Electrophilic substitution in the benzene ring

Mechanisms of basic electrophilic substitution reactions

The chemical behavior of aromatic hydrocarbons is based on the mechanism of electrophilic substitution. Let us examine this process using two major reactions as examples: nitration and alkylation.

The nitration of benzene involves its interaction with nitric acid ($HNO_3$). A prerequisite for this reaction to proceed is the presence of sulfuric acid ($H_2SO_4$). In this process, sulfuric acid acts as a catalyst. Its main function is to facilitate the formation of active electrophilic species—nitronium cations ($NO_2^+$). It is these cations that attack the benzene ring, resulting in the formation of nitrobenzene. The formation of benzenesulfonic acid belongs to similar reactions.

Another classic example is the alkylation of benzene, also known as the Friedel–Crafts reaction. The essence of this reaction lies in the interaction of benzene with haloalkanes, leading to the formation of benzene homologues. Anhydrous aluminum chloride ($AlCl_3$) is used here as a catalyst. The role of the catalyst is to help generate active electrophilic species—carbocations. For example, the reaction of benzene with chloroethane ($C_2H_5Cl$) yields the ethyl cation ($C_2H_5^+$), and the resulting equation is:

$C_6H_6 + C_2H_5Cl \xrightarrow{AlCl_3} C_6H_5C_2H_5 + HCl$

The product of this reaction is ethylbenzene.

Homologues and derivatives of benzene

Various benzene derivatives are produced based on electrophilic substitution reactions and subsequent transformations. In particular, styrene (vinylbenzene according to the IUPAC nomenclature) is synthesized from ethylbenzene via dehydrogenation. Structurally, the styrene molecule is a benzene ring directly bonded to a vinyl group ($-CH=CH_2$).

Another important homologue is cumene, or isopropylbenzene. Its synthesis is also carried out via the Friedel–Crafts method. To obtain cumene, the starting reactants needed are benzene itself ($C_6H_6$), 2-chloropropane ($CH_3CHClCH_3$) acting as the alkylating agent, and aluminum chloride ($AlCl_3$) as the process catalyst.

Effect of substituents on electrophilic substitution

If a substituent is already present in the benzene ring, the course of electrophilic substitution reactions changes significantly. First, various isomers are formed. Second, the reaction rate begins to differ from that of pure, unsubstituted benzene—it can be either increased or decreased.

The nature of subsequent substitution is entirely determined by the character of the groups already present in the ring. This influence is exerted through two main electronic effects: inductive ($I$) and mesomeric ($M$). Depending on how substituents redistribute electron density, they are divided into two large groups: first- and second-order directing groups.

First- and second-order directing groups: classification and effects

First-order substituents are electron donors. They possess predominantly donor properties and increase electron density in the benzene ring. These include alkyl groups (exhibiting a positive inductive effect, $+I$), as well as halogens, the hydroxyl group ($-OH$), and the amino group ($-NH_2$) (exhibiting positive mesomeric $+M$ and negative inductive $-I$ effects).

Their mechanism of action is that they increase the rate of the substitution reaction compared to unsubstituted benzene. Due to the redistribution of $\pi$-electrons in the conjugated system, the highest negative charge ($-\delta$) is created at the ortho- (o-) and para- (p-) positions. As a result of the reaction, a mixture with a predominance of o- and p- isomers is formed.

Second-order substituents are electron acceptors. They possess acceptor properties, meaning they decrease electron density in the ring by pulling electrons toward themselves. They exhibit a negative inductive ($-I$) and/or negative mesomeric ($-M$) effect. This group includes the nitro group ($-NO_2$), sulfo group ($-SO_3H$), carboxyl ($-COOH$) and aldehyde ($-CHO$) groups, ammonium group ($-NH_3^+$), as well as trichloromethyl ($-CCl_3$) and trifluoromethyl ($-CF_3$).

These substituents slow down substitution reactions (processes proceed more slowly than in pure benzene). The redistribution of $\pi$-electrons leads to a relative increase in electron density (against the background of its overall decrease) exclusively at the meta- (m-) position. The result of the reaction is the formation of m- isomers.

Comparative analysis using nitration as an example

To clearly see the difference in the influence of substituents, let us consider the reaction of introducing a nitro group ($-NO_2$) for phenol and nitrobenzene.

A. Nitration of phenol ($C_6H_5OH$) The phenol molecule contains a hydroxyl group ($-OH$). It exhibits a positive mesomeric effect ($+M$), donating electron density directly into the ring. This activates the aromatic system, so the nitration reaction proceeds rapidly. The incoming nitro group is directed to the ortho and para positions: $C_6H_5OH + HNO_3 \xrightarrow{H_2SO_4} o\text{-nitrophenol} + p\text{-nitrophenol}$

B. Nitration of nitrobenzene ($C_6H_5NO_2$) Here, the initial substituent is the nitro group ($-NO_2$). It exhibits a negative mesomeric effect ($-M$), withdrawing electron density from the ring. This deactivates the molecule, causing the reaction with nitric acid to proceed slowly. The incoming group is directed to the meta position: $C_6H_5NO_2 + HNO_3 \xrightarrow{H_2SO_4} m\text{-dinitrobenzene}$

Class and exam questions

What else to study on this topic

• Acidic properties of benzenesulfonic acid and its interaction with sodium bicarbonate.
• Synthesis of cumene (isopropylbenzene) using 2-chloropropane as an alkylating agent.
• Obtaining styrene (vinylbenzene) via the dehydrogenation of ethylbenzene.
• Detailed analysis of inductive and mesomeric effects during the redistribution of π-electrons.

Other topics in "Chapter 15. Hydrocarbons"

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