ortho

Meaning: Ortho (o-) describes two substituents on adjacent carbons of a benzene ring, a 1,2 relationship. Meta (m-) is 1,3 and para (p-) is 1,4. In electrophilic aromatic substitution, a group already on the ring steers the incoming electrophile. Electron-donating groups such as –OH, –NH₂, –OCH₃ and alkyl groups stabilize the carbocation intermediate when attack occurs at the ortho or para carbon, so they are ortho/para directors. Halogens are also ortho/para directors.

On the MCAT: Expect a passage on nitration, halogenation or Friedel–Crafts reactions of a substituted benzene, asking for the major product or the relative rate. A monosubstituted ring has two ortho positions and one para position, but bulky groups often shift the product toward para because of steric hindrance. Naming also appears: o-xylene is 1,2-dimethylbenzene.

Common confusion: Directing effect and reactivity are separate questions. Halogens deactivate the ring yet direct ortho/para, while electron-withdrawing groups such as –NO₂, –CN and carbonyl groups deactivate the ring and direct meta. Also, ortho means adjacent, not opposite; para is the opposite (1,4) position.