Question

Both ethylene \(\left(\mathrm{C}_{2} \mathrm{H}_{4}\right)\) and benzene \(\left(\mathrm{C}_{6} \mathrm{H}_{6}\right)\) contain the \(\mathrm{C}=\mathrm{C}\) bond. The reactivity of ethylene is greater than that of benzene. For example, ethylene readily reacts with molecular bromine, whereas benzene is normally quite inert toward molecular bromine and many other compounds. Explain this difference in reactivity.

Short answer

Ethylene is more reactive due to localized electrons, while benzene's delocalized electrons stabilize it, reducing reactivity.

Step-by-step solution

Identify the bond structure

Ethylene (\(\mathrm{C}_{2} \mathrm{H}_{4}\)) has a simple double bond between two carbon atoms (\(\mathrm{C} = \mathrm{C}\)). Benzene (\(\mathrm{C}_{6} \mathrm{H}_{6}\)) has a ring structure with alternating single and double bonds, often represented as a resonance hybrid with delocalized electrons.

Compare electron localization

In ethylene, the electrons in the \(\mathrm{C} = \mathrm{C}\) bond are localized between two carbon atoms, making them more accessible for reactions. In benzene, the \(\mathrm{C} = \mathrm{C}\) bonds are part of an aromatic ring with delocalized electrons, which spread the electron density over the entire ring and stabilize the molecule, making it less reactive.

Evaluate stability and reaction likelihood

The localized electron density in ethylene's \(\mathrm{C} = \mathrm{C}\) bond is higher, increasing the likelihood of reacting with electrophiles like bromine. The delocalized electron cloud in benzene stabilizes the molecule and discourages reaction by lowering the energy of the electron cloud, thus needing special conditions or catalysts to react with bromine.

Key concepts

Ethylene Reactivity

Ethylene, a simple organic compound with the formula \(\mathrm{C}_2\mathrm{H}_4\), is known for its high reactivity. This is mainly due to its double bond, represented as \(\mathrm{C} = \mathrm{C}\), where the electrons are tightly held between the two carbon atoms. This localization of electron density means the electrons are more available to participate in chemical reactions.

When ethylene encounters an electrophile, such as bromine, it reacts readily because the localized electron cloud can easily be attacked. This characteristic makes ethylene an essential starting material in various chemical syntheses and polymerization reactions. It can transform into multiple valuable compounds, exemplifying the importance of electron density localization in determining reactivity.

Understanding ethylene's reactivity provides insight into why other compounds with similar structures also exhibit high reactivity, thereby acting as fundamental building blocks in organic chemistry.

Benzene Stability

Benzene, \(\mathrm{C}_6\mathrm{H}_6\), is a fascinating molecule because of its resonance stability. Unlike ethylene, benzene has a cyclic structure with alternating single and double bonds known collectively as a resonance hybrid. This means instead of any specific electrons being tied to a single bond, electrons are shared and delocalized across the entire carbon ring.

This delocalization contributes to benzene's stability, as it lowers the overall energy of the molecule. The electrons are continuously moving within the ring, distributing the potential reactivity evenly across all atoms. This unique property renders benzene less reactive compared to compounds with localized bonds, like ethylene.

Benzene's structural stability is key in its role as a staple in aromatic compounds, enabling it to persist under conditions where less stable compounds would readily react. The stability provided by the shared electron cloud allows benzene to be a cornerstone in the formation of more complex organic molecules.

Delocalized Electrons

The concept of delocalized electrons is fundamental in understanding the stability of aromatic compounds like benzene. Delocalization refers to the sharing of electrons across several atoms, rather than being confined to a pair of atoms as in localized bonds. This characteristic allows the electrons to spread their charge over a larger area, which stabilizes the molecule by decreasing electron repulsion.

In benzene, the electrons in the \(\mathrm{C} = \mathrm{C}\) bonds are not fixed between two carbon atoms but are shared across the entire ring. This electron movement creates an electron cloud above and below the plane of the carbon atoms, providing additional stability and reducing the molecule's reactivity to electrophiles.

Delocalized electrons are a powerful structural feature that explains why some organic compounds exhibit unexpected behaviors, especially their inertness to simple reactions. This property is fundamental in both synthetic applications and the understanding of organic chemistry principles.

Aromatic Compounds

Aromatic compounds are a class of molecules characterized by their circular, planar structures and the presence of delocalized electrons, which confer unusual stability. Benzene is the simplest aromatic compound, serving as a model for understanding how delocalization aids in lowering reactivity.

These compounds obey Huckel's rule, which states that only those with \(4n + 2\) \(\pi\) electrons exhibit aromaticity. This rule helps explain the tendency of these substances to maintain their structural integrity and resist reactions that might break the aromatic system.

The aromatic structure fundamentally changes the way these molecules interact in chemical processes. While they are less reactive towards additions, they often participate in substitution reactions that retain the aromatic ring. Comprehending aromatic compounds illustrates broader chemical concepts of stability and electron management, highlighting their importance in life sciences, pharmacology, and material engineering.