The Isomers of C₄H₁₀O : Fractional Distillation and NMR Spectroscopy

Distillation

Liquids are usually purified by distillation. Simple distillation (Figure 1) will allow you to remove non-volatile impurities (with very high boiling points) from a liquid. Simple distillation works well for separating sodium chloride (bp 1413 ^oC) from water (bp 100.0 ^oC). The sample is heated, allowing some of the molecules to escape from the liquid into the gas phase. Impurities with much higher boiling points will remain in the distillation pot. As the vaporized sample (minus impurities) is cooled in the condenser, it returns to the liquid phase. The purified liquid is collected in the round bottom flask at the bottom of the apparatus.

Figure 1. Simple Distillation Apparatus

If simple distillation was used to separate a 50/50 mixture of cyclohexane (bp 80.7 ^oC) and toluene (bp 110.6 ^o C), the first fraction of distillate would be mostly cyclohexane, but some toluene would be present. The material left in the pot would be mostly toluene. The distillate would have increasing amounts of toluene and decreasing amounts of cyclohexane as the distillation continued. If the distillate is separated into fractions and re-distilled, the early fractions (lower boiling points) will lead to purer cyclohexane, and the late fractions will lead to purer toluene.

In situations like this one, fractional distillation should be used. The apparatus for a fractional distillation (Figure 2) has a column between the pot and the still head. A series of separate distillations will take place in this fractionating column, allowing the substances to be better separated.

Figure 2. Fractional Distillation Apparatus

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 will use distillation to separate a mixture of diethyl ether and 1-butanol. Finally, you will use NMR to determine how well your distillation separated the two isomers.

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 ether and butanol.

The necessary fractional distillation apparatus should be assembled in your hood. (We will be using glass rings as column packing material.) Be sure you note the positions of the clamps, water hoses, and thermometer. Remove the top Keck clamp, separate the distilling head from the fractionating column and condensing column, separate the fractionating column from the 100-mL round bottom flask. Obtain ~40 mL (the exact quantity is not important) of the mixture in a graduated cylinder and pour it into the 100 mL round bottom flask. Add a boiling stone. Reassemble the distillation apparatus. Place an Erlenmeyer flask labeled #1 in an ice bath as the receiving flask. Turn the water on in the cooling hoses (it only has to come out as a steady trickle) and turn on the Variac. All of the Variacs are different, but a good number to start on is 30-40 (the Variac numbers control the amount of voltage, they do not correspond to temperature). The distillation should drip at a constant rate when the head temperature reaches ca. 35-40°C. Collect about 5 mL of that fraction in the first flask. Change receiving flasks (to #2) and collect the rest of the distillate that comes over at this low temperature°C. Be sure to put a cork in #1. If the distillation is going properly, the temperature will drop after the first fraction has been collected. When this drop occurs, change receiving flasks (now #3), turn up the Variac 10 units, and wait for the next fraction. If things seem to be going too slowly, you can wrap the fractionating column in aluminum foil or turn up the Variac a little more. The temperature of the next fraction should be ~115°C. Collect the first few mL in #3 and then switch to #4 when the temperature seems to have stabilized. Do NOT distill to dryness (leave something in the round bottom flask)! Be sure to label and cap the receiving flasks after using them, otherwise the distillate will evaporate. Prepare NMR samples of the two fractions you think are the most pure. After you have run the NMR's, pour all of the remaining distillates and whatever is left in the 100 mL round bottom flask into the container provided.

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 six compounds (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 isomers 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.