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Ethers (Alkoxyalkanes)

CBSE Class 12 & JEE Mains • Module 13 of 20 • R—O—R' — The Solvents of Organic Synthesis

📍 Chapter Overview

Ethers — Complete Mind Map

Topics Covered: Preparation (Dehydration of alcohols, Williamson Synthesis) · Physical Properties · Chemical Reactions (Cleavage of C—O bond by HI, Electrophilic substitution of anisole) · $S_N1$ vs $S_N2$ mechanism in ether cleavage.

🤖 AI Prompt — Chapter Mind Map: Purple-violet themed mind map on dark background. Central node: "ETHERS — R—O—R'" labeled in white. Four main branches: (1) "Preparation" — Dehydration of ROH ($H_2SO_4$, 413K) and Williamson Synthesis ($R-O^-Na^+$ + $R-X$); (2) "Cleavage by HI" — Mechanism box showing attack on smaller alkyl group (for $1^\circ/2^\circ$) vs attack on larger group (for $3^\circ$/benzylic); (3) "Electrophilic Substitution" — 4 boxes for Anisole: Bromination (Br2/CH3COOH), Nitration (HNO3/H2SO4), FC Alkylation, FC Acylation. Show o/p products; (4) "Physical Properties" — Low polarity, low boiling point (compared to alcohols), high solubility in water. High resolution educational poster.

1. Preparation of Ethers

1.1 Dehydration of Alcohols (NCERT Section 7.6.1)

Reaction: $2 C_2H_5OH \xrightarrow{H_2SO_4, 413K} C_2H_5—O—C_2H_5$ (Ethoxyethane) — *NCERT Page 214*

Wait! If temperature is raised to **443K**, ethene is formed (dehydration to alkene). 413K is crucial for Ether.

Mechanism: Dehydration to Ether (NCERT Page 214, Section 7.6.1)

Draw a 3-step mechanism for ethanol to ethoxyethane formation on white background. Step 1 (Protonation): Arrow from O lone pair of ethanol to $H^+$. Product: $CH_3CH_2O^+(H)H$. Step 2 (Nucleophilic Attack): A second ethanol molecule approaches. Arrow from its O lone pair to the $\alpha$-carbon of the protonated group. Arrow from $C—O$ bond to water oxygen. (SN2 attack). Step 3 (Deprotonation): Arrow from $O—H$ bond of the result back to O. Final Product: Diethyl ether. Clear labels (i), (ii), (iii) and bold skeletal structures as per NCERT style.

1.2 Williamson Synthesis (Most Versatile Method)

Reaction Name: Williamson Synthesis (NCERT Page 215, Section 7.6.1)

Condition: Alkyl halide ($R—X$) reacts with sodium alkoxide ($R'—O—Na$).

$R—X + R'—O—Na \rightarrow R—O—R' + NaX$

NCERT Key Rule: The alkyl halide MUST be **Primary (1°)** for good yields. If $2^\circ$ or $3^\circ$ alkyl halide is used, elimination dominates to give alkenes.

2. Chemical Reactions — Cleavage of C—O Bond by HI

This is the most important chemical property of ethers. Reaction with concentrated $HI$ or $HBr$. — *Section 7.6.2*

Mechanism: Cleavage of Ethers by HI (NCERT Page 216)

Draw the mechanism of methoxyethane ($CH_3—O—C_2H_5$) reacting with HI on white background. Step 1 (Protonation): Arrow from O lone pair to H of HI. Step 2 (Attack): Show $I^-$ attacking the SMALLER methyl group. Arrow from $I^-$ to $CH_3$. Arrow from $C—O$ bond to Oxygen. Product: $CH_3I$ + $C_2H_5OH$. Add a second case: $(CH_3)_3C—O—CH_3$ with HI. Show $I^-$ attacking the TERTIARY carbon because the intermediate carbocation is stable (SN1 pathway). Correct products: $(CH_3)_3CI$ + $CH_3OH$. Clear bold labels, textbook mechanisms.

NCERT Summary Rules for HI Cleavage:

If both groups are $1^\circ$ or $2^\circ$: $I^-$ attacks **smaller** group ($S_N2$).
If one group is $3^\circ$: $I^-$ attacks **tertiary** group ($S_N1$).
If one group is **Aryl** (Anisole): Cleavage gives **Phenol + Methyl Iodide**. The $O—C_{\text{aryl}}$ bond is too strong (partial double bond) to break.

3. Electrophilic Substitution in Aromatic Ethers

The $-OCH_3$ group (in Anisole) is **activating** and **Ortho-Para Directing**. — *NCERT Section 7.6.2 (2)*

⚡ QUICK REVISION: Reactions of Anisole

Reaction	Reagents	Major Product
Bromination	$Br_2$ in $CH_3COOH$	p-Bromoanisole (Major)
Nitration	Conc. $H_2SO_4 + HNO_3$	4-Nitroanisole (Major)
F.C. Alkylation	$CH_3Cl$ + anhydrous $AlCl_3$	4-Methylanisole (Major)
F.C. Acylation	$CH_3COCl$ + anhydrous $AlCl_3$	4-Methoxyacetophenone (Major)

Practice Problems

E Q1. Arrange alphabetically the reagents needed for Williamson synthesis of tert-Butyl methyl ether. Should you use t-BuCl + NaOMe or MeCl + NaOt-Bu?

M Q2. Predict products of HI cleavage of: (i) Methoxybenzene (ii) Benzyl ethyl ether. *(NCERT Reference)*

M Q3. Explain the mechanism of methoxyethane formation from propan-1-ol and diazomethane. (Extra info, but relevant for JEE).

H Q4. Why is ethoxybenzene ($C_6H_5OC_2H_5$) less reactive than phenol towards electrophilic substitution? Compare the +M effect.

Using $3^\circ$ alkyl halide in Williamson synthesis — **ALKENE** will be the major product! Always use $1^\circ$ halide.
Assuming $I^-$ attacks phenyl ring in Anisole + HI — **WRONG!** It attacks the methyl group, producing Phenol and $CH_3I$.
Confusing the mechanism of ether cleavage — remember smaller group for $S_N2$, tertiary for $S_N1$.