Tuesday, August 31, 2010

Chemistry Is Like Sex: Coupling Illustrated

G.N. Lewis (I wrote about him back here) was from Berkeley and thus a bit more liberal when defining acids and bases: He gave more general definitions of them than Brønsted and Lowry did. And while more open-minded, Lewis was a bit of a chauvinist when he argued that a base's precious electrons helped "complete the octet" of an acid when they coupled.

Consider the coupling of a simple base, ammonia, with a simple Lewis acid, borane. If you're a jaded chemist who has "seen it all" you might consider just skipping to this link dealing with borane and ammonia making borazane as a hydrogen fuel energy source.

We already "know" what ammonia looks like here--but what about borane?  I wrote a bit about boron the element back here.  Turns out that the word "boron" is etymologically linked to the Arabic tongue as well, via the word borax.

Borane, BH3, is a natural fit for ammonia's lone pair.  Consider its structure:

BH3 looks a bit like NH3 but completely lacks a lone pair.  BH3 has only six surrounding electrons instead of eight and so is electronically unfulfilled. In a sense, it has a big hole in its middle. In the absence of an available lone pair, BH3 readily dimerizes in a head-to-tail fashion with another sister BH3 molecule to form B2H6. Here's an illustration of two BH3's getting it on together:

When NH3 and BH3 prepare to bond, a natural question is where should NH3 put its lone pair?  BH3 has what's called a "virtual orbital" (there's nothing virtuous about it).  A virtual orbital is just an empty electron orbital. Another name is a LUMO. Empty orbitals have metes and bounds, despite there being nothing there there. Here's a lurid depiction of borane's virtual orbital:

It's a bit hard to see in the depiction above but all three of borane's tripodal hydrogen limbs are squished flat into a planar configuration between the two swollen globes. The red and blue empty lobes are equivalent in the eyes of ammonia's incoming lone pair: Borane's empty orbital can be approached from above or below.  As the ammonia approaches one side of borane, one empty lobe enlarges to accept the lone pair while the other shrinks. Also, borane's little hydrogen limbs fold back away from the incoming lone pair to accommodate the fit.  The final coupling product looks like this:

BH3, with the help of ammonia's lone pair, now has an octet of electrons. 


  1. Boron chemistry. I love it.

    Always thought that was an underappreciated field of chemistry. The awesome Borax used in the soap dispensers at Ted's Montana Grill convinced me of it. That stuff just makes your hands feel cleaner, somehow. Unlike the goofy, gloppy and unwisely antibacterialized gels in vogue today, something about Borax makes you feel like there's just enough abrasive action at a microscopic level to get everything clean.

    Wasn't that the premise of Lava soap, BTW (expertly immortalized by Pulp Fiction)? One of the maintenance engineers at my work told me that it was made from pumice.