Which one of the following compounds undergoes substitution...

Which one of the following compounds undergoes substitution at a slower rate than benzene and yet yields predominantly ortho and para products?
(a) phenol
(b) chlorobenzene
(c) nitrobenzene
(d) benzene sulphonic acid

Key Concepts

Electrophilic Aromatic Substitution (EAS)

This is a fundamental reaction mechanism where an electrophile replaces a hydrogen atom on an aromatic ring. The process requires the aromatic system to be sufficiently electron-rich to attract the electrophile, and the mechanism typically proceeds through the formation of a resonance-stabilized carbocation intermediate. The rate of reaction and the distribution of substitution products are greatly influenced by any substituents already present on the ring.

Activating and Deactivating Groups

Substituents on an aromatic ring can either increase the ring's reactivity (activating groups) or decrease it (deactivating groups) toward electrophilic aromatic substitution. Activating groups donate electron density into the ring, often speeding up the reaction, while deactivating groups withdraw electron density, slowing down the reaction relative to benzene. The relative electron-donating or –withdrawing effects of these groups determine how fast the substitution occurs.

Directing Effects

Substituents also influence the position where new groups are introduced during electrophilic aromatic substitution, known as directing effects. Groups can be ortho-, para- directors or meta directors. Even when a substituent is deactivating, it can still favor substitution at specific positions on the ring (e.g., ortho and para positions) due to resonance stabilization of the intermediate formed during the reaction.

Inductive and Resonance Effects

These are two major electronic effects that substituents exhibit on aromatic rings. The inductive effect refers to the electron withdrawing or donating influence transmitted through sigma bonds, generally causing a uniform effect irrespective of position. The resonance effect involves the delocalization of electrons through the pi system of the aromatic ring, which can either stabilize or destabilize transition states or intermediates depending on the substituent. This balance explains how a substituent may both slow the reaction overall (via induction) while still directing substitution to ortho and para positions (via resonance donation).

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