Re: What are the most important chemical discoveries of 20th century?

This is a difficult question to answer because I am not sure if you want
any five important discoveries or the "five most important discoveries".
There have been numerous discoveries over the past one hundred years and
a list of Nobel Prize winners is a simple way to see what it is that
chemists have valued most or thought to be the most significant. A 
historical list of the Chemistry Nobel laureates is available on the web
at "http://www.nobel.se/chemistry/laureates/index.html".

However, if you would like to know what I consider to be the most important 
discoveries in chemistry, then I would suggest the following:

1) 1911 Rutherford experimentally and 1913 Bohr theoretically provide a 
rational explanation for the atom - the solar system model - which 
eventuall evolves into the Quantum mechanical model of the atom during the
1920's due to the work of Schrodinger, Dirac, Heissenberg, Bohr, and 
others.

2) During the 1910's and 20's W.H. and W.L.Bragg (1914), M. von Laue 
(1912), and others devised the technique of x-ray diffraction 
crystallography and refined it to the point that it could be used to 
determine the shape of molecules. More importantly, x-ray crystallography
gives details of bond lengths, bond angles, torsion angles, and all of the
other parameters critical to understanding the shape of molecules.

3) 1916 G.N. Lewis devised an explanation of the chemical bond and how 
molecules were held together. 

4) 1931 J.A. Nieuwland discovered the polymerization of acetylene to give 
"neoprene" - synthetic rubber - and W.H. Carothers discovered the 
artificial polymer "nylon".

5) 1930's the sulfa drugs were discovered and the whole of the 
pharmaceutical industry took off - specifically, the work of Domagk with 
sulfanilamide and Fleming's penicillin.

That is, the discover of the atom, the observation of the atom, the method
of combining atoms to make molecules, the making of BIG molecules, and
the making of small molecules that directly impact upon life. But the
list of things that are missing is a lot larger than those included here.

Take 1939 alone: francium was discovered, DDT synthesized, Vitamin K 
discovered, the role of essential minerals to living organisms was first
elucidated, and the anti-bacterial substance tyrothricin was first 
isolated. All of these had an important impact upon chemistry.

Anyway, I hope that the above list helps. If you would like to refine your
question, I suspect that I could be a bit more useful in answering it.