ultraviolet spectroscopy
Summary
Ultraviolet spectroscopy measures how strongly a sample absorbs UV light (about 200–400 nm) as electrons jump from π or nonbonding orbitals to π* orbitals. Most testable: the more extended the conjugation, the smaller the energy gap and the longer the λmax.
Detail
Meaning: UV spectroscopy measures how much a sample absorbs ultraviolet light, roughly 200 to 400 nm. A photon is absorbed when its energy, E = hc/λ, matches the gap between a filled orbital (π or nonbonding) and an empty π* orbital. More extended conjugation shrinks the gap, so λmax shifts to longer wavelength. Isolated double bonds absorb below about 200 nm and are usually not seen on a standard instrument.
Equation: A = εbc, where A is absorbance (unitless), ε is molar absorptivity (m²/mol; M⁻¹cm⁻¹ in practice), b is path length (m; cm in practice), and c is concentration (mol/m³; mol/L in practice).
On the MCAT: You may be asked to rank compounds by λmax: more conjugation gives a longer λmax. If conjugation is extensive enough to absorb visible light, the compound is colored, and the color you see is complementary to the color absorbed. Beer-Lambert questions use A to find concentration from a calibration curve or to follow a reaction.
Common confusion: UV reports mainly on conjugation, not on the full set of functional groups the way IR does. Also, longer wavelength means lower energy, so a larger conjugated system absorbs lower-energy light, not higher-energy light.
Sources
- Organic Chemistry (Clayden, Greeves, Warren)
- Organic Chemistry (Klein)
- Organic Chemistry (Wade)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.