| 1853 | Alexander Butlerov discovers haloalkane chemistry basics | Foundation of organic halide nomenclature |
| 1896 | Paul Walden discovers inversion of configuration in SN reactions (Walden inversion) | Named after discoverer; complete inversion in SN2 now called "Walden inversion" |
| 1901 | Victor Grignard discovers organomagnesium reagents (Grignard reagents) | Nobel Prize 1912; RMgX from R-X + Mg in ether; revolutionary C-C bond formation tool |
| 1930s | Thomas Midgley Jr. develops Freon (CFCs) as refrigerants | Replaced ammonia/SO2 as refrigerant; initially seemed safe and non-toxic |
| 1935 | Frank Whitmore studies carbocation rearrangements in SN1 reactions | Explains unexpected SN1 products via 1,2-hydride and alkyl shifts |
| 1937 | E.D. Hughes and C.K. Ingold formally classify SN1 and SN2 mechanisms | Rigorous kinetic and stereochemical analysis establishes the modern classification |
| 1939 | Paul Müller discovers DDT's insecticidal properties | Nobel Prize 1948; initially celebrated for malaria control, later found to bioaccumulate |
| 1944 | DDT used widely in WWII to prevent typhus | Saved millions of lives from lice-borne typhus among troops |
| 1962 | Rachel Carson publishes "Silent Spring" — documents DDT bioaccumulation | Catalyzed environmental movement; documented eggshell thinning in raptors |
| 1972 | DDT banned in USA | DDT still used in some countries for malaria vector control (limited) |
| 1974 | Rowland and Molina publish CFC-ozone depletion theory | Nobel Prize 1995; predicted stratospheric ozone damage from CFCs |
| 1985 | Ozone hole over Antarctica discovered by British Antarctic Survey | Confirmed the Rowland-Molina theory; accelerated international action |
| 1987 | Montreal Protocol signed — phaseout of CFC production | 197 countries signed; most successful international environmental treaty |
| 1990s+ | HFCs (no Cl) introduced as CFC replacements | No chlorine → no Cl• radical → no ozone depletion (though HFCs are greenhouse gases) |