Properties Reactions Alcohols

7.6 Physical Properties

All three families — alcohols, phenols and ethers — contain a C–O bond, yet their physical behaviour splits cleanly into two camps depending on whether a hydrogen sits on oxygen. Alcohols and phenols carry an –OH group and are powerful hydrogen-bond donors and acceptors; ethers carry no O–H and can only accept hydrogen bonds, not donate them.

7.6.1 Boiling Points

Intermolecular hydrogen bonding between molecules of an alcohol creates large clusters that must be separated before boiling can occur. The resulting boiling points are far higher than those of alkanes of comparable molar mass, and also much higher than those of the analogous ethers which have no O–H.

7.6.2 Solubility in Water

Lower alcohols (methanol, ethanol, propan-1-ol, tert-butanol) are miscible with water because their –OH inserts into the water H-bond network. As the hydrocarbon chain lengthens the hydrophobic part dominates and solubility falls — decan-1-ol is practically insoluble. Phenol is only moderately soluble (≈ 8.3 g per 100 g H₂O at 20 °C); the large aromatic ring repels water. Phenol dissolves freely in most organic solvents.

7.7 Chemical Properties of Alcohols

The reactions of an alcohol fall into three groups sorted by which bond breaks: the O–H bond, the C–O bond, or the C–H bond α to the oxygen (oxidation/dehydrogenation).

7.7.1 Reactions Involving Cleavage of the O–H Bond

(a) Acidity

Alcohols are very weak acids, roughly as acidic as water (pKa ≈ 16–18). The equilibrium R–OH ⇌ R–O⁻ + H⁺ lies far to the left. Adding an electron-donating alkyl group destabilises the alkoxide anion, so acidity decreases along the series:

(b) Reaction with active metals

Sodium, potassium and aluminium liberate hydrogen gas from an alcohol, giving the corresponding alkoxide:

(c) Esterification

In the presence of concentrated H₂SO₄, an alcohol and a carboxylic acid give an ester and water in an equilibrium that can be driven forward by removing water.

Mechanism: protonation of the carbonyl activates it towards nucleophilic addition by the alcohol; a tetrahedral intermediate then loses water and a proton to deliver the ester. With an acid chloride or anhydride the reaction is much faster and essentially irreversible:

7.7.2 Reactions Involving Cleavage of the C–O Bond

(a) With HX — substitution

Protonation converts the poor leaving group –OH into water (the best leaving group). The halide then displaces water: 3° alcohols go by SN1 (carbocation), 1° alcohols by SN2. The classic Lucas test exploits this rate difference:

(b) With PCl₅, PCl₃, SOCl₂

(c) Dehydration to alkenes

Concentrated H₂SO₄ at 443 K, or passage over alumina (Al₂O₃) at 623 K, converts an alcohol into an alkene by loss of water. Reactivity order of alcohols toward dehydration follows carbocation stability: 3° > 2° > 1°. When more than one alkene is possible, the more substituted (Saytzeff) product dominates.

7.7.3 Oxidation and Dehydrogenation

Primary alcohols climb the "oxidation ladder" in two steps: first to an aldehyde (lose 2 H), then to a carboxylic acid (gain O / lose 2 H). Secondary alcohols stop at the ketone. Tertiary alcohols have no α-hydrogen to remove and therefore resist oxidation under ordinary conditions — strong reagents will cleave a C–C bond and break the carbon skeleton.

Dehydrogenation (passing the alcohol vapour over hot copper, 573 K) removes two hydrogens without adding oxygen:

7.8 Chemical Properties of Phenols

7.8.1 Acidity — The Defining Feature

Phenol (pKa ≈ 10) is a million times more acidic than ethanol (pKa ≈ 16). The phenoxide ion is stabilised by resonance: the negative charge is delocalised onto the ortho and para carbons of the ring, spreading it over four atoms instead of localising it on one oxygen.

Substituent effects on phenol acidity:

• Electron-withdrawing groups (–NO₂, –Cl, –CHO, –CN) especially at the ortho and para positions increase acidity by further stabilising the phenoxide. 4-nitrophenol pKa = 7.15; picric acid (2,4,6-trinitrophenol) pKa ≈ 0.4 — as strong as a mineral acid.
• Electron-donating groups (–CH₃, –OCH₃, –OH, –NH₂) decrease acidity; p-cresol pKa ≈ 10.3.

Because phenol is considerably stronger than carbonic acid, aqueous NaOH — but not aqueous NaHCO₃ — deprotonates it to sodium phenoxide:

7.8.2 Esterification

Phenols react faster with acid chlorides or anhydrides than with carboxylic acids. Acetylation of salicylic acid gives aspirin — one of the biggest-selling pharmaceuticals of the twentieth century.

7.8.3 Electrophilic Aromatic Substitution on the Ring

The –OH lone pair donates electron density into the ring, activating it strongly and directing incoming electrophiles to the ortho and para positions.

(a) Nitration

The two isomers are separated by steam distillation: the ortho isomer forms an intramolecular hydrogen bond (chelation between –OH and –NO₂) so it is volatile and distils over; the para isomer relies on intermolecular H-bonds, forms an associated polymer-like network, and stays behind in the flask.

(b) Halogenation

(c) Kolbe's reaction

Sodium phenoxide — a stronger nucleophile than phenol — attacks CO₂ at the ortho position; workup gives salicylic acid, the starting point for aspirin.

(d) Reimer–Tiemann reaction

Phenol + chloroform in aq. NaOH produces a dichlorocarbene electrophile (:CCl₂). The carbene attacks the ortho position; hydrolysis of the intermediate benzal chloride gives an aldehyde — salicylaldehyde.

7.8.4 Oxidation

Mild oxidation of phenol with Na₂Cr₂O₇/H₂SO₄ or chromic acid gives the bright-yellow dicarbonyl benzoquinone:

Worked Examples