Ethers — Revision Notes

⚡ 30-Second Revision

General Formula: — R-O-R'
Nomenclature: — Alkoxyalkane (IUPAC), Alkyl alkyl ether (Common)
Preparation:

- Williamson Synthesis: $RO^-Na^+ + R'X \xrightarrow{S_N2} R-O-R'$ (R'X must be primary) - Dehydration of Alcohols: $2ROH \xrightarrow{H_2SO_4, 140^circ C} R-O-R + H_2O$ (for primary alcohols)

Reactions:

- Cleavage by HI/HBr: $R-O-R' + HX \rightarrow RX + R'OH$ (then $R'OH + HX \rightarrow R'X + H_2O$ if excess HX) - Mechanism: $S_N2$ (primary/secondary, attacks less hindered C); $S_N1$ (tertiary, forms carbocation) - Aryl alkyl ethers: C(aryl)-O bond not cleaved ($C_6H_5-O-R + HX \rightarrow C_6H_5OH + RX$) - Electrophilic Substitution (Aromatic Ethers): -OR is activating, ortho-para directing.

Physical Properties: — Lower B.P. than alcohols (no H-bonding), soluble in water (H-bond acceptor) for smaller ethers.

2-Minute Revision

Ethers are organic compounds with a C-O-C linkage, represented as R-O-R'. They are named as alkoxyalkanes (IUPAC) or alkyl alkyl ethers (common). Key preparation methods include the Williamson ether synthesis, which is an $S_N2$ reaction between an alkoxide and a primary alkyl halide. Remember, using secondary or tertiary alkyl halides leads to elimination. Another method is the acid-catalyzed dehydration of primary alcohols at $140^circ C$ to form symmetrical ethers.

The most important reaction of ethers is their cleavage by hot concentrated hydrohalic acids (HI, HBr). The mechanism depends on the alkyl groups: if a tertiary group is present, it's $S_N1$ (forming the tertiary alkyl halide); otherwise, it's $S_N2$ (attacking the less sterically hindered carbon).

Crucially, the aryl-oxygen bond in aromatic ethers is never cleaved. Aromatic ethers undergo electrophilic substitution, with the alkoxy group being activating and ortho-para directing. Physically, ethers have lower boiling points than alcohols due to the absence of intermolecular hydrogen bonding, but smaller ethers are water-soluble as they can accept hydrogen bonds from water.

5-Minute Revision

Ethers are characterized by the R-O-R' functional group, where the oxygen is $sp^3$ hybridized with a bent geometry. They are classified as simple (symmetrical) or mixed (unsymmetrical). IUPAC nomenclature names them as alkoxyalkanes (e.g., methoxyethane), while common names list alkyl groups alphabetically followed by 'ether' (e.g., ethyl methyl ether).

Preparation:

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Williamson Ether Synthesis: — This is the most versatile method. It's an $S_N2$ reaction where an alkoxide ($RO^-$) attacks a primary alkyl halide ($R'X$). For example, $CH_3CH_2O^-Na^+ + CH_3Br \rightarrow CH_3CH_2OCH_3 + NaBr$. The critical point is that $R'X$ must be primary; using secondary or tertiary alkyl halides will lead to E2 elimination products (alkenes) due to the alkoxide's basicity.
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Acid-catalyzed Dehydration of Alcohols: — Symmetrical ethers can be formed from primary alcohols at $140^circ C$ in the presence of concentrated $H_2SO_4$. For example, $2CH_3CH_2OH \xrightarrow{H_2SO_4, 140^circ C} CH_3CH_2OCH_2CH_3 + H_2O$. Higher temperatures favor alkene formation.

Reactions:

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Cleavage by Hot Concentrated HI/HBr: — This is the most significant reaction. The ether oxygen is first protonated. The subsequent attack by the halide ion ($X^-$) depends on the alkyl groups:

* Primary/Secondary Alkyl Groups: $S_N2$ mechanism. $X^-$ attacks the less sterically hindered carbon. Example: $CH_3OCH_2CH_3 + HI \rightarrow CH_3I + CH_3CH_2OH$. If excess HI is used, the alcohol further reacts: $CH_3CH_2OH + HI \rightarrow CH_3CH_2I + H_2O$.

* Tertiary Alkyl Group: $S_N1$ mechanism. The C-O bond breaks to form a stable tertiary carbocation, which then reacts with $X^-$. Example: $(CH_3)_3COCH_3 + HI \rightarrow (CH_3)_3CI + CH_3OH$. * Aryl Alkyl Ethers: The C(aryl)-O bond is never cleaved due to resonance stabilization.

Only the C(alkyl)-O bond breaks. Example: $C_6H_5OCH_3 + HI \rightarrow C_6H_5OH + CH_3I$.

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Electrophilic Aromatic Substitution (for Aromatic Ethers): — The -OR group is a strong activating and ortho-para directing group due to resonance. For example, nitration of anisole ($C_6H_5OCH_3$) yields ortho- and para-nitroanisole.

Physical Properties: Ethers have lower boiling points than alcohols of comparable molecular mass because they cannot form intermolecular hydrogen bonds with themselves. However, smaller ethers are somewhat soluble in water as they can act as hydrogen bond acceptors with water molecules. They are generally inert and excellent solvents for many organic reactions, especially those involving Grignard reagents.

Prelims Revision Notes

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Definition & Structure: — Ethers are R-O-R'. Oxygen is $sp^3$ hybridized, bent geometry, C-O-C angle $\approx 110-112^circ$. Polar molecules.
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Nomenclature:

* IUPAC: Alkoxyalkane (e.g., $CH_3OCH_2CH_3$ is methoxyethane). * Common: Alkyl alkyl ether (e.g., $CH_3OCH_2CH_3$ is ethyl methyl ether).

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Isomerism: — Metamerism (e.g., methoxypropane and ethoxyethane are metamers of $C_4H_{10}O$).
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Preparation Methods:

* Williamson Ether Synthesis: $RO^-Na^+ + R'X \xrightarrow{S_N2} R-O-R'$. Crucial: $R'X$ must be primary to avoid E2 elimination. If making a tertiary ether, the tertiary group must be the alkoxide (e.

g., $(CH_3)_3CO^-Na^+ + CH_3Br$). * Dehydration of Alcohols: $2ROH \xrightarrow{H_2SO_4, 140^circ C} R-O-R$. Limited to primary alcohols for symmetrical ethers. Higher temperatures ($170^circ C$) favor alkenes.

* Alkoxymercuration-Demercuration: Alkene + $Hg(OAc)_2$ in $R'OH$, then $NaBH_4$. Markovnikov addition of $R'O-H$.

1
Reactions of Ethers:

* Cleavage by Hot Concentrated HI/HBr: * General: $R-O-R' + HX \rightarrow RX + R'OH$. If excess HX, $R'OH + HX \rightarrow R'X + H_2O$. * Mechanism: * Primary/Secondary R, R': $S_N2$ attack by $X^-$ on the less hindered carbon.

* One Tertiary R: $S_N1$ mechanism, forms stable tertiary carbocation, then reacts with $X^-$. * **Aryl Alkyl Ethers ($C_6H_5-O-R$):** C(aryl)-O bond is never cleaved due to resonance. Products are phenol ($C_6H_5OH$) and alkyl halide ($RX$).

* Electrophilic Aromatic Substitution (Aromatic Ethers): -OR group is activating and ortho-para directing (e.g., nitration, halogenation).

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Physical Properties:

* Boiling Point: Lower than alcohols of comparable molecular mass (no intermolecular H-bonding). * Solubility: Smaller ethers are soluble in water (can act as H-bond acceptors with water). Solubility decreases with increasing alkyl chain length. * Density: Lighter than water. * Solvent Properties: Excellent aprotic solvents, especially for Grignard reagents (stabilize Mg via coordination).

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Distinguishing Tests: — Do not react with active metals like Na (unlike alcohols/phenols).

Vyyuha Quick Recall

For Williamson Ether Synthesis, remember 'P-SN2': Primary alkyl halide for SN2 reaction. If you use a secondary or tertiary alkyl halide, you'll get an Elimination product instead of an Ether. So, 'P-SN2, no E for T!'