We hope, you have reached the same conclusion that Hoffmann and Arthur Eichengrün did, that adding an acetyl group to salicylic acid might produce a pain reliever that doesn't hurt people's stomachs. Hoffmann didn't know why adding an acetyl group to a drug's molecular structure might make it less irritating. But he knew it worked for other drugs, so it was possible that it could make salicylic acid less irritating, too.

So Hoffmann went into his laboratory at the Bayer research facility in Leverkusen, Germany, and proceeded to add an acetyl group to salicylic acid. The result was a new compound that we call acetylsalicylic acid. Look at the picture below and you can see the difference between salicylic acid and acetylsalicylic acid (ASA).

Hoffmann simply changed the molecular structure of salicylic acid so that it would contain an acetyl group, the carbon, hydrogen, and oxygen atoms shown in red.

Once Hoffmann had made acetylsalicylic acid, he was not yet done. He hypothesized that this new drug, soon to be known as aspirin, would relieve pain and not irritate the stomach. But he hadn't actually tried the drug on a patient yet, and until he had, he could not say that he had solved his problem, nor that he knew his drug would work.

Salicylated cotton for wound dressing: Courtesy Johnson & Johnson.