For example:
* happy -> unhappy (not happy)
* lock -> unlock (the opposite of lock)
* important -> unimportant (not important)
The word "acute" is an adjective meaning sharp, severe, or intense, and it doesn't have a specific prefix. There is no word "iacute".
It's possible you're thinking of the accented letter "í", which is called an acute accent. This accent mark is used in some languages, including Spanish and Portuguese, to indicate a change in pronunciation.