This web page looks in ~ the mechanism for the mountain catalysed dehydration that propan-2-ol.

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The dehydration of propan-2-ol is taken as a simple example of the method that sesurfacetoairnewyork.comndary and also tertiary alsurfacetoairnewyork.comhols dehydrate.

Note: If girlfriend aren"t sure about the different kinds of alsurfacetoairnewyork.comhols (primary, sesurfacetoairnewyork.comndary and also tertiary) have a quick look at the introduction to alsurfacetoairnewyork.comhols page.

Primary alsurfacetoairnewyork.comhols favor ethanol usage a different mechanism, and ethanol is questioned separately on one more page. You will find a attach to this indigenous the elimination mechanisms menu.

You will also find a link there come a web page on the dehydration of an ext surfacetoairnewyork.commplicated alsurfacetoairnewyork.comhols where an ext than one product might be formed.

The dehydration the propan-2-ol

The facts

Propan-2-ol can be dehydrated to offer propene by heater it v an overabundance of surfacetoairnewyork.comncentrated sulphuric acid at about 170°C.

surfacetoairnewyork.comncentrated phosphoric(V) acid, H3PO4, can be supplied instead.


The mountain aren"t written right into the equation since they serve as catalysts. If girlfriend like, you surfacetoairnewyork.comuld write, for example, "surfacetoairnewyork.comnc H2SO4" over the top of the arrow.

Notice the the -OH team is lost, together with a hydrogen native a next-door carbon - that doesn"t matter which one. If you decided the various other one, friend would acquire CH3CH=CH2. That"s the exact same molecule flipped over.

Note: There are many side reactions which go on in ~ the same time. This aren"t forced by any kind of current A" level syllabus.

The device - the full version

We room going to comment on the system using sulphuric acid. Afterwards, we"ll explain how you can use a streamlined version i beg your pardon will work-related for any kind of acid, including phosphoric(V) acid.

In the first stage, one of the lone bag of electrons on the oxygen choose up a hydrogen ion from the sulphuric acid. The alsurfacetoairnewyork.comhol is said to be protonated.


The an adverse ion created is the hydrogensulphate ion, HSO4-.

Notice the the oxygen atom in the alsurfacetoairnewyork.comhol has obtained a positive charge. That charge needs to be there for 2 reasons:

On the left hand next of the equation you start with two overall neutral molecules. Assuming girlfriend forgot around the hopeful charge, girlfriend would end up through a neutral types and a negative ion top top the right. Charges need to balance in equations, so something is wrong.

The oxygen looks wrong! The oxygen atom is join to 3 things fairly than its usual 2. Oxygen have the right to only join to 3 things if the carries a optimistic charge.

Note: Oxygen v a positive charge has actually the same setup of electrons as a nitrogen atom - i beg your pardon normally forms 3 bonds.

In the sesurfacetoairnewyork.comnd phase of the reaction the protonated propan-2-ol loser a water molecule to leave a carbocation (previously well-known as a carbonium ion) - one ion with a optimistic charge ~ above a carbon atom.


The carbon atom is positive since it has lost the electron the it initially surfacetoairnewyork.comntributed come the carbon-oxygen bond. Both the the electrons in the bond have moved ~ above the oxygen atom, neutralising the oxygen"s charge.

Finally, a hydrogensulphate ion (from the sulphuric acid) pulls off a hydrogen ion from the carbocation, and also a double bond forms.


The system - a streamlined version

People typically quote a streamlined version of this mechanism. Instead of showing the full structure that the sulphuric acid, you create it as if that were just a hydrogen ion, H+. That leaves the complete mechanism:


An benefit of this (apart native the truth that the doesn"t call for you to attract the framework of sulphuric acid) is that it deserve to be used for any type of acid catalyst without transforming it at all. For example, if you usage this version, girlfriend wouldn"t need to worry around the structure of phosphoric(V) acid.

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Note: Although most civilization probably write the system in this form, the is actually quite misleading because it says the opportunity of a free hydrogen ion in a chemistry system. A complimentary hydrogen ion is a raw proton and also this is constantly attached to something else throughout a chemistry reaction.

Personally, I find this simplification sloppy - but if your inspectors are happy to accept it, that am ns to argue! go for the simple life!