Column I Column II (a) Electrophilic substitution (p) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~N}_{2} \mathrm{Cl}+\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH} \stackrel{\mathrm{NaOH}}{\longrightarrow}\) (b) Nucleophilic substitution (q) Toluene + \(\mathrm{NBS} \longrightarrow\) (c) Free radical substitution (r) Dow's process (d) Coupling reaction (s) Phenol \(\longrightarrow\) picric acid (t) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{O} \mathrm{Na}+\mathrm{CH}_{3} \mathrm{Br} \longrightarrow\)

Electrophilic substitution reactions are a cornerstone of organic chemistry, involving the replacement of an atom or group of atoms in an aromatic ring with an electrophile. An electrophile is an electron-loving species with a deficiency of electrons, seeking to pair up with a location rich in electrons, such as an aromatic ring.

To visualize this, imagine the benzene ring as a welcoming host for guests with a strong desire for negative charge. During the electrophilic substitution, the stable arrangement of electrons in the benzene ring is temporarily disrupted to accommodate the electrophile, leading to the formation of an intermediate complex. This intermediate arenium ion eventually reforms the original stable electron arrangement, but now includes the electrophile.

A famous example is the nitration of benzene, where a nitro group replaces one of the hydrogen atoms. The transformation of phenol to picric acid is another key example, where multiple electrophilic substitution steps are involved to introduce nitro groups into the aromatic ring.

Nucleophilic substitution is the converse of electrophilic substitution. In this type of reaction, a nucleophile, which is a species with a pair of electrons to share, replaces a leaving group in a molecule. Nucleophiles are often negatively charged or have a lone pair of electrons that can be used to form a new bond with a positively charged or partial positive atom.

A simple way to consider a nucleophilic substitution is to imagine it as a dance where the nucleophile cuts in, pushing out the leaving group. This can be seen in the reaction of sodium phenoxide with methyl bromide - an SN2 mechanism is a possibility here; the nucleophile (sodium phenoxide) attacks the electrophilic carbon atom in methyl bromide, replacing the bromide ion.

This type of reaction is fundamental for creating new connections in molecules, significantly altering their structure and properties, and is common in various organic syntheses.

Free radical substitution is a reaction that typically proceeds via a chain mechanism involving highly reactive species known as free radicals. Free radicals are atoms or groups of atoms with an unpaired electron, making them quite reactive.

The reaction begins with initiation, where free radicals are generated, often by the decomposition of a starting material called the initiator (in the presence of heat or light). This is followed by the propagation steps, where the generated free radical reacts with a stable molecule to extract an atom and create a new radical, perpetuating the chain. Finally, termination occurs when two radicals combine, ending the chain reaction.

An illustrative example of free radical substitution is the halogenation of toluene using N-Bromosuccinimide (NBS), where a hydrogen atom on the methyl group is replaced by a bromine atom. This type of reaction is pivotal for creating points of functionalization on molecules that can be further used in chemical synthesis.

A coupling reaction is a unique and powerful tool in organic chemistry that enables the connection of two separate chemical entities, frequently resulting in a more complex and valuable product.

One of the distinctive features of coupling reactions is their ability to link together two molecules, often catalyzed by metals. The reaction typically involves a species with a diazonium group, which can be thought of as a molecular bridge that facilitates the bonding process between two separate aromatic rings.

For instance, when a diazonium salt reacts with phenol in the presence of a base like NaOH, the result is a newly formed azo compound. This is what occurs in the combination of Diazonium Salt with Phenol, which falls under azo coupling reactions. Azo compounds are characterized by a nitrogen-to-nitrogen double bond (N=N) and are widely used as dyes in the textile and printing industries due to their vivid colors.