Absorption of Light by Organic Molecules

The electromagnetic spectrum is a vast continuum stretching from the very short wave cosmic, gamma, and x-rays through the ultraviolet (uv), visible, and infrared to the long wavelength radio waves (Fig. 4-29). When electromagnetic radiation falls on a molecule, whether organic or inorganic, it may or may not be absorbed, depending upon the wavelength of the radiation and the structure of the molecule. If the molecule absorbs light in the visible region, it will appear to be colored; if not, the molecule will be colorless if it transmits the light (like water) or white if it reflects the light. Even colorless compounds will absorb ultraviolet light; for example, clear suntan oil absorbs some uv from the sun and so protects the skin. In this section we will discuss the absorption of visible and ultraviolet light by organic compounds, and show how this absorption may be used to identify and characterize the compounds.

Figure 4-29. Visible light comprises only a tiny fraction of the spectrum of electromagnetic radiation.

When ultraviolet or visible light is absorbed by an atom or molecule, an electron is excited-in other words, it is raised to an orbital of higher energy than the one it usually occupies. In an atom, for example, an electron might move from an s to a p orbital upon absorption of light; in a molecule an electron moves from one molecular orbital to another, more energetic one. This excitation is possible because light in the visible and ultraviolet regions of the spectrum contains a great deal of energy-of the order of 50 to 200 kcal/mole, or even more at very short wavelengths. Since all matter contains electrons, there will be some portions of the visible-ultraviolet spectrum in which light absorption will occur for any molecule.

For practical reasons the ultraviolet-visible region of the spectrum is divided into three parts. Light whose wavelength is shorter than 200 nm (2000 Å) is referred to as the vacuum ultraviolet; here air and most solvents absorb, and measurements must be carried out, with great difficulty, in a vacuum. Every organic molecule absorbs light very strongly in the vacuum ultraviolet part of the spectrum. The region from 200 nm to 360 nm is called the near ultraviolet (or simply the ultraviolet), and from 360 to about 700 nm the visible. Light in these regions has less energy and is only able to excite relatively loosely bound electrons, such as those in pi bonds. Note the absorption spectrum of benzene, phenanthrene, and naphthacene in Fig. 4-30. Colorless benzene does not absorb in the visible part of the spectrum, while the yellow naphthacene absorbs in the violet and blue part of the visible spectrum. The light we see when we look at a sample of naphthacene is white light from which the violet and blue portions have been removed by absorption; that is, yellow light is reflected. It is important to remember this complementary nature of color: The color observed on reflection is always white light from which the absorbed color has been removed. An object that appears green actually absorbs purple light, for example, while a compound that absorbs green light will appear red. The relationship between absorbed and observed color is shown in Fig. 4-29.

As a general rule, molecules containing conjugated systems of pi electrons absorb light closer to the visible region than saturated molecules or those with isolated double or triple bonds. The longer the conjugated system, the longer the wavelength of the light absorbed.

Ethylene absorbs light of wavelength in the far ultraviolet part of the spectrum, at 180 nm. Butadiene, with two conjugated double bonds, absorbs at 217 nm, a wavelength closer to the visible zone than that absorbed by ethylene. 1,3,5-Hexatriene absorbs still closer to the visible region, at 258 nm. All three compounds are colorless; however, as the number of conjugated double bonds increases, the position of absorption falls nearer the visible region, and with enough conjugation, the molecules are colored. The pigment betacarotene (Fig. 4-31) contains 11 conjugated double bonds and is the dye mainly responsible for the color of carrots. It absorbs blue light strongly, so that carrots appear orange.

Figure 4-30. The absorption spectrum of benzene, phenanthrene, and naphthacene in the ultraviolet and visible regions of the spectrum.

Figure 5-31. Compounds containing extended conjugated systems of pi electrons absorb visible light and so are colored.

Not only the wavelength of light absorbed but the absorption intensity is characteristic of a given molecule. Note that both benzene and naphthacene absorb light in the near ultraviolet but that the latter does so much more intensely. A solution of naphthacene will absorb almost 100 times as much light at 250 nm. as a solution of benzene of the same molar concentration. The extinction coefficient E is defined as the logarithm of the ratio of the light intensity (I0) that enters a 1 M solution of the compound in a cell 1 cm long to the intensity of the light that emerges after absorption (I). Water, alcohol, and hexane, which do not absorb in the near uv or visible, are typical solvents. The value of E can be very large or very small, so it is often plotted as log e to get several spectra or different parts of one complex spectrum on a small graph (as in Fig. 5-30).

The absorbance (log (I0/I)), concentration (c, in moles per liter) and length of cell (1, in cm) are related by Beer's Law: log (I0/I)=Ecl.

Ultraviolet-visible light may sometimes be used to identify a molecule by using its absorption spectrum as a "fingerprint" and comparing it with the spectrum of an authentic sample of the material. Even if an authentic spectrum is not available for comparison, clues to the structure of an unknown compound can often be gained from its uv spectrum. A hydrocarbon that does not absorb in the uv, for example, is either saturated or contains only nonconjugated multiple bonds.

Sample cells used in measuring visible and UV spectrum

Figure 4-32. Typical apparatus for measuring the uv-visible absorption spectrum. The prism or grating is used to select a particular wavelength; the absorption coefficient (E) at that wavelength is given by log (I0/I) for a 1 M solution 1 cm long.

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