Final answer:
Deactivating groups are electron-withdrawing and decrease electron density on a benzene ring, making the ortho and para positions less reactive towards electrophiles and thus cannot be ortho-para directors. Instead, they direct electrophiles to the meta position.
Step-by-step explanation:
In the context of organic chemistry, ortho-para directors are substituents on a benzene ring that donate electrons through resonance or induction, which increases the electron density on the ortho and para positions of the ring, leading these positions to be more reactive towards electrophilic aromatic substitution. On the contrary, deactivating groups are typically electron-withdrawing groups which decrease electron density on the benzene ring through induction or resonance. Because the ortho and para positions are less electron-rich when a deactivating group is attached, they are less likely to attract electrophiles, making deactivating groups meta directors instead.
For example, a nitro group is a deactivating group due to its strong electron-withdrawing nature. When it is attached to a benzene ring, it decreases the reactivity of the ring and directs incoming electrophiles to the meta position, absent from the increased electron density that ortho-para directors would provide.