The Isomers of C₄H₁₀O: NMR Spectroscopy

The structures of the six possible molecules with molecular formula C₄H₁₀O are shown below. They are constitutional isomers: they have the same molecular formula, but have different atom connectivity. This experiment is an introduction to two important techniques: Fractional Distillation and NMR Spectroscopy. You will use NMR spectroscopy to structurally characterize the some of the six isomers with molecular formula C₄H₁₀O. You are using distillation to separate a mixture of two compounds. Later, you will use NMR to determine how well your distillation separated the mixture.

General Instructions:

• We will have a brief lecture on NMR spectroscopy before any experimental work.
• Work in pairs for this experiment. Take separate laboratory notes and turn in separate lab reports.
• The NMR uses a powerful magnet: leave credit cards and large metal objects (keys) outside the room.
• Do not dispose of any chemicals down the sink: we will be recycling the cyclohexane and toluene.

B. ¹³C NMR Spectroscopy of the C₄H₁₀O Isomers

While one partner monitors the distillation, the other will join the professor in the NMR room. We will discuss how samples are prepared for NMR spectroscopy, the general features of the instrument, and the basics of the software. We will then run the ¹³C NMR on some of the five molecules (time permitting) above. Copies of the spectra will be available online. After this, partners will switch and the process will be repeated.

Once the fractional distillations are complete, you will prepare samples in CDCl₃ (deuterated chloroform) of the two fractions you think are the most pure and run the ¹³C NMR on them. You will be provided with a copy of the spectrum of the original mixture for comparison. After your samples have been run, discard the chloroform solution in the container provided.

You should discuss how well (based on the temperature and NMR evidence) your fractional distillation separated the two compounds and what steps you could take to improve the distillation in the future. In addition, you need to analyze each NMR spectrum and discuss how it relates to the structure(s) of the molecule(s).

As an exercise, draw out the structures of the isomers of C₅H₁₂ and state how many signals that each isomer would give in a ¹³C NMR spectrum.