Final answer:
Salvarsan is the arsenic compound that proved highly effective in treating syphilis.
Step-by-step explanation:
The arsenic compound that proved highly effective in treating syphilis was called Salvarsan. In the early 1900s, Paul Ehrlich and Sahachiro Hata discovered this compound after screening more than 600 arsenic-containing compounds. Salvarsan successfully cured syphilis in rabbits and was marketed as a remedy for the disease in humans.