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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
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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
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
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
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.