Ether Preparation: Definition, Williamson's Synthesis, Dehydration of Alcohols

Ethers are hydrocarbon derivatives in which an alkoxy or aryloxy group replaces a hydrogen atom. Ethers are organic compounds that contain an ether group, which is an oxygen atom linked to two alkyl or aryl groups. They are represented by the general formula R–O–R′, where R and R′ represent alkyl or aryl groups. Ethers are further classified into two types: simple or symmetrical ethers, which have the same alkyl groups on both sides of the oxygen atom, and mixed or unsymmetrical ethers, which have different alkyl groups on both sides of the oxygen atom. The solvent and anesthetic diethyl ether, also known simply as "ether" (CH₃–CH₂–O–CH₂–CH₃), is an example of the first group. Ethers are common in organic chemistry and even more so in biochemistry because they serve as common linkages in carbohydrates and lignin. Let’s discuss the ether preparation in detail along with some important questions.

Key Takeaways: Ether, R–O–R′, C-O-C, Williamson's synthesis, Dehydration of alcohols, Oxygen, Atom, Alcohols, Organic chemistry, Solvent, Biochemistry, Alkyl groups

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What are Ethers?

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Ethers are referred to as a class of organic compounds that mostly contain an ether group in which the oxygen atom is bonded to two alkyl or aryl groups. C-O-C is the functional group. The central atom, oxygen, is in the sp³ hybridised state. The remaining two completely sp³ orbitals are free; they form no bonds with any orbital. As a result, the C-O-C bond angle is around 110°, which is close to the normal tetrahedral angle (109° 28') but different from that in water (105°).

Ethers

The difference is due to the fact that in ethers, the repulsion between lone electron pairs is counterbalanced by the repulsion between bulky alkyl groups, resulting in a value close to 109° 28' being retained.

Ethers can be classified as symmetrical or simple (when both alkyl groups are identical) or unsymmetrical or mixed (when the two alkyl groups are different). Ethers exhibit functional isomerism (when combined with alcohols) and metamerism. The most common example is diethyl ether (ethoxyethane), also known as ether.

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Preparation of Ether

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Williamson’s synthesis

This is the most common method of ether formation (nucleophilic substitution reaction), also known as Williamson's synthesis. It consists of an alkoxide ion S_N2 attack on an alkyl halide, alkyl sulphate, or alkyl sulphonate.

Williamson’s synthesis

In an S_N2 reaction, the alkoxide ion reacts with the substrate to produce ether. A good leaving group must be present in the substrate. Halides, sulphates, and sulphonates are important leaving groups. For good yield, the substrate must contain a primary alkyl group. Elimination occurs in the case of the tertiary substrate. Low temperature is another factor that contributes to substitution.

Preparation of ethers by dehydration of alcohols

Alcohols dehydrate in the presence of protic acids (sulphuric acid) to produce alkenes and ethers under different conditions. Dehydration of ethanol at 443 K, for example, yields ethane in the presence of sulphuric acid, whereas it yields ethoxyethane at 413 K. This is an excellent method of preparation via primary alcohols. The dehydration of alcohol to produce ethers is a nucleophilic substitution reaction. The alcohol involved in the reaction serves two functions: one as a substrate and the other as a nucleophile.

Ethers by dehydration of alcohols

It can operate using either an S_N1 or an S_N2 mechanism. The mechanism chosen is determined by whether the protonated alcohol loses water before or after the attack of a second alcohol molecule. In general, secondary and tertiary alcohols use the S_N1 mechanism, whereas primary alcohols use the S_N2 mechanism.

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Chemical Properties of Ether

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Ethers are less polar and thus less reactive, and they do not react with active metals (Na, K), cold dilute acid, oxidising and reducing agents, or other chemicals.

The reason is that they do not have an active functional group.

• Due to the presence of a lone pair on the oxygen atom, ether has a basic nature and behaves like a Lewis base. Oxonium ion is formed when ethers react with cold conc. acid. Ethers, such as BF₃, AICI₃, and RMgX, form a dative bond with Lewis acid.
• There is a significant difference between the boiling points of ethanol and dimethyl ether. Ethanol has a much higher boiling point of 78 degrees Celsius, whereas dimethyl ether has a negative 25 degrees boiling point. And this explains the molecules' state of matter.
• Ethanol is, of course, a liquid at room temperature and pressure because its boiling point is higher than room temperature. Dimethyl ether, on the other hand, has already turned into a gas due to its much lower boiling point.

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Things to Remember

• Ethers are derivatives of hydrocarbons in which a hydrogen atom is replaced by an alkoxy or an aryloxy group.
• Ethers may be symmetrical or simple (when both the alkyl groups are identical) and unsymmetrical or mixed (when the two alkyl groups are different).
• Ethers show functional isomerism (with alcohols) and metamerism. Diethyl ether (ethoxyethane), commonly known as ether, is the most important example.
• Ethers are less polar so less reactive and do not react with active metals (Na, K), cold dilute acid, oxidizing and reducing agents.

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