He was already very interested in Chemistry - too much so for the professor of that subject at Besancon, whom Pasteur used to embarrass with unanswerable questions.

Explanation

The error is in the pronoun "whom", which should be replaced with "who" because it is the subject of the verb "used to embarrass".The correct sentence would be: "He was already very interested in Chemistry - too much so for the professor of that subject at Besancon, who Pasteur used to embarrass with unanswerable questions."