Benzene Reactions


Benzene is a colourless organic compound, although it resembles a pale-yellow liquid at room temp. It has the chemical formula $\mathrm{C_{6}H_{6}}$. Especially in comparison to addition reactions, it is very cooperative in electrophilic substitution reactions. This could be because benzene starts to lose its aromaticity throughout most of the addition reactions. Because it has a pair of delocalized electrons which spans all of the C atoms within the ring, it has a high binding affinity for electrophiles while also being extremely stable to electrophilic substitutions. Whenever benzene has been involved, the electrophilic aromatic substitution reaction includes 3 important stages. Such stages are as follows − Formation of the electrophile, production of the intermediate carbocation, as well as the elimination of the proton from its transitional carbocation to establish stabilisation. The substitution reactions have been categorised as benzene-specific reactions. Pharmacists started to comprehend the rising information of the overall composition of benzene but also how it may affect the chemical characteristics as well as derivatives just after the late 1930s. The following are the most frequent benzene reactions −

  • Nitration of Benzene

  • Sulfonation of Benzene

  • Halogenation of Benzene

Nitration of Benzene

It is usually a reaction of conc. $\mathrm{HNO_{3}}$ (Nitric acid) well as conc. $\mathrm{H_{2}SO_{4}}$ (sulfuric acid) at such a temp of not more than 50 degrees Celsius in the reaction nitration of benzene. Even as temp raises, there seems to be a greater possibility of producing with over 1 nitro group, $\mathrm{-NO_{2}}$, which is replaced over the ring as well as leads in the synthesis of nitrobenzene. Inside this process, the strong $\mathrm{H_{2}SO_{4}}$ functions as a catalyst. The "nitronium ion" as well as "nitryl cation," $\mathrm{NO_{2}^+}$, is indeed the electrophile in this case. This would be formed by the reaction of $\mathrm{HNO_{3}}$ with $\mathrm{H_{2}SO_{4}}$. Nitrobenzene is formed when benzene interacts with conc. $\mathrm{HNO_{3}}$ at 323 to 333k within the existence of intense $\mathrm{H_{2}SO_{4}}$. This one is referred to as benzene nitration.

The Mechanism for Nitration of Benzene

  • $\mathrm{HNO_{3}}$ separates the H atom from $\mathrm{H_{2}SO_{4}}$ to produce the $\mathrm{NO_{2}^+}$ ion.

  • The $\mathrm{NO_{2}^+}$ ion works as such an electrophile within the mechanism, interacting using $\mathrm{C_{6}H_{6}}$ to generate the arenium ion.

  • The H of the arenium ion is lost towards the Lewis base, leading to the manufacturing of $\mathrm{C_{6}H_{5}NO_{2}}$.

Sulfonation of Benzene

It refers to the electrophilic aromatic substitution reaction among $\mathrm{C_{6}H_{6}}$ as well as $\mathrm{H_{2}SO_{4}}$. This method involves heating benzene at 40 degrees Celcius for several hours underneath the presence of fuming $\mathrm{H_{2}SO_{4}}$. Such a reaction produces benzenesulfonic acid as a byproduct. Because of its higher electronegativity, the O atom in $\mathrm{H_{2}SO_{4}}$ binds an electron, culminating in an electrophile. When the organic compound is damaged, benzenesulfonic acid is formed. Throughout this situation, the electrophile is sulphur trioxide ($\mathrm{SO_{3}}$). Based on the nature of the acid involved, the $\mathrm{SO_{3}}$ electrophile could be generated in one of 2 methods. This might be produced by detaching conc. $\mathrm{H_{2}SO_{4}}$ using residues of sulphur trioxide. $\mathrm{H_{2}S_{2}O_{7}}$, also known as fuming sulfuric acid, is indeed a sulphur trioxide solution in $\mathrm{H_{2}SO_{4}}$, providing it with a much better supply of sulphur trioxide. It is highly electrophilic even though this seems to be a polar molecule with such a significant no. of positive charges here on the S atom. This attracts it to the ring electrons.

The Mechanism for Sulfonation of Benzene

The oxygen in $\mathrm{H_{2}SO_{4}}$ tends to attract an electron due to its enhanced electronegativity, leading to the synthesis of an electrophile. It reacts with the phenyl ring to form benzenesulfonic acid.

Halogenation of Benzene

Halogenation happens when an H atom within the benzene ring has been displaced by such a halogen atom (F, Cl, Br, and I). Lewis acids like $\mathrm{FeCl_{3}}$ as well as $\mathrm{AlCl_{3}}$ have been utilised as catalysts but also halogen carriers. This reaction takes place via an electrophilic substitution process.

The Mechanism for Halogenation of Benzene

  • Since $\mathrm{FeBr_{3}}$ is quite a Lewis acid, this works to develop electrophile Br ions, simply reacting using the attacking reagent.

  • The Br ion functions as such an electrophile throughout the reaction, interacting using $\mathrm{C_{6}H_{6}}$ to produce an arenium ion that subsequently changes to bromobenzene.


Since benzene only contains C as well as H atoms, it is categorised as a hydrocarbon. Along with its tremendous degree of unsaturation, it's indeed particularly reactive. To synthesise benzene, H atoms have been usually replaced with another atom as well as radical in most of its reactions. When $\mathrm{C_{6}H_{6}}$ reduces its aromaticity throughout addition reactions, electrophilic substitution reactions seem to be more prevalent. Sulfonation is often a reversible reaction that produces benzenesulfonic acid by combining $\mathrm{SO_{3}}$ with $\mathrm{H_{2}SO_{4}}$. The benzene ring interacts using halogens under the influence of Lewis acids to form aryl halides during the halogenation of benzene. Whenever a powerful withdrawing group, including a halide, is moved on a phenyl ring through the nucleophile, a nucleophilic radical substitution takes place. Such a reaction is accomplished through the addition-elimination reaction mechanisms.


Q1. What does benzene nitration perform?

Ans. Nitration adds N to a benzene ring, which can subsequently undergo substitution reactions. The $\mathrm{NO_{2}}$ group disables the ring. The N element in such a ring is extremely useful because it can function also as a directing group as well as a latent amino group.

Q2. Why is it necessary to use an abundance of one of its reactants?

Ans. To sustain the moderately rapid general rate of the reaction as well as to meet the stoichiometric criteria of the nitration reaction, one of its reactants is frequently generated in surplus.

Q3. Why could sulfonation be reversed?

Ans. The benzene sulfonation reaction is spontaneous. $\mathrm{SO_{3}}$ interacts quickly with water to form $\mathrm{H_{2}SO_{4}}$ as well as heat. As a result, by including heat with benzenesulfonic acid into $\mathrm{H_{2}SO_{4}}$ reverses the process.

Q4. Which molecule is highly susceptible to sulphonation?

Ans. Toluene has been the most easily sulphonated molecule among them since the substituent donates electrons (+ I effect), activating the ring structure for electrophilic aromatic substitution.

Q5. In benzene reactions, which position is often more reactive: ortho, para, or meta?

Ans. Ortho, as well as para coordinators, have units containing an O and N connected to such an aromatic ring because the oxygen, as well as nitrogen, may drive electrons further into the ring, rendering the ortho as well as para positions highly reactive but also stabilising the arenium ion which then develops. As a result, the ortho, as well as para constituents, develop quicker than the meta products.

Q6. In an electrophilic substitution reaction, which would be more reactive, $\mathrm{C_{6}H_{6}}$ versus $\mathrm{C_{6}H_{5}Cl}$

Ans. Because of the - I effect, $\mathrm{C_{6}H_{5}Cl}$ becomes less reactive than $\mathrm{C_{6}H_{6}}$ in electrophilic substitution reactions. Because Cl is an electron-withdrawing element, which disables the benzene ring as well as decreases the electron density also on the benzene ring, rendering the aromatic ring less reactive towards the forthcoming electrophile.

Q7. Why is benzene halogenation performed mostly in the dark?

Ans. Immediate halogenation requires darkness for electrophilic substitution to occur rather than free radical addition.

Updated on: 04-Apr-2023


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