Question

Describe dehydration of alcohols to obtain an alkene.

Short answer

Dehydration of alcohols is an elimination reaction where a water molecule is removed from an alcohol molecule, resulting in the formation of an alkene. This process requires a strong acid catalyst, such as sulfuric acid or phosphoric acid, and high temperature (140 to 180 °C) to promote the reaction. The reaction mechanism can follow either E1 (common with secondary and tertiary alcohols) or E2 (common with primary alcohols) pathways, involving protonation of the hydroxyl group and subsequent elimination of water to form the alkene. An example is the dehydration of ethanol to ethene using sulfuric acid as a catalyst at 180 °C.

Step-by-step solution

Understanding the dehydration reaction

Dehydration of alcohols is an elimination reaction in which a water molecule is removed from an alcohol molecule, leading to the formation of an alkene. This process typically requires a strong acid catalyst and high temperature.

Write down the general chemical equation

The general chemical equation for the dehydration of alcohol to form an alkene can be represented as follows: Alcohol + strong acid catalyst \(\xrightarrow[]{heat}\) Alkene + Water

Explain the role of acid catalyst

A strong acid catalyst, such as sulfuric acid (\(H_2SO_4\)) or phosphoric acid (\(H_3PO_4\)), is required to protonate the hydroxyl group (-OH) in alcohol, enabling the elimination of water molecule. The protonation of the hydroxyl group leads to a good leaving group, due to its positive charge, ensuring a smooth reaction.

Mechanism of the reaction

The dehydration of alcohols follows the E1 or E2 mechanism, depending on the type of alcohol (primary, secondary, or tertiary) and reaction conditions. In both mechanisms, the first step involves protonation of the hydroxyl group. The difference lies in the order of the remaining steps: E1 Mechanism (common with secondary and tertiary alcohols): 1. Formation of a carbocation by the leaving group, water. 2. Elimination of a proton from the carbocation by a base to form the alkene. E2 Mechanism (common with primary alcohols): 1. Simultaneous removal of a proton from the adjacent carbon by a strong base and leaving of the water molecule 2. Formation of the alkene through rearrangement.

Describe the reaction conditions

The temperature plays a crucial role in this reaction. Generally, the dehydration of alcohols requires high temperatures (140 to 180 °C) to shift the equilibrium towards the formation of alkene. If the temperature is not sufficiently high, the reaction might not proceed to completion.

Example of dehydration reaction

As an example, let's consider the dehydration of ethanol (ethyl alcohol) to form ethene (ethylene): Ethanol + \(H_2SO_4\) \(\xrightarrow[]{180 ^{\circ}C}\) Ethene + Water \[CH_3CH_2OH + H_2SO_4 \xrightarrow[]{180 ^{\circ}C} CH_2=CH_2 + H_2O\] In this reaction, sulfuric acid (\(H_2SO_4\)) acts as the acid catalyst, and the reaction occurs at 180°C. The hydroxyl group in the ethanol molecule is protonated, followed by the elimination of water, resulting in the formation of ethene and water.

Key concepts

Elimination Reaction

An elimination reaction is a process where elements within a molecule are removed, resulting in the formation of a new compound with fewer atoms. In the context of alcohol dehydration, this reaction involves the removal of a water molecule from the alcohol, creating an alkene. This transformation is key in organic chemistry, as it allows the conversion of simple alcohols into more reactive and versatile alkenes.
During the dehydration process, the alcohol undergoes a structural change as a water molecule (one oxygen and two hydrogen atoms) is eliminated. This reaction is favored and can progress efficiently with the presence of an acid catalyst and heat.
The typical general equation can be symbolized as:

• Alcohol + Acid Catalyst (with heat) → Alkene + Water

Understanding the concept of elimination reactions is crucial because it enhances the knowledge of how organic molecules can be manipulated to create different types of chemical structures.

Acid Catalyst

An acid catalyst is essential for the dehydration of alcohols. It substantially speeds up the chemical reaction without being consumed or altered permanently in the process. Commonly used acid catalysts include sulfuric acid (\( H_2SO_4 \)) and phosphoric acid (\( H_3PO_4 \)). These acids work by protonating the hydroxyl group (-OH) in alcohols, making it a better leaving group.
This protonation step is crucial because it transforms the hydroxyl into water (\( OH^- \rightarrow H_2O \)), a better leaving group due to its stability. Once the hydroxyl is converted into water, it can dissociate, setting the stage for alkene formation.
The role of the acid catalyst is to lower the energy barrier for the reaction. It ensures that the elimination reaction can proceed under milder conditions than would otherwise be possible.

E1 Mechanism

The E1 mechanism, or unimolecular elimination, is a two-step process typically favored by secondary and tertiary alcohols during dehydration. The mechanism begins with the protonation of the alcohol's hydroxyl group, converting it into a good leaving group (water).

• The first step involves the departure of the water molecule, resulting in the formation of a carbocation. This carbocation is an intermediary that is positively charged and quite reactive.
• In the second step, a base removes a proton from the carbocation, resulting in the formation of the alkene.

This process is called "unimolecular" because the rate-determining step depends on the concentration of only one molecular entity: the starting alcohol. The intermediate carbocation makes E1 reactions sensitive to factors like temperature and the stability of the carbocation formed.

E2 Mechanism

The E2 mechanism, or bimolecular elimination, differs primarily in that it involves a single concerted step, making it often preferred for primary alcohols. In this mechanism, the elimination of the leaving group (water) and the removal of a proton occur simultaneously, with a strong base playing a crucial role.

• Both the proton abstraction by the base and the departure of water happen at the same time, leading directly to the formation of the alkene.

This "bimolecular" reaction depends on the concentration of both the alcohol and the base, making it influenced by the nature and strength of the base used. It is typically simpler because it skips the carbocation intermediate, offering a direct route to forming the alkene. However, it requires powerful and effective base conditions to proceed smoothly.