THE DEHYDRATION OF ETHANOL
This page looks at the mechanism for the acid catalysed dehydration of a simple primary alcohol like ethanol to give an alkene like ethene.
This isn't as straightforward as the dehydration of a secondary or tertiary alcohol, and it is important that you read the page about the dehydration of propan-2-ol before you continue with this page.
The dehydration of ethanol
Ethanol can be dehydrated to give ethene by heating it with an excess of concentrated sulphuric acid at about 170°C.
Concentrated phosphoric(V) acid, H3PO4, can be used instead.
The acids aren't written into the equation because they serve as catalysts. If you like, you could write, for example, "conc H2SO4" over the top of the arrow.
Note: There are many side reactions which go on at the same time. These aren't required by any current A' level syllabus.
You will find two versions of the mechanism for the dehydration of primary alcohols on the web and in various textbooks.
One of these is exactly the same as the mechanism for the reaction involving propan-2-ol and other secondary or tertiary alcohols (known technically as an E1 mechanism), but the other is different (known as an E2 mechanism).
The more reliable sources give the E2 mechanism for the dehydration of primary alcohols including ethanol. I am going to treat this as the "correct" version, and have added a note as to why I think is better following the mechanism.
The correct version in full
If you have read the page on the dehydration of propan-2-ol, you will know that it involves the formation of a carbocation (a carbonium ion).
If ethanol used the same mechanism, you would get a primary carbocation formed, CH3CH2+, but this is much less stable than a secondary or tertiary carbocation.
That would lead to a very high activation energy for the reaction.
Note: You will find a discussion about carbocations by following this link.
The alternative mechanism avoids the formation of the carbocation, and so avoids the high activation energy.
We are going to discuss the mechanism using sulphuric acid.
In the first stage, one of the lone pairs of electrons on the oxygen picks up a hydrogen ion from the sulphuric acid. The alcohol is said to be protonated. That is exactly the same as happens with propan-2-ol and the other secondary and tertiary alcohols.
In the mechanism we have already looked at with propan-2-ol, the next thing to happen was loss of water to form a carbocation, followed by removal of a hydrogen ion from the carbocation and the formation of a double bond.
In this case, instead of happening in two separate steps, this all happens at the same time in one smooth operation. By doing that, you avoid the formation of an unstable primary carbocation for primary alcohols.
No simplified version of this!
I am not giving a simplified version of this mechanism just in terms of hydrogen ions. If you don't show something removing the hydrogen ion from the protonated alcohol, you are really missing an important feature of the reaction.
Why do I think this is most probably the correct version?
There is experimental evidence to support it. While I was researching this, I came across two academic papers (here and here). I don't have access to the full articles, but the abstracts suggest that there is experimental evidence for both propan-1-ol and ethanol dehydrating using a mechanism which doesn't involve a carbocation as the intermediate. I haven't been able to find any experimental evidence for a dehydration mechanism involving carbocations in the case of primary alcohols.
Reliable sources quote the mechanism above. For example, the first page of a Google search for dehydration ethanol mechanism includes two university sources from LibreTexts from the University of California, and from the University of Calgary.
Interestingly, if you read sources giving this mechanism, they will usually explain why it is different for primary alcohols rather than secondary or tertiary ones - in terms of the stability of the carbocations. Sources quoting the other mechanism tend to quote it without comment.
Note: If you have expert knowledge of this topic, and can quote experimental evidence that shows that primary alcohols can dehydrate using a mechanism involving a carbocation, I would like to know. Any other information giving more evidence for the mechanism above would also be very welcome. Please contact me via the address you will find by following this link.
The other mechanism that you might come across
This is the mechanism that looks justs like the one involving propan-2-ol, and involves the formation of an unstable primary carbocation.
This is the version that you would have found on Chemguide up to December 2012, and was originally included because it was the one wanted by one of the UK Exam Boards (AQA) at the time I wrote the page in 2000.
It is possible that your examiners may still want it. As far as I am aware, the only syllabuses which I track which still ask this topic are IB and AQA. An old AQA mark scheme gave the version below. IB and AQA students need to read the notes that follow. If you need to know about this, you need to check what your examiners are currently expecting.
Note for IB students:
I contacted IB about this to find out exactly what they wanted, and received a prompt and helpful reply:
"Although the IB syllabus does not identify different elimination mechanisms (E1 and E2 to give only the most common ones), it is generally assumed that teachers will teach their students the proper mechanism (as you do so) depending of the substrate. It is therefore questionable to have published a mark scheme with an E1 mechanism since the E2 mechanism was then the most probable. We apologize for the confusion. Nevertheless, during the marking process following the little syllabus guidance, examiners did allow the use of any one of the elimination mechanisms and thus, for example, did not penalize the use of an E1 mechanism for the dehydration reaction of a primary substrate/alcohol (this is of course not what we want to see as an answer)."
Essentially they are saying that they expect you to have been taught the correct version of the mechanism (the first mechanism on this page), but that they will accept either version.
I understand that there is no consistency in the versions which the various IB textbooks give on this either. If you aren't sure what to do, I suggest you talk to your teacher about it. The safest thing to do is to learn the correct version, of course - and if you intend to carry on with chemistry at a higher level, this is what you should do.
Note for AQA students:
Final update (I hope!)
A very old AQA mark scheme penalised students for giving the correct mechanism by having one mark specifically awarded to showing the structure of an intermediate carbocation in the dehydration of ethanol. If you give the first mechanism above, there is no carbocation, and so you can't get this mark.
I queried this and the current position (March 2018) is, quoting from AQA's response:
"You are correct that when this was on a previous specification, the mechanism expected was the version called E1 which involves a carbocation, even if this is unlikely for primary alcohols. This is still the case; we still expect this E1 mechanism because most examples will involve secondary or tertiary alcohols and we wish to keep things simple. Similarly for nucleophilic substitutions, we expect candidates to give SN2 mechanism rather than the alternative SN1, but we would allow either for equal marks."
They go on to say
"I can confirm that either mechanism will score the same mark . . . and on any future papers we will give equal marks to both mechanisms if this comes up."
Note: If I was teaching this to a class, I would want to teach the correct mechanisms - one for secondary and tertiary alcohols, and a different one for primary ones. It doesn't seem to me that teaching this properly adds much in the way of difficulty, and it avoids the need for students who go on to do chemistry at a higher level having to unlearn an incorrect mechanism.
Since there is no penalty now in quoting the correct mechanisms, I think that is what should be taught. We shouldn't be in the business of teaching incorrect material.
The same thing, incidentally, applies to the nucleophilic substitution mechanisms mentioned in AQA's comments. You cannot reasonably use the same SN2 mechanism for all kinds of halogenoalkane.
© Jim Clark 2013 (last updated March 2018)