Question

Which of the following products is formed when \(n\)-heptane is passed over \(\left(\mathrm{Al}_{2} \mathrm{O}_{3}+\mathrm{Cr}_{2} \mathrm{O}_{3}\right)\) catalyst at \(773 \mathrm{~K} ?\) (a) Benzene (b) Toluene (c) Polyheptane (d) Cycloheptane

Short answer

The product formed when n-heptane is passed over an \(\mathrm{Al}_2 \mathrm{O}_3 + \mathrm{Cr}_2 \mathrm{O}_3\) catalyst at 773 K is benzene.

Step-by-step solution

Understanding the process

The process described is known as catalytic reforming, where a straight-chain hydrocarbon like n-heptane is converted into a more complex one with a ring structure or branching. The catalysts mentioned, \(\mathrm{Al}_2 \mathrm{O}_3\) and \(\mathrm{Cr}_2 \mathrm{O}_3\), are used to facilitate such chemical reactions.

Identifying the product

When n-heptane is passed over \(\mathrm{Al}_2 \mathrm{O}_3\) and \(\mathrm{Cr}_2 \mathrm{O}_3\) catalyst at \(773 \mathrm{K}\), it undergoes dehydrogenation and cyclization. This converts the straight chain of n-heptane into a ring structure, forming an aromatic hydrocarbon due to the conditions favoring dehydrocyclization.

Recognize the probable outcome

n-heptane consists of 7 carbon atoms which allows it to form a six-carbon ring plus one hydrogen molecule, resulting in benzene, an aromatic hydrocarbon, upon the removal of one hydrogen atom from the chain due to dehydrocyclization. So the correct answer would be benzene.

Key concepts

n-Heptane Dehydrogenation

When discussing the transformation of n-heptane through dehydrogenation, we are looking at a chemical process that strips hydrogen atoms from the molecule. In the context of catalytic reforming, n-heptane is passed over a catalyst at high temperatures—in our case, 773 K—to form other, often more value-added hydrocarbons.

Through this process, the seven carbon atoms in n-heptane (C_{7}H_{16}) can undergo a rearrangement to form a new structure with a six-carbon ring, a process referred to as cyclization. The removal of hydrogen atoms, specifically, results in the formation of an aromatic hydrocarbon such as benzene (C_{6}H_{6}). This fundamental reaction is central to the petrochemical industry, as it helps to convert less valuable straight-chain hydrocarbons into more valuable, highly energetic, and versatile aromatic compounds.

Aromatic Hydrocarbon Synthesis

The synthesis of aromatic hydrocarbons is particularly important in the manufacturing of chemicals and fuels. In the case of the textbook problem, we see the formation of benzene from n-heptane, which involves both dehydrogenation and cyclization. The process starts with the linear n-heptane molecule and, through the application of heat and catalysts, forms a six-carbon cyclic structure characteristic of aromatics.

The resulting benzene molecule is a versatile building block in chemistry. It is not only used as a solvent but also serves as a precursor to a variety of chemicals, including plastics, resins, and synthetic fibers. Moreover, benzene itself can be further alkylated to produce other compounds, like toluene and xylenes, which have their applications in industry.

Catalyst Function in Organic Chemistry

In organic chemistry, catalysts play the role of facilitators in chemical reactions without being consumed in the process. They work by providing an alternative reaction pathway with a lower activation energy. For the conversion of n-heptane into benzene, the catalysts Al_{2}O_{3} and Cr_{2}O_{3} are employed to assist in breaking the original carbon-hydrogen bonds and forming new carbon-carbon bonds, vital for the cyclization to occur.

The function of these catalysts is twofold: they speed up reactions that would otherwise occur slowly or not at all, and they dictate the nature of the products formed. Without such catalysts, the production of many compounds like aromatic hydrocarbons would be much less efficient or economically unfeasible—all crucial elements when understanding the significant role catalysts hold within the realm of organic chemistry.