What exactly is "B-strain"?

Question

On factors that influence the stability of carbocations, I came across this peculiar one: > Bulky groups attached to the positively charged $C$-atom (of the carbocation), stabilize it by B-strain, which prevents the C-atom for returning to the $sp^{3}$ state. \n> _{**Source**: Pearson's *Organic Chemistry (7e)*, Morrison and Boyd\n>}\n\n The issue? I've **never** heard of "B-strain" before. I mean, it's possible that bulky groups stabilize the positive $C$-atom via inductive (and other) effects...but what's with "B-strain"? I ran a cursory Google search, but I couldn't find any reliable Chemistry-literature that deals with this. In fact, it raised more questions than answers, most notably, the presence of *another* effect called the "F-strain". So I must ask: **What is the "B-strain", and how does it contribute to stabilization of carbocations?** **Additionally, is there such a thing as the "F-strain"? If so, what's it all about?**

Answer 1

B-strain is back-strain. If you have a sp3 alkyl halide with three tert-butyl groups, there's quite a bit of steric strain (back-strain, relative to the halogen, the strain is at the back of the molecule) as the bond angles of the sp3 carbon are pushed to 109.5°. As the carbonation forms the carbon becomes sp2 hybridized and has bond angles closer to 120°. This release of b-strain to the larger bond angles is a stabilizing force for carbonation formation.

F-strain is front strain and is less relevant for the same situation (tri tert-butyl alkyl halide) as the strain is all on the back relative to the halogen LG.