12.2.3. Limitations

IR spectroscopy is excellent for identifying the presence or absence of specific functional groups, but it does not give information about the molecular formula, molecular mass, or the exact skeletal structure of the molecule.

IR spectroscopy is a great tool for detecting functional groups in organic compounds. It tells us whether certain groups, like alcohols or carbonyls, are present or absent in a molecule. However, it has its limitations. Firstly, it doesn’t provide information about the molecular formula, meaning we can't determine the actual number of atoms of each element in the compound. Secondly, we can't find out the molecular mass from IR data, which is crucial for understanding the size of the molecule. Lastly, IR doesn't give a clear picture of the skeletal structure—the arrangement of atoms within the molecule—so we might know what functional groups are there but not how they connect to each other.

Think of IR spectroscopy like a team member who can spot different colors of lights in a traffic jam but can't count the number of cars or figure out how many people are in each car. They can clearly say, 'There's a red car, a blue car, and a yellow car,' but they can't tell you how many cars are there in total or how they’re arranged on the road.

IR spectroscopy does not provide information regarding the exact skeletal structure of the molecule.

While IR spectroscopy can tell us which functional groups are present, it cannot elucidate the precise arrangement or connectivity of these groups within a molecule. Essentially, you might know about various 'rooms' in a house, but you would have no idea how they connect, or where the doors are that link them. Without this information, if we have two different molecules that share the same functional groups, IR won't help us distinguish between them based on structure.

Imagine you walked into a new school building. You can clearly see the art room, the science lab, and the gym, but you have no idea how to get from one room to the other or how they are connected—perhaps there are doors, or perhaps it's a long hallway. This is the limitation of IR spectroscopy; while it can identify what’s in the 'building' (molecule), it can't show you the layout (structure).

It does not offer information regarding the molecular formula or molecular mass.

When we analyze a compound using IR spectroscopy, we fail to obtain crucial details such as the molecular formula (which reveals how many atoms of each element are present) and the molecular mass (the total weight of the molecule). This information is essential because knowing the molecular mass helps chemists deduce the compound's identity and reactivity. For example, two compounds might show similar functional groups in their IR spectra, but their different molecular formulas can result in completely different physical and chemical properties.

Consider IR spectroscopy like having a menu at a restaurant. You can see what dishes are available (like seeing the functional groups), but the menu doesn't tell you the portion size (molecular mass) or how many ingredients are in each dish (molecular formula). If you want to know if a meal is enough for you or how many calories it has, you need more than just the menu—you need additional nutritional information.

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