Dehydration of Alcohols to Alkenes - Reactions and Mechanism

In this article we will learn about Dehydration Reactions of alcohols to alkenes. We will discuss few reactions and mechanism.

Dehydration is the process of removal of water molecule from alcohol compound to produce alkene compound. It is an example of elimination reaction in organic chemistry.

Key words: Alcohol, Dehydration, Alkene, E1-elimination, E2-elimination.

Introduction

In general dehydration reaction takes place in acidic medium. The alcohols have tendency to react with acids to undergo dehydration reaction and produce alkene compound. The reaction proceeds through E1 –reaction mechanism. Also the tertiary and secondary alcohols are best substrates for dehydration reaction.

Generally acids like Sulphuric acid (H₂SO₄) and Phosphoric acid (H₃PO₄) are used for dehydration of alcohols.

Mechansm:

The reaction proceeds through E1 –reaction mechanism. It consists of three steps:

1. First step is the protonation of alcohol; to make hydroxide as a good leaving group.

2. Second step is removal of water molecule to form carbocation intermediate. 

3. Final step is deprotonation to produce alkene compound.

Example 1:

Reaction of cyclohexanol in presence of sulphuric acid under heating condition gives cyclohexene as dehydration product.

         

The reaction preferentially gives highly substituted alkene product. Also trans-alkene product is preferred than cis-alkene because trans-isomer is thermodynamically stable as compare cis-isomer.

Example 2:

3-methylbutane-2-ol undergoes dehydration reaction in presence of sulphuric acid (H₂SO₄) to produce 2-methylbut-2-ene as major and 3-methylbut-1-ene as minor product.

In this case after formation of carbocation; there is choice of removal of H_a or H_b. Deprotonation of Ha gives highly substituted alkene; hence it will be major product. Similarly, removal of Hb gives least substituted alkene so it will be minor product. 

Since the dehydration reaction proceeds through carbocation intermediate so there are chances of rearrangement of carbocation. The carbocation prefers to undergo rearrangement to form more stable and highly substituted carbcation. Hence there will be possibility of more than one product in dehydration reaction.

Example 3:

Dehydration reaction of 2,2-dimethylcyclohexanol gives 3,3-dimethylcyclohexene as minor product. Whereas 1,2-dimethylcyclcohexene forms as major product. How this is possible?

The answer is that deprotonation of carbocation intermediate results in the formation of 3,3-dimethylcyclohexene. Additionally the carbocation undergoes rearrangement by “methyl shift” to form most stable tertiary carbocation. Finally deprotonation step gives 1,2-dimethylcyclcohexene  as major product. 

Example 4:

Answer:

In this example after carbocation formation, rearrangement in the molecule takes place to form more stable carbocation. Then final deprotonation step gives is alkene product. 

To overcome the problem of multiple products due to selectivity and rearrangement there is another method available for dehydration of alcohols.

Dehydration of alcohol by phosphoryl chloride (POCl₃)

Dehydration of alcohol by phosphoryl chloride (POCl₃) and pyridine gives alkene product without any rearrangement in the molecule. 

Mechanism:

Step 1: Nucleophilic addition of alcohol to Phosphoryl chloride (POCl₃) to

Step 2: Removal of chloride ion as leaving group.

Step 3:  Proton transfer from organophosphorous species to pyridine.

Step 4: E2 elimination gives alkene as main product and pyridinium phosphorodichloridate as by product.

The reaction follows E2 mechanism therefore there will not be rearrangement in the intermediates.

Let’s see example 5:

2,2-dimethylcyclopentanol when heated with H₂SO₄ it gives dehydration product but there will be rearrangement takes place.

Same starting material if we use with the reagent POCl₃ /pyridine; this gives alkene product and there will not be rearrangement in the molecule.

Mechanism:

Phosphoryl chloride (POCl₃) / pyridine reagent gives highly substituted alkene product as major product.

Example 6:

2-methylpentan-3-ol when treated with POCl₃ / pyridine it gives highly substituted alkene as major product.

Examples for Practice:

To summarize this topic, dehydration of alcohol can be done in acid catalyst like sulphuric acid (H₂SO₄) or phosphoric acid (H₃PO₄). In this case highly substituted alkene product is formed as major product. This method has disadvantages also, it gives mixture of product or sometimes rearrangement product is possible.

There is another method for dehydration of alcohol in which the alcohol is treated with phosphoryl chloride. This method is quite better and it provides highly substituted alkene product. Also there will not be any rearrangement in the final product.

That's all for this topic. If you have any questions please feel free to ask me in the comment box. Thank you..!