# Color of methoxy substituted phenyl rings when heated in the presence of acid

Anyone who's familiar with organic synthesis will probably have came across the DMB and PMB protecting groups.

When TLC-ing compounds containing these protecting groups using common stains like vanillin or cerium ammonium molybdate (CAM, the Hanessian stain), its not uncommon for the compounds to appear bright pink/purple. This is a useful phenomenon, but one which I have no explanation for.

I'd assumed that this was related to the stain itself, however even when the TLC plate is simply heated in methanolic hydrochloric acid, the pink color still appears (quite vividly in fact).

I realise that all of the stains mentioned are made up with some acid, however, heating PMB/DMB containing compounds under acidic conditions without the TLC plate (in a flask, for example), doesn't afford this colour.

Any insight into what's forming the color? Specifically when PMB/DMB compounds show it, but simple benzyl ethers etc do not.

• many phenol derivitive are relatively easily oxidized into bright colored products. Jun 28 '16 at 22:50
• Well, anisaldehyde is technically PMB-carbonyl and colours the TLC pink … And DMB is des-methyl-vanillin, isn’t it?
– Jan
Jun 29 '16 at 13:16
• @Jan: Not necessarily. PMB is p-methyoxybenzyl whil DMB is dimethoxybenzyl. Both are used mostly in hydroxy protection. Jul 13 at 22:42

TLC Visualization by the color formation when used p-Anisaldehyde or vanillin in sulfuric acid as the developing agent is known in Natural Products Chemistry for long time. OP's suggestion of color development is due to inclusion of p-methoxybenzyl group (PMB) or dimethoxybenzyl group (DMB) not correct. It seems like these staining compounds make colored derivatives with certain compounds with or without PMB and\or DMB protecting reagents. For instance, when p-Anisaldehyde visualizing agent (p-Anisaldehyde/ conc. $$\ce{H2SO4}$$), which is a universal derivatization agent that can be used to detect a wide range of nucleophilic compounds (Ref.1), is used in series of plant extracts (tea samples) containing the polyphenol natural products, it gives bright orange spot on the TLC plate with asplathin, a flavonoid (Ref.2):

As you have seen in the structure, there was no PMB or DMB group to give this specific orange color. The basis for this kind of detection is the derivatization of the solute molecules with an organic compound containing one or more conjugated electron system such as that in p-anisaldehyde. This kind of electron system can be absorb either visible or ultraviolet part of the spectrum to give corresponding color (Ref.3).

Some other visual color developments with p-Anisaldehyde/ conc. $$\ce{H2SO4}$$ detecting system are shown in following images (Ref.4):

Note that although some compounds have given multicolor with the system none of them contained either PMB or DMB protecting groups. As a matter of fact all of them are natural products. As noted by the authors of Ref.4, the color of monoterpenes was different than that for triterpenes, and color for steroids are different than either of them. For instance, after revelation and heating, monoterpenes, triterpenes, and steroids appeared as blue, purple, and gray spots, respectively. None of these colors can be interpreted by simple theory.

Also keep in mind that the color development by p-Anisaldehyde/ conc. $$\ce{H2SO4}$$ is not limited to natural products. The synthetic compounds also gived color compounds when derivatized by p-Anisaldehyde/ conc. $$\ce{H2SO4}$$ on TLCs.

References:

1. Hellmut Jork, Werner Funk, Walter Fischer, and Hans Wimmer, In Thin-Layer Chromatography - Reagents and Detection Methods, Volume 1a: Physical and Chemical Detection Methods, Fundamentals, Reagents I; Translated by Frank Hampson, Wiley-VCH Verlag GmbH: Weinheim, Germany, 1990 (ISBN13: 9783527278343).
2. Emily Amor Stander, Wesley Williams, Fanie Rautenbach, Marilize Le Roes-Hill, Yamkela Mgwatyu, Jeanine Marnewick, and Uljana Hesse, “Visualization of Aspalathin in Rooibos (Aspalathus linearis) Plant and Herbal Tea Extracts Using Thin-Layer Chromatography,” Molecules 2019, 24(5), 938 (11 pages) (DOI: https://doi.org/10.3390/molecules24050938).
3. Bernard Fried and Joseph Sherma, In Practical Thin-Layer Chromatography: A Multidisciplinary Approach; CRC Press: Boca Raton, FL, 1996 (ISBN: 0-8493-2660-5; ISBN 13: 9781351422420).
4. Alice da Cruz Lima Gerlach, Alice Gadea, Rosa Mara Borges da Silveira, Philippe Clerc, Françoise Lohézic-le Dévéhat, “The Use of Anisaldehyde Sulfuric Acid as an Alternative Spray Reagent in TLC Analysis Reveals Three Classes of Compounds in the Genus Usnea Adans. (Parmeliaceae, lichenized Ascomycota),” Preprints 2018, 2018020151 (DOI: 10.20944/preprints201802.0151.v1).
• Nice answer, but you may have misinterpreted my question somewhat. I’m aware that reagents give colours with many compounds, including those not containing the PMB/DMB group. My query was specifically relating to the colour when these are present (eg using CAM can very clearly tell a PMB protected alcohol from the parent merely by colour in the vast majority of cases) Jul 19 at 21:18
• @NotEvans: As I mentioned in the answer, the reason for the color is unexplainable, according to most of authors (al most all of them are natural product chemists). I think it is very complicated and nobody wanted to investigate. Kind of split-light phenomena. :-) Jul 19 at 21:26