MCAT Organic Chemistry Review

Spectroscopy

11.2 Ultraviolet Spectroscopy

Although you will never have to interpret ultraviolet (UV) spectroscopy data on the MCAT, it is fair game for discussion. A basic understanding of how it works and when it is used will suffice. UV spectra are obtained by passing ultraviolet light through a sample that is usually dissolved in an inert, nonabsorbing solvent, and recording the absorbance. The absorbance is then plotted against wavelength. The absorbance is caused by electronic transitions between orbitals. The biggest piece of information we get from this technique is the wavelength of maximum absorbance, which tells us the extent of conjugation within conjugated systems: the more conjugated the compound, the lower the energy of the transition and the greater the wavelength of maximum absorbance.

ELECTRON TRANSITIONS

UV spectroscopy works because molecules with π-electrons or nonbonding electrons can be excited by ultraviolet light to higher-energy antibonding orbitals. Molecules with a lower energy gap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are more easily excited and can absorb longer wavelengths (lower frequencies) with lower energy.

MCAT EXPERTISE

UV spectroscopy is most useful for studying compounds containing double bonds and/or heteroatoms with lone pairs that create conjugated systems. For the MCAT, that is all you need to know.

CONJUGATED SYSTEMS

Conjugated molecules, or molecules with unhybridized p-orbitals, can also be excited by ultraviolet light. Conjugation shifts the absorption spectrum, resulting in higher maximum wavelengths (lower frequencies). For example, benzene has three broad absorbances, which mark the energy level transitions; these are found at 180, 200, and 255 nm wavelengths. Larger conjugated molecules may even absorb light in the visible range, leading to color. Because the technique for UV spectroscopy can also be used at visible wavelengths, it is sometimes called UV–Vis spectroscopy.

MCAT Concept Check 11.2:

Before you move on, assess your understanding of the material with these questions.

1.    What types of molecules can be detected by UV spectroscopy?

2.    In UV spectroscopy, what is the HOMO? What is the LUMO? How are they related to absorption wavelength?

·        HOMO:

·        LUMO:

·        Relation to absorption wavelength: