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DBL%20Hendrix%20small.png College chemistry, 1983

Derek Lowe The 2002 Model

Dbl%20new%20portrait%20B%26W.png After 10 years of blogging. . .

Derek Lowe, an Arkansan by birth, got his BA from Hendrix College and his PhD in organic chemistry from Duke before spending time in Germany on a Humboldt Fellowship on his post-doc. He's worked for several major pharmaceutical companies since 1989 on drug discovery projects against schizophrenia, Alzheimer's, diabetes, osteoporosis and other diseases. To contact Derek email him directly: Twitter: Dereklowe

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May 10, 2011

A Complete Diversion: Purple Compounds

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Posted by Derek

I had to use some potassium permanganate a little while back - first time in years I'd had any of it out in the lab, and I was reminded of just what a spectacular purple color the stuff has.
There's some of it dissolving in water, via Flickr, and it's hard to beat for sheer purplelosity. But the solid doesn't look as impressive; it's quite dark (which is probably how it makes such an intense color on dissolution). So what's the best purple solid in the lab?

I have to promote my personal favorite, chromium (III) chloride (image courtesy of the Wikipedia entry).

That's a pretty good shot, but it really should be experienced in person. The stuff is metallic purple flakes, weirdly reflective - it looks like it should be the color of a custom racer's hood, rather than anything you'd actually order from a chemical supply house. Now all I have to do is find a use for it in the lab. . .

Comments (50) + TrackBacks (0) | Category: Life in the Drug Labs


1. Bunsen Honeydew on May 10, 2011 11:26 AM writes...

Purple is cool but I think my favourite is the one that mother nature struggles with most- blue. There are plenty of copper compounds but outside of nitroso compounds, there are virtually no organic blue compounds. I never cease to find this fascinating, given that purple, green, yellow, orange and red are fairly accessible.

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2. Rick on May 10, 2011 11:35 AM writes...

Talk about coincidences, I just did the potassium permanganate-glycerol redox reaction for my chemistry class today. Nothing except thermite tops it for sheer suspense, excitement and high school student delight!

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3. AndrewD on May 10, 2011 11:41 AM writes...

You obviously do not work in Dyestuffs, an old list I have goes to C.I.Disperse Blue 390, and this doesn't include the solvent, acid,base, direct and smoke blues (actually there is some overlap amoung these dye types.)

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4. opsomath on May 10, 2011 11:44 AM writes...

2,2'-bithiophene crystallizes from hexanes to yield lovely flat colorless crystals, but slowly oxidizes in air to become contaminated with a lovely blue compound, and I was never sure what the nature of this compound was. It is polar enough that elution over silica with hexanes traps all the blue stuff at Rf = 0 (I would clean up big batches by filling a Buchner with silica and pouring a solution of dirty TT in hexanes through it) It is also nonpolar enough to _remove_ by recrystallizing the aged material from hexanes, though, so there's that. Perhaps it's the 5,5' bis-oxo product, the diketone? Anyone have a suggestion?

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5. ag on May 10, 2011 11:46 AM writes...

I remember the pleasant surprise of treating an alpha-chloro oxime with base and seeing the cool blue color of the unsaturated nitroso compound. Too bad the chemistry didn't work out :-/

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6. Dave on May 10, 2011 11:51 AM writes...

I remember once using 2-phenyl benzaldehyde and it being a lovely blue-violet color. Nothing inorganic there, but there was some suspicion in the lab that the color was due to an impurity. Anyone else have experience with this stuff?

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7. Hap on May 10, 2011 11:55 AM writes...

Azulene, obviously, is a cool blue (the TAs in my second organic class worked for a professor who made azulenes and they made some for class), and when I started at my job, I read about people who made azulenium sandwiches (cyclophanes, not metal complexes) that were green, purple, blue, etc. I would have liked to see those.

I also tried to make addition compounds from a dione with an amidine and got rainbow colored foams as products, which, unfortunately, weren't stable (and which I wasn't competent enough or able enough to purify).

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8. barry on May 10, 2011 11:59 AM writes...

for blue in the lab, nothing beats a Birch reduction. That nearly magical system of solvated electrons in anhydrous ammonia still turns heads

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9. DannoH on May 10, 2011 12:26 PM writes...

Keeping it simple...elemental Iodine. Has a nice deep purple crytal with an m-Endo type metallic sheen.

Yes, even us lowly microbiologists frequent this board!=D

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10. Hap on May 10, 2011 12:32 PM writes...

The vapor pressure sucks, though - the purple vapor is pretty if you don't breathe it, but then all the color is gone.

I've never seen a Birch, and the color has to be pretty, but the quench probably sucks, too.

4: If you Google "bithiophene air oxidation", you get a 1978 JACS that might talk about it (and there's some other papers). Maybe you're getting oligothiophenes?

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11. old man on May 10, 2011 12:40 PM writes...

The wispy blue tails of benzophenone ketyl streaming off the back of planetary balls of molten pottasium as a new THF still comes up is a sight I will always remember fondly.

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12. lost academic on May 10, 2011 12:44 PM writes...

We're using fluorescent markers for ROS in our lab a lot right now, and so my color picks are my compound of 'choice', DCFH (2',7' dichlorofluorescein) that oxidizes to a nice spring green (now if only it would oxidize when I told it to and not, you know, whenever it felt like it). We're also exploring Amplex Red, which is coming out a nice neon pink. We've noted that our waste containers look like the liquids they use in stock photos for labs.

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13. OldLabRat on May 10, 2011 12:46 PM writes...

A few decades ago I made tetraphenylfuran. The crystals are almost black and the CHCl3 solution was an amazing deep purple.

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14. GladToMoveToProcess on May 10, 2011 12:56 PM writes...

#13: Never made that, but tetraphenylcyclopentadienone is another pretty one.

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15. Baltic on May 10, 2011 1:07 PM writes...

1,3-Indandione and bindone show off pretty purple and blue colors sometimes - after a while you grow tired of the fact that everything that comes in contact with them gets at least some purple stains that don't come off too easily, though.

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16. MTK on May 10, 2011 1:08 PM writes...

It's actually amazing how much color has played a role in chemistry. I'm sure that many of us, including myself, first got interested in chemistry due to the colors. Synthetic chemistry as a real discipline basically started with the dye industry and the manufacture of artificial dyes. And of course, the German dye industry led directly to the first pharmaceutical companies.

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17. canchem on May 10, 2011 1:27 PM writes...

Tetraphenylcyclopentadienone. There's a conjugated system for ya. Brilliant purple plates, just oodles of product, dead easy synthesis. It was the workhorse reaction for 3rd year org chem when I TA'd.

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18. canchem on May 10, 2011 1:28 PM writes...

@14 - just saw you made the stuff too...

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19. milkshake on May 10, 2011 1:33 PM writes...

Possible use: You can reduce CrCl3 in situ to CrCl2 for Nozaki-Hiyama-Kishi coupling. It is probably cheaper that way when done on large scale.

By the way KCr(SO4)2 hydrate is also beautifully purple and isomorphic with KAl(SO4)2 hydrate, known for its chunky rhombs. I was growing layered clear-and purple crystals and also purple crystals of varying hues as a kid, by adding the Cr salt to the mix. It co-crystallized beautifully.

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20. Anonymous on May 10, 2011 1:47 PM writes...

"...first time in years I'd had any of it out in the lab...."

TLC staining apart, right?
Or maybe LC and HPLC have obviated TLC in medchem labs nowadays....

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21. psi*psi on May 10, 2011 1:49 PM writes...

Bunsen: PENTACENES. Diketopyrrolopyrrole thin films! If you insist that it has to be natural, anthocyanins in the right pH range.

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22. opsomath on May 10, 2011 1:58 PM writes...

Turns out the bithiophene thing is 5-hydroxy-2,2'-bithiophene oxidatively dimerizing into an analogue of indigo - which brings up the one we all obviously missed, indigo! It is the blue jean color and produced by oxidative dimerization of indole.

Thanks for the pointer on the paper, Hap.

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23. Josh on May 10, 2011 2:21 PM writes...

Methylene Blue never ceases to amaze me. Something like one molecule of the stuff pretty much turns a small lake blue.

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24. Kiwi on May 10, 2011 2:34 PM writes...

And if we're looking at purples there is always my childhood favorite nitrogen triiodide. Beautiful metallic purple color as a solid, and a puff (or two!) of purple vapor when detonated.

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25. microbiologist on May 10, 2011 2:50 PM writes...

Crystal violet, methylene blue, trypan blue, toluidine blue, thiazolyl blue, iodophenol...Every lab I've worked in has a box of these dyes, often dating back several decades. Some are purple, others are really blue.

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26. DannoH on May 10, 2011 2:50 PM writes...

Per one of the previous e-book references made by Derek:

John D. Clarke's "Ignition", Page 78, references nickel acetylacetonate as "...a pity too, because the nickel complex gave the fuel a particularly beautiful purple color, and somehow I'd always wanted a purple propellant..."

Always fun to read about the joy to be had when a fuel detonates every third attempt to use it.

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27. Bruce Hamilton on May 10, 2011 3:12 PM writes...

Not a strong blue, but liquid oxygen always generates a strong feeling of respect and awe.

My vote is another one for anthocyanins. They produce the autumn colours of leaves.

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28. milkshake on May 10, 2011 3:32 PM writes...

many nitroso compounds are deep blue. When you chlorinate ketone oximes you get alpha chloronitroso compounds R2C(NO)Cl which are particularly gorgeous. A friend of mine was making these compounds - once walked away from a large-scale distillation of such stuff made from acetone oxime. He did it in the right moment too - when his ears stopped ringing it was no longer blue, there was just a big hole blown up in the hood.

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29. jack on May 10, 2011 4:30 PM writes...

hydroxocobalamin (vitamin B12) is the best red color followed by the colchicine color red.

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30. Anonymous on May 10, 2011 5:05 PM writes...

As for uses of CrCl3, if you mix it with a salen ligand, then oxidize it (IBX works), the resulting complex can catalyze hindered Diels-Alder reactions, though the ee is less than ideal. But then, it looses it's pretty color in the process.

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31. Mark on May 10, 2011 5:10 PM writes...

I always thought the most pretty color I've seen in chemistry is a copper-ammonia complex. If you get the concentrations right, you get an incredibly rich purple-blue color. Reminds me of the sky or ocean.


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32. Jordan on May 10, 2011 7:06 PM writes...

The material known as Pigment Violet 23 is a pretty dramatic purple colour and forms deeply coloured metallic-looking crystals.

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33. Anonymous BMS Researcher on May 10, 2011 8:14 PM writes...

When my father was in High School a fellow student got a little too enthusiastic with a thermite reaction. It burned through several floors of a concrete building and ended up embedded in the floor of the basement. Fortunately nobody was in its path.

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34. ChemistA on May 10, 2011 9:04 PM writes...

I just used a protected guanine that was lilac purple and turned hydrangea blue after bicarb workup. It was refreshing after all the whites, yellows and browns.

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35. leftscienceawhileago on May 10, 2011 10:48 PM writes...

Are the "purples" for a lot of these compounds we are talking about actually spectral violet or "purple" proper (a mix of red and blue being reflected to the eye)?

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36. Vinod on May 10, 2011 11:42 PM writes...

#11, I totally agree with you, though I have used sodium in my THF stills. The first wisps of blue wafting through the solution are simply delightful to watch.
The Birch reduction is also another delight to watch.
I also love acid / base titrations using phenolphthalein as the indicator - feels great watching the solution turn colorless to pale pink.

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37. lapsed chemist on May 11, 2011 1:18 AM writes...

If we are talking purple, let's not forget the original Grubbs catalyst. When properly made, it has a magnificent colour as a solid; commercial product always looked grotty in my day. No idea what the successor catalysts look like because my ring-closing metathesis days are 11 years behind me. Happy times...

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38. porphyrin on May 11, 2011 1:23 AM writes...

Porphyrins. Good ole tetraphenylporphyrin exists as lovely purple crystals, and in DCM solution, can be green or red. Even more fun is tuning the colour by metallating the porphyrin.
Beautiful stuff, until you want to run them on a column!

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39. IAmDave on May 11, 2011 1:47 AM writes...

Quinones are purple if electron rich enough - start fusing on hetrocycles and you get some wonderful colors from that part of the spectrum. The simplest blue organic compound I'm aware of is thiobenzophenone, which is blue even when pure due to pi -> pi* transitions across its tiny HOMO -LUMO gap.

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40. Mayhem on May 11, 2011 5:33 AM writes...

My favourite was something I cooked up in a test tube that shot out in shiny metallic jade green flakes all over the one guy in class I never got along with. Never was able to work out what it was - we were doing flame tests and got bored and started mixing liquids to see what would happen in finest student tradition.

Beautiful colour though. The green contrasted nicely with the purple of his face :)

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41. Anonymous on May 11, 2011 6:08 AM writes...

I thought this was the most famous purple:

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42. IrII on May 11, 2011 9:37 AM writes...


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43. processchemist on May 11, 2011 10:02 AM writes...

The purplest of the purples in my experience comes from antocyanidins: cyanidine and pelargonidine chlorides are a nightmare when you have to wash glassware, if you're not experienced. And nature use plenty of these compounds in all the red/blue/purple flowers and fruits....

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44. barney on May 11, 2011 11:14 AM writes...

my favorite thing about this blog? that a post on awesome purple stuff gets more comments than the industry meltdown, etc, etc. i heart u chemists...

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45. Anonymous on May 11, 2011 4:02 PM writes...

I've got some bottles of 2,6-dialkyl anilines which are lovely shades of lilac. Probably need to be redistilled...

When I TA'd o-chem lab, the students performed an extraction of trimyristin from nutmeg and saponified it to myristic acid. Without fail, every semester, one student's saponification would turn purple. Lavender purple. It wasn't nail polish, they had been using the same glassware the entire semester, and all the reagents came from common stocks. Damnedest thing.

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46. Simon on May 13, 2011 9:55 AM writes...

[Ni(ethylenediamine)3]2+ is a very nice purple. A spectacular violet is obtained when you oxidise Nyholm's 'diars' complex [Ni(diars)2]2+ with aqua regia - it gives a Ni(IV) complex, [NiCl2(diars)2]2+. If I remember correctly, the corresponding bromo-complex is a dark velvet green.

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47. Myma on May 15, 2011 7:56 AM writes...

Plain old NBS recrystallizing, watching the yellows and oranges swirl around, and the crystals gently floating up and down on bubbles of gas.

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48. Ben on May 17, 2011 1:01 PM writes...

We made ReO3 crystals in the teaching labs as an undergrads that were a beautiful metallic purple-red colour before we ground them up for a powder x-ray sample.

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49. Torquill on June 5, 2011 4:10 PM writes...

I have a great fondness for crystal violet, which was mentioned in passing; I'm not a chemist (plant pathologists, represent!) but the final for my last semester in ochem was to synthesize crystal violet, a rather fickle compound for undergrads. I gave it my best shot and got a sort of muddy brown liquid, sighed, and chucked it in my drawer. When cleaning the drawer out ten days later on the last day of class, I discovered that the equilibrium had settled and it had turned a pure deep amethyst color. I re-analyzed the spectrum, submitted a revised report, and found I had a better product than our reference compound. The gorgeous color was icing on the cake. :)

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50. anonymous on April 8, 2012 12:12 AM writes...

Bistrifluoromethyl nitroxide is a rather exotic free radical that is isolable as a purple gas.

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