The most outstanding milestones in the History of Chemistry are the following: (there are many others but these are my favorite)
At the end of the XVIII Century
The Organic Chemistry enigma: The VITAL FORCE.
It is already realized that organic compounds are formed by a very limited number of different elements.
Urea synthesis: a bridge is laid down between inorganic and organic chemistry.
The precision of elemental analyses is improved.
An unsuspected ravel unravels: ISOMERISM.
The organic radicals are proposed as en ordering principle.
Discovery and profusion of organic radicals.
Establishing further order amidst organic radicals: the concept of substitution.
Definition of derived radicals.
Ordering by compound type.
Unifying radicals and compound types.
Internal structure of radicals: The discovery of carbon tetravalency and its capacity to form endless chains.
Tetrahedral structure of carbon: optical isomerism.
First modern formulatons.
Haxagonal structure of benzene.
Set up of resonance theory.
Development of x-ray diffaction and mass spectrometry.
Conformational analysis: cyclohexane stereochemistry.
Discovery and development of Nuclear Magnetic Resonance.
This summary has been extracted in part from the book From Vital Force to Structural Formulas
O. Theodor Benfey, ACS, 1975
Most part of the biographic references have been extracted from the Larousse enciclopedia.