Spectroscopy and Synthesis Quizzes

Hey All!

If the site has had a consistent weak spot over the past few years, it’s been in the realms of spectroscopy and synthesis.

I’m happy to announce that this hole has at least been partially filled with a new selection of quizzes on these topics.

Spectroscopy Quizzes

(link)

The section on Spectroscopy starts with some exercises on Index of Hydrogen Deficiency (IHD), and then moves on to UV-Vis, IR, Mass Spectrometry and finally to ¹³C and ¹H NMR.

When teaching this topic, I’ve always been of the mind that students should start by calculating IHD first. With a bit of practice, it’s a simple calculation that sets the stage for the remaining topics. There is a lot of logic in structure determination and a quick IHD calculation helps you quickly rule out certain possibilities.

Next comes UV-Vis. The focus here is just on the key lesson that increasing conjugation results in a narrowing of the energy gap ΔE  between the highest-energy occupied molecular orbital (HOMO) and the lowest-energy unoccupied molecular orbital (LUMO). Since E = hν and λ = c / ν , this means the wavelength of light absorption increases as ΔE decreases. The goal of the quizzes is to learn to quantify conjugation lengths and correlate these with λ_max.

The section on infrared spectroscopy focuses on training students to recognize the main diagnostic bands in an IR spectrum – the OH stretch around 3200-3400 cm^-1 and the C=O stretch around 1700 cm^-1. Students who spend the first 15-30 seconds of IR analysis on looking at these two regions will be well-primed to then move on to looking at areas of secondary importance, such as the C-H stretch around 3000 cm^-1  and the acetylene C-H stretch around 3400 cm^-1. There are also exercises on recognizing nitriles, amides, amines, and carboxylic acids.

The exercises on mass spectroscopy focus on identifying the presence of halides (Cl and Br) by looking for their characteristic M+2 peaks, and roughly approximating the number of carbons in a sample by analyzing the strength of the M+1 peak.

¹³C NMR spectroscopy begins with quizzes on predicting the number of signals each molecule will generate. The second section then tries to prime students to look for the presence of sp² – hybridized carbon in the region above 100 ppm, and sp³ – hybridized carbon below 80 ppm. Questions on identifying molecules given the ¹³C spectrum are all multiple choice.

Finally, the section on ¹H NMR spectroscopy begins with identifying the number of signals a molecule will produce and then asks students to predict multiplicity and chemical shift. The final section provides some multiple choice questions on identifying which molecule is represented by each spectrum.

The “click to flip” quiz format is good for training students on key patterns and in applying logic, but it has its limitations. It does not lend itself well to classic structure determination problems that provide multiple pieces of information (IR, ¹³C, ¹H). Students who want more practice with these sorts of problems should probably check out these problems from CU Boulder.

Synthesis

The Practice Quizzes section now has over 100 new problems on Synthesis, divided roughly into Org 1 topics (alkenes, alkynes, nucleophilic substitution) and Org 2 topics (everything else up to amino acids).

These have a “roadmap” format where students must fill in the blanks to get to the desired product. To make things slightly easier, condensed formulae are provided.

Of course there can be multiple ways to solve each particular synthesis problem and I’ve tried to note situations where multiple alternatives exist.

I hope the instructor community and MOC Members will find these new resources useful.

Please let me know via this form if you spot any typos or items that need correction.

– James