Final answer:
Halides are ortho-para directors because of their lone pairs that stabilize the carbocation intermediate, but they are mildly deactivating due to their electronegativity that reduces electron density in the aromatic ring, making it less reactive towards electrophiles.
Step-by-step explanation:
Halides act as ortho-para directors in electrophilic aromatic substitution reactions because they possess lone pairs of electrons that can be donated into the aromatic ring system, thereby stabilizing the carbocation intermediate that occurs at the ortho and para positions. However, halides are also mildly deactivating due to their electronegativity, which pulls electron density away from the ring and thus, slightly hinders the reaction with electrophiles by making the ring less electron-rich.
However, when involved in coupling reactions, as mentioned with palladium catalysis, halides such as iodide can activate positions on electron-deficient pyrimidines for the reaction with electron-rich heterocycles due to the effective catalyst systems used. This illustrates how halides can still partake in chemical reactions even when they are mildly deactivating.