3/30/2007

I Hate O-Chem Terminology Sometimes

Since it was brought up in an earlier post that I mixed up the terms "stereoselective" and "stereospecific," I busted out my phys org notes to relearn the terms, so I don't get pwn3d by professors on it later. I'll post it here for everyone else's benefit too.

Stereoselectivity: A reaction is stereoselective when one stereoisomer is formed in preference to another.
Lindlar hydrogenation of alkynes is stereoselective. The cis alkene is generated preferential to the trans isomer.


Stereospecificity: A reaction is stereospecific when one stereoisomer of a reactant gives a stereoisomer of a product, while another stereoisomer of that reactant gives a stereoisomer of that product.
Bromination of alkenes is stereospecific. trans-2-butene gives a meso product, while cis-2-butene gives a chiral product plus its enantiomer. The meso product and the chiral product (and its enantiomer) are diastereomers.

Stereospecific reactions are necessarily stereoselective. However, not all stereoselective reactions are stereospecific. In the above example, Lindlar hydrogenation of alkynes is not stereospecific because, quite simply, there are no stereoisomers of alkynes. Bromination of alkenes are stereoselective as well as stereospecific. However, because a reaction is stereospecific does NOT mean it is 100% stereoselective.

I think a good rule of thumb is if you want to use the term "stereospecific," don't, unless you think REALLY hard about it beforehand. And if you do, you better be talking about two different reactions where the reactants are stereoisomers and the products are stereoisomers.

There are nearly countless numbers of terms that refer to some aspect of stereochemistry, whether referring to orientation or symmetry of a molecule (woo group theory), comparing one molecule to another molecule, distinguishing particular sites in a molecule, or distinguishing its reactivity. The current terminology is extremely precise in regards to these subjects; one need only pick up a copy of Eliel's 1,200 page bible of stereochemistry to see what I mean. Nevertheless, people understandably (owing to the massive number of terms and their occasional close proximity in meaning) mangle up a lot of the terms. Some people (especially professors) are very picky about the terminology, so be careful when asking them why a particular reaction is stereospecific if it, in fact, is not; not only will you have to endure a long explanation of the CORRECT terminology, but you might even forget you had a different question in the first place. The terminology is in place to be precise about meaning, even though you can usually get away with explaining yourself correctly using mangled terminology, assuming the person you're communicating with is either sympathetic to the mangled terminology or mangles the terminology in similar ways. Meaning is subjective like that, try as we scientists do to avoid it. It's a fun conundrum: we create terminology to avoid confusion, then confuse the terms. Organic chemistry is a lot like that.

6 comments:

Ψ*Ψ said...

Yet another reason to love nice, flat aromatic chemistry. Who needs that pesky third dimension, anyway?

milkshake said...

The advantage of working with polyaromatic compounds is that no matter how badly you mess up, the atoms will at least stay within the same plane...

Ψ*Ψ said...

...Unless, of course, you are working on twistacenes.

Liquidcarbon said...

Heard about helical theory of chirality?

Wang, D.Z. Tetrahedron, 2005, 61, 7125; 7134

Matt Jenks said...

Excellent post, Excimer.

In grad school, at our group meetings, we would go through this once about every six months. And Rich would always tell us "just don't use stereospecific" as a general rule of thumb.

As chemists, we're just following the general pattern of usage of the English language. We have terms and then misuse them to the point where the original meaning is lost and we just go with what is convenient for the vernacular. Eventually, if we keep bastardizing stereospecific and stereoselective, they'll mean the same thing!

Ψ*Ψ said...

It seems that someone out there is actually learning useful things from this blog. We just got a google for "stereoselective stereospecific term."