Question

Arrange in order of decreasing trend towards \(\mathrm{S}_{\mathrm{E}}\) reactions: (i) chlorobenzene (II) benzene (III) anilinium chloride (IV) toluene (a) \(\mathrm{IV}>\mathrm{II}>\mathrm{I}>\mathrm{III}\) (b) \(\mathrm{I}>\mathrm{II}>\mathrm{III}>\mathrm{IV}\) (c) \(\mathrm{II}>\mathrm{I}>\mathrm{III}>\mathrm{IV}\) (d) \(\mathrm{III}>\mathrm{I}>\mathrm{II}>\mathrm{IV}\)

Short answer

Option (a)  ext{IV}> ext{II}> ext{I}> ext{III}.

Step-by-step solution

Understand Electrophilic Substitution in Aromatic Compounds

Electrophilic substitution reactions ( ext{S}_{ ext{E}}) in aromatic compounds involve an electrophile attacking the aromatic ring. The direction and reactivity of such reactions are influenced by substituents on the ring.

Identify the Groups on Each Compound

Chlorobenzene (I) has a chlorine atom, which is electron-withdrawing but deactivating for electrophilic substitution. Benzene (II) has no substituents. Anilinium chloride (III) possesses a positively charged nitrogen, which is electron-withdrawing and deactivating. Toluene (IV) carries a methyl group, which is electron-donating and activating.

Determine the Activating or Deactivating Nature of Substituents

Evaluate the substituents: the methyl group in toluene (IV) increases reactivity due to its electron-donating nature. Benzene (II) is neutral. The chlorine in chlorobenzene (I) is slightly deactivating. The anilinium group in (III) is strongly deactivating.

Rank Compounds Based on Reactivity

Arrange the molecules from most active to least for electrophilic substitution. Toluene (IV), due to the activating methyl group, will be most reactive, followed by benzene (II), then chlorobenzene (I), and lastly, anilinium chloride (III) due to the strong deactivation.

Select the Correct Answer

From evaluation, the correct order is toluene (IV) > benzene (II) > chlorobenzene (I) > anilinium chloride (III), which matches option (a)  ext{IV}> ext{II}> ext{I}> ext{III}.

Key concepts

Aromatic Compounds

Aromatic compounds are fascinating molecules characterized by their stability and unique structure. They contain one or more planar rings of carbon atoms that follow Huckel's rule, which states that aromatic rings must have \(4n+2\) π (pi) electrons, where \(n\) is a non-negative integer. This electron arrangement offers aromatic rings the ability to delocalize electrons, resulting in considerable stability.

One of the most common aromatic compounds is benzene, which has a six-carbon ring with alternating single and double bonds. The delocalized electrons in benzene make it relatively unreactive towards addition reactions, which would disrupt its aromaticity. However, benzene and other aromatic compounds readily undergo electrophilic substitution reactions to preserve their stable electron configuration.

These reactions involve the replacement of a hydrogen atom in the aromatic ring with an electrophile, a process that can be modulated by different substituents on the ring.

Activating and Deactivating Groups

Substituents attached to aromatic rings can significantly influence the reactivity of these compounds during electrophilic substitution reactions. Depending on their electronic nature, substituents can either increase or decrease the ring's reactivity. These are known as activating and deactivating groups respectively.

• **Activating Groups:** Electron-donating groups such as alkyl groups, like the methyl group on toluene, enhance electron density on the aromatic ring. This increase in electron density attracts electrophiles more effectively, accelerating the substitution reaction.
• **Deactivating Groups:** Conversely, electron-withdrawing groups, like the chlorine atom in chlorobenzene or the anilinium group, reduce the electron density on the ring. This makes the ring less attractive to electrophiles, hindering the reaction.

While an electron-donating group intensifies the reactivity of the aromatic ring, a strongly deactivating group can make the ring significantly less receptive to substitution. The key to determining the reactivity in electrophilic substitution reactions is understanding these effects and their impact on the electron cloud of the aromatic ring.

Reactivity Order in Chemistry

In chemistry, understanding the reactivity order helps predict how different compounds will interact under similar conditions. For electrophilic substitution reactions in aromatic compounds, the nature of the substituents greatly influences this order.

Compounds with electron-donating groups, such as toluene with its methyl group, are typically more reactive compared to those with neutral or electron-withdrawing groups. Benzene, lacking substituents, serves as a neutral benchmark for reaction rates. Substituted compounds with moderately electron-withdrawing groups, like chlorobenzene, have reduced reactivity. Finally, strongly electron-withdrawing groups, seen in anilinium chloride, confer the least reactivity.

Therefore, when you arrange compounds in order of reactivity towards electrophilic substitution, consider how activating and deactivating groups alter the electron density of the aromatic ring. This understanding guides chemists in predicting and manipulating chemical reactions to achieve desired outcomes efficiently. By observing these patterns, you can master the subtleties of aromatic chemistry and improve your problem-solving skills in organic reactions.