Ethers

Overview and Structure of Ethers

Ethers are organic compounds in which an oxygen atom is bonded to two carbon atoms (or carbon-containing groups). Their characteristic functional group can be written as:

$$
\mathrm{R{-}O{-}R'}
$$

where $R$ and $R'$ are alkyl, aryl, or other carbon-based groups. The C–O–C unit defines an ether linkage.

Typical subclasses:

• Symmetrical ethers: $R = R'$ (e.g. $\mathrm{CH_3{-}O{-}CH_3}$, dimethyl ether)
• Unsymmetrical ethers: $R \neq R'$ (e.g. $\mathrm{CH_3{-}O{-}C_2H_5}$, methyl ethyl ether)
• Dialkyl ethers: both $R$ and $R'$ are alkyl
• Aryl alkyl ethers: one group is aryl, the other alkyl (e.g. anisole, $\mathrm{C_6H_5{-}O{-}CH_3}$)
• Diarylethers: both $R$ and $R'$ are aryl (e.g. diphenyl ether)

The oxygen atom in ethers is $sp^3$-hybridized, and the C–O–C angle is somewhat larger than the tetrahedral angle (about $110^\circ$), giving most simple ethers a bent, non-linear geometry at oxygen.

Unlike alcohols, ethers have no O–H bond, which has major consequences for their physical properties and reactivity.

Nomenclature of Ethers

Two main naming systems are important.

Common (trivial) names

Common names are widely used in practice, especially for simple ethers:

1. Name each $R$ group as an alkyl (or aryl) group.
2. List them in alphabetical order.
3. Add the word “ether”.

Examples:

• $\mathrm{CH_3{-}O{-}CH_3}$: dimethyl ether
• $\mathrm{CH_3{-}O{-}C_2H_5}$: ethyl methyl ether
• $\mathrm{C_2H_5{-}O{-}C_2H_5}$: diethyl ether
• $\mathrm{C_6H_5{-}O{-}CH_3}$: anisole (traditional name; “methyl phenyl ether” as a descriptive common name)

For symmetrical diaryl ethers, names like “diphenyl ether” are common.

IUPAC nomenclature

In IUPAC naming for more complex molecules, ethers are usually treated as alkoxy-substituted hydrocarbons:

1. Choose the longest carbon chain as the parent hydrocarbon.
2. The ether-containing substituent is named as an alkoxy group:
   – $\mathrm{CH_3O{-}}$: methoxy
   – $\mathrm{C_2H_5O{-}}$: ethoxy
   – $\mathrm{C_3H_7O{-}}$: propoxy, etc.
3. Number the chain to give substituents the lowest possible locants.
4. Combine: locant + alkoxy + parent name.

Examples:

• $\mathrm{CH_3{-}O{-}CH_2CH_3}$
  Parent: ethane, substituent: methoxy on C-1 → 1-methoxyethane
  (Common: ethyl methyl ether)
• $\mathrm{CH_3CH_2{-}O{-}CH(CH_3)_2}$
  Parent: propane, substituent: ethoxy on C-2 → 2-ethoxypropane
• $\mathrm{CH_3O{-}CH_2CH_2CH_3}$ → 1-methoxypropane
• $\mathrm{C_6H_5{-}O{-}CH_3}$ → methoxybenzene (IUPAC; “anisole” commonly used)

When the ether is part of a more complex functionalized molecule, higher-priority functional groups (like carbonyls, carboxylic acids, etc.) determine the parent name; the ether side becomes an alkoxy substituent in that context.

Physical Properties of Ethers

Polarity and hydrogen bonding

• The C–O bond is polar, so ethers are polar molecules.
• Ethers can accept hydrogen bonds via the oxygen’s lone pairs (e.g. from water, alcohols).
• Ethers cannot donate hydrogen bonds because they lack an O–H bond.

Consequences:

• Boiling points:
  For similar molecular mass, ethers typically have:
• higher boiling points than alkanes (due to dipole–dipole interactions)
• lower boiling points than isomeric alcohols (no intermolecular hydrogen bonding between ether molecules)
• Solubility:
• Low-mass ethers (like dimethyl ether, diethyl ether) are moderately soluble in water, thanks to hydrogen bonding with water.
• Solubility decreases with increasing hydrophobic carbon content.
• Ethers are very good solvents for many nonpolar and moderately polar organic compounds.

Volatility and flammability

Many common ethers (e.g. diethyl ether, THF) are:

• Volatile (low boiling points and high vapor pressures)
• Highly flammable; vapors can form explosive mixtures with air

This has direct practical implications in laboratory safety.

Preparation of Ethers

Only methods particularly associated with ethers are summarized here; general reactivity of alcohols, alkyl halides, and substitution reactions are developed elsewhere.

Dehydration of alcohols (acid-catalyzed)

Primary alcohols can form ethers via condensation in the presence of strong acids such as concentrated $ \mathrm{H_2SO_4} $, typically at moderate temperatures.

General form (for symmetrical ethers):

$$
2 \,\mathrm{R{-}OH} \;\xrightarrow[\Delta]{\text{acid}} \;\mathrm{R{-}O{-}R} + \mathrm{H_2O}
$$

Example: Diethyl ether from ethanol:

$$
2 \,\mathrm{C_2H_5OH} \;\xrightarrow[\Delta]{\text{conc. } H_2SO_4} \;\mathrm{C_2H_5{-}O{-}C_2H_5} + \mathrm{H_2O}
$$

Competing dehydration to alkenes occurs at higher temperatures, so conditions must be controlled. This method is most practical for simple symmetrical ethers of primary alcohols.

Williamson ether synthesis

The Williamson ether synthesis is the most important laboratory method to make ethers.

It is a nucleophilic substitution reaction between an alkoxide ion and a primary alkyl halide (or similar derivative):

1. Prepare an alkoxide from an alcohol and a strong base:
   $$
   \mathrm{R{-}OH + NaH \rightarrow R{-}O^-Na^+ + H_2}
   $$
   or
   $$
   \mathrm{R{-}OH + Na \rightarrow R{-}O^-Na^+ + \tfrac{1}{2}H_2}
   $$
2. React the alkoxide with an appropriate alkyl halide:
   $$
   \mathrm{R{-}O^- + R'{-}X \rightarrow R{-}O{-}R' + X^-}
   $$

Conditions favor an $S_\mathrm{N}2$ reaction and work best with:

• Primary alkyl halides
• Unhindered alkoxides

Example: Synthesis of ethyl tert-butyl ether:

• Start with tert-butoxide ion $\mathrm{(CH_3)_3CO^-}$ and ethyl bromide $\mathrm{C_2H_5Br}$:
  $$
  \mathrm{(CH_3)_3CO^- + C_2H_5Br \rightarrow (CH_3)_3COC_2H_5 + Br^-}
  $$

To avoid elimination, the halide partner is usually the less hindered part of the final ether.

Williamson synthesis also enables the preparation of aryl ethers, using phenoxide ions and appropriate alkyl halides.

Other methods (brief overview)

Depending on context, other routes to ethers include:

• Addition of alcohols to alkenes in the presence of acid, giving alkyl alkyl ethers (or alkyl aryl ethers if an aromatic alcohol is used).
• Intramolecular cyclization of appropriately substituted dihalides or diols to give cyclic ethers (e.g. epoxides, tetrahydrofuran). Details of cyclic ethers are often treated separately.

Chemical Properties and Reactions of Ethers

Ethers are generally less reactive than many other functional groups. Their relative inertness and solvent properties make them useful in synthesis and as reaction media. Still, they undergo some characteristic reactions.

Cleavage by strong acids

One of the key reactions of ethers is cleavage of the C–O bond by strong acids like hydrogen iodide ($\mathrm{HI}$) or hydrogen bromide ($\mathrm{HBr}$).

General pattern:

$$
\mathrm{R{-}O{-}R' + HX \rightarrow R{-}X + R'{-}OH}
$$

With excess acid and suitable conditions, both sides can be converted to alkyl halides:

$$
\mathrm{R{-}O{-}R' + 2\,HX \rightarrow R{-}X + R'{-}X + H_2O}
$$

Mechanistic tendencies:

• For primary and secondary alkyl ethers, cleavage often proceeds via $S_\mathrm{N}2$ at the less hindered carbon.
• For tertiary alkyl ethers, $S_\mathrm{N}1$ pathways (via carbocations) become important upon protonation of the ether oxygen.

Example:

$$
\mathrm{C_2H_5{-}O{-}C_2H_5 + HI \rightarrow C_2H_5I + C_2H_5OH}
$$

Under more forcing conditions:

$$
\mathrm{C_2H_5OH + HI \rightarrow C_2H_5I + H_2O}
$$

Overall:

$$
\mathrm{C_2H_5{-}O{-}C_2H_5 + 2\,HI \rightarrow 2\,C_2H_5I + H_2O}
$$

Aryl–alkyl ethers (like anisole) typically cleave at the alkyl–oxygen bond, not the aryl–oxygen bond, because direct $S_\mathrm{N}2$ or $S_\mathrm{N}1$ on an aromatic ring is unfavorable:

$$
\mathrm{C_6H_5{-}O{-}CH_3 + HI \rightarrow C_6H_5OH + CH_3I}
$$

Here the result is a phenol and an alkyl iodide.

Oxidation and peroxide formation

In normal conditions, ethers are relatively resistant to mild oxidizing agents, but they slowly react with oxygen in air to form peroxides:

$$
\mathrm{R{-}O{-}R + O_2 \rightarrow \text{ether peroxides}}
$$

These peroxides are often explosive, especially when concentrated (e.g. during solvent evaporation). This is particularly important for:

• Diethyl ether
• Tetrahydrofuran (THF)
• Dioxane
• Other ethers with easily abstractable hydrogens adjacent to oxygen

Practically, this leads to safety measures such as:

• Storing ethers in tightly closed, often dark containers
• Using inhibitors in commercial solvent grades
• Testing old ether solvents for peroxides before distillation or concentration

Coordination and complex formation

The lone pairs on the oxygen atom allow ethers to act as Lewis bases, coordinating to Lewis acids or metal cations. Examples:

• Complexes with $\mathrm{BF_3}$ (e.g. $\mathrm{BF_3 \cdot OEt_2}$, where $\mathrm{OEt_2}$ is diethyl ether)
• Solvation of cations (e.g. $\mathrm{Na^+}$, $\mathrm{K^+}$) in solutions of organometallic reagents (e.g. Grignard reagents in diethyl ether or THF)

This donor ability is central to the use of ethers as solvents in many organometallic and coordination chemistry contexts, stabilizing reactive species without strongly participating in the chemical reaction itself.

Cyclic Ethers (Brief Introduction)

Cyclic ethers are rings containing one or more oxygen atoms, such as:

• Epoxides (oxiranes): three-membered cyclic ethers
• Tetrahydrofuran (THF): five-membered ring
• Tetrahydropyran: six-membered ring

They share the C–O–C functional motif but have properties influenced by ring size (e.g. ring strain in epoxides makes them considerably more reactive than open-chain ethers). Detailed discussion of their reactivity (ring-opening, etc.) is usually treated separately.

Ethers as Solvents and in Applications

Because of their combination of moderate polarity, low reactivity, and good solvating power, ethers play a central role as solvents and auxiliaries in organic and coordination chemistry.

Typical uses:

• Reaction solvents for:
• Grignard reagent formation and reactions (commonly in diethyl ether or THF)
• Many organolithium and organosodium reactions
• Various reduction and substitution reactions where protic solvents would interfere
• Extraction solvents for nonpolar and moderately polar compounds
• Phase-transfer and complexation roles where ether oxygen coordinates to cations, improving solubility

In everyday and industrial contexts, certain ethers have specific roles, for example as:

• Fuel additives (e.g. MTBE, methyl tert-butyl ether)
• Anesthetics (historically, diethyl ether)
• Components in specialized solvents and formulations

Safety Aspects Specific to Ethers

Several safety issues are particularly important for ethers:

• High flammability:
• Low flash points and high vapor pressures
• Vapors are heavier than air and can travel to ignition sources
• Peroxide formation:
• Storage over long periods, especially in partially filled containers, increases peroxide risk
• Concentration (e.g. by distillation or evaporation) can concentrate peroxides and lead to explosions
• Narcotic effects:
• Inhalation of ether vapors can cause dizziness, drowsiness, or unconsciousness

Good practice includes:

• Working with ethers in well-ventilated areas or fume hoods
• Avoiding open flames near ether use
• Labeling containers with opening dates and testing old stocks for peroxides
• Disposing of aged peroxide-forming ethers according to appropriate safety protocols

These properties make ethers useful but demand careful handling in laboratory and industrial environments.