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MHT-CET Chemistry · Alcohols, Phenols and Ethers

Ethers: Preparation and Reactions

Ethers R–O–R' are made by the Williamson synthesis — a sodium alkoxide displacing a halide by SN2, so the halide must be primary or methyl and never aryl — and are cleaved by hot HI at the alkyl–oxygen bond, an aryl alkyl ether giving the phenol plus the alkyl iodide; cold concentrated H₂SO₄ merely protonates them to oxonium salts.

All Alcohols, Phenols and Ethers notesMHT-CET Chemistry strategy

Why this matters

13 PYQs, none HARD. Five are Williamson — which method makes ethers, which halide will NOT work (chlorobenzene), what methyl bromide gives with sodium tert-butoxide; eight are reactions — ethers in cold concentrated H₂SO₄ (oxonium salts, three times), anisole or ethoxybenzene with HI or dilute acid (phenol plus the alkyl fragment), anisole with bromine in acetic acid (para), and which heterocycle lacks oxygen. Two cards.

Concept 1 of 2

Williamson Synthesis: Alkoxide Plus Primary Halide

Intuition

The alkoxide is a nucleophile; the halide is its target. Because the step is SN2, put the crowding on the ALKOXIDE side and keep the halide primary: sodium tert-butoxide + methyl bromide gives methyl tert-butyl ether cleanly, whereas tert-butyl bromide + methoxide would eliminate. An aryl halide cannot be the target at all — its C–X bond has double-bond character and resists SN2.

Definition

  • R-ONa+R’-X→R-O-R’+NaX\text{R-ONa} + \text{R'-X} \to \text{R-O-R'} + \text{NaX} (SN2). Method of choice for unsymmetrical ethers.
  • Does NOT work with C6H5Cl\text{C}_6\text{H}_5\text{Cl} (aryl halide); poor with tertiary halides (elimination). Ethyl chloride, propyl chloride, methyl bromide are fine.
  • CH3Br+(CH3)3CONa→(CH3)3C-O-CH3\text{CH}_3\text{Br} + (\text{CH}_3)_3\text{CONa} \to (\text{CH}_3)_3\text{C-O-CH}_3, 2-methoxy-2-methylpropane (MTBE) — not isobutylene.
  • Anisole from sodium phenoxide + CH3I\text{CH}_3\text{I} (the phenoxide is the nucleophile, the methyl halide the target).
  • Symmetrical ethers also from two alcohols with conc. H2SO4\text{H}_2\text{SO}_4 at 413 K. Methoxymethane is the simplest ether; benzenol and benzene-1,2-diol are phenols, propan-2-ol an alcohol.

Williamson synthesis

R-O−Na++R’-X→SN2R-O-R’+NaX(R’X primary; never aryl)\text{R-O}^-\text{Na}^+ + \text{R'-X} \xrightarrow{\text{SN2}} \text{R-O-R'} + \text{NaX} \quad (\text{R'X primary; never aryl})

Worked example

Plan a Williamson synthesis of ethyl tert-butyl ether and say which pairing must be avoided.
Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 1Alcohols, Phenols and EthersEASY
Which among the following is the method for the preparation of ethers?

[Q89 · 9th May Shift 1 · 2023]

Expecting isobutylene from methyl bromide and tert-butoxide

Elimination needs a β-hydrogen on the HALIDE; methyl bromide has none. The bulky base simply attacks the unhindered methyl carbon: MTBE forms.
Drill 4 more on williamson synthesis: alkoxide plus primary halide

Concept 2 of 2

Reactions of Ethers: Oxonium Salts, HI Cleavage, Anisole

Intuition

The ether oxygen has lone pairs, so cold concentrated H₂SO₄ simply protonates it — the ether dissolves as an oxonium salt. Hot concentrated HI breaks the bond: for a dialkyl ether the smaller alkyl leaves as iodide (SN2); for an aryl alkyl ether the aryl–O bond cannot break, so anisole gives PHENOL and iodomethane. Dilute acid hydrolyses anisole to phenol and methanol. On the ring, OCH₃ is an activating o/p director and bromine in acetic acid gives mainly p-bromoanisole.

Definition

  • Cold conc. H2SO4\text{H}_2\text{SO}_4: R2O+H2SO4→[R2OH]+HSO4−\text{R}_2\text{O} + \text{H}_2\text{SO}_4 \to [\text{R}_2\text{OH}]^+\text{HSO}_4^-, an oxonium salt — not an alkanol, acid or alkyl hydrogen sulphate.
  • Hot HI, 398 K: C6H5OCH3→C6H5OH+CH3I\text{C}_6\text{H}_5\text{OCH}_3 \to \text{C}_6\text{H}_5\text{OH} + \text{CH}_3\text{I}; ethoxybenzene → phenol + ethyl iodide. Never iodobenzene.
  • Dilute H2SO4\text{H}_2\text{SO}_4, heat: anisole → phenol + methanol.
  • Electrophilic substitution on anisole: Br2\text{Br}_2/acetic acid → p-bromoanisole (major); nitration → o/p-nitroanisole; Friedel–Crafts → o/p products.
  • Oxygen heterocycles: furan, THF, pyran; pyrrole has nitrogen instead.

Cleavage by HI

Ar-O-R+HI→ΔAr-OH+R-I(aryl-O bond survives)\text{Ar-O-R} + \text{HI} \xrightarrow{\Delta} \text{Ar-OH} + \text{R-I} \quad (\text{aryl-O bond survives})

Worked example

Give the products of (i) ethyl methyl ether and (ii) ethyl phenyl ether with excess hot HI.
Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 2Alcohols, Phenols and EthersMODERATE
Identify the product obtained when ethoxybenzene reacts with hot and concentrated HI.

[Q82 · 12th May Shift 1 · 2024]

Cleaving the aryl side with HI

The aryl–oxygen bond has partial double-bond character and does not break. Anisole gives phenol + CH₃I; 'iodobenzene + methanol' is the planted option.
Drill 7 more on reactions of ethers: oxonium salts, hi cleavage, anisole

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

  • Williamson Synthesis: Alkoxide Plus Primary Halide

    Williamson synthesis

    R-O−Na++R’-X→SN2R-O-R’+NaX(R’X primary; never aryl)\text{R-O}^-\text{Na}^+ + \text{R'-X} \xrightarrow{\text{SN2}} \text{R-O-R'} + \text{NaX} \quad (\text{R'X primary; never aryl})
  • Reactions of Ethers: Oxonium Salts, HI Cleavage, Anisole

    Cleavage by HI

    Ar-O-R+HI→ΔAr-OH+R-I(aryl-O bond survives)\text{Ar-O-R} + \text{HI} \xrightarrow{\Delta} \text{Ar-OH} + \text{R-I} \quad (\text{aryl-O bond survives})

Watch out for (2)

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