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Alcohol Elimination Reaction

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Alcohol Elimination Reaction

Plastic plays a big role in our everyday lives. We find it everywhere, from bottles and packaging to acrylic counter tops and clothing. However, our insatiable desire for the material leads to problems. Plastic is made from crude oil, a finite resource. This means that overall, plastics are non-renewable, and when they are produced, processed and disposed of, they release carbon dioxide into the air.

However, there might be a solution. If we produce plastics from a renewable source such as alcohols, we might be able to create plastics that have no net carbon dioxide output. This is where alcohol elimination reactions come in.

An elimination reaction is an organic reaction in which two atoms or groups of atoms are removed from a molecule, forming two new molecules in the process.

Alcohol elimination reactions produce alkenes, the starting point for many plastics. For example, if we eliminate ethanol produced naturally in fermentation, we get ethene.

This could be a way of making carbon-neutral plastics, as the carbon dioxide released when these plastics are burnt is counteracted by the carbon dioxide taken in when the ethanol is produced. Alcohol elimination reactions are a simple solution to a prevailing problem.

  • This article is about alcohol elimination reactions in organic chemistry.
  • We'll start by providing an overview of alcohol elimination reactions, including the reactants, conditions, and products.
  • We'll then take a deep dive into their mechanisms.
  • Finally, we'll explore isomeric products produced in alcohol elimination reactions.

What are alcohol elimination reactions?

As we explored above, elimination reactions are reactions in which two atoms or groups of atoms are removed from a molecule, forming two new molecules in the process. In alcohol elimination reactions, a hydroxyl group and a hydrogen ion are lost from an alcohol. These react together to form water. Because of this, alcohol elimination reactions are known as dehydration reactions. A C=C double bond forms in the remaining molecule, producing an alkene.

Here is the overall word equation:

Let's look at it in more detail.

Reactants in alcohol elimination reactions

In alcohol elimination reactions, the reactant is - you guessed it - an alcohol. However, not just any alcohol can react; you'll see why more clearly later on. In order to react, the alcohol needs a hydrogen atom on one of the carbons adjacent to the carbon bonded to the -OH group. This is because that hydrogen atom is lost from the alcohol as a hydrogen ion in the elimination reaction. The carbon bonded to the -OH group is known as the alpha carbon and any adjacent carbons are known as beta carbons.

The Greek letters alpha and beta refer to the carbon atom's position relative to the molecule's functional group. In this case, the functional group is the hydroxyl group, -OH. The alpha carbon is the carbon atom bonded directly to the functional group. In other words, it is the first carbon in the hydrocarbon chain joined to the functional group. You can probably guess what the beta carbon is - it is the second carbon in the hydrocarbon chain. We carry on naming each carbon in the chain in turn, using gamma, delta, epsilon and so forth.

To see if an alcohol is suitable, follow these steps:

  1. Locate the hydroxyl group, -OH, and the carbon atom it is bonded to. This is the alpha carbon.
  2. Find any adjacent carbon atoms - there might be multiple. These are beta carbons.
  3. See if any of the beta carbons are attached to any hydrogen atoms.

If they are, your alcohol is suitable for dehydration.

Sounds confusing? Here are a few examples.

Alcohol Elimination Reaction dehydration suitable alcohol StudySmarterAn alcohol suitable for dehydration. Anna Brewer, StudySmarter Original

Here, the hydroxyl group is shown in pink and the alpha carbon is shown in turquoise. The beta carbon, adjacent to the alpha carbon and shown in blue, is part of a -CH3 group. This -CH3 group contains hydrogen atoms. Therefore, this alcohol is suitable for elimination.

How about this next alcohol?

Alcohol Elimination Reaction dehydration suitable alcohol StudySmarterAn alcohol unsuitable for dehydration. Anna Brewer, StudySmarter Original

Once again, the hydroxyl group is shown in pink, the alpha carbon is shown in turquoise, and the beta carbon is shown in blue. However, this time the beta carbon is bonded to three methyl groups. It isn't attached to any hydrogen atoms. Therefore, this alcohol is unsuitable for elimination.

Products in alcohol elimination reactions

The products of alcohol elimination reactions are an alkene and water.

Alkenes are unsaturated hydrocarbons with the general formula CnH2n. They contain a C=C double bond.

However, not just any old alkene is made. The C=C double bond is always found between the alpha carbon and the beta carbon that lost a hydrogen ion. Sometimes this results in just one alkene, but in some cases, you can form multiple different isomeric alkenes. Isomerism occurs if the original alcohol is a secondary or tertiary alcohol. Don't worry - we'll look at this in more detail later on.

Conditions for alcohol elimination reactions

Finally, let's look at the conditions needed for alcohol elimination reactions. You might have noticed a hydrogen ion, H+, in the word equation. We'll show it to you again down below:

The hydrogen ion represents an acid. It shows that we need an acid catalyst for the reaction. We most commonly use concentrated sulphuric or phosphoric acid, but as an alternative to an acid, you can also use hot aluminium oxide. The mixture requires heating to about 170 °C.

A catalyst is a substance that increases the rate of a chemical reaction without being used up in the process. You can find out more in Catalysts.

The mechanism for alcohol elimination reactions

Alcohol elimination reactions take place via two different mechanisms, depending on the type of alcohol involved.

  • Primary alcohols use something called an E2 mechanism.
  • Secondary and tertiary alcohols use something called an E1 mechanism.

Here, the number refers to how many species the rate of reaction is dependent on - in other words, the order of the reaction. E2 mechanisms are dependent on both the concentration of the alcohol and the concentration of the acid catalyst. E1 mechanisms, on the other hand, are dependent on just the concentration of the alcohol.

You might not have come across orders before. They're explored in much more depth in Rate Equations. If you're not sure about the differences between primary, secondary, and tertiary alcohols, check out Alcohols for more information.

For your exams, you don't need to know the exact mechanism, simply the reactants, products, and conditions. However, we've included the mechanism as a deep dive. Learning exactly how chemical reactions take place often helps you understand the topic a little better. If you're ready, we'll explore it now.

We'll first look at E1 mechanisms. Remember, this is the mechanism that secondary and tertiary alcohols use in alcohol elimination reactions. We'll show the steps now, using propan-2-ol as an example.

Alcohol Elimination Reaction E1 mechanism StudySmarterAn example of an E1 mechanism in alcohol elimination. Anna Brewer, StudySmarter Original

  1. In the first step, one of the lone pairs of electrons from the hydroxyl group's oxygen attacks a hydrogen ion from the acid catalyst, H+. The oxygen is said to be protonated and becomes positively charged.
  2. The protonated oxygen breaks off from the alcohol, taking the two hydrogen atoms attached to it with it. This forms a water molecule. The carbon the oxygen was attached to - the alpha carbon - now becomes a carbocation - a carbon with a positive charge.
  3. One of the C-H bonds from a beta carbon breaks and its electrons are used to create a C=C double bond. This releases a hydrogen ion, H+, which is used to regenerate the catalyst.
  4. The overall products are an alkene and water. Here we have produced propene.

Look at step 3. This is why only certain alcohols can react. There needs to be a hydrogen atom attached to the carbon adjacent to the alpha carbon in order for an elimination reaction to occur. The C-H bond breaks and provides the electrons that form the C=C double bond, forming an alkene.

We'll now turn our attention to E2 mechanisms. They are very similar. However, two of the steps happen simultaneously. In the E1 mechanism, a water molecule is first lost from the alcohol, leaving a carbocation behind, and then a hydrogen ion is eliminated. In the E2 mechanism, these two steps happen at the same time, avoiding the need to form a carbocation.

Alcohol Elimination Reaction E2 mechanism StudySmarterAn example of an E2 mechanism in alcohol elimination. Anna Brewer, StudySmarter Original

  1. As before, a lone pair of electrons from the oxygen atom attacks a hydrogen ion from the acid catalyst, protonating the oxygen.
  2. The protonated oxygen breaks off from the alcohol, taking its two hydrogen atoms with it. At the same time, an adjacent C-H bond breaks and the electrons are used to form a C=C double bond. A hydrogen ion is released which is used to regenerate the acid catalyst.
  3. The overall products are an alkene and water.

E2 mechanisms happen because using an E1 mechanism would mean forming a primary carbocation. This is a carbocation attached to just one methyl group and is a lot less stable than a secondary or tertiary carbocation. We won't go into why this is, but it means that the reaction's activation energy is a lot higher. An E2 mechanism is more energetically favourable.

Isomeric products of alcohol elimination reactions

Do you remember how we said that alcohol elimination reactions can form isomeric products? Let's take a look at how.

First of all, let's consider what happens when you dehydrate butan-1-ol. The 1 in its name indicates that the hydroxyl group is attached to the first carbon in the chain. This is the alpha carbon. The molecule is an example of a primary alcohol, meaning the alpha carbon is bonded to just one other alkyl group. Here's what it looks like:

Alcohol Elimination Reaction butan-1-ol diagram StudySmarterButan-1-ol. Anna Brewer, StudySmarter Original

You can see that the alpha carbon is bonded to just one other carbon atom. This is the only beta carbon. Remember that the hydrogen ion is always lost from a beta carbon. The C=C double bond forms between the alpha carbon and this beta carbon. That means that in this molecule, the C=C double bond can only form in one place, producing just one alkene. In this case, the alkene formed is but-1-ene. Here, we've highlighted the hydroxyl group, the alpha carbon, the beta carbon, and the C=C double bond that forms.

Alcohol Elimination Reaction butan-1-ol elimination but-1-ene diagram StudySmarterDehydrating butan-1-ol to produce but-1-ene. Anna Brewer, StudySmarter Original

But what do you think will happen if you dehydrate butan-2-ol? Let's look at it together.

In butan-2-ol, the hydroxyl group is bonded to the second carbon atom in the chain. This is the alpha carbon. The alpha carbon is bonded directly to two other carbon atoms, making butan-2-ol an example of a secondary alcohol. These are the beta carbons. Notice how both of these beta carbons contain hydrogen atoms:

Alcohol Elimination Reaction butan-2-ol diagram StudySmarterButan-2-ol. Anna Brewer, StudySmarter Original

The hydrogen ion eliminated could come from either beta carbon - the first carbon (on the left) at the end of the chain, or the third carbon (on the right) in the middle of the chain. As before, the C=C double bond forms between this beta carbon and the alpha carbon. This means that in this alcohol, the C=C double bond can form in multiple different places. We'll form three different isomeric products.

  • If the hydrogen ion comes from the beta carbon on the left, we'll produce but-1-ene.
  • If the hydrogen comes from the beta carbon on the right, we can produce either E-but-2-ene or Z-but-2-ene.

Alcohol Elimination Reaction butan-2-ol elimination but-1-ene but-2-ene isomer diagram StudySmarterThe isomeric products of the elimination reaction of butan-2-ol. Anna Brewer, StudySmarter Originals

If you're not too confident about isomers, go and take a quick look at Isomerism.

Examples of alcohol elimination reactions

Finally, let's look at some specific examples of alcohol elimination reactions using named alcohols.

First up, let's take methylpropan-1-ol. Heating this alcohol with concentrated sulphuric acid produces methylpropene and water. Once again, we've highlighted the hydroxyl group, the alpha carbon and the beta carbon.

Alcohol Elimination Reaction methylpropan-1-ol elimination methylpropen 1 elimination reaction diagram StudySmarterThe elimination reaction of methylpropan-1-ol. Anna Brewer, StudySmarter Original

Another example is pentan-2-ol. Here you can see that there are two beta carbons. Heating this alcohol with an acid catalyst therefore produces a mixture of isomeric products: pent-1-ene, E-pent-2-ene and Z-pent-2-ene.

Alcohol Elimination Reaction pentan-2-ol elimination pent-1-ene pent-2-ene isomer diagram StudySmarterThe elimination of pentan-2-ol. Anna Brewer, StudySmarter Original

Alcohol Elimination Reaction - Key takeaways

  • An elimination reaction is an organic reaction in which two atoms or groups of atoms are removed from a molecule, forming two new molecules in the process.
  • In alcohol elimination reactions, also known as dehydration reactions, a hydrogen ion and a hydroxyl group are lost from an alcohol. The overall products are an alkene and water.
  • Only certain alcohols can react in elimination reactions.
  • Alcohol elimination reactions take place via either an E1 or E2 mechanism.
  • Alcohol elimination reactions can produce a mixture of isomeric alkenes.

Frequently Asked Questions about Alcohol Elimination Reaction

Yes - alcohols undergo elimination reactions, forming an alkene and water.

There are two main types of elimination reaction: E1 and E2. The number represents how many species the rate of reaction is dependent on. However, two other types of elimination reaction also exist: E1CB and Ei

Yes - dehydration of alcohols is an example of an elimination reaction.

Elimination reactions are useful because they generally transform a saturated molecule into one with a double bond. Alcohol elimination reactions are particularly useful because they produce alkenes, the starting point of many polymers.

Alcohol elimination reactions are also known as dehydration reactions and turn an alcohol into an alkene and water. 

Final Alcohol Elimination Reaction Quiz

Question

What are the products of alcohol elimination reactions?

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Answer

An alkene

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Question

What conditions are required for alcohol elimination reactions?

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Answer

Heat

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Question

True or false? Alcohol elimination reactions can produce isomeric products.

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Answer

True

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Question

What are alcohol elimination reactions also known as?

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Answer

Dehydration reactions

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Question

Name the alkene produced when propan-1-ol is dehydrated.

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Answer

Propene

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Question

Name the alkene produced when methylpropan-2-ol is dehydrated.

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Answer

Methylpropene

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Question

Name the alkene produced when pentan-1-ol is dehydrated.

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Answer

Pent-1-ene

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Question

Name the three isomers produced when butan-2-ol is dehydrated.

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Answer

But-1-ene, E-but-2-ene, Z-but-2-ene

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Question

What is an elimination reaction?

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Answer

An elimination reaction is an organic reaction in which two atoms or groups of atoms are removed from a molecule, forming two new molecules in the process.

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Question

Which atoms or groups of atoms are lost from an alcohol in an elimination reaction?

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Answer

-H

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Question

Which functional group is formed in an alcohol elimination reaction?

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Answer

C=C

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Question

What makes an alcohol suitable for dehydration?

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Answer

It must have a hydrogen atom joined to its beta carbon. This is the carbon atom next to the carbon directly attached to the -OH group.

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Question

Explain why some alcohols form isomeric products when dehydrated.

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Answer

Secondary and tertiary alcohols have two or three beta carbons respectively. A hydrogen atom can be lost from any of these carbons in dehydration, forming multiple different isomeric alkenes.

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Question

True or false? Elimination reactions involve joining two molecules together. 

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Answer

False

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