Lewis Acid/Base Chemistry in formation of Sulfuric Acid

[Qube]

Homework Statement

How does sulfur trioxide react with water to form sulfuric acid

Homework Equations

This is clearly a Lewis acid/base problem.

The Attempt at a Solution

Okay, I tried drawing sulfur trioxide out with three double bonds (as it is traditionally drawn). I know the oxygens are nucleophilic and the hydrogens on the water are electrophilic. Nucleophile attacks electrophile but that gives the central sulfur two lone pairs of electrons. This clearly can't be right because I have never seen a sulfuric acid Lewis structure with two lone pairs.

I went to Wikipedia and it says the sulfur trioxide molecule has 2 dative bonds and one double bond. This seems to make a little bit more sense. I had the datively bonded oxygens attack the hydrogens.

My question is how does the extra oxygen get appended onto the sulfur? I guess the sulfur, being electrophilic, attacks the nucleophilic oxygen too?


Also, on a side note, would this reaction be an example of why sulfur isn't hypervalent - i.e. the above reaction wouldn't make sense if sulfur were hypervalent?

 

[alingy1]

I'm just going to shout out my n00b knowledge: a proton shift apparently occurs (zumdahl, chemistry). Hope that helps!

 

[Qube]

Yes, that's clear, but what is the mechanism? This is my proposed "mechanism" if we stick with tradition - i.e. sulfur trioxide and sulfuric acid are both hypervalent. I end up with two lone pairs on the sulfur (??).

 

[abitslow]

Your set up implies the belief that there is one reaction mechanism. This is clearly wrong. There seem to be three homogenous cases: bimolecular gasseous, bimolecular aqueous, and trimolecular. You didn't specify temperature or pressure (concentration). I wasn't able to find any clear answer to your question, regardless of the context. It seems to be clear that SO3.H2O complex exists in the atmosphere, based on the literature. Another paper used MD DFT to model the bimolecular reaction between SO3 and H2O in solution, where they claim concerted reaction at both the S and the O (with the -O-H of the water). Another paper claims that the reaction occurs between two water molecules and the SO3 (I don't recall if this was in solution or gasseous at low pressure). In many real world situations, the reaction is inhomogenous (occurring on a surface). I doubt if THREE reaction centers H→O, H→O, O→S is a likely mechanism. Nor is some O(-) ion going to be floating around. So, neither of your proposed mechanisms seem reasonable to me. (HSO4(-) exists in the atmosphere, hint hint.).

 

[DeeNos]

Yes, this is indeed a Lewis acid/base problem. The reaction between sulfur trioxide and water to form sulfuric acid involves the transfer of a proton (H+) from the water molecule to the sulfur trioxide molecule. This proton transfer is facilitated by the Lewis base property of water, which can donate its lone pair of electrons to the electrophilic sulfur atom in sulfur trioxide. This results in the formation of a dative bond between the oxygen atom in water and the sulfur atom in sulfur trioxide, forming a sulfuric acid intermediate.

The extra oxygen atom in sulfuric acid is formed through a similar proton transfer process. The oxygen atom in the water molecule that initially bonded to the sulfur atom in sulfur trioxide now becomes the proton donor, while the oxygen atom in sulfur trioxide becomes the proton acceptor, forming another dative bond. This results in the formation of a stable sulfuric acid molecule with a total of four oxygen atoms bonded to the central sulfur atom.

As for the question about sulfur being hypervalent, the Lewis structure of sulfur trioxide does not necessarily indicate that sulfur is hypervalent. The dative bonds in the Lewis structure simply represent a sharing of electrons between two atoms, rather than a formal transfer of electrons as in covalent bonds. Therefore, the Lewis structure does not necessarily reflect the true electronic structure of the molecule. Additionally, the concept of hypervalency is still debated and there is no clear consensus on its applicability to sulfur compounds.