Final answer:
Heteroatoms with one or more pairs of unshared electron pairs are known as ortho/para directors. These atoms donate electron density to conjugated π-systems like benzene rings, making ortho and para positions more reactive towards electrophiles.
Step-by-step explanation:
Heteroatoms with one or more pairs of unshared electron pairs are ortho/para directors. These heteroatoms possess lone pairs that can be donated into a system of conjugated π-bonds, which is typically the case with benzene rings in organic chemistry. When a substituent with a lone pair is attached to a benzene ring, it tends to activate the ortho and para positions, relative to itself, for further substitution.
The electron-pair geometry influences the distribution of electron density around a central atom, affecting properties like electronegativity, which is the tendency of an atom to attract electrons in a bond to itself. Given that atoms with lone pairs usually have higher electronegativity, these atoms can draw electron density towards themselves, impacting how the benzene ring reacts with electrophiles. Formal charge considerations are also important when determining the directing effects of substituents.
Example of Ortho/Para Directing
An example of an ortho/para directing group is the hydroxyl group (-OH) in phenol. The oxygen atom has lone pairs that can interact with the π-system of the benzene ring, making the ortho and para positions more nucleophilic and reactive towards electrophilic aromatic substitution reactions.