Alcohols, Phenols, and Ethers

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:

1. Find the longest carbon chain containing the –OH group
2. Replace the -e ending of the alkane with -ol
3. Number the chain to give the –OH the lowest possible number
4. 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 (forming a 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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