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

In the realm of organic chemistry, electron-donating groups (EDGs) play a critical role in the chemistry of aromatic compounds. EDGs, such as the hydroxyl group in phenol, donate electron density through resonance or induction to the aromatic ring, thereby increasing its electron cloud density.

This heightened electron density makes the aromatic ring more reactive towards electrophiles, as it now possesses a stronger ability to stabilize the positive charge of the electrophilic intermediate formed during the electrophilic aromatic substitution (EAS) process. EDGs make the aromatic ring an 'activated' system, which reacts more readily with electrophiles, thus increasing the substitution rate compared to benzene.

Moreover, EDGs also dictate where on the aromatic ring the electrophilic attack will occur. They primarily direct electrophiles to the ortho and para positions relative to themselves. This directionality is a consequence of the specific resonance structures that can be drawn when an EDG is attached to the ring, which show increased electron density at these positions.

Conversely, electron-withdrawing groups (EWGs) have an altogether different influence on the reactivity of aromatic compounds. EWGs, such as the nitro group in nitrobenzene or the sulfonic acid group in benzene sulfonic acid, pull electron density away from the aromatic ring through resonance or inductive effects.

As a result, they decrease the electron cloud density around the ring, which in turn makes it less reactive to electrophilic attack, hence 'deactivating' the aromatic system. When a compound is adorned with an EWG, it generally undergoes electrophilic aromatic substitution at a slower rate than benzene itself.

Regarding selectivity, EWGs typically direct electrophilic substitution to the meta position of the ring. This phenomenon is attributed to the resonance stabilization being less favorable or completely absent at the ortho and para positions due to the presence of the EWG.

The reactivity of aromatic compounds in electrophilic aromatic substitution (EAS) reactions is intricately linked to the nature of substituents already present on the ring. EDGs, by pushing electron density towards the ring, increase its reactivity, leading to a faster reaction rate compared to benzene. In contrast, EWGs pull electron density away, causing the aromatic compounds to be less reactive and hence react at a slower rate.

However, there are exceptions, such as sulfonic acid groups, which, although strong EWGs, can direct substitutions to the ortho and para positions. This occurs because of the dual nature of these groups—through their ability to participate in resonance, they can provide resonance structures that place negative charge on the ortho and para positions, albeit to a lesser degree than the meta position, thus facilitating EAS at these sites albeit at a slower overall rate compared to an unsubstituted benzene ring.

Specific substituents on an aromatic ring not only affect the rate of reaction but also the position where an electrophile will add. Ortho and para directors are primarily electron-donating groups that increase electron density at the ortho and para positions, making them more favorable sites for electrophilic attack.

This preferential positioning is due to the stability of the resulting carbocation intermediates when the electrophile adds to these positions. Resonance structures can be drawn showing the positive charge delocalized over the aromatic system, particularly at the ortho and para locations, when an EDG is attached.

These resonance structures are not possible with meta substitution, therefore, EDGs generally do not facilitate electrophilic attack at the meta position. In typical reactions, these directing effects lead to the formation of products where the new substituent group is placed at the positions next to (ortho) or opposite (para) to the existing EDG on the aromatic ring.