induction

Meaning: Electron density in sigma bonds is pulled toward the more electronegative atom. Electron-withdrawing groups (F, Cl, O, N, NO₂, C=O) draw density through the carbon chain. This stabilizes a nearby negative charge, such as a conjugate base, and destabilizes a nearby positive charge. Alkyl groups are weakly electron-donating, so they help stabilize carbocations (tertiary > secondary > primary), although hyperconjugation also contributes. Each additional bond weakens the effect, and it is small beyond about three or four bonds.

On the MCAT: The usual setup is ranking acidity. Trichloroacetic acid is a stronger acid than dichloroacetic, then chloroacetic, then acetic acid, because more electronegative atoms stabilize the carboxylate. A halogen closer to the COOH, or a more electronegative halogen (F > Cl > Br), gives a lower pKa. Induction also explains why a carbonyl carbon next to an electron-withdrawing group is more electrophilic, and why alkyl substitution raises amine basicity and carbocation stability.

Common confusion: Students mix induction with resonance. Induction works through sigma bonds and falls off with distance. Resonance works through pi systems and lone pairs, delocalizing charge over a conjugated system. A group can do both in opposite directions: on a benzene ring, -OH and -OCH₃ withdraw by induction but donate by resonance, and the net effect depends on which dominates.