Ethers

Full Notes

Preparation of Ethers

Ethers are organic compounds in which two alkyl or aryl groups are bonded to the same oxygen atom. Their general formula is R–O–R′, and they exhibit distinct physical and chemical properties compared to alcohols.

Ethers By Dehydration of Alcohols

Symmetrical ethers can be obtained by dehydrating primary alcohols under acidic conditions. Depending on the conditions used, an alkene can also be formed.

For Example:
2CH₃CH₂OH → CH₃CH₂–O–CH₂CH₃ + H₂O
(Ethanol → Diethyl ether)

Note: Secondary and tertiary alcohols undergo elimination (to give alkenes), not ether formation.

Williamson Synthesis

Williamson synthesis is a widely used laboratory method for preparing both symmetrical and unsymmetrical ethers. It involves the reaction of an alkyl halide (R–X) with a sodium alkoxide (R′O⁻Na⁺), producing an ether (R–O–R′) and a salt (NaX).

Reagents: Sodium alkoxide (RO⁻Na⁺) + alkyl halide (R′–X)

The reaction proceeds through an S_N2 mechanism, where the alkoxide ion acts as a nucleophile and attacks the electrophilic carbon of the alkyl halide.

Mechanism: S_N2
Best when R′–X is primary (bulky halides undergo elimination)

For Example:
CH₃CH₂ONa + CH₃I → CH₃CH₂–O–CH₃ + NaI
(Sodium ethoxide + Methyl iodide → Methoxyethane)

Note - For best results, the alkyl halide should be primary. This is because secondary and tertiary alkyl halides are more prone to elimination reactions under basic conditions, due to steric hindrance. In such cases, elimination competes with or even dominates over substitution.

For Example: When sodium methoxide reacts with tert-butyl bromide, the main product is 2-methylpropene (an alkene) rather than the expected ether, as the bulky tert-butyl group favours elimination (E2) rather than substitution.

Aryl halides (like bromobenzene) do not undergo Williamson reaction.

Phenols can also react by the same mechanism forming phenolic ethers.

Physical Properties of Ethers

Boiling Point

• Ethers have lower boiling points than alcohols due to lack of intermolecular H-bonding.
• Comparable to alkanes of similar molecular mass.
• Boiling point increases with chain length and molecular mass.

Solubility

• Ethers can form hydrogen bonds with water due to lone pairs on oxygen.
• Soluble in water to some extent.
• Solubility decreases with increasing hydrophobic alkyl chains.

Polarity

• Due to the electronegativity of oxygen, ethers are slightly polar.
• Useful as solvents for non-polar and moderately polar compounds.

Chemical Reactions

Cleavage of C–O Bond in Ethers

Ethers are broken apart by concentrated hydrohalic acids (HI, HBr) to form alcohols and alkyl halides.

If both R and R′ are alkyl groups, cleavage gives two products. With excess HX, both alkyl groups are converted to halides.

Example:CH₃–O–CH₃ + HI

Order of reactivity:
HI > HBr >> HCl (breaking of ethers using HI or HBr uses high temperatures)

Note that tertiary alkyl ethers undergo S_N1 as a more stable cation intermediate can be formed, primary/methyl ethers undergo S_N2.

Electrophilic Substitution in Aromatic Ethers

Phenyl alkyl ethers (like anisole) undergo electrophilic substitution reactions similar to phenol. The alkoxy group (–OR) is an activating, 2,4-directing (ortho/para) group. This is due to resonance, where lone pairs on the oxygen atom delocalize into the aromatic ring, increasing electron density especially at the ortho and para positions.

This increased electron density makes the ring more reactive toward electrophiles and directs substitution to these positions.

Halogenation

Anisole reacts with bromine in ethanoic acid without requiring a Lewis acid catalyst (like FeBr₃), due to the activating effect of the –OCH₃ group.

• The 4-bromo (para) isomer (p-bromoanisole) is formed as the major product (90% yield)
• The 2-bromo (ortho) isomer (o-bromoanisole) is formed in smaller amounts

This confirms the ortho/para-directing influence of the methoxy group in anisole.

Nitration

When anisole is treated with a mixture of concentrated sulphuric acid and nitric acid, it undergoes nitration to form a mixture of 2-nitro (ortho-) and 4-nitro (para-) -anisole.

Friedel–Crafts Alkylation/Acylation

In the Friedel–Crafts reaction, anisole reacts with alkyl or acyl halides in the presence of anhydrous aluminium chloride (a Lewis acid), resulting in the introduction of alkyl or acyl groups at the 2nd (ortho) and 4th (para) positions of the aromatic ring.

Note - alkylation refers to the substitution of an alkyl (−R) group and acylation refers to the substitution of an acyl (−COR) group.

Summary

• Ethers are R–O–R′ compounds prepared by alcohol dehydration and Williamson synthesis.
• Williamson synthesis is best with primary halides due to the S_N2 mechanism.
• Ethers have lower boiling points than alcohols and limited water solubility.
• HI and HBr cleave ethers to give halides and alcohols with HI most reactive.
• Aromatic ethers are activated and give ortho and para electrophilic substitution products.