CHEMCA
EXAM MASTER REVIEW SHEET
Organic Chemistry: Ethers
1. Classification & Properties
Ethers ($R-O-R'$) are functional isomers of alcohols but differ significantly in physical behavior.
Ethers have much lower B.P. than isomeric alcohols because they cannot form intermolecular H-bonds.
B.P. Order: Alcohol > Ether > Alkane
Solubility in water is comparable to alcohols of similar mass as they can form H-bonds with water molecules.
Ethoxyethane solubility $\approx$ Butan-1-ol solubility.
2. Methods of Preparation
Williamson's Synthesis
Reaction of an alkyl halide with sodium alkoxide ($R-X + R'-ONa \rightarrow R-O-R' + NaX$).
Best results with Primary ($1^\circ$) Alkyl Halide.
With $2^\circ$ or $3^\circ$ halides, Elimination competes or dominates (forming Alkenes).
Acidic Dehydration of Alcohols
$2CH_3CH_2OH \xrightarrow{H_2SO_4, 413K} CH_3CH_2OCH_2CH_3 + H_2O$
• Critical Temp: At 443K, Ethane (Alkene) is formed instead.
3. Cleavage of C-O Bond by HI
Reaction with cold Conc. $HI$ is the most important chemical property. The products depend on the nature of alkyl groups.
| Ether Nature | Mechanism | Product Outcome |
|---|---|---|
| $1^\circ$ or $2^\circ$ Alkyl Groups | $S_N2$ | Iodide forms with the smaller/less bulky group. |
| One Group is $3^\circ$ | $S_N1$ | Iodide forms with the $3^\circ$ group (via stable carbocation). |
| Anisole ($Ph-O-CH_3$) | $S_N2$ | Phenol + Methyl Iodide (Ar-O bond is too strong to break). |
4. Aromatic Ethers (Anisole)
The $-OCH_3$ group is Activating and Ortho-Para Directing due to resonance (+M effect).
Friedel-Crafts Reaction
Anisole reacts with $CH_3Cl/AlCl_3$ to give 2-methoxytoluene and 4-methoxytoluene (Major).
5. Peroxide Formation & Epoxides
Auto-oxidation
Ethers form explosive peroxides when exposed to air and light for long periods.
Test: Using $FeSO_4$ and $KCNS$ (red color indicates peroxides).
Epoxides (Oxiranes)
Cyclic ethers with 3-membered rings. Highly reactive due to ring strain.
Open readily with nucleophiles in both acidic and basic conditions.
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