Dehydrating Agents: How do they work and what is their role in dehydration?

[jd12345]

Why is conc H2SO4 , KOH and phosphorus pentoxide dehydrating agents?
I don't understand how they work. Do they adsorb water , react with water to from new compounds or anything else ? How do they dehydrate?

 

[Borek]

Do they adsorb water , react with water to from new compounds

Both ways possible. Although this is a thin ice - the difference between physical and chemical change is blurry, so it is not always easy say what is happening.

Classic approach is that first two of those you listed absorb (not adsorb) water without a chemical reaction, third reacts with water.

 

[morrobay]

The SO^--₄ anion has a very high negative charge density and so
will very strongly attract the Hydrogen's end of the polar H₂O molecule.
If you pour conc. H₂SO₄ on paper the water is removed with
only Carbon left.

Note: I am not sure if you were aware that SO4 is very hydrophilic for the above reasons . If so ,then Borek answered your question.

 

[Borek]

The SO^--₄ anion has a very high negative charge density and so will very strongly attract the Hydrogen's end of the polar H₂O molecule.

That's not the way it works. Sulfuric acid is very strong and it protonates water converting it to ionic H₃O⁺. H₃O⁺ is no longer volatile.

Edit: changed H₃⁺ to H₃O⁺, obvious mistake by me.

 

[morrobay]

That's not the way it works. Sulfuric acid is very strong and it protonates water converting it to ionic H₃⁺. H₃⁺ is no longer volatile.

Well Sulfuric acid being a strong acid and protonating water does not exclude the
following : The Sulfate ion forms a hydration shell consisting of a symmetrical
arrangement of 16 Hydrogen bonded water molecules -
SO₄^-- (H₂O)₁₆

So are you saying that it is
SO₄^--(H₃O⁺)₂
If so then it looks like we are about back where we started

 

[Borek]

Well Sulfuric acid being a strong acid and protonating water does not exclude the
following : The Sulfate ion forms a hydration shell consisting of a symmetrical
arrangement of 16 Hydrogen bonded water molecules -
SO₄^-- (H₂O)₁₆

So are you saying that it is
SO₄^--(H₃O⁺)₂
If so then it looks like we are about back where we started

I guess it must be a function of how much water is added, but if you start with the concentrated acid I am sure first stage is protonation. Note that the 16 water molecules arrangement means 16 moles of water per mole of sulfuric acid - that's 3M solution, so diluted its drying properties are negligible (compare http://www.generalchemical.com/assets/pdf/Sulfuric_Acid_Water_Vapor_Concentration_22.pdf ).

 

[morrobay]

I think we are talking about different stages of a process:
Sulfuric acid as a dehydrating acid, followed by a diprotic acid. Sucrose plus conc. Sulfuric acid.
C₁₂H₂₂O₁₁ --> conc.--> 12C + 11H₂O
+ mixture: H₂SO₄ + H₂O --> H₃O⁺ + HSO₄^-
HSO₄^- + H₂O > H₃O⁺+ SO₄^--

Then it is this Sulfate ion that is hydrated by hydrogen bonded water molecules
SO₄^--(H₂O)₁₆

(thanks)

 

[Borek]

I think we are talking about different stages of a process:

Yes and no - we are talking about different stages of dilution, but after we are past the first stage sulfuric acid is hardly a dehydrating agent. It is an effective dehydrating agent only when it is concentrated.

Sulfuric acid as a dehydrating acid, followed by a diprotic acid.

Can you elaborate, I am not sure what you mean. Sounds to me like you are separating its dehydrating properties from its acidic properties. Water protonation is already an acid base reaction, so dehydrating properties are the effect of the sulfuric acid acidity.

 

[morrobay]

I think we are talking about different stages of a process:
Sulfuric acid as a dehydrating acid, followed by a diprotic acid. Sucrose plus conc. Sulfuric acid.
C₁₂H₂₂O₁₁ --> conc.--> 12C + 11H₂O
+ mixture: H₂SO₄ + H₂O --> H₃O⁺ + HSO₄^-
HSO₄^- + H₂O > H₃O⁺+ SO₄^--

Then it is this Sulfate ion that is hydrated by hydrogen bonded water molecules
SO₄^--(H₂O)₁₆

(thanks)

This is getting complicated. Is it correct to just say that the three above processes:
dehydration , diprotonation and hydration can occur together.And in this order:dehydration first ,that is when the Sulfuric acid is most concentrated .
Followed by diprotonation, followed by hydration. Just the same sequence in the above post.
If it is possible to analyze the exact sequence here in terms of rates and changing concentrations ( the water produced from sucrose )
then I am going to let you do it.

 

[Borek]

I don't think you can treat dehydration and protonation as separate processes. Protonation is the driving force behind dehydration, so they occur simultaneously. My bet is that in the case of sugars "free" proton from the sulfuric acid protonates -OH group, making it possible to remove water molecule from the sugar molecule (leaving a carbocation behind). Then this water molecule is protonated again.

 

[morrobay]

I don't think you can treat dehydration and protonation as separate processes. Protonation is the driving force behind dehydration, so they occur simultaneously. My bet is that in the case of sugars "free" proton from the sulfuric acid protonates -OH group, making it possible to remove water molecule from the sugar molecule (leaving a carbocation behind). Then this water molecule is protonated again.

Thanks - I was wondering what the mechanism was on that.