional group — it determines most of the chemistry.
Physical properties and hydrogen bonding
Alcohols have significantly higher boiling points than corresponding alkanes and alkyl halides because they can form intermolecular hydrogen bonds (O-H···O). To boil an alcohol, you must supply enough energy to break these hydrogen bonds. Methanol (bp 65°C) vs methane (bp -162°C) — a difference of 227°C! Small alcohols (up to C₄) are water-soluble because the -OH group can hydrogen bond with water molecules. As the carbon chain increases, the nonpolar alkyl part dominates and solubility decreases. All alcohols are soluble in organic solvents. Phenol (C₆H₅OH) has a high melting point (40.5°C) and moderate water solubility due to its ability to form hydrogen bonds with water. Hydrogen bonding also explains why alcohols have higher viscosity than alkanes.
Methods of preparation of alcohols
(1) Hydration of alkenes: acid-catalyzed addition of H₂O across the double bond (follows Markovnikov's rule). CH₂=CH₂ + H₂O (H⁺) → CH₃CH₂OH. (2) Hydroboration-oxidation: BH₃ then H₂O₂/OH⁻ gives anti-Markovnikov alcohol — alkene → primary alcohol (if terminal alkene), with no rearrangement. (3) Reduction of carbonyl compounds: aldehydes → primary alcohols, ketones → secondary alcohols (using NaBH₄, LiAlH₄, or catalytic hydrogenation). (4) Grignard synthesis: RMgX + formaldehyde → 1° alcohol, + other aldehydes → 2° alcohol, + ketones → 3° alcohol. (5) From alkyl halides via SN2: R-X + OH⁻ → R-OH + X⁻ (aqueous NaOH or KOH). (6) Fermentation: C₆H₁₂O₆ (glucose) → 2C₂H₅OH + 2CO₂ (zymase enzyme from yeast). Industrial ethanol: hydration of ethene (400°C, 70 atm, H₃PO₄ catalyst) or fermentation of molasses.
Reactions of alcohols — dehydration to alkenes
Alcohols undergo dehydration (elimination of H₂O) to form alkenes in the presence of an acid catalyst (conc. H₂SO₄ or H₃PO₄) and heat. The reaction follows E1 mechanism (carbocation intermediate): R-OH + H⁺ → R-OH₂⁺ (protonated alcohol, good leaving group), then H₂O leaves → R⁺ (carbocation), then loss of β-H → alkene. Reactivity: 3° > 2° > 1° alcohols (more stable carbocation = easier). For 2° and 3° alcohols, carbocation rearrangements (hydride shift, alkyl shift) can occur, giving rearranged alkene products. Dehydration of primary alcohols requires higher temperatures (170-180°C for ethanol → ethene). At lower temperatures (~140°C), ether formation occurs via intermolecular dehydration: 2 R-OH → R-O-R + H₂O. Distinguishing test: Lucas test — alcohol + ZnCl₂/HCl (conc.) at room temperature. Tertiary alcohols react immediately (cloudy in ~1 min), secondary in ~5 min, primary do not react at room temperature.
Oxidation of alcohols
Oxidation of alcohols gives different products depending on whether the alcohol is primary, secondary, or tertiary. Primary alcohol [O] → aldehyde [O] → carboxylic acid. Mild oxidizing agents (PCC, pyridinium chlorochromate, in CH₂Cl₂) stop at the aldehyde. Strong oxidizing agents (K₂Cr₂O₇/H₂SO₄, KMnO₄) go all the way to the carboxylic acid. Secondary alcohol [O] → ketone (does not oxidize further without breaking C-C bonds). Tertiary alcohol — no α-hydrogen → cannot be oxidized under normal conditions. Examples: Ethanol (1°) + K₂Cr₂O₇/H⁺ → ethanal (CH₃CHO) then ethanoic acid (CH₃COOH). 2-Propanol (isopropyl alcohol, 2°) + K₂Cr₂O₇/H⁺ → acetone (CH₃COCH₃). The color change from orange (Cr₂O₇²⁻) to green (Cr³⁺) is a diagnostic test for alcohols (breathalyzer test for drunk driving!).
Phenols — structure, acidity, and preparation
Phenol (C₆H₅OH): the -OH group is attached directly to a benzene ring. Structure: planar aromatic ring with -OH in the plane. The O-H group can rotate freely. Phenol is more acidic (pKa ≈ 10) than alcohols (pKa ≈ 16-18) but less acidic than carboxylic acids (pKa ≈ 4-5). Reason for higher acidity: the phenoxide ion (C₆H₅O⁻) is stabilized by resonance — the negative charge delocalizes into the benzene ring (especially at ortho and para positions). In an alcohol molecule, the alkoxide ion (RO⁻) has the negative charge localized on oxygen, with no resonance stabilization. Preparation: (1) Dow process — chlorobenzene + NaOH (high T, P) → sodium phenoxide + HCl → phenol. (2) From cumene (cumene hydroperoxide process — important industrial route): cumene (isopropylbenzene) + O₂ → cumene hydroperoxide → H⁺ → phenol + acetone. This is efficient because both phenol and acetone are valuable products. (3) From aniline: aniline + NaNO₂/HCl (0-5°C) → diazonium salt + H₂O (warm) → phenol + N₂ + HCl.
Reactions of phenols — electrophilic substitution
The -OH group in phenol is strongly activating and ortho/para-directing for electrophilic aromatic substitution. The lone pair on oxygen donates electron density into the ring by resonance. This makes the ortho and para positions especially electron-rich, so electrophiles attack there. (1) Nitration: dilute HNO₃ → ortho and para nitrophenols. Conc. HNO₃ + H₂SO₄ → picric acid (2,4,6-trinitrophenol — a yellow explosive! Picric acid was used in WWI as an explosive and dye). (2) Halogenation: Br₂ in water at room temperature → 2,4,6-tribromophenol (white precipitate!) — this reaction is so fast and quantitative that it is used as a test for phenol. No catalyst needed! (3) Sulfonation: conc. H₂SO₄ → phenol-2-sulfonic acid (at room temperature) and phenol-4-sulfonic acid (at 100°C). (4) Kolbe-Schmitt reaction: sodium phenoxide + CO₂ (125°C, pressure) → then H⁺ → salicylic acid (ortho-hydroxybenzoic acid), which is acetylated to aspirin! (5) Reimer-Tiemann reaction: phenol + CHCl₃ + NaOH → salicylaldehyde (ortho-hydroxybenzaldehyde). (6) Coupling reaction: phenol + diazonium salt → azo dye (bright orange/red precipitate). Ferric chloride test: phenol + FeCl₃ → violet/purple color — a distinguishing test for phenols.
Ethers — preparation and reactions
Ethers: R-O-R' (symmetrical) or R-O-R (unsymmetrical). Nomenclature: alkoxyalkane — methoxymethane (CH₃-O-CH₃), ethoxyethane (C₂H₅-O-C₂H₅, diethyl ether), methoxybenzene (C₆H₅-O-CH₃, anisole). Preparation: (1) Williamson's ether synthesis: R-O⁻Na⁺ + R'-X → R-O-R' + NaX. This is an SN2 reaction — works best with 1° alkyl halides (no elimination). Cannot use 3° alkyl halides (elimination dominates). The alkoxide is prepared from alcohol + Na or NaH. (2) Dehydration of alcohols: 2 R-OH (conc. H₂SO₄, 140°C) → R-O-R + H₂O. Only works for symmetrical ethers. (3) Alkoxymercuration-demercuration: similar to oxymercuration for alkenes. Physical properties: polar but no O-H → no intermolecular hydrogen bonding → lower boiling points than alcohols of same molecular mass. Higher boiling points than alkanes due to dipole-dipole interactions. Reactions: (1) Cleavage with HI/HBr: R-O-R + HI → RI + ROH (initially). With excess HI: 2 RI + H₂O. The reaction involves protonation of ether oxygen followed by SN1 or SN2 displacement. (2) Auto-oxidation: ethers form explosive peroxides on exposure to air! R-O-R + O₂ → ROOR (peroxide). This is why old ether bottles should never be distilled to dryness — the peroxides are concentrated and explode. Peroxide test: Fe²⁺ + SCN⁻ → red color if peroxides present.
Key Points
- •Alcohols: R-OH; classification 1°, 2°, 3° based on carbon bearing -OH
- •Hydrogen bonding → high boiling points, water solubility for small alcohols
- •Dehydration: conc. H₂SO₄, 170°C → alkene (E1); 140°C → ether
- •Oxidation: 1° → aldehyde → carboxylic acid; 2° → ketone; 3° → no reaction
- •Lucas test: 3° alcohol reacts instantly, 2° in 5 min, 1° no reaction
- •Phenol (Ar-OH): more acidic than alcohols (pKa ~10 vs ~16-18)
- •Phenoxide ion resonance-stabilized → deactivates ring? No, -OH activates ring for EAS
- •FeCl₃ test: phenol → violet/purple color (distinguishes from alcohols)
- •Kolbe-Schmitt: phenol + CO₂ → salicylic acid → aspirin
- •Br₂/H₂O test: phenol → 2,4,6-tribromophenol (white precipitate) — no catalyst needed
- •Williamson's ether synthesis: R-O⁻ + R'-X → R-O-R' (SN2, use 1° RX)
- •Ether cleavage: HX → R-X + R-OH (protonation then SN1/SN2)
- •Ethers form explosive peroxides on air exposure — never distill to dryness
Practice Questions
- Explain the acidity of phenol. Why is phenol more acidic than ethanol but less acidic than acetic acid?
- Describe with mechanisms: (a) Dehydration of ethanol to ethene (b) Williamson's ether synthesis.
- How are alcohols classified? Describe the oxidation of 1°, 2°, and 3° alcohols with K₂Cr₂O₇/H₂SO₄.
- Write reactions: (a) Kolbe-Schmitt reaction (b) Reimer-Tiemann reaction (c) Br₂/H₂O with phenol.
- Explain Lucas test. How can it be used to distinguish between 1°, 2°, and 3° alcohols?
- What is the iodoform reaction? Which class of alcohols gives a positive iodoform test?
- Prepare the following: (a) Ethanol from ethene (b) Phenol from cumene (c) Anisole from methyl iodide.
- Explain why ethers have lower boiling points than alcohols. Why should old ether samples not be distilled to dryness?