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DDT is one of the most famous chemicals in history: praised for saving lives from malaria, then blamed for harming birds and wildlife. Both parts of the story come from the same property. This post covers how Paul Müller found DDT's power, a short chemistry calculation, and what happened next.
Who was Paul Müller?
His Nobel Prize facts page gives his dates: born on 12 January 1899 in Olten, Switzerland, and died on 12 October 1965 in Basel. He won the 1948 Nobel Prize in Physiology or Medicine "for his discovery of the high efficiency of DDT as a contact poison against several arthropods".
His Nobel biography says he started as a laboratory assistant at 17, earned his doctorate from the University of Basel in 1925 and joined the dye company J. R. Geigy that May. He worked on tanning agents, a seed disinfectant and moth-proofing before, in 1935, he began searching for new contact insecticides.
How did he search for an insecticide?
In his Nobel lecture, Müller explains that he first wrote down what an ideal insecticide should be:
Great insect toxicity.
Rapid onset of toxic action.
Little or no mammalian or plant toxicity.
No irritant effect and little or no odour.
As wide a range of action as possible.
Long, persistent action, meaning good chemical stability.
Low price.
He tested substances himself, spraying blowflies in a large glass chamber. After hundreds of failures he told himself, "Now, more than ever, must I continue with the search." Reading a 1934 paper describing a related compound, he wondered what effect its CCl₃ group would have. He prepared it in September 1939, then made a version with two more chlorine atoms. That compound, first made in 1873 by an Austrian student, showed a contact action stronger than anything he had seen. His fly cage became so toxic that untreated flies fell after touching the walls, even after cleaning; he had to take it apart and leave it in the open air for about a month.
The biography adds that a Swiss patent followed in 1940 and two products went on sale in 1942. During the Second World War, when supplies of the plant insecticide pyrethrum ran short, DDT was produced in Britain and the United States and used against typhus, carried by lice, and malaria, carried by mosquitoes.
A worked example: DDT's formula and chlorine content
The name dichlorodiphenyltrichloroethane tells you the formula.
Ethane gives a two-carbon backbone. One carbon carries three chlorines (CCl₃). The other carries one hydrogen and two phenyl rings.
Each ring is C₆H₄Cl: six carbons, four hydrogens and one chlorine ("dichloro" across the two rings).
Count: carbon 2 + 6 + 6 = 14; hydrogen 1 + 4 + 4 = 9; chlorine 3 + 1 + 1 = 5. So DDT is C₁₄H₉Cl₅.
Molar mass: 14 × 12.01 + 9 × 1.008 + 5 × 35.45 = 168.14 + 9.07 + 177.25 = 354.46 g/mol.
Chlorine by mass: 177.25 ÷ 354.46 × 100 = 50.0 per cent.
Common mistakes to avoid:
Counting six hydrogens per ring. Each ring has lost one hydrogen to chlorine and one to the bond with the backbone, leaving four.
Using 35 for chlorine. Use 35.45, or your answer drifts.
Saying Müller invented DDT. He discovered what it could do; the compound dated from 1873.
What went wrong later?
The Nobel facts page puts it plainly: DDT "became concentrated in the food chain and injured other animals and people". Our post on Rachel Carson and Silent Spring shows how concentrations rose from plankton to birds.
Today the Stockholm Convention lists DDT in Annex B. Production and use are restricted to disease-vector control, when no equally effective alternative exists and in line with WHO guidance. The Convention page names malaria and visceral leishmaniasis as the diseases where DDT is still used, and reports that in 2020 its expert group concluded it was time to move towards a focused phasing out.
For the mosquito side of the story, read our post on Ronald Ross and malaria.
What can a student take from Müller's way of working?
Müller wrote his goals down before he started, ran his own tests, and kept going through hundreds of failures. His list also shows the limit of any checklist: it judged the insect and the person spraying, not what happens to a chemical over years in the wider world.
When you design an answer or an experiment, list your criteria and then ask what your list leaves out. If molar mass and percentage composition still slow you down, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning.
Next step: calculate the percentage of carbon in DDT and check that carbon, hydrogen and chlorine add up to 100 per cent.
