Key Methods for the Synthesis of Ethers Explained
Ether is a class of organic compounds consisting of an oxygen atom that is chemically bonded to two alkyl groups or aryl groups. Ether, alcohol, and water have similar chemical structures. In alcohol, a single hydrogen atom of a water molecule is replaced by the alkyl functional group. In ether, two hydrogen atoms of the water molecule are replaced either by alkyl or aryl groups. In subsequent sections, we will also look at the formation of ether.
In normal temperature conditions, ether exists as a colourless liquid that has a pleasant smell. Unlike alcohols, ethers have low density, lower boiling points, and low solubility in water. Ethers by themselves are not too reactive and are hence used as solvents for other chemical compounds such as oils, perfumes, waxes, fats, gums, dyes, and resins. In the gaseous state, ethers are used as fumigating agents, pesticides, and insecticides to keep the soil healthy.
Application as Solvents
As mentioned above, ethers are good solvents. They are used in a variety of extraction processes and other chemical reactions. Since they are volatile, they are used to start diesel and gasoline engines in places where the weather is cold.
A type of ether called MTBE is added to gasoline to increase the level of octane and decrease the level of nitrogen oxide pollutants. Dimethyl ether is used as a refrigerant while ethylene glycol is used in the formation of plastic.
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Applications in Pharmacology
Ethers have their applications in the field of medicine too. Ethyl ether was traditionally used as an anesthetic. The first application of ether as an anesthetic can be traced back to 1842. Ethers are also used in pain relief medications. Codeine which is a well-known pain relief medicine is an etherized form of morphine.
Properties of Ethers
Ethers are similar in structure to alcohols. While alcohols have a chemical bond between the hydrogen and the oxygen atom which is highly polarized, ethers do not. Due to this, ethers cannot form hydrogen bonds with each other.
The oxygen atoms in ether, however, have non-bonded electron pairs which can be used to form hydrogen bonds with other molecules that are not ethers. For instance, hydrogen bonds can be formed with alcohols or amines, i.e., O―H or N―H bonds.
The ability to create hydrogen bonds with other compounds gives ether its application as solvents for other organic and inorganic compounds. Ethers cannot form hydrogen bonds among themselves. This is the reason why they have a lower boiling point than alcohols of the same molecular weight.
Preparation of Ethers
In this section, we will take a closer look at various methods of synthesis of ethers. There are two methods by which ethers can be synthesized:
Williamson ether synthesis
Dehydration of alcohols
Let us study the reaction mechanisms of both these methods.
Williamson Synthesis
This is one of the most flexible methods of ether synthesis. It was discovered by and named after chemist Alexander Williamson. In the Williamson synthesis reaction, the alkoxide ion reacts with the alkyl halide to substitute the (―O―R) group with the halide. For this method, the alkyl halide should be unhindered for the substitution to occur instead of an elimination reaction.
Williamson Ether Synthesis Mechanism
The Williamson synthesis mechanism occurs in the following steps:
The reaction of the nucleophile with alkyl halide from the back to form an ether.
The entire reaction happens in one go
Cleavage of the molecule and formation of the bond takes place simultaneously
The products depend on whether elimination or substitution reaction occurs.
Crown Ether Synthesis
Crown ethers are synthesized by a modified version of the Williamson ether synthesis reaction. These compounds are created using the same steps as in a Williamson ether reaction when a templating cation is present.
Dehydration of Alcohol
If a protic acid reacts with alcohol then its two molecules lose water to form either ether or alkene, the product formed depends on the temperature conditions. Mostly, the dehydration of a single molecule of alcohol competes with the dehydration of two molecules. The dehydration of a single molecule is easier and leads to the formation of alkenes. The loss of two molecules can create ethers with primary alkyl groups.
Fun Facts
Ethers should be stored in small quantities in airtight containers and used as soon as possible. This is because when ethers are exposed to the air, they explode. The reason behind this is the process of autoxidation. Air contains oxygen. Ethers react with oxygen present in the air to form dialkyl peroxides or hydroperoxides. If these compounds exist at a concentrated level or if they are exposed to heat, they lead to an explosion.
FAQs on Synthesis of Ethers: Preparation, Mechanisms & Examples
1. What are the two primary methods for the synthesis of ethers covered in the Class 12 CBSE syllabus?
As per the NCERT and CBSE syllabus for 2025-26, the two main laboratory methods for the synthesis of ethers are:
Dehydration of Alcohols: This method involves the removal of a water molecule from two alcohol molecules using a protic acid like concentrated sulphuric acid (H₂SO₄) under controlled temperature conditions. It is mainly suitable for preparing symmetrical ethers from primary alcohols.
Williamson Ether Synthesis: This is a more versatile method where a sodium alkoxide (R-ONa) reacts with an alkyl halide (R'-X) to form an ether (R-O-R'). It is highly effective for preparing both symmetrical and unsymmetrical ethers.
2. How are ethers prepared by the dehydration of alcohols? Explain the mechanism involved.
The preparation of ethers by dehydration of alcohols involves heating excess primary alcohol with a protic acid, such as concentrated H₂SO₄ or H₃PO₄, at a specific temperature. For example, heating ethanol at 413 K yields diethyl ether. The reaction follows an Sₙ2 mechanism:
Step 1: Protonation of the alcohol molecule by the acid to form a protonated alcohol (oxonium ion).
Step 2: A second, unprotonated alcohol molecule acts as a nucleophile and attacks the protonated alcohol, displacing a water molecule.
Step 3: Deprotonation of the resulting intermediate to form the ether.
This method is generally not suitable for secondary or tertiary alcohols as they tend to undergo elimination to form alkenes.
3. What is Williamson ether synthesis and why is it considered a superior method for preparing unsymmetrical ethers?
Williamson ether synthesis is a reaction where a sodium alkoxide reacts with a primary alkyl halide via an Sₙ2 mechanism to form an ether. It is considered superior for preparing unsymmetrical ethers because it offers better control over the final product. For an unsymmetrical ether (R-O-R'), the choice of reactants is crucial. The reaction works best when the alkyl halide is primary to minimise competing elimination reactions. The alkoxide can be primary, secondary, or tertiary. This versatility allows for the strategic synthesis of complex, unsymmetrical ethers that cannot be easily prepared via the dehydration of alcohols.
4. What is the mechanism of Williamson ether synthesis? Provide a clear example.
The Williamson ether synthesis follows a straightforward bimolecular nucleophilic substitution (Sₙ2) mechanism. The alkoxide ion (RO⁻) acts as a potent nucleophile that attacks the electrophilic carbon atom of the alkyl halide (R'-X). In a single step, the nucleophile forms a new C-O bond while the halide ion (X⁻) departs as the leaving group.
Example: The synthesis of ethyl methyl ether.
Reactants: Sodium ethoxide (CH₃CH₂O⁻Na⁺) and methyl bromide (CH₃Br).
Mechanism: The ethoxide ion (CH₃CH₂O⁻) attacks the methyl group of methyl bromide, displacing the bromide ion.
Product: Ethyl methyl ether (CH₃CH₂-O-CH₃) and sodium bromide (NaBr).
5. Why can't tertiary alkyl halides be used in Williamson ether synthesis to prepare ethers?
Tertiary alkyl halides cannot be effectively used in Williamson ether synthesis because the alkoxide, which is a strong base as well as a nucleophile, will favour an elimination (E2) reaction over a substitution (Sₙ2) reaction. Due to the significant steric hindrance around the tertiary carbon, the alkoxide finds it easier to act as a base and abstract a proton from a beta-carbon. This results in the formation of an alkene as the major product instead of the desired ether.
6. How does temperature control the outcome of the reaction when synthesising ether from ethanol?
Temperature is a critical factor that determines whether the dehydration of ethanol produces an ether or an alkene. There is a competition between intermolecular substitution and intramolecular elimination.
At a lower temperature (around 413 K or 140°C), the intermolecular substitution (Sₙ2) reaction is favoured, where one ethanol molecule attacks another, leading to the formation of diethyl ether.
At a higher temperature (around 443 K or 170°C), the intramolecular elimination (E2) reaction becomes dominant, where a single ethanol molecule eliminates water to form ethene.
Therefore, precise temperature control is essential for maximising the yield of ether.
7. What happens if you try to synthesise an ether using a secondary alcohol via the acid-catalysed dehydration method?
When attempting to synthesise an ether from a secondary alcohol using acid-catalysed dehydration, a mixture of products is typically formed. While some ether may be produced via an Sₙ1 or Sₙ2 pathway, the primary product is often an alkene due to a competing elimination (E1) reaction. The secondary carbocation formed after protonation is relatively stable and can easily lose a proton to form a double bond. This makes the dehydration of secondary alcohols an inefficient and unreliable method for preparing ethers, with low yields and difficult purification.
