Vivian Marlowe
Ethers are organic compounds that can be prepared from alcohols through various methods. The two most common methods are alcohol dehydration and Williamson ether synthesis. The former is used for the industrial preparation of ethers, while the latter is a two-step process that is particularly useful for preparing asymmetrical ethers. The reaction generally follows the SN2 mechanism for primary alcohol, where the alcohol acts as a nucleophile. The Williamson synthesis exhibits higher productivity with primary alkyl halides. The preparation of ethers by dehydration of alcohol is a nucleophilic substitution reaction, and it is limited to symmetrical ethers derived from primary alcohols.
| Characteristics | Values |
|---|---|
| Ether classification | symmetrical ethers, asymmetrical ethers |
| Ether preparation methods | Alcohol dehydration, Williamson ether synthesis, alkoxymercuration |
| Alcohol dehydration | Involves acid-catalyzed dehydration of alcohols via the SN1 or SN2 mechanism |
| Williamson ether synthesis | Alkoxide ion reacts with a primary alkyl halide or tosylate in an SN2 reaction |
| Alkoxymercuration | Reaction of an alkene with an alcohol in the presence of a trifluoroacetate mercury (II) salt |
| Ether reactivity | Relatively unreactive except when breaking the carbon-oxygen bond |
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What You'll Learn
Alcohol dehydration
Ethers are classified into two categories: symmetrical ethers and asymmetrical ethers. Symmetrical ethers are formed when two identical groups are attached to the oxygen atom, while asymmetrical ethers are formed when two different groups are attached.
The alcohol involved in the reaction plays two roles: one alcohol molecule acts as a substrate, while the other acts as a nucleophile. The reaction generally follows the SN2 mechanism, where the nucleophile attacks the substrate, resulting in the displacement of a water molecule and the formation of a new C-O bond.
While alcohol dehydration is a versatile method for preparing symmetrical ethers, it is limited to primary alcohols. When secondary or tertiary alcohols are used, they tend to dehydrate to form alkenes instead of the desired ethers. For the preparation of asymmetrical ethers, the Williamson ether synthesis is a more suitable method.
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Williamson ether synthesis
The Williamson ether synthesis is a widely used organic reaction that forms an ether from an organohalide and a deprotonated alcohol (alkoxide). This reaction was developed by Alexander Williamson in 1850 and is important in the history of organic chemistry as it helped prove the structure of ethers.
The Williamson ether synthesis is a substitution reaction, where a new C-O bond is formed, and a bond is broken between the carbon and the leaving group (LG) on the same carbon atom. This typically occurs when the leaving group is a halide or sulfonate. The reaction proceeds through an SN2 mechanism (nucleophilic substitution, bimolecular), where the nucleophile approaches the carbon atom from the backside of the carbon-leaving group bond. The nucleophile donates a pair of electrons into the antibonding orbital of the C-leaving group bond.
The Williamson ether synthesis is particularly effective when the nucleophile is an alkoxide ion (RO-) and the leaving group is a primary carbon on an alkyl halide. This combination ensures the desired SN2 reaction occurs, as secondary and tertiary carbons tend to favour elimination reactions due to the steric hindrance of nucleophiles navigating through alkyl groups. The alkoxide ion can be generated from primary, secondary, or tertiary alcohols with the addition of a strong base, such as sodium hydride (NaH) or potassium hydride (KH).
The Williamson ether synthesis is versatile and widely used in both laboratory and industrial settings. It is valuable for preparing both symmetrical and asymmetrical ethers and is especially useful in the production of aromatic ethers. The reaction is typically conducted at temperatures between 50 and 100 °C and can take between 1 to 8 hours to complete. To improve yields and speed up reaction times, microwave-enhanced technology has been introduced, increasing the yield of ether synthesized from a range of 6-29% to 20-55%.
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Using trifluoroacetate mercury (II) salt
The process of creating an ether from two alcohols using trifluoroacetate mercury (II) salt is called alkoxymercuration. This process is similar to oxymercuration, which involves converting an alkene to an alcohol.
In the first step of the alkoxymercuration reaction, one of the alcohols is protonated to become a good leaving group. This is followed by the second alcohol displacing water from the protonated alcohol during an SN2 reaction, yielding a protonated ether. Finally, this intermediate is deprotonated to yield the symmetrical ether. This process is known as the Williamson Ether Synthesis.
The Williamson Ether Synthesis involves the SN2 reaction of an alkoxide nucleophile with a primary alkyl halide or tosylate. Alkoxides are commonly created by deprotonating an alcohol with a strong base, such as sodium hydride (NaH). Simple alcohols can be used as a solvent during this synthesis.
Alkoxymercuration involves reacting an alkene with an alcohol in the presence of trifluoroacetate mercury (II) salt [(CF3CO2)2Hg]. This reaction produces an alkoxymercuration product. Demercuration, using sodium borohydride (NaBH4), then yields an ether product. This reaction allows for the Markovnikov addition of an alcohol to an alkene to create an ether.
It is important to note that the alcohol reactant is used as the solvent in this process. Trifluoroacetate mercury (II) salt is preferred over mercuric acetate because the trifluoroacetate anion is a poorer nucleophile than acetate. Most 1o, 2o, and 3o alcohols can be successfully used for this reaction. The mechanism of alkoxymercuration is similar to oxymercuration, with the electrophilic addition of the mercuric species to the alkene.
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Alkoxymercuration
The first step of the mechanism involves the reaction of an alkene with an alcohol in the presence of a mercury(II) salt, such as mercuric acetate, to form an alkoxymercury intermediate. This intermediate is then reduced with sodium borohydride to yield the ether product. The mercury ion acts as a bridge, stabilising the carbocation intermediate so that it cannot rearrange. The reaction follows an electrophilic addition mechanism, with the nucleophile attacking the more substituted carbon of the three-membered ring via an SN2 reaction.
An example of this reaction is the formation of 2-methoxy-2-methylpropane from 2-methylpropene and methanol in the presence of an acid catalyst.
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Oxymercuration
The oxymercuration reaction can be divided into three steps:
- Initiation: The nucleophilic double bond attacks the mercury ion, ejecting an acetoxy group. Simultaneously, the electron pair on the mercury ion attacks a carbon on the double bond, forming a mercurinium ion with a positive charge on the mercury atom.
- Nucleophilic Attack: A nucleophilic water molecule attacks the more substituted carbon, releasing the electrons involved in its bond with mercury.
- Product Formation: A negatively charged acetate ion deprotonates the alkyloxonium ion, forming the waste product HOAc. The electrons participating in the bond between oxygen and the attacked hydrogen collapse into the oxygen atom, neutralizing its charge and creating the final alcohol product.
It is important to note that the oxymercuration step is stereoselective, but the subsequent demercuration step is not. The stereochemistry established during oxymercuration is scrambled during demercuration, allowing the hydrogen and hydroxy groups to be cis or trans to each other. The demercuration step can be achieved with sodium borohydride (NaBH4), which breaks the C-Hg bond and forms a new C-H bond.
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Frequently asked questions
Ethers are chemical compounds that are commonly used as solvents and extractants in various industries. They are formed by the combination of two alcohol molecules.
Ethers are classified into two types: symmetrical ethers and asymmetrical ethers. Symmetrical ethers are formed when two identical groups are attached to the oxygen atom, while asymmetrical ethers are formed when two different groups are attached.
The Williamson ether synthesis is a two-step process commonly used to prepare asymmetrical ethers. It involves reacting an alkoxide ion with a primary alkyl halide or tosylate in an SN2 reaction.
Alcohol dehydration is an industrial process used to prepare symmetrical ethers. It involves the acid-catalyzed dehydration of primary alcohols via the SN2 mechanism. The reaction occurs by displacing water from a protonated ethanol molecule by the oxygen atom of another ethanol molecule.
