Defining Anhydrides: What Is the Most Accurate Definition?

[JeffEvarts]

A question about an anhydride that has nothing to to with acetic acid! How novel!

I am not entirely sure I understand what an anhydride is.

I found a reference to disulfuric acid, H2S2O7, and thought "Ah! Sulfuric Anhydride", but when I went to my other references, specifically the oxoacid entry on wikipedia, they listed SO3 as the anhydride.

I get it to one of three possibilities:
1) The anhydride is what you get when you take water out of the acid (SO3)
2) The anhydride is what you get when you link two acid residues via an oxygen (H2S2O7)
3) The anhydride of something without a C-O-H group (such as H2SO4) does not exist, so the question is moot

What sayeth the authorities?
-Jeff Evarts

 

[maajdl]

Anything that can be obtained by removing a (chemically bond) molecule of water can be called anhydrite.
Another example is gypsum that when heated transforms to "anhydride":

CaSO4·2H2O + heat → CaSO4·½H2O + 1½H2O (steam)

http://en.wikipedia.org/wiki/Calcium_sulfate

 

[Borek]

I am not sure if any formal definition exists. Unfortunately, IUPAC goldbook site at the moment is not working, so I can't check to be sure.

I would say acid anhydride is a substance that will produce acid on the reaction with water - so SO₃ classifies, while disulfuric acid doesn't.

 

[epenguin]

I too am not sure there is an a confining formal definition but I think your definition (a) is as good a definition as any. And better than some other nomenclature used for such things. I mean H₂S₂O₇ is called persulphuric but the equivalent with phosphoric acid is called pyrophosphoric acid. You can call the molecules of an acid with various numbers of H₂O removed anhydrides, and except for the one with water completely removed you can all them all oxyacids or poly oxyacids. (I worried about this a bit I remember at school, and noticed there were a number of these pairs coming up which I tried to learn and fix in mind, then I had the inspiration you didn't need to stop at two and proudly announced you must have an infinite number of oxyacids of any element! This was at first ridiculed and then congratulated.http://www.puma-project.eu/yacs/smileys/images/medal_full.gif)

Anyway I just googled "phosphoanhydride" and exactly the right kind of molecules tumble out. In the lab you use acetic anhydride for acetylatinng, in your cells you use phosphoanhydride (mainly triphosphates) for phosphorylating, or pyrophosphphorylating or for adenylylating etc. - no molecules are more important!

 

[JeffEvarts]

Three authorities, 2.5 answers. :)

Perhaps I shall just avoid the term when I can. That sounds like the smartest path.

Thank you,
-Jeff

 

[Borek]

That's because reality doesn't care about tight definitions

I don't think there is a serious difference between what I suggested and your (a) definition - sure, there are compounds that will fit one, but won't fit the other, but in the majority of cases molecule that you get by removing water molecule from the acid will - given a chance - happily react with water to produce acid back, so both approaches are more or less equivalent.

It is also a matter of a personal preference - when there is no agreed on definition, everyone will tell you what they think when they hear the word (actually that happens even sometimes where the definition exists).