Question

The symbol: is often used to represent benzene. It is also the structural formula of cyclohexatriene. Are benzene and cyclohexatriene the same substance? Explain.

Short answer

No, benzene and cyclohexatriene are not the same substance. Despite having the same molecular formula, their structures are different. Benzene is an aromatic compound with delocalized \(\pi\) electrons, whereas cyclohexatriene is non-aromatic with localized double bonds.

Step-by-step solution

Understand the Compounds

Begin by analyzing the structures of both benzene and cyclohexatriene. Benzene is a six-membered ring of carbon atoms, each bonded with one hydrogen atom, and contains conjugated double bonds. Cyclohexatriene, on the other hand, is a six-membered ring of carbon atoms with alternating single and double bonds.

Compare the Bonding

Compare the bonding in benzene and cyclohexatriene. Benzene actually has a resonance structure, which means the double bonds are delocalized and evenly distributed around the ring, leading to six equal bonds. In cyclohexatriene, there are three distinct double bonds and three single bonds.

Conclusion

Based on the analysis, it can be concluded that benzene and cyclohexatriene are not the same. Even though they have the same molecular formula, their chemical structures and bonding are different; benzene is aromatic with delocalized electrons, while cyclohexatriene is non-aromatic with localized electrons.

Key concepts

Chemical Structure

Understanding the chemical structure of a molecule is crucial to grasp its properties and behavior. Chemical structure determines the placement of atoms and the type of bonds between them. For benzene, the structure is more than a simple arrangement of atoms. Each carbon atom in benzene is bonded to two other carbons and a hydrogen atom, creating a hexagonal ring. However, the true nature of benzene's structure isn’t apparent until you consider the concept of resonance—it's actually a hybrid of multiple structures with delocalized electrons.

The textbook exercise hints at the importance of considering resonance when evaluating the structure of benzene, as opposed to the localized bonds in cyclohexatriene. The similar formulas of benzene and cyclohexatriene (both C6H6) can lead to confusion, so it’s vital to recognize that chemical structure involves more than just the atoms present; it’s about how those atoms are connected and the electron distribution within the molecule.

Resonance Structures

Resonance structures are a way to represent molecules with delocalized electrons that can't be adequately described by a single Lewis structure. In the case of benzene, the molecule is often depicted with alternating single and double bonds. However, this is just a simplified representation.

In reality, the electrons involved in the double bonds are not confined to two adjacent carbon atoms but are shared evenly across the whole carbon ring. This results in what is called resonance stabilization, where the actual molecular structure is a hybrid of the possible resonance forms. The textbook exercise illustrates that recognizing the resonance in benzene's structure is key to understanding its stability and unique characteristics, distinguishing it from non-aromatic compounds such as cyclohexatriene, which lack this stabilization.

Aromatic Compounds

Aromatic compounds, such as benzene, have a unique stability due to their ring-like structure and delocalized electrons. The term 'aromatic' originally referred to the distinct smells of these compounds, but now it's more closely associated with their chemical properties.

To be classified as aromatic, a compound must fulfill certain criteria, including having a planar ring structure, a certain number of delocalized π (pi) electrons, and must follow Hückel's rule—having 4n+2 π electrons, where n is a non-negative integer. Benzene, possessing 6 π electrons (where n=1), meets these requirements and exemplifies aromaticity. As the exercise solutions clarify, aromatic stability is absent in cyclohexatriene, which does not exhibit the same delocalization of electrons. This distinct difference markedly affects the chemical reactivity and physical properties of aromatic compounds.

Bond Delocalization

Bond delocalization is the phenomenon where electrons are shared among more than two atoms, creating a cloud of electrons that can move freely within a certain region of a molecule. This is a key feature of aromatic compounds and contributes to their enhanced stability and unique chemical behavior.

In benzene, rather than having localized double bonds, all six carbon atoms share the electrons equally, creating a delocalized electron system. This dispersal of electron density over the entire ring means that benzene doesn't react as typical alkenes do. The textbook exercise underscores the importance of this delocalization in defining benzene's identity, contrasting it with cyclohexatriene, where electrons are localized in distinct π bonds. Because of delocalization, benzene exhibits lower reactivity compared to other unsaturated hydrocarbons, and this affects how it participates in chemical reactions.