Dehydration of Alcohols: Definition, Properties, Reaction

One of the ways to synthesize alkenes is by dehydration of alcohols, a process wherein alcohols undergo E1 or E2 mechanisms to lose water and form a double bond. This dehydration reaction of alcohols to generate alkene is made by heating the alcohols at high temperatures and in the presence of strong acids like sulphuric or phosphoric acid. Let’s have a closer look at the topic along with important questions. 

Chemistry is divided into three parts named organic chemistry, inorganic chemistry, and physical chemistry. Organic chemistry consists of the study of organic compounds containing carbon and hydrogen bonds. These study the properties, exhibit the reactions they undergo, and how the synthesis of such compounds is understood through organic chemistry. Alcohol, too, is a part of organic chemistry.

Alcohol is an organic compound containing one or more hydroxyl groups directly attached to carbon atoms or atoms. Alcohols can be divided into the following parts like mono, di, tri, or polyhydric compounds. This division depends on the number of hydroxyl groups attached to these structures.

Key Terms: Alcohol, Dehydration of alcohol, reaction mechanism, secondary dehydration, tertiary alcohol dehydration, Organic chemistry, organic compound, hydroxyl groups

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Nomenclature of Alcohol

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The most common way of naming alcohol depends on the alkyl group and adding alcohol to it. The last letter ‘e’ in the word alkene is replaced with ‘ol’. The one which contains the longest carbon chain is treated as the parent chain and determines the numerals and the position of the groups, and the other substituent are indicated.

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Properties of Alcohol

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• Boiling Point

The boiling point of alcohol increases with the increase in the number of carbon atoms. This is because, with the increase in the number of carbon atoms, Vander wall forces increase. The boiling point also decreases with the branching in the carbon chain.

• Solubility

Alcohols are soluble in the water. This is because alcohols form hydrogen bonds with water. Therefore, the solubility in water decreases with increasing the size of the alkyl group.

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Dehydration of Alcohol

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This is a simple elimination reaction. This happens when alcohols react with protic acid to undergo dehydration and lose a molecule of water to form an alkene. Such reactions are categorized as dehydration reactions. The rate of word dehydration might vary depending on the type of alcohol, like primary, secondary, or tertiary. It is the highest in tertiary alcohols because the carbonation is very stable in such alcohols, especially when compared to primary and secondary alcohol.

There are three ways in which dehydration can be performed:-

• With the help of protonated alcohol
• Through carbonation
• Through alkenes

Dehydration reactions, when conducted in the presence of oxygen, transform alcohols into aldehydes. Such reactions are endothermic and required a sufficient amount of heat as an input.

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Reaction Mechanism of Dehydration of Alcohol

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The process of dehydration of alcohols generally tries to follow the E1 or E2 Mechanism. Therefore, primary alcohol mostly tries to follow the E2 mechanism, while tertiary and secondary alcohols follow the E1 mechanism.

The entire reaction mechanism is further divided into three basic steps:-

1. Formation of Protonated Alcohol

In this first step, the alcohol undergoes a reaction with protic acid. This step is straightforward and is also reversible.

2. Formation of Carbocation

In the second step, the (C–O) bond is broken, resulting in a carbocation formation. Out of the three steps, this is the slowest one in the mechanism of dehydration of alcohol. This is also said to be the rate-determining step.

3. Formation of Alkene

The last and final step is when the proton is eliminated with the help of a base. Then, the carbon atom, nearest to the carbo anion, breaks the C-H bond and helps in forming the C-C bond resulting in the formation of alkene.

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Example of Alcohol Dehydration Mechanism

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The dehydrogenation of alcohol followed the E2 mechanism shows a different selectivity known as anti and syn elimination. Anti eliminations are when the products have resulted from the groups removed from opposite sides, while syn elimination is when the products have been formed from the groups removed from the same side. In addition, the E2 mechanism also helps eliminate a proton and the hydroxyl group, which is present in the alcohol without forming an ionic intermediate. In the E1 mechanism, the initial and the first step of dehydration is forming the carbonation, in which the C-H bond present in the compound is broken very initially.

The three types of alcohols donor's primary, secondary grocery undergo nucleophilic substitution reactions with HI, HBr, and HCl to form alkyl halides. Such reactions are known as SN1 reactions for primary alcohols SN² reactions for secondary and tertiary alcohols.

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Reaction of Alcohol Dehydration

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A dehydration reaction is a form of chemical reaction in which water is formed from the extraction of the components of water from a single reactant. When dehydration of alcohol is performed, an alkene is formed. 
The structural equation of alcohol dehydration is:

C₂H₅OH → C₂H₄ + H₂O
Alcohol dehydration exemplifies the elimination reaction which differs from substitution and addition reaction. An elimination reaction refers to a reaction in which 2 groups or 2 atoms on neighboring carbon atoms are eliminated or removed from a molecule which leaves numerous bonds between the carbon atoms.

Moreover, the dehydration of secondary as well as tertiary alcohols in acidic conditions also follows the E1 method. The protonation of the hydroxyl group successfully transforms the leaving group from hydroxide ions into the water. The hydronium H₃O+ is comparatively stronger than H₂O, and the conjugate surface of the former H₂O is a better leaving group as compared to that of a latter OH. by the dehydration of a protonated alcohol when a relatively stable carbocation is produced, an E1 elimination can occur. And when a proton is lost from carbon at the same time as water is lost from the neighboring carbon, an E2 reaction takes place. 

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Protonated Primary Alcohol

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Dehydration can take place easily when a neighboring double bond is formed. Alcohol that carries a carbonyl group, two carbons away readily undergoes through a dehydration process and finally yields α, β- unsaturated carbonyl compound.

By the E1cB mechanism, the location of the carbonyl group to the hydroxyl group in β hydroxyl carbonyl compounds opens the way for elimination under general conditions.

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Secondary Alcohol Dehydration

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The process of alcohol dehydration requires cleavage of the C-O bond along with loss of a proton from the beta position, whose consequences will be either alkenes or a mixture of alkenes. And the order of dehydration will be first tertiary, then secondary, and at last primary. 

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Tertiary Alcohol Dehydration

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The easiest way to dehydrate is the tertiary form of alcohol as the carbocations are more stable and therefore easier to form than that of the secondary and primary carbocations.

The alcohol has to be heated to roughly 50°C in 5% H₂SO₄ for the dehydration to take place. For the secondary alcohol, the dehydration needs about 100°C in 75% H₂SO₄, and primary can only be dehydrated at 170°C in 95% H₂SO₄ which are under extreme conditions. The dehydration process of both secondary and tertiary alcohols comprises a product formation called the carbocation intermediate, where the carbocation gets rearranged if the fallout is a more stable carbocation.

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Things to Remember

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• Primary alcohols undergo the E2 mechanism, while secondary and tertiary undergo the E1 mechanism.
• The order of reactivity in case of dehydration reaction of alcohol is

Methanol < primary < secondary < tertiary

• When alcohol reacts with protic acids, it loses a molecule of water in order to form alkenes.
• Dehydration of alcohol is a basic example of an elimination reaction.

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Sample Questions

Ques: The hydration of alcohol belongs to which category of reactions? (2 marks)
(a) Substitution reaction
(b) Elimination reaction
(c) Addition reaction
(d) Both (a) and (b)

Ques: Out of the given compounds, which is the most easily dehydrated and why? (3 marks)

Ques: Work on the correct order the rate of dehydration when the compounds are treated with concentrated H2SO4? (3 marks)

Ques: What are the properties of alcohol? (2 marks)

Ques: Why is tertiary alcohol dehydration the easiest way to dehydrate? (3 marks)

Ques: Tertiary alcohols under dehydration by: (1 mark)
(a) SN¹ mechanism
(b) E₂ mechanism
(c) E₁ mechanism
(d) SN² mechanism

Ques: During the dehydration of alcohols to alkenes by heating with concentrated H2SO4, the initiation step is: (1 mark)
(a) Formation of carbocation
(b) Protonation of alcohol
(c) Formation of carbanion
(d) Elimination of water

Ques: Primary alcohol undergoes dehydration by: (1 mark)
(a) E₂ mechanism
(b) E₁ mechanism
(c) SN¹ mechanism
(d) SN² mechanism

Ques: Which one of the following is the correct order of relative reactivities of alcohols in the dehydration reaction: (1 mark)
(a) 1° < 2° < 3°
(b) 2° < 1° < 3°
(c) 3° < 2° < 1°
(d) 3° < 1° < 2°

Ques: The dehydration of alcohol is an example of: (1 mark)
(a) Biomolecular elimination reaction
(b) Nucleophilic substitution reaction
(c) Unimolecular elimination reaction
(d) Internal substitution reaction 

Ques: Show the mechanism of dehydration steps of ethanol: (Comptt. Delhi 2015)

Ques: Explain the mechanism of acid dehydration of ethanol to yield ethene. (Comptt. All India 2015)

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