Content Curator
The common Benzene Reactions involve halogenation, sulfonation and nitration of benzene. Benzene is an organic compound which is made up of six carbon atoms arranged in a planar ring, each containing one hydrogen atom. The molecular formula of Benzene is C6H6 and was first discovered by Michael Faraday in the year 1825.
| Table of Content |
Keyterms: Benzene, halogenation, sulfonation, nitration, organic compound, carbon, atom, planar ring
Also Read: Types of Chemical Reactions
Benzene
[Click Here for Sample Questions]
- Benzene is classified as a hydrocarbon since it solely has carbon and hydrogen atoms.
- It is extremely reactive due to its high degree of unsaturation.
- In the majority of its reactions, hydrogen atoms are substituted by another atom or radical to form benzene.
- Aromatic chemicals include benzene.
- The term aromatic was originally used to describe benzene and its derivatives because of its various aromas or odours. Later, benzene was classified based on its structure and chemical reactivity, rather than its fragrance.
- Aromatic compounds are currently used to categorise compounds that are highly unsaturated and unusually stable in the presence of chemicals that aggressively react with alkenes.
Benzene
Structure of Benzene
[Click Here for Sample Questions]
- A six-carbon ring (shown by a hexagon) with three double bonds is the most common structural depiction for benzene.
- Each of the carbons in a corner is linked to at least one hydrogen atom.
- In benzene, single bonds divide the double bonds; due to this its configuration is known as conjugated double bonds.
- To symbolise the six pi electrons, one alternate sign that is a circle inside the hexagon is also used.
Reactions of Benzene
[Click Here for Sample Questions]
Electrophilic Substitution Reaction of Benzene
Electrophilic substitution reactions are more common in benzene than addition reactions because it loses its aromaticity during addition reactions.
Electrophiles are attracted to benzene because it includes delocalized electrons that cross carbon atoms in the ring. It is also very stable to electrophilic replacements. In general, the electrophilic substitution reaction of benzene consists of three steps:
- The electrophile is created.
- Intermediate carbocation formation.
- A proton is removed from the carbocation intermediate.
Electrophilic Substitution Reaction of Benzene
Halogenation of Benzene
- In the presence of Lewis acids, such as FeCl3 and FeBr3, benzene interacts with halogens to produce aryl halides.
- An electrophilic aromatic substitution reaction replaces one hydrogen atom of an arene with a halogen atom.
- Lewis acid contains mostly nonbonding electrons, making it merely an electron pair acceptor.
Chlorination of Benzene
Mechanism of Bromination of Benzene
Mechanism of Bromination of Benzene
Nitration of Benzene
[Click Here for Sample Questions]
- In the nitration of benzene, concentrated nitric acid and concentrated sulfuric acid are used to treat benzene at a temperature not exceeding 50°C.
- As the temperature rises, there is a greater possibility of generating several nitro groups like -NO2, which are replaced onto the ring and lead to the creation of Nitrobenzene.
- In this process, concentrated sulfuric acid acts as a catalyst.
- The interaction between nitric acid and sulphuric acid produces "nitronium ion" or "nitrol cation," NO+2.
Nitration of Benzene
Mechanism
Step 1: Nitric acid takes a proton from sulphuric acid and dissociates to create the nitronium ion.
Step 2: In the procedure, the nitronium ion functions as an electrophile, reacting with benzene to create the arenium ion.
Step 3: The arenium ion loses its proton to the Lewis base, resulting in the formation of nitrobenzene.
Sulfonation of Benzene
[Click Here for Sample Questions]
- The electrophilic substitution reaction between benzene and sulfuric acid is known as the sulfonation of benzene.
- This procedure includes heating benzene at 40°C for many hours under the influence of strong fuming sulfuric acid.
- Benzenesulfonic acid is the end product of this reaction.
- Sulphuric acid's oxygen attracts an electron to itself due to its greater electronegativity, resulting in an electrophile.
- The benzene ring is attacked, resulting in the production of benzene sulphonic acid.
- Sulfur trioxide, or SO3, is the electrophile in this case. The sulphur trioxide electrophile can be produced in either of two ways, depending on the type of acid used. It may be made by dissociating concentrated sulfuric acid with traces of SO3 to generate it.
H2SO4 → H2O + SO3
- H2S2O7, or fuming sulfuric acid, may be thought of as an SO3 solution in sulfuric acid, making it a considerably richer source of SO3. Because it is a highly polar molecule with a considerable amount of positive charge on the sulphur atom, sulphur trioxide is electrophilic in nature. It is drawn to the ring electrons by this.
Sulfonation of Benzene
Mechanism of Sulfonation of Benzene
Mechanism of Sulfonation of Benzene
Also Read: Chemical Reactions of Haloalkane and Haloarene
Alkylation and Acylation of Benzene
[Click Here for Sample Questions]
Friedel-Crafts Alkylation
- Alkylbenzenes are produced through the Friedel-Crafts Alkylation process from alkyl halides.
- This reaction has limited use because it might be difficult to halt the reaction at a single alkylation.
- In Step 1, a carbocation intermediate is formed, which opens the door to carbocation rearrangements.
Friedel-Crafts Alkylation
Mechanism
- To produce the electrophile, a carbocation is formed. The haloalkane is activated by these processes. In this phase, only secondary and tertiary halides form the free carbocation.
- The resonance stabilised alkylbenzene ion is formed when the pi electrons from benzene react with the electrophile.
- Lewis Base interacts with the alkyl benzene ion to take up the hydrogen and rebuild the aromatic ring.
The two products are illustrated in the last step above.
As you advance up the periodic table and increase polarity, the reactivity of haloalkanes rises. This indicates that the most reactive haloalkane is RF, followed by RCl, RBr, and lastly RI. This means that the Lewis acids employed as catalysts in Friedel-Crafts Alkylation processes tend to contain similar halogen combinations, such as BF3, SbCl5, AlCl3, SbCl5, and AlBr3, which are all widely used.
Friedel-Crafts Acylation
Friedel-Crafts Acylation requires an extra reaction step. The product's acyl group forms a compound with aluminium chloride. Water is used to isolate the acyl benzene end product.
Friedel-Crafts Acylation
Mechanism
Step 1: The production of acylium ions is the first step.
Step 2: Benzene pi electrons react with the acylium ion to create the sigma complex, which is resonance stabilised acyl benzenium intermediate:
Step 3: Restoring aromaticity by deprotonation of the sigma compound.
AlCl4 returns in the third stage to remove a proton from the benzene ring, allowing the ring to revert to aromaticity. The initial AlCl3 and HCl are both recreated as a result of this process. Most crucially, we have the first half of the reaction's end result, a ketone. As illustrated below, the product forms a complex with aluminium chloride.
2nd Reaction: Water is introduced in the second reaction to release the end product as acyl benzene:
Also Read:
| Related Articles | ||
|---|---|---|
| Iodoform Test | Fehling Test | Clemmensen Reduction |
| Transesterification | Aldol condensation | Cannizzaro Reaction |
Nucleophilic Aromatic Substitution
[Click Here for Sample Questions]
A nucleophilic aromatic substitution occurs when a strong leaving group, such as a halide, is relocated on an aromatic ring by the nucleophile. This response is mostly governed by addition-elimination reaction mechanisms.
The essential concept of this reaction is that the substituted H atoms leave in the form of a proton, formally the electrons in the C-H bond are left behind and the bonding with the electron-deficient electrophile must be completed.
Nucleophilic Aromatic Substitution
Mechanism
When a nucleophile fails to replace hydrogen, the electrons must also leave, necessitating the use of a more typical leaving group such as halide. Electrons have a difficult time separating themselves from groupings.
Things To Remember
- Benzene is an organic compound which is made up of six carbon atoms arranged in a planar ring, each containing one hydrogen atom.
- Electrophilic substitution reactions are more common in benzene than addition reactions because it loses its aromaticity during addition reactions.
- In electrophilic substitution reaction, electrophiles are attracted to benzene because it includes delocalized electrons that cross carbon atoms in the ring.
- In halogenation of Benzene, the benzene ring reacts with halogens in the presence of Lewis acids to produce aryl halides.
- A nucleophilic aromatic substitution occurs when a strong leaving group, such as a halide, is relocated on an aromatic ring by the nucleophile. This reaction is carried out by the addition-elimination reaction mechanisms.
Also Read: Elimination Reaction Mechanism
Important Questions
Ques: What is a substitution reaction? Give an example of the substitution reaction of benzene. (2 Marks)
Ans: A substitution reaction involves replacing an atom, ion, or group of atoms or ions in a molecule with another atom, ion, or group.
For example:
Substitution reaction
In the presence of Lewis acids, such as FeCl3 and FeBr3, benzene interacts with halogens to produce aryl halides. An electrophilic aromatic substitution reaction replaces one hydrogen atom of an arene with a halogen atom.
Ques: Why is it important that the nitration of benzene by nitric acid occurs in the presence of sulfuric acid? (3 Marks)
Ans: Sulfuric acid is required for the formation of a good electrophile. The nitronium ion is formed when sulfuric acid protonates the nitric acid (water molecule is lost). The nitronium ion is an excellent electrophile and is susceptible to benzene attack. The reaction would not take place if sulfuric acid was not present.
Ques: What are the limitations of Friedel-Crafts alkylations? (3 Marks)
Ans: Limitations of Friedel-Crafts Alkylations:
- Carbocations cannot be formed with vinyl or aryl halides.
- The aromatic substrate must not include any strongly deactivating groups, such as NH2, NHR, or NR2, that form complexes with the Lewis acid catalyst and become severely deactivated as a result.
- Polyalkylation is a problem that can be solved by utilising a lot of aromatic substrates.
- Any reaction involving a carbocation can result in carbocation rearrangements.
Ques: Write the addition elimination mechanism of nucleophilic aromatic substitution reaction? (5 Marks)
Ans: A nucleophilic aromatic substitution occurs when a strong leaving group, such as a halide, is relocated on an aromatic ring by the nucleophile.
In this reaction, the substituted H atoms leave in the form of a proton and the electrons in the C-H bond are left behind. The bonding with the electron-deficient electrophile must be completed to carry out this reaction.
When a nucleophile fails to replace hydrogen, the electrons must also leave, necessitating the use of a more typical leaving group such as halide.
Ques: What is the mechanism of bromination of Benzene? (5 Marks)
Ans: Mechanism of bromination of Benzene:
Step 1: Bromine combines with the Lewis acid to generate a compound that increases the electrophilicity of bromine.
Step 2: The π electrons of the aromatic C=C act as a nucleophile, attacking the electrophilic Br, and displacing iron tetrabromide. This step destroys the aromaticity giving the cyclohexadienyl cation intermediate.
Step 3: The active catalyst is regenerated by removing the proton from the sp3 C carrying the Bromo- group, which reforms the C=C and aromatic system, creating HBr and regenerating the active catalyst.
Ques: Write a short note on the mechanism of nitration of benzene. (5 Marks)
Ans: The source of the nitronium ion is through the protonation of nitric acid by sulfuric acid, which causes the loss of a water molecule and the formation of a nitronium ion.
Step 1: Nitric acid takes a proton from sulphuric acid and dissociates to create the nitronium ion.
Step 2: In the procedure, the nitronium ion functions as an electrophile, reacting with benzene to create the arenium ion.
Step 3: The arenium ion loses its proton to the Lewis base, resulting in the formation of nitrobenzene.
Ques: Write a detailed mechanism for the sulfonation of benzene, including all resonance forms. (5 Marks)
Ans: Sulfonation is a reversible process in which sulphur trioxide and sulfuric acid are combined to form benzenesulfonic acid.
- To create benzene, the process is reversed by adding hot aqueous acid to benzenesulfonic acid.
- Fumigating sulfuric acid and sulphur trioxide are used to make benzenesulfonic acid from benzene.
- A concentrated solution of dissolved sulphur trioxide in sulfuric acid is known as fuming sulfuric acid, sometimes known as oleum.
- As oxygen is relatively electronegative, the oxygens in sulphur trioxide draw electrons away from it, making the sulphur electrophilic.
- To make benzenesulfonic acid, benzene attacks sulphur (and subsequent proton transfers occur).
Ques: What are EDG and EWG? Write the difference between the two. (5 Marks)
Ans: EDG: Electron-donating groups are known as EDG. They are also referred to as electron releasing groups (ERG). These are organic compound substituents that can give a part of their electron density to a conjugated pi system. This is accomplished by the resonance or inductive effect. This increases the nucleophilicity of the pi-electron system. When EDG is linked to a benzene ring, for example, the benzene ring undergoes electrophilic substitution processes. This is because the EDG boosts the benzene ring's electron density.
Electron-withdrawing groups are abbreviated as EWG. On an aromatic ring, it exerts the opposite action as EDG. As a result, electron density is removed from a pi-electron system. The pi-electron system becomes more electrophilic as a result of this. As a response, when these groups connect to benzene rings, they slow down electrophilic substitution processes. EWG may also deactivate aromatic rings. This is accomplished by the resonance or inductive withdrawal effect. These groups can reduce the nucleophilicity of the ortho and para locations in benzene. As a result, in meta locations, the benzene ring is prone to electrophilic addition processes. Some examples of EWG include trihalides, sulfonates, ammonium, aldehydes, ketones, esters, etc.
For Latest Updates on Upcoming Board Exams, Click Here: https://t.me/class_10_12_board_updates
Check-Out:
Comments