Alcohols, Phenols and Ethers

Alcohols (R-OH) have -OH attached to an sp³ hybridised alkyl carbon. Phenols (Ar-OH) have -OH attached directly to an sp² hybridised aromatic ring carbon. Ethers (R-O-R′) have oxygen joined to two carbon groups with no free -OH. Oxygen itself is sp³ hybridised with two lone pairs in all three, so the C-O-H or C-O-C bond angle deviates from the ideal tetrahedral angle (109.5°) in opposite directions depending on how many bulky alkyl groups sit on oxygen: in alcohols the C-O-H angle (~108-109°) is slightly compressed below tetrahedral because lone pair-lone pair repulsion dominates, similar to water; in ethers the C-O-C angle (~111-112°) is slightly expanded beyond tetrahedral because steric (bond pair-bond pair) repulsion between the two bulky alkyl groups now dominates over the lone pair effect.

In phenol, the oxygen lone pair is delocalised into the ring by resonance, giving the C-O bond partial double-bond character. This is why the C-O bond in phenol (~136 pm) is shorter than the C-O bond in methanol (~142 pm), and this same resonance is the structural root of phenol's very different chemistry from that of alcohols (see Sections 3 and 5).

Classifying alcohols: by the carbon bearing -OH, an alcohol is primary (1°) if that carbon holds one other carbon, secondary (2°) if it holds two, and tertiary (3°) if it holds three. By the number of -OH groups, alcohols are monohydric, dihydric (e.g. ethylene glycol) or trihydric (e.g. glycerol). Phenols are similarly mono-, di- or trihydric. Ethers with identical R groups on both sides are simple/symmetrical; with different R groups they are mixed/unsymmetrical.

IUPAC nomenclature: alcohols take the suffix -ol on the longest chain containing the -OH carbon, numbered to give -OH the lowest locant (e.g. propan-2-ol). Phenols are named as substituted phenols (e.g. 4-methylphenol). Ethers are named as alkoxyalkanes, the smaller R group cited as the alkoxy substituent on the larger parent chain (e.g. CH₃-O-C₂H₅ is methoxyethane).

Worked example: Classify and name (CH₃)₂CH-CH₂-CH₂-OH. Step 1: the longest chain with -OH is four carbons → butan-1-ol skeleton. Step 2: a methyl branch sits on C3, giving 3-methylbutan-1-ol. Step 3: the -OH carbon (C1) is attached to only one other carbon (C2), so this is a primary alcohol.

From alkenes: Acid-catalysed hydration (dilute H₂SO₄, H₂O) adds water across the double bond following Markovnikov's rule — the carbocation intermediate forms on the more substituted carbon, so -OH ends up there. Hydroboration-oxidation (B₂H₆ then H₂O₂/NaOH) instead gives the anti-Markovnikov product, with -OH on the less substituted carbon, via a concerted syn addition that avoids any carbocation.

From carbonyl compounds: Aldehydes and ketones are reduced to alcohols by LiAlH₄ or NaBH₄, or by catalytic hydrogenation (H₂/Ni or Pt). Aldehydes give primary alcohols; ketones give secondary alcohols.

From Grignard reagents: An alkyl/aryl magnesium halide (RMgX) adds to the carbonyl carbon of an aldehyde or ketone, and acidic hydrolysis (H₃O⁺) of the resulting magnesium alkoxide releases the alcohol. Worked example: CH₃MgBr reacting with (i) methanal (HCHO) then H₃O⁺ gives ethanol (primary, one new C-C bond added to a 1-carbon carbonyl); (ii) ethanal (CH₃CHO) then H₃O⁺ gives propan-2-ol (secondary); (iii) propanone ((CH₃)₂CO) then H₃O⁺ gives 2-methylpropan-2-ol (tertiary). This pattern — formaldehyde gives 1°, other aldehydes give 2°, ketones give 3° alcohols — is a reliable exam shortcut.

Preparation of phenols: (a) Haloarenes fused with NaOH at 623 K and 300 atm give sodium phenoxide, acidified to phenol (Dow's process). (b) Benzenesulphonic acid fused with NaOH gives sodium phenoxide, then acidified. (c) Benzenediazonium salts hydrolysed by warming with water/dilute acid liberate N₂ and give phenol. (d) Industrially, the cumene process is used: benzene + propene (H₃PO₄ catalyst) gives cumene (isopropylbenzene); air oxidation gives cumene hydroperoxide; treatment with dilute acid cleaves this to phenol plus acetone as a valuable by-product.

Boiling points and solubility: -OH groups form intermolecular hydrogen bonds, so alcohols and phenols boil far higher than alkanes, haloalkanes or ethers of comparable molar mass, and lower alcohols mix freely with water. As the hydrocarbon (hydrophobic) chain lengthens, solubility in water falls steadily because the nonpolar part increasingly dominates the molecule.

Acidity of alcohols: Alcohols are very weak acids (they turn litmus neither red nor blue) but react with reactive metals such as Na or K, liberating H₂ and forming alkoxides. Among alcohols, acidity falls in the order 1° > 2° > 3°, because more alkyl groups push electron density onto oxygen (+I effect, destabilising the conjugate base) and also sterically hinder solvation of the alkoxide ion.

Acidity of phenols: Phenol (pKa ≈ 10) is roughly a million times more acidic than a typical alcohol (pKa ≈ 16) or even water (pKa ≈ 15.7). The reason is that the phenoxide ion formed on loss of the O-H proton is stabilised by resonance: the negative charge is delocalised onto the ortho and para ring carbons, spreading it over the ring in addition to oxygen. No such delocalisation is possible for an alkoxide ion.

Ring substituents change phenol's acidity predictably. Electron-withdrawing groups (e.g. -NO₂, -Cl) at ortho/para positions further stabilise the phenoxide by resonance and/or induction, so acidity rises (lower pKa); electron-donating groups (e.g. -CH₃, -OCH₃) destabilise the phenoxide, so acidity falls (higher pKa). Worked example: Rank phenol, p-cresol (4-methylphenol) and p-nitrophenol by acidic strength. p-Nitrophenol is most acidic (NO₂ withdraws electron density by resonance, spreading the negative charge further and stabilising the anion strongly); phenol is next; p-cresol is least acidic (the methyl group's +I/hyperconjugative donation destabilises the phenoxide). Order: p-nitrophenol > phenol > p-cresol.

Reaction with metals: 2 R-OH + 2 Na → 2 R-O⁻Na⁺ + H₂↑, showing the weakly acidic O-H bond.

Esterification: Alcohols react reversibly with carboxylic acids in the presence of a mineral acid catalyst (Fischer esterification) to give an ester and water; being an equilibrium, excess reactant or removal of water drives it toward the ester.

Reaction with hydrogen halides (Lucas test): ROH + HX (conc. HCl with anhydrous ZnCl2, the Lucas reagent) → RX + H₂O. Reactivity follows 3° > 2° > 1° because the mechanism is SN1: tertiary alcohols give an immediate turbidity (stable 3° carbocation forms instantly at room temperature), secondary alcohols turn turbid within about five minutes, and primary alcohols show no visible reaction at room temperature. This is a standard qualitative test to distinguish the three classes.

Acid-catalysed dehydration (E1 mechanism): With conc. H₂SO₄ and heat, alcohols eliminate water to form alkenes, and when more than one alkene is possible the more substituted (Zaitsev) alkene is the major product. Worked mechanism — dehydration of butan-2-ol: Step 1: the -OH oxygen is protonated by H₂SO₄, converting a poor leaving group into a good one (water). Step 2: water leaves, generating a secondary carbocation at C2. Step 3: a base (HSO₄⁻) removes a proton from an adjacent carbon (C1 or C3); removal from C3 gives the more substituted but-2-ene (major, Zaitsev product), while removal from C1 gives but-1-ene (minor).

Oxidation: Primary alcohols are oxidised by mild oxidants such as PCC (pyridinium chlorochromate) to aldehydes only, since PCC cannot push the reaction further; strong oxidants such as acidified KMnO₄ or K₂Cr₂O₄ carry the same primary alcohol all the way to a carboxylic acid. Secondary alcohols are oxidised to ketones, which resist further oxidation under normal conditions since it would require breaking a C-C bond. Tertiary alcohols have no H on the carbinol carbon, so they resist oxidation under normal (mild) conditions altogether; vigorous conditions cause C-C bond cleavage instead.

Acidic reactions: Phenol reacts with NaOH to form soluble sodium phenoxide and water, confirming its acidity (see Section 3). It gives a characteristic violet/purple colouration with neutral FeCl₃ solution, a simple test for the phenolic -OH group.

Electrophilic aromatic substitution: The -OH group is a strong activator and an ortho/para-director because its oxygen lone pair delocalises into the ring, raising electron density especially at the ortho and para positions. Consequently phenol undergoes electrophilic substitution far more readily than benzene: it reacts with bromine water at room temperature with no catalyst to give a white precipitate of 2,4,6-tribromophenol (benzene needs a Lewis acid catalyst for bromination at all), it nitrates readily with dilute HNO₃ at room temperature to give a mixture of ortho- and para-nitrophenol, and it sulphonates with conc. H₂SO₄.

Kolbe's reaction: Sodium phenoxide is treated with CO₂ at about 400 K and 4-7 atm pressure; the electrophilic carbon of CO₂ is attacked at the ortho position (favoured by the ionic, more nucleophilic phenoxide oxygen directing ortho), giving sodium salicylate, which on acidification yields salicylic acid (2-hydroxybenzoic acid).

Reimer-Tiemann reaction: Phenol is treated with chloroform and aqueous NaOH at about 340 K. NaOH generates dichlorocarbene (:CCl₂) from CHCl₃, which reacts electrophilically at the ortho position of the phenoxide ion; subsequent hydrolysis converts the resulting dichloromethyl intermediate to an aldehyde group, giving salicylaldehyde (2-hydroxybenzaldehyde) as the major product.

Why does phenol react faster than benzene toward electrophiles? The ring in phenol is electron-rich because one of oxygen's lone pairs is in resonance with the pi system, generating extra electron density that specifically sits at the ortho and para carbons; this both speeds up electrophilic attack and fixes its position, unlike benzene where all six positions are equivalent and less reactive.

Williamson synthesis: An alkyl halide reacts with a sodium alkoxide (R-O⁻Na⁺) via an SN2 mechanism: R′-O⁻ attacks the back side of the C-X bond, displacing halide and forming R-O-R′. Because it is SN2, the alkyl halide should ideally be primary (unhindered); if a tertiary or bulky secondary halide is used, the strongly basic alkoxide instead favours E2 elimination, giving an alkene as the major product rather than the ether. Worked example: To make tert-butyl methyl ether, the correct combination is CH₃Br (a primary/methyl halide) plus sodium tert-butoxide, (CH₃)₃CO⁻Na⁺ — not tert-butyl bromide plus sodium methoxide, since the bulky tertiary halide would undergo elimination with the basic methoxide instead of substitution.

Physical properties: Ethers are polar (C-O-C bond dipoles do not cancel) but, lacking an O-H bond, cannot hydrogen-bond to each other, so their boiling points are close to those of alkanes of similar molar mass and much lower than isomeric alcohols. They can still accept hydrogen bonds from water via their oxygen lone pairs, giving them modest water solubility similar to alcohols of comparable size.

Cleavage by hydrogen halides: With excess HI (or HBr), the C-O bond of a dialkyl ether is cleaved to give an alkyl halide and an alcohol, which with excess acid reacts further to a second alkyl halide. For a mixed ether where both alkyl groups are simple primary groups, the nucleophile (I⁻) attacks the less hindered carbon (SN2). For aromatic ethers such as anisole, the aryl-oxygen bond has partial double-bond character (resonance, as in phenol) and an aryl cation is very unstable, so this bond never breaks: anisole with excess HI always gives phenol + CH₃I, with iodide attacking the methyl carbon.

Electrophilic substitution in aromatic ethers: In anisole, the -OCH₃ group, like -OH in phenol, is a strong activator and ortho/para-director because its oxygen lone pair conjugates into the ring. This gives fast nitration, halogenation and sulphonation predominantly at the ortho and para positions, by the same resonance-donation logic used for phenol in Section 5.