A man who pleaded guilty to charges of conspiracy to commit extortion in connection with a bizarre kidnaping case was sentenced yesterday to five years in a maximum security prison by a U.S. District Court judge in Alexandria.

Kenneth B. Krohn, a Harvard Ph. D., was convicted in connection with the abduction of Mexican businessman Raul Hernandez Bustamente at Dulles International Airport on Jan. 20, 1973.

Bustamante's body allegedly was dismembered and dumped in the Chesapeake Bay. The body was never found. Kidnaping charges against Thurston Drew Shrader, a second defendant, were dropped by the U.S. prosecutor in return for a guilty plea on a charge of concealing evidence.

Shrader was sentenced to a maximum of three years in prison and yesterday Krohn's attorney, Philip A. Mazon, argued that it would be unfair to sentence one "coconspirator" to more time than the other.

Mason told U.S. District Judge Albert V. Bryan Jr. that his client had had a distinguished carrer in the field of appied mathematics and no previous criminal record. He also urged Bryan, and the government concurred, to send Krohn to a minimum security prison.

But the judge imposed a five-year term for conspiracy to commit extortion and a concurrent five years for crossing state lines to commit extortion.