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Electrophilic substitution of Benzene occurs in a series of processes employing compounds with benzene rings - the arenas. Electrophilic aromatic substitutions include aromatic nitration, aromatic sulfonation, aromatic halogenation, and Friedel-Crafts reaction alkylation and acylation. Benzene has the molecular formula C6H6 and is a highly combustible chemical with a sweet odour. It is either a colourless or light yellow liquid which is found in a liquid state at room temperature and evaporates instantly when exposed into the air.
Key Terms: Electrophilic Substitution, Formation Of Carbocation, Deprotonation, Aromatic Compounds, Benzene
Electrophilic Substitution
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In chemistry, an electrophile is a chemical molecule that accepts an electron pair and thereby forms a bond with nucleophiles. When a hydrogen atom is displaced from a functional group, the constituent of a molecule initiates its major chemical processes, which are known as electrophilic substitution reactions in a compound.
Electrophilic Process and Properties of Aromatic Compounds
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Electrophilic aromatic substitution processes produce aromatic compounds, which are essential for adding functional aromatic ring groups. The other type of electrophilic replacement reaction is an electrophilic aliphatic substitution reaction.Electrophilic substitution reactions usually follow a three-step procedure that includes the steps below.
- An electrophile's appearance.
- A carbocation's appearance (which is intermediate).
- The reduction of a proton charge from an intermediates.
Electrophilic Substitution of Benzene
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Substitution reactions occur when the chemical reactivity of benzene is comparable to that of alkenes in the terms of priority for addition reactions. Because the reagents and parameters used in these reactions are electrophilic, they are referred to as electrophilic aromatic substitution.
The catalysts and co-reagents are utilized to generate the strong electrophilic species needed for the first substitution step. Experiments have revealed that benzene ring substituents can have a significant impact on reactivity. This stimulation or inactivation of the benzene ring against electrophilic substitution can be related to the electron-donors or acceptor’s impact of the substituents, as measured by molecular dipole moments.
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Mechanism of Electrophilic Substitution Reaction of Benzene
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Addition of electrophiles takes place in two-step process followed by deprotonation which is nothing but the mechanism of electrophilic aromatic substitution.An important element of this process is that we can define the electrophile if we know the product, which is the atom or group that substitutes the H+.
If we know the electrophile, we can predict the structure of the result. The catalyst's purpose is to form a bond with the departing group and help them leave in a better state. Substitution reactions of compounds with antagonistic substituent orientations are studied in greater depth.
If the substituents are similar, the molecule's symmetry will simplify the selection once more. The product determining power is commonly used when a substituent has a pairing of non-bonding electrons that can be used for neighboring charge stabilization. The three steps involved in the process of electrophilic substitution reaction are
Generation of Electrophiles
Anhydrous aluminium chloride is an especially valuable Lewis acid for generating electrophiles from the chlorination, alkylation, and acylation of an aromatic ring. The presence of Lewis acid causes the synthesis of electrophiles. The Lewis acid accepts the electron pair from the attacking reagent. The electrophiles that result are Cl+, R+, and RC+O, respectively (from the combination of anhydrous aluminium chloride and the attacking reagent).
Carbocation Formation (Formation of Arenium Ion)
The electrophile attacks the aromatic ring by producing a sigma complex or an arenium ion. sp3 is one of the hybridised carbons in this uranium ion. This arenium ion finds stability in a resonance configuration. Because electron delocalization ends at the sp3 hybridised carbon, the sigma complex or arenium ion loses its aromatic property.
Deprotonation Process (Elimination of positive charge from intermediate carbon)
The third stage in electrophilic substitution is deprotonation. Deprotonation is the reaction's driving force, allowing it to proceed energetically. The activation energy for this phase is substantially lower, and the reaction occurs very quickly.
Things to Remember
- Aromatic nitration, aromatic sulfonation, aromatic halogenation, and Friedel-Crafts reaction alkylation and acylation are all electrophilic aromatic substitutions.
- Electrophilic substitution reactions usually undergo three-step procedures: they are the appearance of electrophiles, carbocations, and reduction of proton charge from intermediates.
- The mechanism of electrophilic substitution reaction takes place in three steps: generation of electrophiles, formation of arenium ions, and deprotonation process.
Sample Questions
Ques. Write a note on Benzene Sulfonation reaction. (3 Marks)
Ans. Benzene sulfonation is the process of heating benzene with sulphuric acid (H2SO4 + SO3) to produce benzene-sulfonic acid. The reaction is completely reversible. The following reaction shows the sulfonation of benzene
Ques. Write down the reaction for nitration of benzene. (3 Marks)
Ans. The source of the nitronium ion causes the loss of a water molecule and the production of a nitronium ion by protonation of nitric acid with sulfuric acid. Here, In the presence of sulphuric acid, benzene combines with nitric acid to generate nitrobenzene around 323-333K. The following reaction shows the nitration of benzene
Ques. Write down the reaction for halogenation of benzene. (3 Marks)
Ans. Benzene interacts with halogens to generate aryl halides in the presence of Lewis acid, such as FeCl3, FeBr3. Benzene halogenation is the name for this reaction. The following reaction shows the halogenation of benzene
Ques. Write down the reaction for Sulfuric Acid Activation of Nitric Acid. (3 Marks)
Ans. The initial stage in benzene nitration is to stimulate HNO3 with sulfuric acid to produce a stronger electrophile, the nitronium ion. The following reaction shows the sulfuric acid activation of nitric acid
Ques. Write a short note on types of electrophilic substitution reaction. (3 Marks)
Ans. Organic molecules go through two kinds of electrophilic substitution reactions. They are as follows:
- Aromatic Substitution Reactions with Electrophiles
- Aliphatic Substitution Reactions with Electrophiles
Ques. Which of the following is most reactive towards electrophilic substitution reaction? (2 Marks)
A. Phenol
B. Anisole
C. Nitrobenzene
D. Benzene
Ans.A. The rate of electrophilic substitution reaction is directly proportional to the nucleophilicity of the Benzene ring. The group which increases the electron density in the benzene ring i.e. electron-donating group increases the nucleophilicity of the Benzene ring which results in an increase in the rate of reaction towards electrophilic substitution Reaction. In the series of activating groups −OH comes first then −OCH3, hence, phenol is most reactive towards electrophilic substitution reaction.
Ques. Arrange the following products in the order of the percent yield achieved from t-butylbenzene nitration. Justify their order. (3 Marks
Ans.
T-butylbenzene will direct to the ortho and para locations because alkyl groups are poor electron donors and hence modestly activating. The enormous size of the t-butyl group, on the other hand, will sterically restrict both ortho locations, favouring para substitution over ortho. As a result, iii > iii
Ques. What are the main products of phenyl ethanoate and ethyl benzoate reactions with HNO3/H2SO4/heat? (3 Marks)
Ans. Both of the starting components are ester, but the aromatic ring is linked to the carboxylate on opposing sides.
- The aromatic ring of ethyl benzoate is connected to a -CO2CH2CH3 group, which is an electron withdrawing group. As a result, the electrophilic aromatic substitution process at the meta site occurs, yielding 3-nitrophenyl ethanoate.
- The aromatic ring of phenyl ethanoate contains a -OC(=O)CH3 group connected to it. Because the -O- atom is an electron donor due to its lone pairs close to the ring, ortho- and para- substitution occurs. The para-product will benefit from the steric effects.
Ques. Polycyclic aromatics such as naphthalene and phenanthrene have higher resonance energies than benzene, which may explain why they are more reactive. (2 Marks)
Ans. Because naphthalene has a higher resonance energy (61 kcal/mol) than benzene (36 kcal/mol), one of the rings is less aromatic and thus more reactive.
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