Theo Nash
When two alcohol molecules come together, they can form an ether molecule by eliminating a water molecule. This reaction is known as the dehydration of alcohols and can be achieved by treating the alcohol with a strong acid, such as sulfuric acid, and heating it. The general formula for ethers is R—O—R, where the hydrocarbon groups (R) may be the same or different. Ethers have similar solubility in water compared to their isomeric alcohols. Diethyl ether, a primary example of an ether, is a colorless, flammable liquid that was once used as an anesthetic.
| Characteristics | Values |
|---|---|
| Mechanism of dehydration | Proton transfer from sulfuric acid to the alcohol |
| Oxonium salt or alkyl hydrogen sulfate may react by SN displacement mechanism with excess alcohol | |
| Phosphoric acid may be used in place of sulfuric acid | |
| Dehydration at high temperatures in the presence of solid catalysts like silica gel or aluminum oxide | |
| Products | Ethylene (intramolecular dehydration) |
| Diethyl ether (intermolecular dehydration) |
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What You'll Learn
Dehydration of alcohols to form alkenes
Alcohols can be dehydrated to form alkenes. This process involves the removal of water from the alcohol molecule to create a double bond. The dehydration reaction of alcohols to generate alkenes can be achieved by heating the alcohols in the presence of a strong acid, such as sulfuric or phosphoric acid, at high temperatures. The required temperature range decreases with increasing substitution of the hydroxy-containing carbon.
The dehydration reaction of alcohols involves multiple steps. Firstly, the acid protonates (adding a proton or H+) the alcohol on the oxygen atom, which is the most electronegative atom. Subsequently, the protonated alcohol loses water to form a positively charged species known as a carbonium ion or carbocation. Finally, the carbonium ion loses a proton to yield the alkene.
Different types of alcohols may undergo dehydration through slightly different mechanisms. However, the fundamental principle behind each dehydration reaction is that the –OH group in the alcohol donates two electrons to H+ from the acid reagent, forming an alkyloxonium ion. This ion serves as an excellent leaving group, departing to create a carbocation. The deprotonated acid, acting as the nucleophile, subsequently attacks the hydrogen adjacent to the carbocation to form a double bond.
Primary alcohols typically undergo bimolecular elimination (E2 mechanism), while secondary and tertiary alcohols generally undergo unimolecular elimination (E1 mechanism). The ease of dehydration of alcohols follows the order: tertiary > secondary > primary. Secondary and tertiary alcohols are more effectively dehydrated using dilute sulfuric acid.
It is important to note that if the reaction temperature is insufficient, the alcohols may not dehydrate to produce alkenes. Instead, they may react with each other to form ethers, such as in the Williamson Ether Synthesis.
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Tertiary alcohols react with sulfuric acid
Alcohols are classified as primary, secondary, or tertiary depending on the position of the hydroxy group. The position of the hydroxy group affects the reactivity of the molecule. When two alcohols come together in the presence of sulfuric acid, an alcohol can be formed. A hydrogen atom from the sulfuric acid adds to the carbon of the double bond with the most hydrogen atoms. Then, a hydroxy group from the water will add to the other carbon of the double bond, as predicted by Markovnikov's rule. This is known as the hydration reaction of an alkene.
Tertiary alcohols do not have a hydrogen atom attached to the carbon. To form a carbon-oxygen double bond, two particular hydrogen atoms need to be removed. Tertiary alcohols do not react with acidified sodium or potassium dichromate(VI) solution. However, they react with sulfuric acid. In the reaction of an alcohol with hot concentrated sulfuric acid, the alcohol is dehydrated to an alkene. This is the reverse of the acid-catalyzed hydration of alkenes.
The dehydration of tertiary alcohols involves proton transfer from sulfuric acid to the alcohol, followed by an E2 reaction of hydrogen sulfate ion or water with the oxonium salt of the alcohol. Alternatively, the alkyl hydrogen sulfate could be formed and eliminate sulfuric acid by an E2 reaction. At lower temperatures, the oxonium salt or the alkyl hydrogen sulfate may react by an Sn1 displacement mechanism with excess alcohol in the mixture, forming a dialkyl ether.
Sulfuric acid is a strong reagent for the dehydration of tertiary alcohols. Potassium hydrogen sulfate, copper sulfate, iodine, phosphoric acid, or phosphorus pentoxide may give better results by causing less polymerization and oxidative degradation. The Sn1-E1 behavior of tertiary alcohols in strong acids can be used to prepare tert-butyl ethers. For example, when a mixture of tert-butyl alcohol and methanol is heated in the presence of sulfuric acid, the tertiary alcohol reacts rapidly but reversibly to produce 2-methylpropene.
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Formation of hemiacetals and acetals
Alcohols (R-OH) can add to carbonyl groups, which are characterized by a carbon-oxygen double bond. The addition of alcohols to carbonyl groups can form hemiacetals and acetals.
Hemiacetals and acetals are formed through the addition reactions of oxygen-based nucleophiles (water and alcohols) to aldehydes and ketones. A hydrate is formed when water is added to an aldehyde or ketone, and a hemiacetal is formed when an alcohol is added. Acetals are formed when two equivalents of an alcohol are reacted with an aldehyde or ketone. This reaction produces water as a byproduct.
The formation of a hemiacetal is the first step in the formation of an acetal. To effectively form hemiacetals and acetals, an acid catalyst must be used because alcohol is a weak nucleophile. The water produced with the acetal must also be removed from the reaction by a process such as a molecular sieve or a Dean-Stark trap.
The mechanism for the formation of a cyclic hemiacetal is the same as that of an acyclic acetal. In both cases, C-O is formed, C-O (pi) is broken, and O-H is formed. The only difference is that the OH and carbonyl groups are connected through a carbon chain. Intramolecular hemiacetal formation is common in sugar chemistry, such as in the common sugar glucose.
Acetals are tetrahedral compounds where two alkoxy (OR) groups are bonded to the central carbon atom. They are stable and unreactive in neutral to strongly basic environments.
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Ethers from alcohols
Symmetrical ethers can be formed from the acid-catalyzed dehydration of primary alcohols. For example, ethanol can be heated to 130-140 °C to yield diethyl ether. The reaction involves protonation of a hydroxyl group to produce the conjugate acid, followed by an SN2 reaction that results in the symmetrical ether. This method is particularly effective for producing symmetrical ethers from primary alcohols.
Another example of ether synthesis from alcohols is the single-stage synthesis of diisopropyl ether, an alternative octane enhancer for lead-free petrol. This process involves the formation of symmetrical ethers from secondary alcohols, such as isopropanol. However, the mechanism is more complex due to the competition between bimolecular dehydration and other pathways like SN1 or elimination-addition reactions. Diisopropyl ether is a useful solvent but requires careful handling and storage due to its propensity to form explosive peroxides.
The reaction of phenols and alcohols over thoria can also lead to ether formation. Under forcing conditions, phenol can dehydrate to diphenyl ether through an unusual mechanism. Additionally, the dehydration of 3,3-dimethyl-2-butanol, a secondary alcohol, can result in the formation of a tertiary carbocation through the rearrangement of a secondary carbocation. This reaction typically employs phosphoric acid instead of sulfuric acid as the dehydrating agent because it is less destructive and a weaker oxidizing agent.
In general, the dehydration of alcohols to form ethers can be achieved using various acids and reagents. For instance, sulfuric acid is commonly used, but it can be strenuous for the dehydration of tertiary alcohols. Alternatives like potassium hydrogen sulfate, copper sulfate, iodine, phosphoric acid, or phosphorus pentoxide may yield better results by reducing polymerization and oxidative degradation. The choice of reagents and reaction conditions depends on the specific alcohol substrates and the desired ether product.
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Alcohols as organic compounds
Alcohols are organic compounds with one or more hydroxyl (―OH) groups attached to a carbon atom of an alkyl group (hydrocarbon chain). They are derivatives of water (H2O) where one of the hydrogen atoms has been replaced by an alkyl group. For example, in ethanol (ethyl alcohol), the alkyl group is the ethyl group, ―CH2CH3.
Alcohols can be classified as primary, secondary, or tertiary, depending on which carbon atom is bonded to the hydroxyl group. If the hydroxyl group is bonded to a primary carbon atom (bonded to only one other carbon atom), the compound is a primary alcohol. Secondary and tertiary alcohols have hydroxyl groups bonded to carbon atoms with two and three other carbon bonds, respectively. Alcohols are further classified as allylic or benzylic if the hydroxyl group is bonded to an allylic carbon atom (adjacent to a C=C double bond) or a benzylic carbon atom (next to a benzene ring).
The term alcohol originally referred to the primary alcohol ethanol, which is commonly used as a drug and is the main alcohol in alcoholic drinks. The suffix '-ol' is used in the IUPAC chemical name of substances where the hydroxyl group is the functional group with the highest priority. Methanol and propanol are also simple alcohols that occur in nature and are industrially synthesized for various applications. Many other alcohols are found in organisms, such as in sugars like fructose and sucrose, and in amino acids like serine.
Alcohols are valuable intermediates in the synthesis of other compounds and are among the most abundantly produced organic chemicals in industry. They are used in the production of toiletries, pharmaceuticals, fuels, and perfumes, and they are also used to sterilize hospital instruments.
One important reaction involving alcohols is dehydration, which can be achieved using sulfuric acid or phosphoric acid. Dehydration of alcohols produces alkenes or ethers, and this process is often used to produce ethanol. Another reaction involving alcohols is the Grignard reaction, where a new C-C bond is formed, and a C-O bond is broken, resulting in the formation of a neutral alcohol product.
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Frequently asked questions
The products are an ether and water.
Ether formation can be enhanced by distilling the ether away as soon as it forms.
Diethyl ether, tertiary-butyl methyl ether (MTBE), and ethylene are some examples of ethers.
Ethers are used as solvents for gums, fats, waxes, and resins. MTBE is also used as an additive for gasoline.
Ethers can be formed by the dehydration of alcohols using sulfuric acid or phosphoric acid.
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