Question

When \(\mathrm{n}\) -hexane is passed over heated alumina or chromic oxide catalyst under pressure, the product formed is (a) Iso hexane (b) cyclohexane (c) butane and ethane (d) Benzene

Short answer

Answer: The product formed is cyclohexane.

Step-by-step solution

Understand the given reaction

We are given that n-hexane is passed over a heated alumina (Al2O3) or chromic oxide (Cr2O3) catalyst under pressure. These catalysts are typically used in dehydrogenation reactions. Dehydrogenation is a process where molecules lose hydrogen atoms and usually form a more unsaturated or cyclic compound.

Recognize the structure of n-hexane

n-Hexane is a straight-chain alkane with 6 carbon atoms. Its molecular formula is C6H14, and its structure is CH3-CH2-CH2-CH2-CH2-CH3.

Analyze the possible products of the reaction

We have four possible products: (a) Iso hexane: This would require the shifting of one hydrogen between carbon atoms, but this wouldn't lead to a more unsaturated compound. (b) Cyclohexane: This product would be formed if a carbon-carbon bond is made between the first and the last carbon of n-hexane, and if two hydrogen atoms are lost. This would be a logical outcome of a dehydrogenation reaction. (c) Butane and Ethane: These products would be formed if the n-hexane carbon chain breaks into two smaller alkanes. However, this reaction would not involve a loss of hydrogen atoms, and it would not be consistent with the dehydrogenation process. (d) Benzene: This product would be formed if n-hexane loses six hydrogen atoms and forms three new carbon-carbon double bonds. While possible, this reaction generally requires more severe conditions than that typically used in dehydrogenation reactions involving alumina or chromic oxide catalysts.

Determine the final product

Based on our analysis, the product likely formed when n-hexane is passed over alumina or chromic oxide catalyst under pressure is (b) cyclohexane. This product is a reasonable outcome of the dehydrogenation process and is consistent with the reaction conditions.

Key concepts

n-Hexane

n-Hexane is a simple, straight-chain alkane, which means it is a saturated hydrocarbon. It is composed of 6 carbon atoms arranged in a linear structure and holds a molecular formula of C6H14. In simpler terms, its structure can be visualized as CH3-CH2-CH2-CH2-CH2-CH3.
This compound is important in various chemical reactions, particularly in the context of catalytic transformations. Since it is a saturated molecule, reactions often aim to transform n-hexane into more complex or unsaturated compounds through processes like dehydrogenation.

Alumina Catalyst

Alumina, chemically represented as Al2O3, serves as an effective catalyst in many industrial processes. Its role in reactions involving n-hexane primarily revolves around facilitating transformations such as dehydrogenation.
Catalysts like alumina speed up reactions by providing an alternative pathway with a lower activation energy. This means that less energy is required to break and form bonds, making the reactions occur more easily under the given conditions.

• Alumina is valued for its thermal stability.
• It can withstand high temperatures, which is crucial for such reactions.

In context, when n-hexane is passed over a heated alumina catalyst, it can lead to the formation of cyclic compounds such as cyclohexane.

Chromic Oxide Catalyst

Chromic oxide, represented as Cr2O3, is another potent catalyst used in facilitating dehydrogenation reactions. Its effectiveness is notable in its ability to convert alkanes like n-hexane into cyclic compounds.
This catalyst is chosen for its properties that aid in dehydrogenation:

• It creates an environment where hydrogen atoms are more easily shed from the alkane.
• It supports the formation of a closed carbon ring, characteristic of products like cyclohexane.

Chromic oxide's robust nature under high temperatures makes it a suitable choice alongside alumina in driving these chemical transformations.

Cyclohexane Formation

Cyclohexane formation is a classic outcome in dehydrogenation reactions involving catalysts like alumina or chromic oxide. When n-hexane is subjected to these conditions, it undergoes structural changes that result in cyclization.
This process involves:

• Loss of hydrogen atoms – n-hexane must lose two hydrogen atoms.
• Formation of a new carbon-carbon bond – the first and last carbon atoms in n-hexane connect, creating a ring structure.

Cyclization transforms the linear n-hexane into a more stable, six-membered ring known as cyclohexane, which is an unsaturated hydrocarbon, adding both practical and industrial value to the reaction process.