4.8 • +11k Ratings
Save
Print
Edit
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.
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.
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:
If they are, your alcohol is suitable for dehydration.
Sounds confusing? Here are a few examples.
An 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?
An 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.
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.
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.
Alcohol elimination reactions take place via two different mechanisms, depending on the type of alcohol involved.
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.
An example of an E1 mechanism in alcohol elimination. Anna Brewer, StudySmarter Original
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.
An example of an E2 mechanism in alcohol elimination. Anna Brewer, StudySmarter Original
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.
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:
Butan-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.
Dehydrating 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:
Butan-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.
The 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.
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.
The 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.
The elimination of pentan-2-ol. Anna Brewer, StudySmarter Original
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.
60%
of the users don't pass the Alcohol Elimination Reaction quiz! Will you pass the quiz?
Start Quiz
Be perfectly prepared on time with an individual plan.
Test your knowledge with gamified quizzes.
Create and find flashcards in record time.
Create beautiful notes faster than ever before.
Have all your study materials in one place.
Upload unlimited documents and save them online.
Identify your study strength and weaknesses.
Set individual study goals and earn points reaching them.
Stop procrastinating with our study reminders.
Earn points, unlock badges and level up while studying.
Create flashcards in notes completely automatically.
Create the most beautiful study materials using our templates.
Sign up to highlight and take notes. It’s 100% free.
