Alcohols, Phenols, and Ethers
Alcohols, phenols, and ethers are three fundamental classes of oxygen-containing organic compounds that appear throughout the HAAD syllabus. Their importance extends far beyond academic chemistry — these functional groups are the structural basis of countless pharmaceutical agents, solvents, disinfectants, and anesthetics. For instance, ethanol is both a recreational beverage and a pharmaceutical solvent; phenol is the parent of aspirin and many disinfectants; ether gave its name to a class of anesthetics (diethyl ether was the first general anesthetic); and glycerol is the backbone of triglycerides and phospholipids in human biochemistry. Understanding the structure, properties, nomenclature, and reactions of these compounds is therefore essential for every HAAD candidate.
Alcohols: Structure and Classification
Alcohols contain the hydroxyl (-OH) functional group bonded to a saturated carbon atom (a carbon that is not doubly or triply bonded). The general formula of alcohols is R–OH, where R is an alkyl or substituted alkyl group.
Classification
Alcohols are classified based on the number of carbon atoms attached to the carbon bearing the –OH group:
Primary (1°) alcohols: The –OH is on a carbon attached to only one other carbon: CH₃–CH₂–OH (ethanol — C attached to one C and two H)
Secondary (2°) alcohols: The –OH is on a carbon attached to two other carbons: CH₃–CH(OH)–CH₃ (propan-2-ol — C attached to two C and one H)
Tertiary (3°) alcohols: The –OH is on a carbon attached to three other carbons: (CH₃)₃C–OH (tert-butanol — C attached to three C and no H)
This classification is critically important because the reactivity of alcohols in elimination and oxidation reactions depends directly on their degree of substitution.
Nomenclature
IUPAC naming of alcohols:
- Find the longest carbon chain containing the –OH group
- Replace the -e ending of the alkane with -ol
- Number the chain to give the –OH the lowest possible number
- Name substituents as prefixes
Examples:
- CH₃OH = Methanol (no number needed — the –OH is at C1)
- CH₃–CH₂–OH = Ethanol
- CH₃–CH₂–CH₂–OH = Propan-1-ol (n-propanol)
- CH₃–CH(OH)–CH₃ = Propan-2-ol (isopropanol)
- CH₃–CH₂–CH₂–CH(OH)–CH₃ = Pentan-2-ol
Common names are frequently used: methyl alcohol (methanol), ethyl alcohol (ethanol), isopropyl alcohol (2-propanol), glycol (ethane-1,2-diol), glycerol (propane-1,2,3-triol).
Physical Properties of Alcohols
The physical properties of alcohols are dominated by the hydrogen-bonding capability of the –OH group:
Hydrogen bonding: The –OH group can form strong hydrogen bonds with other alcohol molecules (intermolecular H-bonding) and with water (when R is small). This significantly elevates the boiling points of alcohols compared to alkanes and halides of similar molecular weight.
| Compound | Molecular Weight | Boiling Point (°C) |
|---|---|---|
| CH₃OH (methanol) | 32 | 64.7 |
| CH₃CH₃ (ethane) | 30 | -88.6 |
| C₂H₅OH (ethanol) | 46 | 78.3 |
| C₃H₇OH (propan-1-ol) | 60 | 97.2 |
| CH₃(CH₂)₂CH₃ (butane) | 58 | -0.5 |
Solubility: Alcohols with small R groups (methanol, ethanol, propan-1-ol) are completely miscible with water because the –OH group can hydrogen-bond with water. As the size of R increases, the hydrocarbon portion increasingly disrupts the water structure, reducing solubility. Alcohols with more than about four carbons are practically insoluble in water.
Acid-base properties: Alcohols are weak acids (pKa ≈ 15–18 for most alcohols — methanol pKa = 15.5, ethanol pKa = 15.9). They are much weaker acids than water (pKa = 15.7 for the reverse of water’s autoionization). This means alcohols do not react with NaOH to form water and a salt (the equilibrium lies far to the left). However, alcohols do react with sodium metal and with sodium hydride (NaH) to evolve hydrogen gas: 2CH₃CH₂OH + 2Na → 2CH₃CH₂ONa + H₂↑
Alcohols are also weak bases (pKb ≈ 15–18) and can be protonated by strong acids (e.g., HCl, H₂SO₄) to form oxonium ions (R–OH₂⁺).
Chemical Reactions of Alcohols
Oxidation Reactions
Alcohols can be oxidized by oxidizing agents including KMnO₄, K₂Cr₂O₇ (dichromate), and PCC (pyridinium chlorochromate). The product depends on the degree of substitution:
Primary alcohols oxidize first to aldehydes and then to carboxylic acids: CH₃CH₂OH →(oxidation) CH₃CHO (acetaldehyde) →(further oxidation) CH₃COOH (acetic acid)
Secondary alcohols oxidize to ketones (no further oxidation is possible): CH₃–CH(OH)–CH₃ →(oxidation) CH₃–CO–CH₃ (acetone/propan-2-one)
Tertiary alcohols do NOT oxidize (no hydrogen on the carbon bearing the –OH to be removed) — they are resistant to oxidation.
The Lucas Test: ZnCl₂ + concentrated HCl is used to differentiate primary, secondary, and tertiary alcohols based on the speed of reaction. Tertiary alcohols react immediately (形成 cloudy layer), secondary alcohols react within 5–10 minutes, and primary alcohols react very slowly (or only on heating). This is an SN1 reaction — the rate depends on the stability of the carbocation intermediate.
Elimination Reactions (Dehydration)
Alcohols undergo dehydration (elimination of H₂O) under acidic conditions (H₂SO₄ at 140–180°C) to form alkenes. The mechanism is E1 (unimolecular elimination via a carbocation). For unsymmetrical alcohols, Zaitsev’s Rule applies — the more substituted alkene is the major product:
CH₃–CH₂–CH(OH)–CH₃ →(H₂SO₄, 140°C) → CH₃–CH=CH–CH₃ (but-2-ene) + H₂O But-2-ene (more substituted) predominates over but-1-ene.
Ester Formation
Alcohols react with carboxylic acids (or acid derivatives like acyl chlorides and anhydrides) under acidic conditions to form esters — a condensation (addition-elimination) reaction: R–COOH + R’–OH →(H⁺) R–COOR’ + H₂O
Example: Acetic acid + Ethanol → Ethyl acetate (an ester with a fruity smell)
Alcohols also react with inorganic acids (HCl, HNO₃, H₂SO₄, H₃PO₄) to form inorganic esters: Ethanol + HNO₃ →(H₂SO₄) → Ethyl nitrate Glycerol + HNO₃ →(H₂SO₄) → Trinitroglycerin (nitroglycerin — a powerful explosive and also a drug used in angina treatment)
Reactions with Active Metals
Alcohols react with sodium, potassium, and other alkali metals to produce alkoxides (the conjugate base of the alcohol) and hydrogen gas: 2CH₃OH + 2Na → 2CH₃ONa + H₂↑ Sodium methoxide
The alkoxides are strong bases (stronger than NaOH) and are used in organic synthesis as deprotonating agents.
Phenols: Properties and Reactions
Phenols have the –OH group directly attached to an aromatic ring (Ar–OH). The simplest phenol is simply phenol (C₆H₅OH).
Distinction between Alcohols and Phenols
| Property | Alcohols | Phenols |
|---|---|---|
| pKa | 15–18 (very weak acids) | 10 (weakly acidic) |
| Reaction with NaOH | No (not acidic enough) | Yes (forms phenoxide salt) |
| Reaction with NaHCO₃ | No | No (distinguishes from carboxylic acids) |
| FeCl₃ test | No color change | Violet/purple color (phenoxide complex) |
| Acidity | Very weak acids | Weak acids |
Electrophilic Aromatic Substitution on Phenol
The –OH group on an aromatic ring is a strongly activating ortho-para director (+M effect). Phenol undergoes EAS reactions much more readily than benzene:
- Halogenation: Phenol + Br₂/H₂O → 2,4,6-tribromophenol (white precipitate) — even without a catalyst; this is a highly sensitive test for phenol
- Nitration: Phenol + dilute HNO₃ → a mixture of ortho- and para-nitrophenol
Kolbe’s Reaction (Decarboxylative Coupling)
Sodium phenoxide (from phenol + NaOH) reacts with CO₂ under pressure at 125°C to give sodium salicylate, which upon acidification yields salicylic acid (2-hydroxybenzoic acid). Salicylic acid is the precursor to aspirin (acetylsalicylic acid).
Ethers: Structure and Properties
Ethers have the general formula R–O–R’ (two alkyl or aryl groups attached to oxygen). The functional group is the ether linkage (C–O–C).
Nomenclature
- Common names: Name the alkyl groups alphabetically, followed by “ether”
- CH₃–O–CH₃ = Dimethyl ether
- CH₃–O–C₂H₅ = Ethyl methyl ether
- C₂H₅–O–C₂H₅ = Diethyl ether
- IUPAC names: Use the alkoxy prefix
- CH₃–O–CH₃ = Methoxymethane
- CH₃–O–C₂H₅ = Methoxyethane
Physical Properties
- Ethers are polar molecules (the C–O–C bond angle is approximately 110°, giving ethers a dipole moment)
- They cannot hydrogen-bond with themselves (no O–H or N–H bonds)
- Their boiling points are therefore similar to alkanes of similar molecular weight
- Ethers are slightly more water-soluble than alkanes (the oxygen can accept hydrogen bonds from water)
- Ethers are excellent solvents for organic reactions because they dissolve a wide range of organic compounds without reacting
Chemical Properties
Ethers are the least reactive of the common organic functional groups — they do not react with sodium, alkali metals, oxidizing agents, reducing agents, or cold dilute acids. This inertness makes them useful as solvents.
Reaction with strong acids: Ethers react with HI (hydriodic acid) or HBr under heated conditions to cleave the C–O bond: R–O–R’ + HI →(heat) → R–I + R’–OH
If excess HI is present, the alcohol product is further converted to alkyl iodide: R’–OH + HI → R’–I + H₂O
Ether as anesthetics: Diethyl ether (C₂H₅–O–C₂H₅) was the first general anesthetic used in surgery (first demonstrated by Crawford Long in 1842 and popularized by William Morton in 1846). Modern anesthetic ethers include isoflurane, sevoflurane, and desflurane — these have largely replaced diethyl ether due to their better safety profiles (lower flammability, faster recovery).
Glycols and Glycerol
Ethane-1,2-diol (ethylene glycol, HO–CH₂–CH₂–OH) is a diol (two –OH groups) used as:
- Antifreeze (car engine coolant) — its high boiling point (197°C) and low freezing point (-13°C) make it ideal
- Precursor to polyester fibers (PET) It is toxic if ingested (metabolized to oxalic acid → kidney failure). Antidote: ethanol (competes for the same enzyme, alcohol dehydrogenase).
Propane-1,2,3-triol (glycerol/glycerin) is a triol with three –OH groups. It is:
- A component of triglycerides (fats and oils)
- Used in pharmaceuticals (as a solvent, humectant, and laxative — glycerol enemas)
- Used in cosmetics (as a moisturizer)
- Used in nitroglycerin (trinitroglycerin) production
⚡ Exam tip: Primary alcohols → aldehydes → carboxylic acids on oxidation; secondary alcohols → ketones (no further oxidation); tertiary alcohols → no oxidation. Phenols give violet color with FeCl₃ test; alcohols do not. Ethers are chemically inert and are excellent organic solvents. Glycols (2 –OH groups) are toxic (ethylene glycol = antifreeze). Glycerol (3 –OH groups) is non-toxic and is a component of fats.
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