Ethers — Preparation, Reactions and Commercial Uses

7.9 Some Commercial Uses of Alcohols

7.9.1 Methanol (wood spirit)

Methanol was once made by destructive distillation of wood — hence the old name "wood spirit". Today it is produced industrially by catalytic hydrogenation of carbon monoxide over a Cu/ZnO/Al₂O₃ catalyst:

Uses: solvent for paints, varnishes and shellac; feedstock for formaldehyde (→ bakelite), acetic acid and methyl tert-butyl ether (petrol additive); antifreeze; emerging as a clean liquid fuel for direct-methanol fuel cells. Toxicity: methanol is metabolised to formaldehyde and formic acid; as little as 10 mL can cause blindness, 30 mL can be fatal.

7.9.2 Ethanol

Made industrially by (i) acid-catalysed hydration of ethylene, and (ii) fermentation of sugars — zymase enzymes of yeast convert glucose into ethanol:

Uses: alcoholic beverages; organic solvent for tinctures and perfumes; industrial feedstock for diethyl ether, ethyl acetate, acetaldehyde; automotive fuel additive (gasohol); hand sanitisers at 60–70% v/v (denatures viral envelope proteins and bacterial membranes). Ordinary ethanol is denatured with methanol or pyridine to make it non-potable for industrial purposes.

7.10 Ethers

7.10.1 Structure Recap

An ether — R–O–R' — has a bent oxygen (C–O–C ≈ 112°) and is sp³-hybridised. Because there is no O–H, ethers cannot hydrogen-bond to themselves; they can, however, accept hydrogen bonds from water. Diethyl ether is therefore slightly soluble in water (≈ 8 g per 100 g at 20 °C) but boils at only 35 °C — lower than an alcohol of the same molar mass.

7.10.2 Preparation of Ethers

(a) Dehydration of an alcohol

Heating ethanol with conc. H₂SO₄ at 413 K forms diethyl ether (condensation — two alcohol molecules + acid); at 443 K the acid drives the reaction further to ethylene + water. So temperature is the key switch between "ether" and "alkene".

The mechanism: one alcohol is protonated, a second alcohol attacks its carbon by SN2 and expels water, giving a protonated ether that deprotonates to the neutral product. This method is practical only for symmetrical ethers of primary alcohols. For 2° and 3° alcohols, elimination to the alkene dominates even at 413 K.

(b) Williamson synthesis

This is the general laboratory method — it works for both symmetrical and unsymmetrical ethers. An alkoxide ion displaces halide from an alkyl halide in an SN2 step:

Because the step is SN2, the alkyl halide partner must be primary — with a 2° or 3° halide the alkoxide acts as a base instead of a nucleophile and elimination (E2) dominates.

7.10.3 Physical Properties of Ethers

• Boiling points much lower than those of isomeric alcohols (dimethyl ether –25 °C vs ethanol 78 °C) because no O–H means no H-bond donation.
• Miscibility with water comparable to that of the isomeric alcohol for low molar mass (ether O accepts an H-bond from water).
• Excellent solvents for Grignard reagents, LiAlH₄ and many organic reactions — inert, volatile and weakly coordinating.

7.10.4 Chemical Reactions of Ethers

(a) Cleavage by HX

Ethers are otherwise inert, but strong hydrogen halides (HI > HBr >> HCl) cleave the C–O–C linkage at high temperature to give an alcohol and an alkyl halide. HI is most reactive because I⁻ is the best nucleophile and HI is the strongest acid. HF does not cleave ethers.

Mechanism & regioselectivity:

• Ether of only 1°/2° alkyl groups: proton adds to ether O; halide attacks by SN2 on the less hindered carbon, expelling the more-substituted alcohol. Example: CH₃–O–C₂H₅ + HI → CH₃I + C₂H₅OH (I⁻ prefers the less hindered methyl).
• Ether with a 3° or benzylic group: protonation then SN1 — the tertiary C–O cleaves to give a stable 3° carbocation and the smaller alcohol; halide then traps the cation. Example: (CH₃)₃C–O–CH₃ + HI → (CH₃)₃C–I + CH₃OH.
• Aryl alkyl ethers (e.g., anisole, C₆H₅–O–CH₃ + HI): cleavage always gives phenol + alkyl halide, never aryl iodide + methanol. Reason: the C(aryl)–O bond has partial double-bond character and cannot undergo SN1 or SN2; only the alkyl–O bond breaks. Product: C₆H₅OH + CH₃I.

(b) Electrophilic aromatic substitution on anisole

The –OCH₃ group is a strong ortho/para director (like –OH) because its oxygen lone pair donates into the ring. Anisole undergoes:

• Halogenation: with Br₂ in ethanoic acid → p-bromoanisole (major) + o-bromoanisole.
• Nitration: with dilute HNO₃ → o- and p-nitroanisole.
• Friedel–Crafts alkylation with R–X / anhyd. AlCl₃ → o/p-alkylanisole.
• Friedel–Crafts acylation with CH₃COCl / AlCl₃ → p-methoxyacetophenone (major).

Worked Examples

Chapter 7 — Summary

• Alcohols R–OH (sp³ C–O–H), phenols Ar–OH (sp² C–O–H) and ethers R–O–R' (sp³ C–O–C) are the three oxygen functional-group families derived formally from water.
• IUPAC: alcohol = alkaneol; ether = alkoxy-alkane; phenol has its own parent name.
• Alcohols are prepared from alkenes (Markovnikov hydration; anti-Markovnikov hydroboration–oxidation), from aldehydes/ketones/acids/esters (reduction with LiAlH₄, NaBH₄, H₂/Ni), and from Grignard addition to carbonyls.
• Phenols — four routes: chlorobenzene + NaOH (Dow); benzenesulphonic acid + NaOH fusion; diazonium salt + H₂O; cumene + O₂ (industrial).
• Alcohols acidity order: H₂O > 1° > 2° > 3°. Phenols (pKa ≈ 10) are a million times stronger than alcohols because the phenoxide is resonance-stabilised. Electron-withdrawing substituents (–NO₂) raise acidity further (picric acid pKa 0.4).
• Key reactions of alcohols: with Na → alkoxide + H₂; with R'COOH/H₂SO₄ → ester; with HX/PCl₅/SOCl₂ → alkyl halide; dehydration (conc. H₂SO₄ or Al₂O₃) → alkene; oxidation (PCC, K₂Cr₂O₇) → aldehyde/ketone/acid depending on class; dehydrogenation (Cu, 573 K) → carbonyl.
• Key reactions of phenols: with NaOH → phenoxide; with Br₂/H₂O → 2,4,6-tribromophenol; with HNO₃ → nitrophenols and picric acid; Kolbe → salicylic acid; Reimer–Tiemann → salicylaldehyde.
• Ethers are made by alcohol dehydration (413 K) or — better — Williamson (alkoxide + 1° R'–X). They are cleaved by HI/HBr: SN2 on 1°/2° ethers, SN1 on ethers containing a 3° group, and selectively at the alkyl–O bond in aryl alkyl ethers. Anisole undergoes ortho/para electrophilic substitution.
• Methanol: industrial feedstock, solvent, fuel; TOXIC. Ethanol: beverages, solvent, fuel additive, sanitiser.

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