Question

Describe the addition reaction of benzene with hydrogen and chlorine.

Short answer

Benzene doesn't favorably undergo addition reactions; it converts to cyclohexane with hydrogenation and performs substitution with chlorine.

Step-by-step solution

Understanding Benzene Structure

Benzene is a hydrocarbon with a chemical formula of \(C_6H_6\). It is composed of a six-carbon ring with alternating double bonds, known for its stability due to resonance.

The Addition Reaction Concept

Typically, an addition reaction involves adding atoms to an unsaturated molecule containing double or triple bonds. For benzene, an addition reaction would involve hydrogen or halogens adding across these double bonds.

Adding Hydrogen to Benzene (Hydrogenation)

To add hydrogen to benzene, a catalyst like platinum or nickel is required under high temperatures and pressures. This process converts benzene into cyclohexane, which doesn’t have any double bonds: \[ C_6H_6 + 3H_2 \Rightarrow C_6H_{12} \]

Attempting Addition with Chlorine

Normally, benzene does not undergo addition with chlorine directly due to its stability. Instead, an aromatic substitution occurs, forming chlorobenzene in the presence of a catalyst like aluminum chloride (AlCl₃). An actual addition reaction adding chlorine across benzene’s double bonds does not typically occur.

Key concepts

Benzene Structure

Benzene is a unique hydrocarbon with the molecular formula \(C_6H_6\). It is best described as a ring-shaped molecule composed of six carbon atoms, each bonded to one hydrogen atom. The fascinating part of benzene's structure is the arrangement of its carbon atoms which form a planar hexagonal shape.
Benzene's most distinctive characteristic, perhaps, is the presence of alternating double bonds, which in truth, are not typical single or double bonds. Instead, this "alternating" pattern represents a concept known as resonance.
Resonance means that benzene's electrons are delocalized over the entire ring.

• This electron delocalization imparts notable stability to benzene, a concept further solidifying its place as an aromatic compound.
• The remarkable stability that arises from resonance causes benzene to behave differently from many other hydrocarbons.

As a result, benzene often doesn’t react in ways we might first expect, such as in classic addition reactions.

Hydrogenation of Benzene

The hydrogenation of benzene refers to the process by which hydrogen molecules are added across the benzene ring, effectively saturating it. This process requires specific conditions to overcome benzene's inherent stability.
Hydrogenation is typically achieved using a catalyst, such as platinum or nickel, under high temperature and pressure. The reaction transforms benzene to cyclohexane, a saturated hydrocarbon, without double bonds:
\[ C_6H_6 + 3H_2 \Rightarrow C_6H_{12} \]
This reaction illustrates how hydrogen atoms add up over benzene's ring, fully saturating it, something only achievable under these rigorous conditions.

• This transformation changes not just the structure, but also the chemical properties of the compound.
• Instead of a stable aromatic system, cyclohexane acts as a typical alkane with no resonance stabilization.

The need for exhaustive conditions highlights benzene’s inherent resistance to change.

Aromatic Substitution

Benzene’s chemical sturdiness leads it to preferentially undergo reactions known as aromatic substitutions, rather than simple additions. An aromatic substitution is a type of reaction where one of the hydrogen atoms on the benzene ring is replaced, or substituted, by another atom or group of atoms.
For instance, when reacting with chlorine, benzene generally forms chlorobenzene, rather than the expected addition product. This takes place in the presence of a catalyst like aluminum chloride (AlCl₃).

• In this setup, the stable aromatic ring remains intact, with only one hydrogen atom being replaced.
• The preserved aromaticity is a key reason for this reaction taking precedence over addition reactions in benzene.

Understanding this concept is crucial, as it explains why benzene doesn’t react readily with certain agents in the way that its structure might initially suggest.

Addition Reaction Concepts

Addition reactions are fundamental chemical reactions where atoms or groups of atoms are added to a molecule, typically one with unsaturated bonds like double or triple bonds. In these cases, the double or triple bonds open up to allow new atoms to attach themselves to the original molecule.
While benzene features alternating double bonds, it does not typically undergo addition reactions like other alkenes or alkynes due to its resonance stability.

• In most instances, attempts to perform an addition reaction on benzene, such as with chlorine, fail because the reaction would disrupt the aromatic ring.
• Instead, the reaction shifts to a substitution pathway, preserving the ring's stability through aromatic substitution.

This deviation from traditional reactions underscores the important role of benzene’s unique structure in determining its reactivity patterns.