Most reactions in organic chemistry are done in solution. After the reaction is finished however, a chemist must remove the solvent in order to isolate the product of the reaction. A rotary evaporator (“rotovap”) is an extremely useful tool for accomplishing this task. The solution containing product starts off in a round-bottomed flask. This flask is attached to the rotovap using an airtight adapter. Then, vacuum is pulled on the system while the round-bottomed flask is spun in a warm-water bath. By pulling a vacuum, the rotovap is able to lower the atmospheric pressure above the solution so that its boiling point gets lowered. The warm-water bath raises the vapor pressure of the solution to facilitate evaporation of the solvent. Spinning the flask increases the solvent’s surface area, which also enhances evaporation. Solvent vapors soon fill the inside of the rotovap and eventually encounter the cooling coils on the left side of the device. When this happens, the solvent condenses and drips into the trap, where it is collected for recycling.
Gas Chromatography (GC) Unit
This device is used to analyze mixtures of compounds. It consists of an injection port, a column suspended in an oven, and a detector. A few microliters of sample are injected into the GC and get separated on the basis of their different boiling points as they pass through the column. Compounds with higher boiling points pass through the column more slowly and reach the detector later than compounds with lower boiling points. Based on the detector’s response, it is also possible to determine the relative amounts of each component of the mixture.
If you had to analyze a sample and determine the chemical formula of the sample, a mass spectrometer is the tool you would use to do it. Mass spectrometers are often connected to a GC, as ours is here. The mass spectrometer is set up to analyze each component of a mixture as it comes off the column of a GC. Thus, as each component of a mixture comes off the column of the GC, it feeds into the mass spectrometer. Our mass spectrometer is set up to bombard sample molecules with a beam of electrons. A high speed electron strikes a sample molecule with enough energy to break the molecule up into fragments—these fragments are then analyzed by a detector. The mass spectrometer gives chemists information about the masses of these fragments. Each molecule fragments in its own unique way; this provides chemists with a type of molecular fingerprint that is useful in determining the identity of an unknown molecule.
An infrared (IR) spectrometer is useful for determining what types of bonds are present in a molecule. A covalent chemical bond can be thought of as a tiny spring; atoms that are bonded together can vibrate in different ways. For analysis, a chemical sample is placed in the path of a beam of infrared light. When infrared light encounters a sample, each of the different types of bonds in the molecule is good at absorbing a specific wavelength of light. After absorption, the light then gets converted into vibrational energy. For instance, a C=O double bond typically absorbs 5800 nm light well. By looking at what wavelengths of light are being absorbed, it is possible for chemists to get an understanding of all of the different types of bonds present in an unknown molecule.
Nuclear Magnetic Resonance (NMR) Spectrometer
If you set up a reaction, how do you prove that you actually made the product you set out to make? How can you determine if other unexpected products formed? An NMR spectrometer is one of the most useful tools available for determining the structure of an unknown molecule. It is very similar in principle to the MRI machines found in hospitals. An NMR spectrometer makes use of radio-wave radiation and a strong magnetic field; a sample is immersed in a magnetic field and pulsed with radio-wave radiation. Each sample responds to this pulse differently, and based on the response, it is possible for a chemist to piece together the structure of an unknown molecule.