Final answer:
Powerful electron-withdrawing groups decrease the reactivity of Friedel-Crafts alkylations and acylations by reducing the electron density of the aromatic ring and decreasing its nucleophilicity. They also increase the rate of tetrahedral intermediate collapse and leaving group departure.
Step-by-step explanation:
Powerful electron-withdrawing groups have a significant effect on the reactivity of Friedel-Crafts alkylations and acylations. These groups, like nitro (-NO2) and carbonyl (e.g., ketone or aldehyde) groups, decrease the electron density of the aromatic ring by withdrawing electrons. This decreases the nucleophilicity of the ring and makes it less reactive towards electrophilic aromatic substitution reactions. The presence of powerful electron-withdrawing groups also increases the rate of tetrahedral intermediate collapse and the leaving group's ability to leave, further affecting the reactivity of Friedel-Crafts reactions.