PeriodicallyPedanticEnglish is such a peach.
Phil KIt’s because the im isn’t a prefix but part of the word. (It was originally spelt empeche)
To be fair it was a prefix in the Latin word that it's ultimately derived from. We still treat it a little like a prefix when we use the im- part of the related "import" like a prefix, as we also have "export" and "transport"