[Will]

Every source I have come accorss tells me that when you do the acid catalysed dehydration of an alcohol that the mechanism is always the protonation of the oxygen atom to eliminate water, which gives you the carbocation, which nothing attacks, but a water molecule takes a hydrogen from a neighbouring carbon atom to give you the alkene.

My question is, why doesn't, with sulfuric acid, you don't get an alkyl hydrogensulfate caused by the attack of the conjugate base on the carbocation. Then to get your alkene I would assume a water molecule or another hydrogensulfate ion takes the hydrogen from the neighbouring carbon atom and the hydrogensulfate gets eliminated.

Hope the question makes sense!

[moussa]

hi
2 things must be taken in consideration:
1- oxygen electron pair can attract proton easily so the proton of H2SO4 is attracted  to the oxygen atom
2- water molecules is a good leaving group so it prefer to leave the compound better tha having + sign ona hetroatom (o)
and hence you'll get your coumpound once the other H leaves
hopping that this would answer your question

[Albert]

why doesn't, with sulfuric acid, you don't get an alkyl hydrogensulfate caused by the attack of the conjugate base on the carbocation

Sulfuric acid catalize this reaction: it gives an H+ to your alcohol. Then HSO4-, an extremely weak nucleophile, attacks the R-OH2+, takes water out and forms the carbocation.
Water, after that, takes a proton and becomes H3O+ (HSO4- is a too strong acid for doing it).

[moussa]

The dehydration of ethanol
The full version of the mechanism[/b]

This full version covers the mechanism using sulphuric acid. Afterwards, we'll look at the simplified version which will work for any acid, including phosphoric(V) acid.

The oxygen atom in the ethanol has two active lone pairs of electrons, and one of these picks up a hydrogen ion from the sulphuric acid. The alcohol is said to be protonated.

[moussa]

watch the arrows

[Will]

I totally agree with you, I'm just confused about what happens to that carbocation once water is eliminated. I am asking why the hydrogensulfate ion which is left behind after indirectly protonating the oxygen atom doesnt attack it. Thanks

[moussa]

The negative ion produced is the hydrogensulphate ion, HSO4-.

Notice that the oxygen atom in the alcohol has gained a positive charge. That charge has to be there for two reasons:

On the left hand side of the equation you start with two overall neutral molecules. Assuming you forgot about the positive charge, you would end up with a neutral species and a negative ion on the right. Charges must balance in equations, so something is wrong.

The oxygen looks wrong! The oxygen atom is joined to 3 things rather than its usual 2. Oxygen can only join to 3 things if it carries a positive charge.

[moussa]

In the second stage of the reaction the protonated ethanol loses a water molecule to leave a carbocation (previously known as a carbonium ion) - an ion with a positive charge on a carbon atom. The carbon atom is positive because it has lost the electron that it originally contributed to the carbon-oxygen bond. Both of the electrons in that bond have moved onto the oxygen atom, neutralising the oxygen's charge.

[moussa]

One of the several things that can now happen to this carbocation is for it to lose a hydrogen ion from the CH3 group. This hydrogen ion is pulled off by a hydrogensulphate ion to regenerate the sulphuric acid catalyst.

[moussa]

The simplified version of the mechanism

Instead of showing the full structure of the sulphuric acid, it is commonly written as if it were simply a hydrogen ion, H+.

An advantage of this (apart from the fact that it doesn't require you to draw the structure of sulphuric acid) is that it can be used for any acid catalyst without changing it at all. For example, if you use this version, you wouldn't need to worry about the structure of phosphoric(V) acid.


In the first stage, the ethanol gets protonated exactly as before - the only difference is that you are writing H+ instead of the full structure of the sulphuric acid.

The second stage is identical to the one in the full version of the mechanism.

The final stage shows a hydrogen ion "falling off" the carbocation - rather than being pulled off. This is seriously misleading, but it's what the examiners want!

[Will]

Why couldn't the hydrogensulfate ion attack the carbocation instead to form an alkyl hydrogensulfate?

[moussa]

see the hydrogen in the carbon next to carbo cation  has its electron pair near to C.C so it is preferable to stabilize it
as the solution having Hso4- can eleminate the proton easily so the alkene is formed. in otherwords  hydrogen with its electrons is nearer to c.c so it is easier to do that and oxygen as we now atrackt hydrogens rather than attacking the neucles CC and  the size heremay be taken in account)  

[Yggdrasil]

Why couldn't the hydrogensulfate ion attack the carbocation instead to form an alkyl hydrogensulfate?

It can and it will.  However, the sulfate group is an extremely good leaving group, so the resulting alkyl sulfate will easily dissociate in an E1 mechanism to reform the carbocation.  In other words, the alkyl sulfate and carbocation are at equilibrium.  Since the formation of the alkene is more thermodynamically favorable, the carbocation will mostly be converted to the alkene.  This shift the equilibrium between the alkyl sulfate and carbocation to favor formation of the carbocation, so there will be insignificant amounts of the alkyl sulfate at equilibrium.

[Will]

Thank you so much for the explanation Yggdrasil  !! I couldn't find anything as useful as that in any of my books or online!

[HP]

Note that its very possible and "full" organic sulphate formation:
R-OH + H2SO4conc--> R-O-SO2-OH
R-OSO2Oh + R-OH--> R-O-SO2-O-R
Sulphates to alkene i dont know the mechanism dehydration to form the alken in heating...Also in this reactions theres always and some R-O-R - ether product formation.