Preparation of Ether: Dehydration of Alcohol & Williamson’s Synthesis

The preparation of ethers can be performed by various methods. 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. 
• They are further classified into two types: simple or symmetrical ethers and mixed or unsymmetrical ethers.
• Symmetrical ether has the same alkyl groups on both sides of the oxygen atom.
• Mixed or unsymmetrical ethers have different alkyl groups on both sides of the oxygen atom. 
• Preparation of ether can be done by the process of Williamson synthesis.
• The solvent and anaesthetic diethyl ether (CH₃–CH₂–O–CH₂–CH₃), is an example of the first group. 
• Ethers are common in organic chemistry and biochemistry because they serve as common linkages in carbohydrates and lignin. 

Key Terms: Preparation of Ether, Ether, Williamson's synthesis, Dehydration of Alcohols, Acid Dehydration, Alkyl Halide, Dimethyl Ether, Atom, Chemical Compound, Oxygen, Carbon, Hydrogen, Electrons, Symmetrical Ether, Asymmetrical Ether

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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.

• 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^o, which is close to the normal tetrahedral angle (109^o 28') but different from that in water (105^o).

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^o 28' being retained.

• It exhibit functional isomerism (when combined with alcohols) and metamerism.
• The most common example is diethyl ether (ethoxyethane), also known as ether.

Ether

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Preparation of Ether

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There are different method involved in the preparation of ether which are as follows:

Williamson’s Synthesis

Williamson’s Synthesis is the most common method of ether formation (nucleophilic substitution reaction). It consists of an alkoxide ion SN₂ attack on an alkyl halide, alkyl sulphate, or alkyl sulphonate.

• In an SN₂ 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 alcohol. 
• 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.

It can operate using either an SN₁ or an SN₂ 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 SN₁ mechanism, whereas primary alcohols use the SN₂ mechanism.

Preparation of Ether by Acid Dehydration

Preparation of ether by acid dehydration involves a reaction between dehydrated ethanol with sulphuric acid. The reaction will result in the formation of dimethyl ether. 

2R−OH+H₂SO₄→R−O−R+H₂O

Preparation of Ether by heating Alkyl Halides

It involves heating the required alkyl halide with silver oxide, which results in the formation of ether.

2R-X + Ag₂O → R-O-R + 2AgX

Preparation of Ether by Diazomethane

There is another for preparation of the required compound. It involves a reaction between diazomethane and alcohol, which results in the formation of ether.

CH₃OH + CH₃N₂ → CH₃ – O – CH₃

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Properties of Ether

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The properties of ether are as follows:

Physical Properties of Ether

The physical properties of ether are as follows:

• Ethers are less polar and, thus, less reactive compounds.
• They do not react with active metals (Na, K), cold dilute acid, oxidising and reducing agents, or other chemicals.
• Due to the presence of a lone pair on the oxygen atom, ether has a basic nature and behaves like a Lewis base. 

Oxonium ions are formed when ethers react with cold, concentrated acid. Ethers, such as BF₃, AICI_3, 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. 
• The compound is liquid at room temperature and pressure because its boiling point is higher than at room temperature. 
• On the other hand, Dimethyl ether has already turned into a gas due to its much lower boiling point.

Chemical Properties of Ether

The chemical propertoes of ether are as follows:

Cleavage of C-O Bond

Ether will form cleavage of the C-O bond when an excess hydrogen halide is added to the ether compound. This results in the formation of alkyl halides.

R-O-R + HX → RX + R-OH

Halogenation

In halogenation, aromatic ether will be treated with a halogen in the presence or absence of a catalyst.

Electrophilic Substitution

In the case of electrophilic substitution, the alkoxy group in the ether compound activates the aromatic ring found at the ortho and para positions.

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Things to Remember

• Ethers are derivatives of hydrocarbons which are formed when an oxygen atom bonds to two alkyl or aryl groups. 
• It may be symmetrical(when both the alkyl groups are identical) and unsymmetrical (when the two alkyl groups are different).
• Friedel Craft’s Reaction is an important example of an electrophilic substitution reaction.
• The compound is used as a refrigerant and a solvent.
• It is used in the synthesis of various compounds.