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 one new large molecule and one new smaller molecule 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.
Alcohol elimination reactions: alcohol to alkene
As we explored above, elimination reactions are reactions in which two atoms or groups of atoms are removed from a molecule, forming one new large molecule and one new smaller molecule in the process. In alcohol elimination reactions, we remove a hydroxide ion (OH-) and a hydrogen ion (H+) from an alcohol. These react together to form water. A C=C double bond forms in the remaining molecule, producing an alkene. The reaction requires a hot concentrated acid catalyst, such as phosphoric acid (H3PO4) or sulphuric acid (H2SO4).
Here's the word equation for an alcohol elimination reaction. Overall, our reactant is an alcohol and our products are an alkene and water.
alcohol → alkene + water
However, not just any old alcohol can take part in an elimination reaction. Likewise, not just any old alkene is formed. Let's take some time to look more closely at the reactants and products of alcohol elimination reactions.
Reactants
In alcohol elimination reactions, the reactant is - you guessed it - an alcohol. However, not all alcohols are suitable for elimination. In order to react, the alcohol needs a hydrogen atom on one of the carbons adjacent to the C-OH bonded carbon. 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 for elimination, follow these steps:
- Locate the hydroxyl group (-OH) and the carbon atom it is bonded to. This carbon atom is the alpha carbon.
- Find any adjacent carbon atoms - there might be multiple. These carbon atoms are beta carbons.
- See if any of the beta carbons are bonded to a hydrogen atom.
If one of the beta carbons is bonded to a hydrogen atom, your alcohol is suitable for dehydration. You could have one beta carbon bonded to three hydrogen atoms, or two beta carbons each bonded to one hydrogen atom - it doesn't matter at all. As long as at least one of the beta carbons has at least one C-H bond, you can dehydrate your alcohol!
Sounds confusing? Here are a few examples.
Ethanol - an example of an alcohol suitable for dehydration. Vaia Originals
The alcohol above is ethanol. 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 at least one hydrogen atom - in actual fact, it contains three. Therefore, ethanol is suitable for elimination.
Dimethylpropan-1-ol - an example of an alcohol unsuitable for dehydration.Vaia Originals
This second alcohol is dimethylpropan-1-ol. 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, dimethylpropan-1-ol is unsuitable for elimination.
Products
The products of alcohol elimination reactions are an alkene and water.
Alkenes are unsaturated hydrocarbons with the general formula CnH2n. They all 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 product, but in some cases, you can form multiple different isomeric alkenes. Isomerism typically occurs if the original alcohol is a secondary or tertiary alcohol. Don't worry - we'll look at why this is the case in more detail later on.
Dehydration of alcohol elimination reactions
Alcohol elimination reactions remove a hydroxide ion and a hydrogen ion from the alcohol molecule. These ions react together to form water. As a result, alcohol elimination reactions are frequently known as dehydration reactions. Other examples of dehydration reactions that you'll come across in organic chemistry include esterification and the reduction of amides. You can read about the two processes in the articles Reactions of Carboxylic Acids and Amide respectively.
Alcohol elimination reaction by strong acid
We mentioned that alcohol elimination reactions use a concentrated acid catalyst - typically phosphoric acid (H3PO4) or sulphuric acid (H2SO4). You wouldn't use hydrochloric acid (HCl) or nitric acid (HNO3) for this purpose because they are not concentrated enough. Using a dilute acid means that there is additional water in the system and decreases the chances of a successful dehydration reaction. On the other hand, it is possible to get essentially pure phosphoric and sulphuric acids, making them much more suitable.
Alternatively, we can dehydrate alcohols by vapourising them, then passing the gases over hot aluminium oxide (Al2O3). In this case, the aluminium oxide acts as the catalyst. However, we'll be focusing on the acid catalyst method when we look at elimination reaction mechanisms, coming up next.
Elimination reaction of alcohol mechanism
Alcohol elimination reactions take place via two different mechanisms, depending on the type of alcohol involved. The mechanisms are similar but they have their differences.
- Primary alcohols use something called an E2 mechanism. The rate of the E2 mechanism is dependent on the concentration of two species: the alcohol and the acid catalyst.
- Secondary and tertiary alcohols use something called an E1 mechanism. The rate of the E1 mechanism is dependent on the concentration of just one species: the alcohol itself.
We can deduce a lot about a mechanism just from its name. For example, the letter E in E2 tells us that this is an elimination reaction, whilst the number 2 tells us that the rate is dependent on the concentration of two species. The number of species responsible for the rate of a chemical reaction is also known as the reaction's order. Reaction orders are explored in much more depth in the article Rate Equations.
If you need a quick recap 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 of elimination reactions, 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. Note that we've represented the acid catalyst using the general formula for an acid (HA).
An example of an E1 mechanism in alcohol elimination using propan-2-ol. The organic molecule's changing functional group is shown in blue and the acid is shown in red to help you understand the process.Vaia Originals
- In the first step, one of the lone pairs of electrons from the hydroxyl group's oxygen atom attacks the acid catalyst's hydrogen atom. The H-A bond breaks heterolytically, and the oxygen atom forms a bond with the resulting positive hydrogen ion. The oxygen becomes positively charged; we say that it is protonated. We're also left with a negative A- ion from the acid.
- The protonated oxygen breaks off from the alcohol, taking the bonding pair of electrons and its two bonded hydrogens with it. This forms a water molecule. The carbon the oxygen was attached to (the alpha carbon) now becomes a carbocation - a carbon atom with a positive charge.
- The negative A- ion attacks a C-H bond on a beta carbon. The bond breaks, releasing a hydrogen ion, and the bond's electrons are used to create a C=C double bond. The A- ion combines with the hydrogen ion to regenerate the acid catalyst.
- 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 the E2 mechanisms. It is very similar to the E1 mechanism. 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 using ethanol. As before, the diagram is colour-coded to help you understand the process.Vaia Originals
- A lone pair of electrons from the oxygen atom attacks a hydrogen ion from the acid catalyst, protonating the oxygen. This is the same as the first step in the E1 mechanism.
- The A- ion attacks a C-H bond on one of the alcohol's beta carbons at the same time as the protonated oxygen breaks off from the molecule, taking the bonding pair of electrons and its two hydrogen atoms with it. The electrons from the breaking C-H bond are used to form a new C=C bond, and the hydrogen ion released regenerates the acid catalyst.
- 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 the reasons behind the stability of carbocations, but it means that the reaction's activation energy is much higher. An E2 mechanism is more energetically favourable.
If you do want to find out more about carbocations, check out Nucleophilic Substitution Mechanism for a more in-depth explanation.
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 this alcohol's name indicates that the hydroxyl group (-OH) is attached to the first carbon in the chain. Remember that the carbon atom with the C-OH bond is the alpha carbon. Because the alpha carbon is the first carbon atom in the chain, butan-1-ol is an example of a primary alcohol - the alpha carbon is bonded to just one other alkyl group. Here's what it looks like:
Butan-1-ol. Vaia Original
The alpha carbon is bonded to just one other carbon atom. Therefore, there is only one beta carbon. Remember that in elimination reactions, the hydrogen ion is always lost from a beta carbon and that 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. Here we form but-1-ene.
The diagram below highlights butan-1-ol's hydroxyl group, alpha carbon, beta carbon, and the C=C double bond that forms in the alkene product.
Dehydrating butan-1-ol to produce but-1-ene.Vaia 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 carbon atoms are the beta carbons.
Butan-2-ol.Vaia Original
Notice how in this molecule, both of the beta carbons contain hydrogen atoms. 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 butan-2-ol, the C=C double bond can form in multiple different places. We form a mixture of three different isomeric products:
- If the hydrogen ion comes from the beta carbon on the left, we produce but-1-ene.
- If the hydrogen comes from the beta carbon on the right, we produce either E-but-2-ene or Z-but-2-ene.
The isomeric products of the elimination reaction of butan-2-ol. Vaia Originals
All primary alcohols result in just one unique alkene. However, not all secondary and tertiary alcohol elimination reactions form isomers!
For example, dehydrating propan-2-ol produces just one distinct alkene: propene. You always end up with the same product, no matter which of its beta carbons loses a hydrogen ion. Try sketching the molecules out yourself to see. The easiest way to determine whether an alcohol elimination reaction produces isomeric products is to draw the reaction in this way and consider all of the different placements of the C=C double bond, and the arrangements of atoms around them. It sounds tricky, but you'll quickly build up your confidence as you get more practice.
Before we try some examples ourselves, make sure your isomer knowledge is up to speed by having 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. Methylpropan-1-ol is a primary alcohol and so produces just one alkene product.
Heating this alcohol with concentrated sulphuric acid produces methylpropene and water. Like in the earlier diagrams, we've highlighted the hydroxyl group, the alpha carbon and the beta carbon.
The elimination reaction of methylpropan-1-ol. Vaia Original
Another example is pentan-2-ol. Pentan-2-ol is a secondary alcohol, and so here you can see that there are two beta carbons. Heating this alcohol with an acid catalyst 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, Vaia 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 a new large molecule and a new smaller molecule in the process.
- In alcohol elimination reactions, also known as dehydration reactions, a hydrogen ion and a hydroxide ion 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.
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