Ethers are an important class of organic compounds characterized by an oxygen atom connected to two alkyl or aryl groups. Their general formula is \( R-O-R' \), where \( R \) and \( R' \) can be the same or different alkyl or aryl groups. Ethers have widespread applications in organic synthesis and industry, and understanding their properties and reactions is crucial for success in JEE Chemistry.
The oxygen atom in ethers is sp³ hybridized and forms two sigma bonds with alkyl or aryl groups. The bond angle around oxygen is approximately 110°, slightly less than the tetrahedral angle (109.5°) due to the lone pairs on oxygen.
The general structure: \[ R - O - R' \]
The oxygen has two lone pairs, making ethers polar molecules but generally less reactive compared to alcohols. Their dipole moment depends on the nature of the groups attached.
There are two common naming systems for ethers:
The names of the alkyl or aryl groups attached to oxygen are written in alphabetical order followed by the word “ether.” Example: \( \mathrm{CH_3-O-CH_3} \) is called dimethyl ether.
Ethers are named as alkoxy derivatives of alkanes. The smaller alkyl group attached to oxygen is named as alkoxy substituent and the longer chain is the parent alkane.
Example:
This is the most important laboratory method for preparing ethers. It involves the reaction of an alkoxide ion with a primary alkyl halide or tosylate.
\[ \mathrm{R-O^- + R'-X \rightarrow R-O-R' + X^-} \]
Conditions:
Primary alcohols usually do not give ethers on acid dehydration. Secondary and tertiary alcohols can give symmetrical ethers under acid catalysis.
\[ \mathrm{2 R-OH \xrightarrow{H^+, \Delta} R-O-R + H_2O} \]
Epoxides (cyclic ethers) can be formed by reaction of alkenes with peracids such as mCPBA.
\[ \mathrm{R-CH=CH_2 + RCO_3H \rightarrow Epoxide} \]
Halohydrins can be treated with a base to give epoxides via intramolecular nucleophilic substitution.
\[ \mathrm{R-CH(OH)-CH_2X + OH^- \rightarrow Epoxide + X^- + H_2O} \]
Ethers are generally stable but can be cleaved by strong acids like HI or HBr at elevated temperatures to give alkyl halides and alcohols. This reaction proceeds via protonation of ether oxygen followed by nucleophilic attack.
\[ \mathrm{R-O-R' + HX \xrightarrow{\Delta} R-X + R'-OH} \]
For unsymmetrical ethers, the cleavage usually occurs at the less hindered side.
Ethers do not react with metals like sodium under normal conditions unlike alcohols.
Ethers burn in oxygen to give carbon dioxide and water.
Ethers are widely used as solvents due to their relative inertness and ability to dissolve a wide range of compounds. Examples: diethyl ether, tetrahydrofuran (THF).
Symmetrical ethers have identical alkyl groups: \( R-O-R \). Unsymmetrical have different groups: \( R-O-R' \).
Epoxides are three-membered cyclic ethers with high ring strain making them very reactive. Important in organic synthesis for ring-opening reactions.
Ethers where one or both groups are aromatic like anisole (methoxybenzene).
It proceeds via an SN2 reaction where alkoxide ion acts as nucleophile attacking the alkyl halide carbon.
\[ \mathrm{R-O^- + R'-X \rightarrow R-O-R' + X^-} \]
Step 1: Protonation of ether oxygen by acid.
\[ \mathrm{R-O-R' + H^+ \rightarrow R-OH^+-R'} \]
Step 2: Nucleophilic attack by halide ion, breaking C-O bond.
\[ \mathrm{R-OH^+-R' + X^- \rightarrow R-X + R'-OH} \]
Acid or base catalyzed ring-opening of epoxides leads to 1,2-diols or other products depending on conditions.
| Reaction | Reagents/Conditions | Product | Notes |
|---|---|---|---|
| Williamson Ether Synthesis | Alkoxide + Primary Alkyl Halide | Ether | SN2 reaction, primary alkyl halide preferred |
| Acid-Catalyzed Dehydration | Alcohol + Acid, Heat | Symmetrical Ether | Secondary/tertiary alcohols |
| Cleavage with HI/HBr | HI or HBr, Heat | Alkyl Halide + Alcohol | Cleavage at less hindered side |
| Epoxidation of Alkenes | Peracids (e.g. mCPBA) | Epoxide (Cyclic Ether) | Syn addition |