Question

Look up the structures of the four standards which are caffeine, ibuprofen, acetaminophen, and aspirin. Which of these compounds would you expect to be visible under the UV lamp? Explain why the compounds are visible.

Answer 1

UV - visualization technique:

It is a non-destructive visualization method for identifying organic compounds in Thin Layer Chromatography (TLC) plates. The compounds that can be viewed under Ultra violet light are those that are UV-active i.e., capable of absorbing near UV light in the range of 250-360 nm wavelength. To know the criteria for a compound to be UV-active, its structure is important. It should be a rich aromatic system or a system that holds greater degree of conjugation (alternate single and double bonds).

A typical TLC plate is coated with silica gel or alumina along with a fluorescent material that will shine green under short-waved UV light(~250 nm). Both short-waved and long-waved (~360 nm) UV light can be shined on the TLC plate. If any UV-active compound is present on the plate, it would absorb all the UV radiation focussed on the spot and would appear as a dark spot in the green base.

The structures of the given organic compounds are shown below with the aromatic ring and extended conjugation highlighted in red.

All the four compounds possess aromatic ring that holds 3 conjugated double bonds. In addition to that, they also have extended conjugation in the form of carbonyl oxygen (2 lone pairs of electrons) and nitrogen (a lone pair of electron). The lone pairs can participate in the double bond shift by extending the conjugation. Thus, all are UV-active compounds.

Compounds	UV - absorbance (nm)
Caffeine	273
Ibuprofen	264, 272
Acetaminophen	243
Aspirin	237

When exposed to UV light, these compounds will absorb the short wavelength UV-light and reflect back the visible light, so that they appear as dark spots on the TLC plate.

Ultra violet light - shorter wavelength / higher energy

Visible light - longer wavelength / lower energy

Greater conjugation needs greater energy for the molecules to undergo excitation ( - ^* electronic transition) and then on returning back to ground state, they emit light of lower energy. This is because some energy is utilized for excitation. Therefore, greater energy (UV light) is absorbed and lower energy (visible light) is emitted. So, all these molecules are visible under UV lamp.