As Pure As the Driven Snow (A Random Musing on the Color of a Chemical Compound)

Yesterday I ran a reaction that behaved beautifully. It's a reaction where I've optimized the conditions to make the process nicely efficient, and now that that groundwork has been laid, all that I have to do is plug various electrophiles and nucleophiles in and it tends to work without a hitch.

I conduct the reaction under microwave irradiation, and the process is similar to how you cook or heat food at home. The microwaves excite molecules that are polar in nature (have a significant dielectric constant), causing them to gain kinetic energy and excite neighboring molecules. This manifests itself as heating your food, and in my case, heating my reaction mixture. Just as it takes a fraction of the time to cook/heat food by microwave compared to, say convectional heating, the same is true for chemical reactions in many cases.

That little bit aside, I could tell that my reaction had worked well by a quick LC-MS of the crude reaction mixture. It was clean enough that all that was necessary was to add the proper solvent (in this case I found acetone to work), which preferentially dissolved the impurities and not the product, and then filter the solid.

What I got was a satisfyingly white powder. Organic compounds typically tend to be some shade of white, so this isn't an uncommon occurrence. And sure, color isn't the best indicator of compound purity. But on some levels it might be the most satisfying (especially when you realize you don't have to run column chromatography).

A little chemistry tangent: inorganic molecules often take on more vibrant colors due to the ease with which electrons within those molecules can be excited or promoted to a higher energy level by light. Light (or a photon) is absorbed by the molecule to excite the electron, and the energy of the photon must match the energy the electron needs to reach that excited state. As you might know, the color of light is determined by its wavelength and is correlated with its energy. Thus, a molecule that absorbs red photons (λ=700 nm, lower energy light) will appear green, and a molecule that absorbs violet photons (λ=400 nm, higher energy light) will appear yellow. It's no surprise that if you look at a color wheel, you'll notice a trend. Organic molecules tend to be white because they don't absorb light in the visible region. The lowest unoccupied molecular orbitals (LUMOs) for organic molecules tend to be of higher energy and require ultraviolet light for promotion of electrons--thus their white color (all colors of the rainbow reflected).

Now, of course, afterwards I use the requisite analytical techniques to truly verify purity; namely, liquid chromatography-mass spectrometry (LC-MS) and proton nuclear magnetic resonance (1H-NMR). But when one sees the off-white heterogeneous crude reaction mixture become a nice, white powder in the filter funnel, one honestly has a good idea what they're going to see when the full and proper analytical workup is given.