Question

Both benzene \(\left(\mathrm{C}_{6} \mathrm{H}_{6}\right)\) and ethene \(\left(\mathrm{C}_{2} \mathrm{H}_{4}\right)\) contain olefinic double bonds. But benzene often undergoes aromatic electrophilic substitution and not addition, while ethylene often undergoes electrophilic addition and not substitution. Why?

Short answer

Answer: Benzene and ethene exhibit different types of reactions due to their distinct electronic structures. Benzene's delocalized π electron cloud provides stability through resonance, making it less reactive towards electrophilic attacks, and undergoes Electrophilic Aromatic Substitution to preserve its aromaticity. In contrast, ethene's localized π electrons in the double bond make it more susceptible to electrophilic attacks, and it undergoes Electrophilic Addition reactions.

Step-by-step solution

Understanding molecular structures and bonding in benzene and ethene

Benzene (C6H6) is an aromatic hydrocarbon, consisting of a planar ring of six carbon atoms, with each carbon sp2 hybridized, and each attached to a hydrogen atom. The double bonds between carbon atoms are not localized; instead, their electrons are delocalized, forming a π electron cloud above and below the ring. This delocalization creates a stable aromatic system known as a resonance hybrid. Ethene (C2H4), on the other hand, is a simple alkene consisting of two double-bonded carbon atoms, each attached to two hydrogen atoms. The double bond between the carbon atoms consists of a σ bond formed by the overlap of two sp2 orbitals and a π bond formed by the side-to-side overlap of two unhybridized p orbitals.

Electrophilic Aromatic Substitution in Benzene

Electrophilic Aromatic Substitution (EAS) is a reaction in which benzene reacts with an electrophile, resulting in the replacement of one hydrogen atom on the ring with the electrophile. This process occurs through the formation of a resonance-stabilized intermediate (sigma complex), which then deprotonates to restore the aromaticity. The driving force for EAS is the high stability of the aromatic ring; thus, reactions that preserve the aromaticity are favored.

Electrophilic Addition in Ethene

Ethene can undergo Electrophilic Addition reaction, whereby an electrophile adds to the double bond, generating a more saturated molecule with a single bond. In this process, the electrophile attacks the π electrons of the double bond, forming a carbocation intermediate, which then reacts with a nucleophile. In this case, the reactivity is due to the presence of the π electrons located in the double bond.

Comparison of reactivity between benzene and ethene

Benzene and ethene differ in their reactivity due to their electronic structures. The delocalized π electron cloud in benzene provides stability through resonance, making it less reactive than ethene towards electrophilic attacks. Consequently, benzene undergoes EAS rather than electrophilic addition, as the substitution preserves the aromaticity. On the other hand, ethene's π electrons in the double bond are more exposed, making them more susceptible to electrophilic attacks. Hence, ethene undergoes electrophilic addition reactions rather than substitutions. In summary, benzene undergoes Electrophilic Aromatic Substitution to preserve its resonance-stabilized aromaticicity, whereas ethene undergoes Electrophilic Addition due to the reactivity of its localized π electrons in the double bond.

Key concepts

Benzene Chemical Structure

Benzene, denoted as \( \mathrm{C}_6\mathrm{H}_6 \), is a fundamental organic compound that has captivated chemists for years due to its unique structure and bonding. Envision benzene as a flat hexagonal ring where each corner represents a carbon atom connected to a hydrogen atom.

What sets benzene apart from other hydrocarbons is the nature of the bonds between its carbon atoms. Unlike chains or rings with single or double bonds, benzene's carbon atoms are linked by bonds that are neither strictly single nor double. This phenomenon arises because the electrons that would form a double bond are not confined to the space between just two carbon atoms; they are delocalized around the entire ring. This forms the \(\text{π} \) electron cloud, which imparts exceptional stability to the molecule—a concept referred to as resonance stability.

Such a setup contributes to benzene's reluctance to undergo reactions that would disrupt this harmonious electron arrangement, like addition reactions that would break the ring's symmetry.

Ethene Reactivity

Ethene, with the formula \( \mathrm{C}_2\mathrm{H}_4 \), is a simple molecule featuring a double bond between its two carbon atoms. This molecule's reactivity fundamentally differs from that of benzene due to its structure.

In ethene, the double bond consists of one \(\text{σ} \) bond and one \(\text{π} \) bond, formed by direct overlap and side-to-side overlap of orbitals, respectively. The presence of an exposed \(\text{π} \) bond makes ethene far more reactive than benzene. This reactive \(\text{π} \) bond is exactly what attracts electrophiles, which can add to the carbon atoms, thus transforming the double bond into a single bond. This process is energetically favorable and results in diverse products, depending on the electrophile involved.

Delocalized Pi Electrons

Delocalized \(\text{π} \) electrons are electrons in a molecule that are not confined to a single atom or a bond between two atoms. In benzene, these electrons are spread out evenly across the ring structure, hovering above and below the plane of carbon atoms like a cloud of negative charge.

This electron cloud encompasses the entire ring, providing energy stabilization to the molecule by allowing the electrons to interact with many nuclei rather than just two. This delocalization also contributes to the overall resilience of benzene to chemically react, especially in ways that would break apart its stable aromatic system.

Pi Electron Cloud

The \(\text{π} \) electron cloud in benzene is a critical feature that imparts unique chemical properties to the molecule. Unlike localized electrons that are restricted to a specific area within a molecule, the \(\text{π} \) electron cloud in benzene is a shared pool of electrons that extend in a synchronized dance above and below the plane of the ring.

This continuous motion and dispersal of electrons over the entire ring increases benzene's stability significantly. The electron cloud also contributes to the molecule’s ability to absorb light at specific wavelengths, which is why benzene and other aromatic compounds have characteristic ultraviolet-visible (UV-Vis) spectra, a property exploited in spectroscopic analysis.

Resonance Stability

Resonance stability is a concept that describes the increased stability of molecules, like benzene, due to the delocalization of \(\text{π} \) electrons. It is often illustrated using resonance forms, which are different Lewis structures that represent the same molecule.

In the case of benzene, resonance stability is conferred by the overlap of p-orbitals, creating a conjugated system where electrons can move freely. This electron delocalization leads to a lower overall energy for the molecule, making it more stable than it would be if its \(\text{π} \) electrons were localized. This stability plays a key role in dictating the types of chemical reactions benzene will engage in, favoring those that preserve this stable electron configuration, like electrophilic aromatic substitution, over those that would disrupt it, such as addition reactions.