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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: orgo814 on April 13, 2016, 11:09:52 AM

Title: F and B strain
Post by: orgo814 on April 13, 2016, 11:09:52 AM

I'm taking a physical organic course and am confused on a topic. Can anyone help explain the difference between "F strain" and "B strain". Thanks for any clarification.

Title: Re: F and B strain
Post by: Dan on April 13, 2016, 02:16:07 PM

I must confess, I'd never heard of it.

I found this paper: http://pubs.acs.org/doi/abs/10.1021/ja01219a007

If I understand correctly, if you have an acid-base reaction:

A + B  ::equil:: AB

Then F-strain is the strain caused by steric repulsion between A and B. B-strain is is the strain induced within B through geometric changes upon complexation of A.

Brown's example is B = trimethylamine. If you complex it with an acid (A), the C-N-C bond angle is compressed, and the increased repulsive forces between the methyl groups (within B) is the B-strain. The F-strain is steric repulsion between B and A.

Title: Re: F and B strain
Post by: Babcock_Hall on April 13, 2016, 04:22:42 PM

Could F and B refer to front and back?
faculty.swosu.edu/tim.hubin/InorganicLects/InorgCh6.1.ppt

F = front strain = direct steric interference at the site of interaction
B = back strain = bulky groups interfere opposite the interaction site upon binding as the molecule adjusts its VSEPR geometry