How the formula of glucose plus fructose plus galactose is derived?

Answer 1

I believe that you are inquiring about the molecular formulae. The molecular formulae of the compounds you asked about, and the molecular formula of any organic compound for that matter, can be easily determined today compared to 100 years ago. In fact, often, nuclear magnetic resonance (NMR) techniques or gas chromatography coupled with mass spectrometry (GC/MS) alone are sufficient to determine with near cetainty the molecular formula of a compound. Sometimes, though, one or more tests in the laboratory may be required, especially if the compound contains an element other than carbon, hydrogen and oxygen.

For compounds that are not excessively large or structurally complex, a H-1, or "proton," NMR spectrum alone is sufficient to elucidate the structure, and thus, the molecular formula to a very high degree of certainty.

For larger and/or complex compounds that are volatile enough, or that can be made sufficiently volitile through chemical derivation, GC/MS can identify the compound provided its GC retention time and fragmentation pattern matches those of a known compound stored in a computer database.

If one wishes to identify a compound that is not volatile or stable enough for a GC, or has probably not yet been discovered such as an essential oil from a rare plant, then NMR is the best tool available for the job. A C-13 NMR spectrum provides the number of carbon atoms in different environments in the molecule. A two-dimensional

C-13/H-1 NMR spectrum indicates which hydrogen atoms are bound to which carbon atoms.

In closing, NMR techniques are the most popular and powerful tools in use today for determining the molecular formula and three-dimensional structure of an unknown compound.