Chemical Structure

Back before instruments, attempts to determine the chemical (atomic) structure of a new substance were very difficult and darn near impossible. How do chemists determine the chemical structure of a substance, which chemists call a molecule?

The first step is to determine the exact number of atoms of carbon, oxygen, hydrogen, nitrogen and other atoms like sulfur in a pure substance. Chemical purity is a slippery slope because this requires chemical purification methods such as fractional distillation, recrystallization, or fused zone refinement. No compound is ever made perfectly pure. It's always a matter of percentage, but if the compound is better than 97% the results will be okay because of the quantitative nature of the method.

The CHNS method is done by combustion. The sample is burned in excess oxygen and using various traps, the carbon dioxide, water and nitric oxide are collected and weighed. From this it's a matter of calculation to determine the amount of each element, and then knowing the atomic weight of each element, one can calculate the atomic formula.
Once a chemist knows the atomic formula of the substance, the next step is to determine how these atoms are arranged. This usually involves using infrared spectroscopy and possibly mass spectroscopy.

Infrared spectroscopy involves illuminating a sample with an infrared light beam when smeared thinly between optical salt crystals and measuring how the sample absorbs specific frequencies of infrared. Salt is used because it doesn't absorb infrared. This allows a chemist to determine the types of chemical structures that are present in the substance. There are plenty of reference infrared charts for many compounds available for comparison. I'm talking mostly about organic chemical substances here.

Another aid in determining the structure or identity of a substance is by use of melting point. There are several compilations of melting points for known compounds. For a liquid this would involve determining the boiling point. If the substance is new and unknown then this idea wouldn't help. In most cases a chemist who is attempting to make a new compound will know what reactions he or she is using to accomplish the synthesis and will have a good idea what to expect. This would not be the case for a chemist trying to determine the structure of a natural product. That's a more daunting task because natural products are quite complex.

These analytical methods are easily done nowadays, but in the distant past they were not available. It's amazing how the early chemists, and even alchemists, were able to deduce the atomic structure of substances.
This may sound boring, but it's a critical part of the science of chemistry. Whether we like it or not, chemicals are part of everything we do in life. Our food, clothing, cars, electronics, household products and plastics are part of modern living and had to be synthesized or isolated and formulated by chemists. I know this because I was a chemist.

Thanks for reading.