Frederic L. Chapin '50 was recently appointed acting ambassador to El Salvador.

Secretary of State Alexander M. Haig named him charge d'affaires to the Central American country, Chapin said yesterday. Chapin replaces Robert E. White--whom President Reagan removed Monday--and will serve until Reagan selects a permanent successor.

Chapin said he will leave for El Salvador this weekend to begin directing the U.S. embassy there, but declined to discuss U.S. involvement in the country's civil war, citing orders from Haig. He also declined to reveal why White lost his post.

Chapin was ambassador to Ethiopia until July, 1980, when he was expelled by Ethiopia's Marxist government. He has been deputy assistant secretary for Inter-American Affairs in the Defense Department since December.

A career foreign service officer, Chapin has served in Brazil, Bolivia and Uruguay.

Chapin--who left Harvard with a magna cum laude degree in history and with Phi Beta Kappa honors--said he became interested in Latin American affairs and fluent in Spanish as a child, while living in Ecuador with his father, also a career foreign service officer.