Q.Write short notes on ortho, para directors in aromatic electrophilic substitution reactions.
[Note: this question is genuinely printed without a serial number in the source textbook, sitting between the numbered Q.32 and Q.33 -- a synthetic number (100) is used here only as a unique database key and is not a book page number.]

Step 1. What an ortho-para director is. A substituent already on a benzene ring is an ortho-para director if it raises electron density specifically at the ortho (1,2-) and para (1,4-) ring positions relative to itself, so that an incoming electrophile in a further substitution attacks preferentially at those positions rather than meta.

Step 2. The activating examples (-OH, -NH2, -NHR, -NHCOCH3, -OCH3, -CH3, -C2H5). For a group like -OH (phenol), the oxygen's lone pair delocalises into the ring by resonance: drawing out phenol's resonance structures places extra negative charge specifically at the ortho and para ring carbons (not at meta). This makes the ring MORE electron-rich overall than plain benzene (an ACTIVATING effect on rate) and SELECTIVELY more electron-rich at ortho/para (the DIRECTING effect on site). The same reasoning, with a lone pair on the directly-attached N or O donating into the ring, applies to -NH2, -NHR, -NHCOCH3 and -OCH3; alkyl groups like -CH3/-C2H5 donate more weakly, by hyperconjugation rather than a lone pair, but with the same net ortho/para-favouring result. …

Draw out the resonance structures of the substituted benzene explicitly for a representative example (phenol for the general case, a halobenzene for the exception) and see directly whic …

• Assuming 'ortho-para director' always means 'activating' -- halogens are the standing counterexample, deactivating by induction yet still ortho-para directing by resonance. …