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Aromatization of Hydrocarbons - Definition, Synthesis Methods, Practice Problems and FAQs

Aromatization of Hydrocarbons - Definition, Synthesis Methods, Practice Problems and FAQs

The most stable organic aromatic homocyclic compound is benzene, as we all know. right????

However, did you know that benzene, a by-product of the burning of tobacco in cigarettes, can reduce red blood cell production and harm bone marrow, putting patients at risk for excessive bleeding. Cancer risk is also raised by exposure to benzene. Therefore, producing benzene via this approach is not very beneficial because it puts our lives in danger.

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Let’s Learn various techniques to produce benzene and its derivative from hydrocarbons!

Table of Contents:

  • Aromatization of hydrocarbons
  • Berthelot Synthesis of Benzene
  • By Catalytic Dehydrogenation
  • Bergman Cyclisation
  • By acyclic alkanes using Al2O3 or Cr2O3
  • Practice Problem
  • Frequently Asked Questions

Aromatization of hydrocarbons:

Let us now look at what aromatization signifies.

Aromatization is a chemical reaction that results in forming an aromatic system from a nonaromatic starting material. Aromatization is usually achieved through the dehydrogenation of existing cyclic compounds, which is demonstrated in the below-given topics. Aromatization is the process by which homocyclic and heterocyclic systems are formed.

There are many methods by which aromatization can take place which is given below:

  1. Berthelot Synthesis of Benzene
  2. By Catalytic Dehydrogenation
  3. Bergman Cyclisation
  4. By acyclic alkanes using Al2Oor Cr2O3

Berthelot Synthesis of Benzene:

Scientist Berthelot discovered that acetylene produced benzene as it passed through a heated tube. This is an illustration of addition polymerization. Cyclic polymerization can be another name for this process. The reaction needs to be carried out at a temperature of 873K. Three ethyne molecules combine to form benzene through cyclic polymerization.

By Catalytic Dehydrogenation:

The conversion of aromatics from naphthenes (cycloalkanes) through dehydrogenation is one of the main reforming processes, which is catalyzed by platinum at a high temperature.

In this given reaction, Methylcyclopentane is dehydrogenated to benzene resulting in the aromatization process.

Dehydrogenation of cyclohexane, cyclohexene, or cyclohexadiene and each of their corresponding derivatives is used to create benzene derivatives.

The same approach probably applies to the synthesis of condensed aromatic compounds like naphthalene.

Bergman Cyclisation:

Alkynes are among the acyclic precursors that are most sensitive to aromatizations because they have undergone partial dehydrogenation. When an enediyne is heated in the presence of an appropriate hydrogen donor, a rearrangement reaction known as the Masamune-Bergman cycloaromatization occurs. A derivative of benzene is the reaction's end product.

A heat process called pyrolysis takes place during the reaction (over 200 °C), creating a para-benzyne biradical species that is both short-lived and extremely reactive. Any hydrogen donor, such as 1,4-cyclohexadiene, will react with it to produce benzene.

Reaction:

Mechanism:

When the enyne moiety is introduced into a 10-membered hydrocarbon ring, the reaction can occur at a much lower temperature of 37 °C due to enhanced ring strain in the reactant.

By acyclic alkanes using Al2O3 or Cr2O3:

The n-alkane is initially treated with aluminium oxide or chromium (III) oxide in the process of aromatization. Using this method produces a cycloalkane. After going through dehydrogenation and aromatization, the cyclohexane formed in the first stage becomes benzene, which is the end product.

The reaction of aromatization of n - hexane to yield benzene is given below:

 

Practice Problem:

Q1. What should be the expected product when Propyne is passed through a Red hot tube at 873 K?

a. Benzene
b. Toulene
c. Mesitylene
d. Xylene

Answer: C

Solution: This method is Berthelot synthesis used for the formation of benzene and its derivatives by passing alkyne through the red hot tube at high temperatures. When propyne is passed through a red hot tube at 873 K, there is a formation of 1,3,5-trimethyl benzene which has a common name of Mesitylene. Hence, the correct answer should be an option (C).

Q2. What should be expected intermediate in the given reaction?

a. Methylcyclohexane
b. Cycloheptane
c. Methylbenzene
d. Heptene

Answer: A

Solution: Aromatization occurs when n-alkane is treated with aluminium oxide or chromium (III) oxide and then heated. This reaction creates methylcyclohexane by treating n-heptane with aluminium oxide or chromium (III) oxide. The first-stage product, methylcyclohexane, undergoes dehydrogenation and aromatization to become the final product, methylbenzene.

Hence, the correct option should be an option (A).

Q3. Predict products A, B and C

a. Cis-Octa-4-en-2,6-diyne, Trans-Octa-4-en-2,6-diyne, 1,2-dimethyl benzene
b. Trans-Octa-6-en-2,4-diyne, Cis-Octa-6-en-2,4-diyne, 1,4-dimethyl benzene
c. Cis-Octa-4-en-2,4-diyne, Trans-Octa-4-en-2,4-diyne, 1,4-dimethyl benzene
d. Trans-Octa-4-en-2,6-diyne, Cis-Octa-4-en-2,6-diyne, 1,2-dimethyl benzene

Answer: D

Solution: In this reaction, When the reactant is treated with Alcoholic KOH, dehydrohalogenation occurs, the product formed is Trans-Octa-4-en-2,6-diyne which is product (A) which is isomerise to form Cis-Octa-4-en-2,6-diyne that is product (B). The product (B) is enediyne. A rearrangement reaction called the Masamune-Bergman cycloaromatization takes place when an enediyne is heated in the presence of a suitable hydrogen donor. The result of the reaction is a benzene derivative. So, the product (C) is 1,2-dimethylbenzene. So, the correct answer is an option (D).

Q4. What will be the expected product formed with cycloheptane treated with Platinum at 873K?

a. MethylCyclohexene
b. MethylCyclohexa-1,3-diene
c. MethylCyclohexa-1,3,5-triene
d. Heptene

Answer: C

Solution: This is one of the main reforming processes of aromatization in which dehydrogenation of cycloalkanes is catalyzed by platinum at high temperatures. So, when cycloheptane is treated with Platinum at 873 K, the product formed should be an option (C) Methylcyclohexa-1,3,5-triene.

Frequently Asked Questions-FAQs:

1. Describe aromaticity.
Answer:
Any planar system that exhibits complete delocalization of the pi-electron in the ring and has (4n+2) pi electrons in the ring is said to be aromatic. The hybridisation of each atom present in the system should be sp or sp2 . This rule is known as the Huckel-rule. Benzene, naphthalene, anthracene, etc. are some examples of aromatic compounds.

2. What exactly are aromatization steroids?
Answer:
Most often, aromatization happens when there is a significant change in reproductive hormone synthesis, such as when anabolic steroids are injected. The body will see the high levels of the hormone as excess, and aromatization will take place in an effort to counterbalance the additional hormone with more estrogen.

3. Why does the benzene ring form?
Answer:
The hexagon formed by the six carbon atoms is absolutely regular. The length of each carbon-carbon bond is exactly the same, falling between single and double bonds. Delocalized electrons are present both above and below the ring's plane. Benzene is especially stable due to the delocalized electrons present, Hence it is formed.

4. Why is benzene referred to as an aromatic hydrocarbon?
Answer:
Because of their distinct scents, benzene-like compounds were previously referred to as aromatic hydrocarbons. Any substance that has a benzene ring or exhibits certain benzene-like characteristics is now considered to be an aromatic chemical (but not necessarily a strong aroma).

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