Spectroscopy Worksheet (Page 2 of 4) in pdf

Spectroscopy Worksheet Page 2

Model 2: UV-Visible Spectroscopy

In Worksheet 3, you saw how a chain of double bonds, each separated by exactly one single bond could be

drawn in different resonance forms. This pattern of double-single-double bonds is called conjugation.

Benzene is the classic example of a conjugated molecule.

In CHEM1101, you learned about molecular orbitals. In conjugated molecules, the energy needed to excite

an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular

orbital (LUMO) is in the ultraviolet or even in the visible region of the spectrum.

For conjugated systems, this LUMO-HOMO energy difference falls in the UV-Vis range.

Critical thinking questions

Which of the molecules below are conjugated?

Circle the molecules above which would show a peak in the UV-Vis spectrum?

How useful do you think a UV-Vis spectrum is in identifying organic molecules?

Model 3:

H NMR Spectroscopy - Chemical Shifts

NMR spectroscopy (most often

C and

H, other nuclei as well) is the most powerful tool for identifying

organic molecules. This is due to its sensitivity. Each unique carbon or hydrogen will give rise to a specific

signal (peak), with a characteristic chemical shift (position) in the NMR spectrum

Nuclei give rise to identical signals only when they are in the same chemical environment. This occurs

when:

(i)

they are attached to exactly the same atoms (e.g. the 4 hydrogen atoms of CH

are equivalent and

give rise to only one peak)

(ii)

are related by symmetry (e.g. the two CH

groups in propane are equivalent).

Critical thinking questions

Indicate each H environment in the molecules below. (Hint: you should first draw them on!)

How many signals would you expect in the

H spectra for these compounds?

For a more in depth analysis of the material in Models 3 – 4, see the online resources on eLearning.

Spectroscopy Worksheet Page 2