Submitted by Sophia E. Hayes / Washington University on Thu, 10/14/2010 - 10:57
Forums

This is the first time I've posted to a forum.  Hope "Research" was the right category.

 

I'm teaching a jr./sr. level Inorganic course using Miessler and Tarr.  I got a question from a student about hydrogen peroxide, H2O2 that I couldn't answer.  It's in  every textbook at  having C2 symmetry, but you'd expect (from a VSEPR picture) that it would be planar.  Apparently, it's not.  

 

Does anyone happen to know the reason why ... any good textbook discussion of this that you know of?

 

Thanks!

Sophia Hayes

Washington University

hayes@wustl.edu

Chip Nataro / Lafayette College
I think it is related to the repulsion of the lone pair on the oxygen atoms. I have seen something about this somewhere...
Thu, 10/14/2010 - 13:28 Permalink
Sibrina Collins / College of Arts and Sciences at Lawrence Technological University

Dear Sophia,

Take a look at the third edition of "Chemical Applications of Group Theory," by Cotton (page 58). There are three examples shown, namely the nonplanar configuration, which has a C2 axis and no planes of symmetry, so the point group is C2. Then, you have the cis-planar configuration, which is C2V. Finally, the trans-planar configuration, which is C2h. Maybe this will help answer your question.

SNC

Thu, 10/14/2010 - 16:15 Permalink
Sophia E. Hayes / Washington University

I did see that section in Cotton, but I appreciate the suggestion.

If you look at the actual structure of H2O2 in gas and solid phases really is C2 and non-planar.  My question is *why* is it non-planar, when from a VSEPR perspective, you'd predict a planar H-O-O-H species?

 Thanks!

 Sophia

Sophia E. Hayes Washington University

Thu, 10/14/2010 - 16:34 Permalink
Kyle Grice / DePaul University

Anslyn and Dougherty to the rescue!!

 Page 122 of "Modern Physical Organic Chemistry" by Anslyn and Dougherty (2006) describes it as a compromise between steric preference and donor-acceptor preference!

I love this book. 

-Kyle 

Thu, 10/14/2010 - 17:06 Permalink
John Gilje / James Madison University

This is sometimes refered to as the gauche effect and it is seen in a number of systems including H2O2, N2H4, the haloalkanes, carbohydrates, etc. There is a recent, quite theoretical discussion of why the fluorines in 1,2-difluoroethane are gauche instead of eclipsed or trans in J. Phys. Chem. A 2005, 109, 1223-1229.

An older, but excellent discussion by the person who coined the term "gauche effect":

Saul Wolfe, "Gauche effect. Stereochemical consequences of adjacnet electron pairs and polar bonds" Acc. Chem. Res., 1972, 5, 102–111.

Wed, 02/09/2011 - 16:50 Permalink
Joanne Stewart / Hope College
I love VIPEr! Thanks for the great answers, Kyle and John! And I love even more that the answer to the question is "a subtle interplay of steric and electronic factors."
Thu, 10/28/2010 - 12:18 Permalink
Adam Johnson / Harvey Mudd College
Joanne, EVERYTHING is "a subtle interplay of steric and electronic factors."  Well, of course, except for EVERYTHING ELSE which is "due to the complex web of kinetics and  thermodynamics."
Thu, 10/28/2010 - 19:37 Permalink