Lena Savoie
Ethers are prepared from organic compounds by various methods. The two most common methods for preparing ethers from alcohols are alcohol dehydration and Williamson ether synthesis. Alcohol dehydration is used for the industrial preparation of ethers. The method involves acid-catalyzed dehydration of alcohols via the SN2 mechanism. Williamson ether synthesis is a two-step process and provides an important route for preparing asymmetrical ethers. The Williamson synthesis exhibits higher productivity in the case of primary alkyl halides. The ether is symmetrical, so each C-O bond of the ether can be cleaved to produce a set of starting materials for consideration.
| Characteristics | Values |
|---|---|
| Ether formation methods | Alcohol dehydration, Williamson ether synthesis, alkoxymercuration of alkenes, acid-catalyzed substitution, acid-catalyzed dehydration |
| Williamson ether synthesis substrates | Prefer methyl or primary alkyl halides |
| Williamson ether synthesis nucleophile | Alcohol |
| Williamson ether synthesis mechanism | SN2 |
| Williamson ether synthesis alternative | Silver oxide (Ag2O) instead of a strong base |
| Alcohol dehydration nucleophile | Another equivalent of the alcohol |
| Alcohol dehydration mechanism | SN2 |
| Alcohol dehydration example | Dehydration of ethanol at 413 K yields ethoxyethane |
| Alcohol dehydration example | Dehydration of ethanol at 443 K yields ethene |
| Alcohol dehydration example | Heating ethanol at 130-140 °C yields diethyl ether |
| Alcohol dehydration catalyst | Protic acids, e.g. sulfuric acid |
| Ether properties | Lower boiling point than alcohols, prone to reacting violently with strong oxidizing agents |
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What You'll Learn
Alcohol dehydration
The alcohol dehydration method is limited to the preparation of symmetrical ethers derived from primary alcohols. Secondary and tertiary alcohols cannot be used because they dehydrate to alkenes. The method is also unfit for preparing asymmetrical ethers because it yields a mixture of ethers.
The process works best for making symmetrical ethers of primary alcohols. A classic example is the heating of ethanol at 130-140 °C in the presence of sulphuric acid to give diethyl ether. The reaction proceeds through protonation of a hydroxyl group to give the conjugate acid followed by an SN2 reaction to give the symmetrical ether. The Williamson ether synthesis, on the other hand, is a more general method for preparing ethers and is suitable for both symmetrical and asymmetrical ethers. It is an important method for the preparation of ethers in laboratories. In this method, an alkyl halide is reacted with sodium alkoxide, which leads to the formation of ether.
The alcohol involved in the reaction plays two roles: one alcohol molecule acts as a substrate while the other acts as a nucleophile. The choice of the mechanism depends on whether the protonated alcohol loses water before or simultaneously upon the attack of a second alcohol molecule. Generally, secondary and tertiary alcohols follow the SN1 mechanism, while primary alcohols follow the SN2 mechanism.
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Williamson ether synthesis
The Williamson ether synthesis is a widely used method for forming ethers from alcohols. It was first developed by Alexander Williamson in 1850 and remains the simplest and most popular method for preparing ethers. The Williamson ether synthesis is a substitution reaction, specifically an SN2 (bimolecular nucleophilic 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. The leaving group is typically a halide or sulfonate ester, and the reaction involves the nucleophilic attack of an alkoxide ion on an electrophilic carbon.
The general form of the reaction is as follows:
Alkoxide ion + primary alkyl halide → ether + halide salt
For example, the reaction of sodium ethoxide with chloroethane forms diethyl ether and sodium chloride:
CH3CH2O^-^ + CH3CH2Cl → CH3CH2OCH2CH3 + NaCl
The Williamson ether synthesis is often used to prepare ethers indirectly from two alcohols. One of the alcohols is first converted to a leaving group, usually tosylate, and then the two molecules are reacted together. The reaction conditions are rather forcing, so protecting groups are used to pacify other reactive parts of the molecules, such as other alcohol or amine groups. The reaction is typically carried out at temperatures between 50 and 100 °C for 1 to 8 hours to ensure complete conversion.
Microwave-enhanced technology has been used to speed up the reaction, reducing the reaction time to a quick 10-minute microwave run at 130 °C, which has significantly increased the yield of the ether product. The Williamson ether synthesis is a versatile and broadly applicable method for preparing ethers and is an important reaction in the history of organic chemistry.
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Alkoxymercuration of alkenes
Alkoxymercuration-demercuration is a reaction used to form ethers from alkenes. This reaction is an alternative to the Williamson Synthesis, which would use a bulky base and secondary alkyl halide. The alkoxymercuration-demercuration reaction is more effective because the reactants in the Williamson Synthesis favour the elimination mechanism.
The first step of the alkoxymercuration reaction involves methyl acrylate reacting with Hg(OAc)2 in methanol. This step is interesting because it goes anti-Markovnikov due to the electron-withdrawing effect of the adjacent methyl ester. The mercury is then replaced with bromine (Br2), and the resulting alkyl halide undergoes an SN2 reaction with NH3, yielding an amino acid.
The overall transformation of the reaction is C=C to H-C-C-OR, with typical reagents being mercury acetate, Hg(OAc)2 in ROH/THF. Mercury compounds are generally quite toxic. The reaction is regioselective, as predicted by Markovnikov's rule, and proceeds via the formation of a cyclic mercurinium ion. The mercurinium ion is formed when the nucleophilic double bond attacks the mercury ion, ejecting an acetoxy group. The electron pair on the mercury ion then attacks a carbon on the double bond.
The final step of the reaction is demercuration, which is achieved by a reduction using sodium borohydride, NaBH4. This step neutralizes the electrons donated to mercury in the previous step.
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Acid-catalyzed substitution
The process of acid-catalyzed ether formation consists of three key steps. Firstly, one equivalent of alcohol is protonated to form its conjugate acid, which acts as a better leaving group. This conjugate acid has the formula OH2 (water), which is a weak base.
In the second step, another equivalent of the alcohol performs a nucleophilic attack at carbon (SN2), resulting in the displacement of OH2 (water) and the formation of a new C-O bond. This is also an SN2 reaction.
Finally, the product is deprotonated by another equivalent of the solvent or another weak base, resulting in the formation of the ether product. This process is particularly important for the synthesis of diethyl ether, which is a valuable commodity chemical and a useful solvent in organic chemistry.
It is important to note that this method is limited to the preparation of symmetrical ethers. Attempting to synthesize unsymmetrical ethers using this process will result in mixtures that need to be separated, leading to low yields of the desired components. Additionally, the temperature must be carefully optimized to minimize side reactions.
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Acid-catalyzed dehydration
\[ \ce {2 CH3CH2-OH + H2SO4 ->[130 \,^oC] CH3CH2-O-CH2CH3 + H2O} \]
The reaction begins with protonation of one equivalent of alcohol to its conjugate acid, which has the good leaving group OH2 (water). This protonation converts hydroxyl, a poor leaving group, into an oxonium ion, a better leaving group.
In the next step, another equivalent of the alcohol performs a nucleophilic attack at carbon (SN2), leading to the displacement of OH2 (water) and the formation of a new C-O bond.
Finally, deprotonation of the product by another equivalent of solvent (or other weak base) results in the ether product.
For example, ethanol can be dehydrated to yield ethoxyethane (also known as diethyl ether) in the presence of sulfuric acid at 413 K. This reaction has been known since the days of Valerius Cordus, who reported the synthesis in 1540. The reaction can be represented as follows:
\[ \ce {2 CH3CH2OH ->[H_2SO_4, 413 K] CH3CH2OCH2CH3 + H2O} \]
It is important to note that the alcohol must be used in excess, and the temperature must be maintained around 413 K. If the temperature is higher, the ethanol will undergo dehydration to yield ethene instead.
A new approach to ether synthesis involves the use of solid acid catalysts, such as DELOXAN ASP or AMBERLYST 15, and supercritical fluid solvents. This method allows for the formation of linear alkyl ethers with high selectivity and little rearrangement to give branched ethers.
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Frequently asked questions
Ether formation is the process of producing ether from an alcohol.
The two most common methods for preparing ethers from alcohols are alcohol dehydration and Williamson ether synthesis.
Alcohol dehydration is an industrial process for preparing ethers. It involves acid-catalyzed dehydration of alcohols via the SN2 mechanism. For example, the dehydration of ethanol in the presence of sulfuric acid yields diethyl ether.
Williamson ether synthesis is a two-step process and an important route for preparing asymmetrical ethers. It involves reacting an alkyl halide with sodium alkoxide, which leads to the formation of ether.
The main point to consider is the ability of the alkyl fragment to form an alkene. The synthesis pathway should also take into account the characteristics of the nucleophile and electrophile.
