Theo Nash
Dehydration of alcohols is a common process in organic chemistry, often used to synthesise alkenes. The dehydration reaction involves heating the alcohol in the presence of a strong acid, such as sulfuric or phosphoric acid, at high temperatures. This process can follow an E1 or E2 mechanism, with primary alcohols typically undergoing E2 elimination and secondary and tertiary alcohols undergoing E1 elimination. During dehydration, the –OH group in the alcohol donates two electrons to H+ from the acid reagent, forming an alkyloxonium ion. This ion then leaves to form a carbocation, and the deprotonated acid attacks the adjacent hydrogen to form a double bond. The relative reactivity of alcohols in dehydration reactions varies, with certain factors influencing the outcome, such as the type of alcohol and the strength of the acid used.
| Characteristics | Values |
|---|---|
| Types of Alcohol | Primary, Secondary, Tertiary |
| Dehydration Mechanism | E1 or E2 |
| E1 Mechanism | Dehydration of alcohols in acidic media at high temperatures |
| E2 Mechanism | Conversion of alcohol functional group into a good leaving group in non-acidic conditions, followed by elimination reaction |
| Primary Alcohol Dehydration | E2 Mechanism |
| Secondary and Tertiary Alcohol Dehydration | E1 Mechanism |
| Reaction Temperature for Dehydration | 170°C for primary alcohol |
| Acid Used | Sulfuric Acid, Phosphoric Acid, Phosphorous Oxychloride (POCl3) |
| Reaction Product | Alkene, Ether |
| Reaction Type | Elimination or Substitution |
| Reaction Rate Determining Step | Formation of Carbocation |
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What You'll Learn
Alcohol dehydration forms alkenes
Alcohols can be dehydrated to form alkenes. This is an example of an elimination reaction, specifically, a β elimination reaction, in which two atoms are removed from neighbouring carbon atoms, resulting in a double bond formation.
The dehydration of alcohols involves heating the alcohol in the presence of a strong acid, such as sulfuric or phosphoric acid, at high temperatures. The reaction temperature depends on the substitution of the hydroxy-containing carbon, with the required temperature decreasing as substitution increases. If the reaction is not sufficiently heated, the alcohols will not dehydrate to form alkenes but will instead react with one another to form ethers.
The dehydration reaction involves the –OH group in the alcohol donating two electrons to H+ from the acid reagent, forming an alkyloxonium ion. This ion is a good leaving group, and it leaves to form a carbocation. The deprotonated acid then attacks the hydrogen adjacent to the carbocation, forming a double bond.
Different types of alcohols may undergo dehydration through slightly different mechanisms. Primary alcohols undergo bimolecular elimination (E2 mechanism), while secondary and tertiary alcohols undergo unimolecular elimination (E1 mechanism). The E2 elimination of 3º-alcohols under relatively non-acidic conditions may be accomplished by treatment with phosphorous oxychloride (POCl3) in pyridine.
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Primary alcohol dehydration
Dehydration of alcohols is a process in which alcohols undergo E1 or E2 mechanisms to lose water and form a double bond. The dehydration reaction of alcohols to generate alkenes proceeds by heating the alcohols in the presence of a strong acid, such as sulfuric or phosphoric acid, at high temperatures.
The general idea 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 acts as a very good leaving group which leaves to form a carbocation. The deprotonated acid (the nucleophile) then attacks the hydrogen adjacent to the carbocation and forms a double bond.
Primary alcohols undergo bimolecular elimination (E2 mechanism) while secondary and tertiary alcohols undergo unimolecular elimination (E1 mechanism). The E2 elimination of 3º-alcohols under relatively non-acidic conditions may be accomplished by treatment with phosphorous oxychloride (POCl3) in pyridine. This procedure is also effective with hindered 2º-alcohols, but for unhindered and 1º-alcohols, an SN2 chloride ion substitution of the chlorophosphate intermediate competes with elimination.
The required range of reaction temperature decreases with increasing substitution of the hydroxy-containing carbon. If the reaction is not sufficiently heated, the alcohols do not dehydrate to form alkenes but react with one another to form ethers (e.g., the Williamson Ether Synthesis). Alcohols are amphoteric; they can act as both an acid and a base. The lone pair of electrons on the oxygen atom makes the –OH group weakly basic.
Oxygen can donate two electrons to an electron-deficient proton. Thus, in the presence of a strong acid, R—OH acts as a base and protonates into the very acidic alkyloxonium ion +OH2.
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Secondary alcohol dehydration
Alcohols can be dehydrated to form alkenes. This process involves the alcohol undergoing E1 or E2 mechanisms to lose water and form a double bond. The dehydration reaction involves heating the alcohol in the presence of a strong acid, such as sulfuric or phosphoric acid, at high temperatures.
Secondary alcohols dehydrate through the E1 mechanism. The dehydration mechanism for a secondary alcohol is analogous to that of a tertiary alcohol. The secondary -OH group forms a relatively unstable secondary carbocation in the intermediate. This is followed by a hydride shift from an adjacent hydrogen to make the carbocation tertiary, which is much more stable. The products are a mixture of alkenes formed with or without carbocation rearrangement.
The general idea 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 then leaves to form a carbocation. The deprotonated acid then reacts with the hydrogen adjacent to the carbocation to form a double bond.
The E1 mechanism involves a carbocation intermediate that can undergo rearrangement, while the E2 mechanism is concerted. The E1 mechanism is associated with a negative entropy of activation, while the E2 mechanism is associated with a positive entropy of activation. This suggests that the E2 mechanism may become less favorable at higher temperatures.
Hydrothermal dehydration of secondary alcohols is an example of an organic reaction that differs from the corresponding chemistry under ambient laboratory conditions. In hydrothermal dehydration, water acts as the solvent and provides the catalyst, and no additional reagents are required.
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Tertiary alcohol dehydration
Alcohol dehydration is a process in which alcohols undergo E1 or E2 mechanisms to lose water and form a double bond. The dehydration reaction of alcohols to generate alkenes involves heating the alcohols in the presence of a strong acid, such as sulfuric or phosphoric acid, at high temperatures.
Different types of alcohols may dehydrate through slightly different mechanisms. However, the general idea 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 acts as a good leaving group, which leaves to form a carbocation. The deprotonated acid (the nucleophile) then attacks the hydrogen adjacent to the carbocation to form a double bond.
Primary alcohols undergo bimolecular elimination (E2 mechanism), while secondary and tertiary alcohols undergo unimolecular elimination (E1 mechanism). The dehydration mechanism for a tertiary alcohol is analogous to that of a secondary alcohol. The E2 elimination of tertiary alcohols under relatively non-acidic conditions may be accomplished by treatment with phosphorous oxychloride (POCl3) in pyridine.
The rate of dehydration is related to the ease of carbohydrate formation, with tertiary alcohols dehydrating faster than secondary or primary alcohols. The dehydration of alcohols can also be influenced by the reaction temperature, with higher temperatures required for primary alcohol dehydration and lower temperatures for secondary and tertiary alcohol dehydration.
Dehydrogenation reactions, conducted in the presence or absence of oxygen using various catalysts, can also transform alcohols into corresponding aldehydes or aromatize substituted cyclohexyl or cyclohexenyl compounds.
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Alcohol dehydration catalysts
The dehydration of alcohols to form alkenes is a common synthesis process in organic chemistry. The dehydration reaction involves the removal of the -OH group and a hydrogen atom from the adjacent carbon atom in the chain.
The dehydration reaction of alcohols to generate alkenes involves heating the alcohols in the presence of a strong acid, such as sulfuric or phosphoric acid, at high temperatures. The reaction temperature depends on the type of alcohol being dehydrated. For example, the dehydration of primary alcohol requires a temperature of 170°C, while the reaction in the source is performed at 25°C, which is insufficient to produce any alkenes.
The general mechanism behind the dehydration reaction involves the –OH group in the alcohol donating two electrons to H+ from the acid reagent, forming an alkyloxonium ion. This ion then leaves to form a carbocation. The deprotonated acid then reacts with the hydrogen adjacent to the carbocation to form a double bond.
Primary alcohols undergo an E2 mechanism (bimolecular elimination), while secondary and tertiary alcohols undergo an E1 mechanism (unimolecular elimination). The E2 mechanism of primary alcohols involves the hydroxyl oxygen donating two electrons to a proton from sulfuric acid (H2SO4), forming an alkyloxonium ion. The nucleophile HSO4– then attacks one adjacent hydrogen, and the alkyloxonium ion leaves, forming a double bond.
The dehydration of secondary and tertiary alcohols involves the protonation of the –OH group to form alkyloxonium ions. The ion then leaves, forming a carbocation as the reaction intermediate. The water molecule then abstracts a proton from an adjacent carbon, forming a double bond. The alkene formed depends on which proton is abstracted.
The dehydration of tertiary alcohols is similar to that of secondary alcohols. The E2 elimination of tertiary alcohols can be achieved under relatively non-acidic conditions using phosphorus oxychloride (POCl3) in pyridine.
Hydrothermal dehydration of alcohols is another method that does not require external reagents or catalysts and is, therefore, an example of green chemistry. In this method, water acts as the solvent and the catalyst.
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Frequently asked questions
Dehydration of alcohol is an elimination reaction that yields an alkene via water elimination. Alcohols undergo E1 or E2 mechanisms to lose water and form a double bond.
The dehydration of alcohol involves three steps. Firstly, the alcohol is acted upon by a protic acid. Secondly, protonation of alcoholic oxygen takes place, which makes it a better leaving group. Finally, the C-O bond breaks, generating a carbocation.
The two types of alcohol dehydration mechanisms are E1 and E2. The E1 method is based on the dehydration of alcohols in acidic media at high temperatures. The E2 method is based on the conversion of the alcohol functional group into a good leaving group in non-acidic conditions followed by the elimination reaction.
Primary alcohols undergo the E2 mechanism, while secondary and tertiary alcohols undergo the E1 mechanism.
Alcohol is a diuretic, which means it increases the production of urine. This can lead to dehydration if fluid intake is insufficient. Additionally, alcohol inhibits the release of the hormone vasopressin, which regulates water retention in the body, further contributing to dehydration.
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