Question

MCAT Mini-Tutorial: UV/Vis Spectroscopy

Answer 1

\({\bf{Basic~Mechanism:}}\) when compounds are irradiated with UV/Vis of a high enough energy, will induce pi to pi* transitions. This is useful for compounds with double/triple bonds.

Recall that an unsaturated does not have the maximum number of hydrogen atoms, thus having a double bond, triple bond, or some other functional group that prevents maximum hydrogen saturation.

degree of unsaturation = (2C+2+N-[H+halogens])/2

C is carbon, N is nitrogen, H is hydrogen, X represents halogens

this gives the collective number of double bonds and rings (double bonds count towards one degree of saturation and triple bonds count towards 2). To distinguish between these three types you will need to be given some information about the compound, such as spectroscopy, or try drawing out structures to see which combinations are feasible given the atoms involved

Answer 2

\({\bf{How~To~Interpret~a~UV/Vis~Spectrum:}}\)



notice how a range of wavelengths are on the x-axis, typically a compound would be irradiated with a low frequency light, increasing up to a higher frequency. A peak (or more) will be observed at the wavelength at which the electron pi to pi* transition(s) will occur. Most unsaturated compounds absorb in the 200nm to 400nm range.

For certain functional groups, an n to pi* transition may occur when an electron from a non-bonding orbital transitions to an antibonding orbital. The energy difference is less than a pi to pi* transition.

Answer 3

Anyway, that's the end of my tutorial, I hope it was a helpful resource. Source material is the 2nd Edition Barron's Prep book for the new MCAT