Analyzing Organic Compounds: Methods & Tools

Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

In this lesson, we will be learning about the basics of how to use infrared spectroscopy, mass spectrometry, and nuclear magnetic resonance spectroscopy to analyze organic compounds.

How Do They Really Know?

Have you ever wondered how the identity of an organic compound is determined? Obviously molecules can't tell us who they are, or really even do anything themselves that would give us hint as to their identity. People have identity cards such as driver's licenses that have your name on it, where you live, your age, and even your height and weight. But again, chemical compounds don't carry 'identification' that we can ask for to determine their identity.

So if molecules don't have I.D. cards, how do chemists determine what (or who) they are? If there wasn't a way to do this, we'd be in big trouble. Luckily though, organic chemists have several tools at their disposal that allow them to determine exactly what the identity of a compound is. In this lesson we will be looking at the three main instrumental methods that are used to find the identity of organic compounds: infrared spectroscopy, mass spectrometry, and nuclear magnetic resonance spectroscopy. Let's dive in!

Infrared Spectroscopy

One of the first (and most important) things we need to be able to do in determining the structure (the identity) of an organic compound is figure out what functional groups are present in the molecule. A functional group is a specific set of atoms or bonds that are responsible for the chemical reactivity of the molecule. The most common identification method organic chemists use to determine functional groups is infrared spectroscopy. Infrared spectroscopy (abbreviated IR spectroscopy) in simplest terms involves analyzing how a molecule interacts with infrared light.

IR spectroscopy leverages the fact that different organic functional groups absorb infrared light at different frequencies. When we shine infrared light on an organic molecule, the individual atoms will vibrate. As you can imagine, there are lots of different ways vibration can occur but the more important modes of vibration include stretching, scissoring, rocking, wagging, and twisting. If we can know which region of an IR spectrum a certain functional group absorbs infrared light, we can readily identify that functional group if a peak is found there. Consider as an example the IR spectrum of ethanol.

IR spectrum of ethanol

Notice the there are a lot of peaks in the spectrum of ethanol, but there is a really broad and deep peak labeled at 3358 wavenumbers (wavenumbers are the units for IR peaks). This region of the spectrum is where hydroxyl groups (-OH) absorb infrared light and as it so happens, ethanol contains a hydroxyl group! Although it can be quite cumbersome to think about memorizing the entire regions of the spectrum, with a little practice it's possible quickly look at an IR spectrum and pick out characteristic peaks associated with a functional group.

Mass Spectrometry

Once we know the functional group of an organic compound, the next step in determining its identity is finding its molecular weight. Mass spectrometry is the go-to tool to obtaining this information. We can also use it to figure out the molecule's chemical formula. Mass spectrometry takes a compound and ionizes it, then sorts the resulting ions based on what's called their mass-to-charge ratio. In simple terms, a mass spectrum gives us the elemental fingerprint of the molecule and allows us to determine what ratio the different atoms are in relative to each other (i.e. the chemical formula). Knowing the chemical formula of a molecule is a vital piece of information in determining its identity and structure.

Nuclear Magnetic Resonance Spectroscopy

So now we know how to determine the functional group(s) and the molecular weight/chemical formula of an organic compound. There's one more key instrumentation method that we need to use to nail down the identity/structure of a molecule. That method is called nuclear magnetic resonance spectroscopy (abbreviated NMR). NMR is by far the most powerful and commonly utilized identification of organic compounds. NMR exploits the magnetic properties of atoms, and very simply it allows us to determine the carbon-hydrogen framework of a molecule. Since organic compounds are primarily composed of carbon and hydrogen, you can hopefully understand why it's so useful.

Carbon-13 NMR

The carbon-13 NMR spectrum of a compound allows us to determine how many chemically distinct carbon atoms are present in the molecule. Take as an example ethyl acetate. What we can see is that there are four different carbon atoms present, and these four atoms we would expect to give rise to four different signals in the spectrum.

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