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Dehydration of Alkanols (alcohols)

Key Concepts

Dehydration of alkanols (alcohols) is an elimination reaction.
Water is eliminated from the alkanol resulting in an alkene.
Any concentrated strong acid can cause the dehydration of an alkanol, but hot, concentrated sulfuric acid is the most commonly used.
The reagent used to dehydrate the alkanol is known as a dehydrating agent.

The dehydration of a primary alkanol (alcohol) produces the 1-alkene

	H \|	H \|
R-	C-	C-	OH
	\| H	\| H

hot conc. H₂SO₄
-------------->

	H \|	H \|
R-	C=	C
	\| H	\| H

+ H-O-H

The dehydration of a secondary alkanol or a tertiary alkanol (alcohol) may produce a mixture of alkene structural isomers^*.
^*Saytseff's Rule predicts that the alkene with the greatest number of alkyl groups on the doubly-bonded carbon atoms will be the major product.

Animated Tutorial

Examples

Dehydration of a Primary Alkanol

Ethanol, C₂H₅OH, (ethyl alcohol) is a primary alcohol.
At 180^oC, concentrated sulfuric acid will dehydrate ethanol to produce ethene, C₂H₄, (ethylene) and water.

ethanol
(ethyl alcohol)

conc. H₂SO₄
------------>
180^oC

ethene
(ethylene)

+ water

CH₃CH₂OH

conc. H₂SO₄
------------>
180^oC

CH₂=CH₂

+ H₂O

	H \|	H \|
H-	C-	C-	OH
	\| H	\| H

conc. H₂SO₄
------------>
180^oC

	H \|	H \|
	C=	C
	\| H	\| H

+ H-O-H

Dehydration of a Secondary Alcohol

2-propanol, (CH₃)₂CHOH, is a symmetrical secondary alcohol.
At 100^oC, concentrated sulfuric acid will dehydrate 2-propanol to produce propene (propylene) and water.

2-propanol

conc. H₂SO₄
------------>
100^oC

propene
(propylene)

+ water

(CH₃)₂CHOH

conc. H₂SO₄
------------>
100^oC

CH₃CH=CH₂

+ H₂O

	H \|	OH \|	H \|
H-	C-	C-	C-	H
	\| H	\| H	\| H

conc. H₂SO₄
------------>
100^oC

	H \|		H \|
H-	C-	C=	C
	\| H	\| H	\| H

+ H-O-H

2-pentanol, CH₃CH₂CH₂CHOHCH₃, is an unsymmetrical secondary alcohol.
Hot concentrated sulfuric acid will dehydrate 2-pentanol to produce water and two possible isomers of pentene, 2-pentene and 1-pentene.

2-pentanol
hot conc. H₂SO₄
-------------> 2-pentene + 1-pentene

CH₃CH₂CH₂CHOHCH₃ hot conc. H₂SO₄
-------------> CH₃CH₂CH=CHCH₃ + CH₃CH₂CH₂CH=CH₂

H
| H
| H
| OH
| H
|

H- C- C- C- C- C- H

|
H |
H |
H |
H |
H

hot conc. H₂SO₄
------------->

H
| H
| H
|

H- C- C- C= C- C -H

|
H |
H |
H |
H |
H

+

H
| H
| H
| H
|

H- C- C- C- C= C

|
H |
H |
H |
H |
H

(major product) (minor product)