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Dehydration of Alcohols (Dehydrogenation) - Mechanism, Examples, FAQs

Dehydration of Alcohols (Dehydrogenation) - Mechanism, Examples, FAQs

Edited By Team Careers360 | Updated on Jul 02, 2025 04:55 PM IST

After reading this article, the reader should be able to answer the following questions- What is formed when a primary alcohol undergoes catalytic dehydrogenation, write the mechanism of dehydration of ethanol to ethene, give examples of dehydrating agent, give mechanism of acid catalyzed dehydration of ethanol to ethene, How can you convert alkene to alcohol, give hydration of alkene mechanism, primary alcohol undergo what reaction to form alkenes, what do you mean by dehydration, what is dehydrogenation, what is dehydrogenation of alcohol, what is dehydrogenation meaning in hindi.

Note: Dehydrate meaning in Hindi is निर्जलीकरण, dehydration meaning in hindi is निर्जलीकरण

Dehydration chemistry is highly useful in industrial processes. The literal meaning of the word ‘dehydration’ is to get rid of water and in this case, molecules of alcohol are getting rid of water. Dehydration of alcohol is an example of an elimination reaction where OH and H bonded to two adjacent carbon atoms get eliminated to form a double bond between the same carbon atoms.

Background wave

Elimination reactions are of two types. E1 type of elimination reaction is considered for tertiary alcohols and E2 type of elimination reaction is considered for Primary alcohols. The dehydration of alcohol gives alkene. There are a few conditions required for dehydrogenation of ethanol reaction like high temperature and presence of Bronsted acids.

  • Dehydration chemistry is highly dependent on the dehydration reagents.
  • Dehydration of alcohol with Al2O3 mechanism is carried out in the presence of solid Al2O3
  • Examples of dehydrating agents are concentrated Sulfuric acid, ZnCl2, and H3PO4, and H2SO4.
  • A few changes in the conditions can lead to the formation of ether from alcohol in place of alkene. The mechanism of dehydration of alcohol to form ether follows a SN2 reaction at lower temperatures.
  • There are about 6-7 methods of dehydration in whole chemistry for different compounds but Dehydration of alcohol is an example of substitution or elimination reaction.

Also read -

Following is an example of acidic dehydration of alcohol.

  • Dehydration of butan-1-ol is given as follow-
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Dehydration of glycerol-

Alcohol to alkene-

Shown below is a general reaction for dehydration of alcohols to alkene (dehydration of ethanol)

General reaction for the conversion of alcohol to alkene-

  • Since dehydration of alcohol is an example of an elimination reaction. Therefore, High temperatures are essential to favor this reaction.
  • Bronsted acids are essential because they are proton donors. We will see how proton donors play a crucial role in the alcohol dehydration mechanism. Bronsted acids are also known as dehydrating agents. H2SO4 and H3PO4are some of the examples of dehydrating agents that are used to carry out the dehydration of alcohols.
  • Acid-catalyzed dehydration of alcohol mechanism depends on the type of alcohol i.e., whether alcohol is primary, secondary or, tertiary will decide the path of mechanism.
    Mechanism of Tertiary alcohol and secondary alcohol-

Dehydration mechanism for tertiary/Secondary alcohols:

3⁰ alcohols are relatively easily dehydrated Under mild conditions which means these will dehydrate even in 20 % aqueous acid solution and a temperature of 85⁰C whereas these conditions change drastically for primary alcohols.

Following is the step-by-step mechanism for acidic Dehydration of alcohol to alkene-

Let us consider the alcohol dehydration reaction of tert-butyl alcohol:

Step 1: In the first step of dehydration of tertiary alcohol oxygen gets protonated from the acid catalyst. We can see how a proton donor is required to initiate the dehydration of alcohol to alkene. Hence it is acid-catalyzed dehydration of ethanol to ethene reaction. Hydronium ions of the acid get transferred to the oxygen atom of the alcohol which has an unshared pair of electrons. A positive charge develops on the oxygen atom and all the oxygen bonds become weaker. H2O will act as a leaving group.

Step 2: H2O leaves as leaving group and carbocation is generated. This carbocation is the intermediate step in this step. This carbocation is reactive because it has only six electrons in its valence shell.

Step 3: Hydrogen atoms are removed from the β-Carbon of the carbocation and an alkene is formed with a hydronium ion.

Dehydration mechanism of Primary alcohols:

Dehydration of ethanol to ethene takes place in drastic conditions. Alcohol dehydration reactions require higher temperatures and concentrated acid solutions. Dehydration of primary alcohol such as ethyl alcohol follows an E2 elimination reaction path because the intermediate formed in this mechanism is secondary carbocation. The mechanism of acid-catalyzed dehydration of ethanol gives ethene.

Dehydration of ethanol reaction proceeds by the formation of carbocation but the conditions required are different since primary carbocation is not more stable than the tertiary and secondary. Dehydration of ethanol follows the mechanism of acid-catalyzed dehydration of ethanol to ethene.

Following is the step-by-step mechanism for acidic dehydration of ethanol-

Step 1: Alcohol mechanism of action is different because it is not a good leaving group. The oxygen of primary alcohol shares its electron with hydronium ions and a protonated alcohol is formed. We know that the -OH group is not a good leaving group but with the protonation of the -OH group H2O+ can leave as a leaving group.

Step 2: In step 2 the H2O+ group leaves and a carbocation is formed. The formation of the carbocation in dehydration of ethanol is the rate-determining step of its mechanism.

Step 3: Carbocation is unstable as it only has six electrons in its valence shell. Therefore, it releases H+ and an alkene i.e., ethane, rate-determining is formed from ethyl alcohol. Dehydration of ethanol mechanism is the general mechanism for the dehydration of alcohol.

Fig- Dehydration of ethanol

Above is the mechanism of acid-catalyzed dehydration of alcohol -

Dehydration of alcohol in case of secondary alcohol-

  • A good example of secondary alcohol is cyclic alcohol such as Cyclohexanol.
  • Secondary alcohols get dehydrated in milder conditions such as 85% Acid solution and 170⁰C temperature.

Fig- Dehydration of cyclohexanol

Order of dehydration of alcohol-

  • Order of dehydration of alcohol will depend upon its structure i.e., tertiary, secondary atom reactions, or primary. The slowest rate-determining step in both the mechanisms in which a carbocation is formed.
  • The type of carbocation formed will determine the ease with which the reaction will follow. E1 reaction is followed by tertiary and secondary alcohols in which the intermediate form is tertiary and secondary carbocation respectively. A tertiary and secondary carbocation is stable as compared to primary hence, the free energy of activation is lower.
  • However, the E2 reaction is followed by primary alcohols. The intermediate form is a primary carbocation which is the least stable carbocation among all the others and the free energy of activation is higher. Dehydration of ethyl alcohol will be the slowest among all.

Fig- The rate of dehydration of alcohol follows the order mentioned above.

Alkene to Alcohol (acid-catalyzed hydration of alkene)-

Reversal of the above reaction will give us alkene. Acid-catalyzed hydration of alkene is an example for an addition reaction. In industries, Reaction to convert alkene to alcohol is very useful to produce low- molecular-weight alcohols.

Hydration of alkene Mechanism is as follows –

Acid is required for the initiation of this reaction and is called the acid-catalyzed mechanism of hydration of alkenes.

  • Markovnikov’s Rule- To convert alkene to alcohol this rule is followed for the addition of water to an alkene. It says that the positive entity of the reagent will add to the carbon atom with the most number of hydrogen and the negative entity of the reagent will add to the carbon with a lesser number of hydrogens.

Following is the mechanism of acid-catalyzed hydration of alkene-

Step 1- The dehydration mechanism starts with the formation of the carbocation is the first step and the rate-determining step. A more stable carbocation is formed in this step.

Step 2- Attack of a water molecule on carbocation to share the lone pair of the oxygen atom. Protonated alcohol is formed.

Step 3- Water present in the solution takes away the proton to form alcohol.

Also, students can refer,

What is Dehydrogenation of alcohol?

Dehydrogenation process in which hydrogen is removed from the substrate. Usually, these reactions are useful for organic molecules such as alkanes, formaldehyde. Dehydrogenations meaning lies in its name i.e., removal of hydrogen from a molecule or addition of oxygen. Since Organic molecules containing carbon are highly stable due to catenation property a large amount of energy is required to remove a wanted molecule from the substrate. Hence, the Dehydrogenation reaction is an endothermic process.

Following are a few types of Dehydrogenation reactions-

  • Dehydrogenation of alkanes-To converts saturated fats to unsaturated fats.
  • Ethylbenzene to styrene.
  • Paraffins and olefins to pentene and iso-pentene.
  • Methanol to formaldehyde (addition of oxygen)

Catalytic dehydrogenation of alcohol can be carried out in the presence of catalysts. These catalysts can be either homogeneous or heterogeneous.

Examples-

  • The catalytic dehydrogenation reaction of alcohol gives aldehydes.
  • Alcohol metabolism in the body is regulated by Alcohol dehydrogenase (ADH)

Alcohol dehydrogenase reaction is given as follows-

Dehydrogenation reaction of alcohol to aldehyde requires the surface of catalysts such as copper. This reaction follows β-elimination with the atom reactions, hydrogen and -OH group.

  • Dehydrogenation reaction of alkanes- Dehydrogenation of alkanes gives olefins. Alkanes are stable and hence not reactive. Therefore, converting alkanes to alkenes is important to obtain essential precursors to form alcohols, aldehyde polymers, etc.

Also check-

NCERT Chemistry Notes:

Frequently Asked Questions (FAQs)

1. Catalytic dehydrogenation of primary alcohol produces?

Catalytic dehydrogenation of 1⁰ alcohol gives an aldehyde. H2 is removed from the substrate. Catalytic dehydrogenation of primary alcohol can be initiated on Ag catalysts in presence of oxygen. Many times catalysts such as Pt, Pd are also used in absence of oxygen.

2. What reaction do primary alcohols undergo to form alkenes?

Primary alcohols undergo an electrophilic elimination reaction to form alkenes. Tertiary and secondary alcohols follow the E1 path and Primary alcohol follows the E2 path.

Protonated alcohol is formed in the E1 pathway and the formation of carbocation takes place. 

3. What do you mean by dehydration?

Dehydration meaning is to remove the molecule of water from a substrate. Alcohols undergo a dehydration reaction.

4. Explain Saytzeff’s rule for elimination reaction.

Dehydration of alkenes takes place in such a way that more substituted alkene is formed. When there are two more carbons from where hydrogen can be eliminated then in that case that hydrogen is removed which yields a more substituted alkene. The intermediate formed in the elimination reaction (E1) is a carbocation. Sometimes to form the most stable carbocation rearrangement of carbocation takes place. This carbocation will decide the alkene formed in the product.

5. Which of the following compounds will be most easily dehydrated out of tertiary, secondary, and primary alcohol?

Tertiary alcohol is most easily dehydrated due to the presence of the -OH group in the tertiary position. In this case, protonated alcohol is formed at a tertiary position to form a tertiary carbocation.

6. Which of the following alkene is highly reactive for hydration and why?

Prop-2-ane, Ethene, But-1-ene

Acid-catalyzed hydration of alkenes follows an electrophilic addition reaction. In the first step, there is the formation of a carbocation. The most reactive alkene will be the one that forms the most stable carbocation. Prop-2-ane will form a tertiary carbocation. Hence, propene will be the most reactive alkene.

7. Can dehydration of alcohols lead to rearrangement products?
Yes, carbocation rearrangements can occur during dehydration, especially with secondary and tertiary alcohols, leading to unexpected alkene products.
8. How does the structure of the alcohol affect the ease of dehydration?
The ease of dehydration increases from primary to secondary to tertiary alcohols. This is due to the increasing stability of the carbocation intermediate formed during the reaction.
9. What is the difference between E1 and E2 mechanisms in alcohol dehydration?
E1 (elimination unimolecular) involves a carbocation intermediate and occurs in two steps, while E2 (elimination bimolecular) occurs in one concerted step without a carbocation intermediate.
10. What is the role of concentration in alcohol dehydration reactions?
Higher concentrations of the acid catalyst typically increase the rate of dehydration by increasing the likelihood of protonation of the alcohol.
11. What is the importance of stereochemistry in alcohol dehydration?
Stereochemistry can influence the product distribution in dehydration reactions, particularly when there are multiple possible alkene products with different geometric isomers (cis/trans or E/Z).
12. Why are acid catalysts used in alcohol dehydration reactions?
Acid catalysts are used to protonate the alcohol's hydroxyl group, making it a better leaving group (water) and facilitating the elimination reaction.
13. What is dehydration of alcohols?
Dehydration of alcohols is a chemical reaction where an alcohol loses a water molecule, forming an alkene. This process typically requires heat and an acid catalyst.
14. What type of reaction is alcohol dehydration classified as?
Alcohol dehydration is classified as an elimination reaction, specifically an E1 or E2 reaction depending on the conditions and the structure of the alcohol.
15. What is the general mechanism for the dehydration of alcohols?
The general mechanism involves protonation of the alcohol by an acid catalyst, followed by the loss of water to form a carbocation intermediate, and finally the loss of a proton to form the alkene product.
16. What is Zaitsev's rule, and how does it apply to alcohol dehydration?
Zaitsev's rule states that in elimination reactions, the major product is the most substituted alkene. In alcohol dehydration, this means that hydrogen is preferentially removed from the carbon with fewer hydrogen atoms.
17. What are some common acid catalysts used in alcohol dehydration?
Common acid catalysts include sulfuric acid (H2SO4), phosphoric acid (H3PO4), and p-toluenesulfonic acid (p-TsOH).
18. How does dehydration of alcohols differ from dehydrohalogenation?
Dehydration removes water from an alcohol to form an alkene, while dehydrohalogenation removes HX (where X is a halogen) from an alkyl halide to form an alkene. Both are elimination reactions but involve different leaving groups.
19. Can dehydration of alcohols be reversed?
Yes, the reverse reaction is called hydration of alkenes. The equilibrium can be shifted towards hydration or dehydration by controlling reaction conditions like temperature and water concentration.
20. What is the role of conjugate bases in alcohol dehydration?
Conjugate bases can act as nucleophiles, potentially leading to substitution reactions instead of elimination. Strong acids with weak conjugate bases are preferred to promote elimination over substitution.
21. How does temperature affect the dehydration of alcohols?
Higher temperatures generally favor dehydration reactions by providing the energy needed to overcome the activation barrier and by increasing the entropy of the system (formation of gaseous water).
22. Why is the term "dehydrogenation" sometimes used instead of "dehydration"?
"Dehydrogenation" is occasionally used because the reaction involves the removal of hydrogen (H) and oxygen (O) atoms, which form water. However, "dehydration" is the more accurate and commonly used term for this specific reaction.
23. What is the industrial significance of alcohol dehydration?
Alcohol dehydration is industrially important for producing alkenes, which are valuable starting materials for many organic syntheses, including the production of polymers and other chemicals.
24. What are some methods to analyze the products of alcohol dehydration reactions?
Common analytical methods include gas chromatography (GC), mass spectrometry (MS), nuclear magnetic resonance (NMR) spectroscopy, and infrared (IR) spectroscopy.
25. How does the dehydration of alcohols relate to the production of biofuels?
Dehydration of alcohols, particularly ethanol, is relevant to biofuel production as it can be used to produce ethylene, a precursor for various petrochemicals and polymers.
26. How does the presence of a leaving group on the β-carbon affect alcohol dehydration?
A leaving group on the β-carbon can lead to competition between elimination (dehydration) and substitution reactions, potentially resulting in a mixture of alkene and substituted products.
27. How can you distinguish between E1 and E2 mechanisms experimentally?
E1 and E2 mechanisms can be distinguished by studying reaction kinetics, solvent effects, and stereochemistry of the products. E1 reactions show first-order kinetics with respect to the alcohol, while E2 reactions show second-order kinetics.
28. How does the dehydration of vicinal diols differ from simple alcohols?
Dehydration of vicinal diols can lead to the formation of aldehydes or ketones through a pinacol rearrangement, rather than simple alkene formation.
29. What is the significance of the E1cB mechanism in alcohol dehydration?
The E1cB (elimination unimolecular conjugate base) mechanism can occur when there's a strong electron-withdrawing group adjacent to the hydroxyl group, leading to deprotonation before elimination.
30. How does isotope labeling help in understanding the mechanism of alcohol dehydration?
Isotope labeling, such as using deuterated alcohols, can provide insights into the reaction mechanism by tracking the movement of specific atoms during the reaction.
31. What is the role of entropy in alcohol dehydration reactions?
Entropy increases during dehydration due to the formation of two molecules (alkene and water) from one (alcohol), which favors the reaction at higher temperatures according to the Gibbs free energy equation.
32. What is the significance of E2 elimination in alcohol dehydration?
E2 elimination can occur with strong bases and good leaving groups, leading to concerted elimination without a carbocation intermediate. This can result in different product distributions compared to E1 pathways.
33. Can primary alcohols undergo dehydration?
Yes, primary alcohols can undergo dehydration, but they typically require more vigorous conditions (higher temperature, stronger acid) compared to secondary or tertiary alcohols.
34. How does the presence of an electron-withdrawing group affect alcohol dehydration?
Electron-withdrawing groups generally make dehydration more difficult by destabilizing the carbocation intermediate formed during the reaction.
35. What is the difference between intermolecular and intramolecular dehydration of alcohols?
Intermolecular dehydration occurs between two alcohol molecules to form an ether, while intramolecular dehydration happens within a single molecule containing two hydroxyl groups to form a cyclic ether or an alkene.
36. How does the presence of a β-hydrogen affect alcohol dehydration?
The presence of a β-hydrogen is crucial for dehydration to occur, as it allows for the formation of the alkene. Without a β-hydrogen, dehydration cannot proceed via the typical elimination mechanism.
37. What are some common side reactions in alcohol dehydration?
Common side reactions include ether formation (especially with primary alcohols), carbocation rearrangements, and in some cases, polymerization of the alkene products.
38. How does ring strain affect the dehydration of cyclic alcohols?
Ring strain can influence the product distribution in cyclic alcohol dehydration, often favoring the formation of less strained products or leading to ring-opening reactions.
39. What is the effect of solvent polarity on alcohol dehydration?
Polar protic solvents generally favor the E1 mechanism by stabilizing charged intermediates, while less polar solvents tend to favor the E2 mechanism.
40. How can you control regioselectivity in alcohol dehydration reactions?
Regioselectivity can be controlled by choosing appropriate reaction conditions, catalysts, and by considering the stability of potential carbocation intermediates and the substitution pattern of the alcohol.
41. What is the role of leaving group ability in alcohol dehydration?
The hydroxyl group is a poor leaving group, which is why protonation (to form water as the leaving group) is crucial for the dehydration reaction to proceed efficiently.
42. What is the significance of syn vs. anti elimination in alcohol dehydration?
Syn elimination occurs when the leaving group and the proton being removed are on the same side of the molecule, while anti elimination occurs when they're on opposite sides. The preferred orientation can affect product stereochemistry.
43. How does the presence of aromatic rings affect alcohol dehydration?
Aromatic rings can stabilize carbocation intermediates through resonance, potentially influencing the reaction rate and product distribution in dehydration reactions.
44. How can you distinguish between primary, secondary, and tertiary alcohol dehydration products?
Primary alcohols typically form terminal alkenes, while secondary and tertiary alcohols form internal alkenes. The degree of substitution of the alkene product can indicate the original alcohol type.
45. What is the importance of carbocation stability in determining the major product of alcohol dehydration?
More stable carbocations (tertiary > secondary > primary) form more readily, often leading to product mixtures that reflect the relative stabilities of possible carbocation intermediates.
46. How does the presence of neighboring group participation affect alcohol dehydration?
Neighboring group participation can lead to unexpected products by stabilizing reaction intermediates or providing alternative reaction pathways, potentially resulting in rearranged or cyclic products.
47. How does the concept of hyperconjugation apply to alcohol dehydration?
Hyperconjugation stabilizes carbocation intermediates and transition states in dehydration reactions, influencing the reaction rate and product distribution.
48. What is the role of Lewis acids in alcohol dehydration?
Lewis acids can catalyze dehydration by coordinating with the oxygen of the alcohol, making it a better leaving group and facilitating the elimination process.
49. What is the significance of the Hammond postulate in understanding alcohol dehydration mechanisms?
The Hammond postulate helps in understanding the structure of transition states in dehydration reactions, particularly in relation to the stability of carbocation intermediates and the energy profile of the reaction.
50. How can you use alcohol dehydration to distinguish between primary, secondary, and tertiary alcohols?
The ease and rate of dehydration, as well as the nature of the products formed, can be used to distinguish between alcohol types. Tertiary alcohols dehydrate most readily, followed by secondary, then primary.
51. What is the importance of understanding alcohol dehydration in organic synthesis?
Understanding alcohol dehydration is crucial for planning synthetic routes, predicting product distributions, and optimizing reaction conditions in organic synthesis, particularly for the production of alkenes and other unsaturated compounds.
52. How does the dehydration of alcohols compare to other elimination reactions in organic chemistry?
Alcohol dehydration is similar to other elimination reactions like dehydrohalogenation, but it typically requires stronger conditions due to the poor leaving group ability of the hydroxyl group compared to halogens.
53. What are some industrial applications of alcohol dehydration reactions?
Industrial applications include the production of ethylene from ethanol, the synthesis of various alkenes used in polymer production, and the manufacture of certain ethers and other organic compounds.
54. How can green chemistry principles be applied to alcohol dehydration reactions?
Green chemistry approaches to alcohol dehydration include using recyclable catalysts, employing microwave irradiation for heating, exploring solvent-free conditions, and utilizing bio-based starting materials.
55. What is the role of transition metal catalysts in alcohol dehydration?
Some transition metal catalysts can facilitate alcohol dehydration through alternative mechanisms, potentially offering milder conditions or improved selectivity compared to traditional acid catalysis.
56. How does the study of alcohol dehydration contribute to our understanding of reaction mechanisms in organic chemistry?
Studying alcohol dehydration provides insights into carbocation stability, leaving group effects, stereochemistry in elimination reactions, and the interplay between kinetic and thermodynamic control in organic reactions, contributing to a broader understanding of reaction mechanisms.

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